The Chemistry of Photography

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UC-NRLF

272 BED

2w.s

REESE LIBRARY .

\

OF THE

UNIVERSITY OF CALIFORNIA.

-r

r

f

THE CHEMISTRY OF

PHOTOGRAPHY.

NATURE SERIES

THE CHEMISTRY OF

PHOTOGRAPHY

BY

RAPHAEL MELDOLA,

F.R.S,

&c

PROFESSOR OF CHEMISTRY IN THE FINSBURY TECHNICAL COLLEGE, CITY AND GUILDS OF LONDON INSTITUTE FOR THE ADVANCEMENT OF TECHNICAL EDUCATION.

MACMILLAN AND AND NEW YORK 1889

CO.

RICHARD CLAY AND SONS, LIMITED, LONDON AND BUNG AY.

PREFACE. THE

lectures

forming the subject of

this little

volume were delivered during the spring term of last

year as a special course

at

the

Finsbury

Technical College, and were addressed to a mixed audience,

of

composed

students

and

In selecting the present

practical photographers. topic, there has

chemical

been no attempt to impart instruc-

tion in the manipulative details of Photography,

since there are already before the public a large

number of

excellent

branch of the

art.

It

works is

mode

with

this

only with the chemical

principles underlying the

tures attempt to deal,

dealing

subject that these lec-

and

of treatment adopted

it

is

may

of convincing photographers

hoped that the

serve the purpose

how

essential

is

some

PREFACE.

vi

knowledge of Chemistry

for the successful carrying

on of

With

their operations.

of technical science,

is

it

intellectual discipline to

respect to students

always a most useful

have presented

in a sys-

tematic form the application of the general scientific

principles

which form the subjects of

every-day studies to

The

technology.

some

their

particular branch

of

interest evinced in the lectures,

both by students and practitioners, has encouraged

me

to offer

them

substantially the

in the present form,

which

same as that adopted

for

In the lecture theatre,

livery.

it

is

de-

of course, im-

is

possible to give authorities for all the statements

made, and such a

series of references

hampered the text

to an extent quite out of pro-

portion with the scope of the work. seen, however, that possible, to

whom

do

I

It

will

have endeavoured, as

be

far as

justice to all those investigators to

the subject

coveries.

would have

is

indebted

for

leading dis-

Free use has been made of the writings

of the late Robert Hunt, of Dr.

J.

M. Eder, Capt.

PREFACE. and

Abney,

Hauptmann

Pizzighelli.

acknowledge the assistance which

I

gladly

have derived

I

from the published works of these well-known authorities.

The

claim of Photography to be recognized as

a branch of Chemical Technology

is

now

generally

admitted, and systematic courses of instruction in this subject are given in

out the country. as though

it

The

this,

institutions through-

operator

who

treats his art

consisted simply of a miscellaneous

collection of recipes in

many

is

bound

to

become superseded

as in all other departments of scientific

industry, unless he has sufficient training in general principles to enable

changing conditions.

him It

to is

meet new and everto

be regretted that

purely scientific chemists have of late years shown a tendency

to neglect the Chemistry of Photo-

graphy as being an uninviting branch of science.

I

their

venture to hope that the present work

may

contribute towards convincing such workers

that

there

are

many

important problems

still

PREFACE.

viii

awaiting solution in this scientific interest

high

is

of research.

field

Its

by a

forcibly expressed

saying of the late Col. Russell's, that Photography

would be a most interesting subject were for the

it

not

pictures.

In order to facilitate the teaching of the scientific

number of ex-

principles of Photography, a

been introduced, hints

have

periments

performance of which are given

in the

for

the

appendix

to each lecture, the bracketed capitals in the text referring to these demonstrations.

that

the

directions

enable

to

explicit

possessed

of

manipulation large to

be

instructions

somewhat

any

to

as

described have

believed

sufficiently

demonstrator

in

photographic

successfully.

A

these experiments are believed

lecture

greater

found

is

lecture

them

repeat

have

be

skill

ordinary

number of

new

will

It

in

demonstrations,

and the

these cases been given

detail.

All

the

in

experiments

been repeatedly performed, and

they have been devised with the object of

illus-

PREFACE. trating the various points

ix

which they are intended

to elucidate with the simplest

the

readiest

great delicacy requiring

have

for this reason

arc light

All

way.

possible

appliances and in

much

experiments of

previous preparation

An

been excluded.

and lantern are indispensable

for

all

many

now such

of the demonstrations, but these are

very general accessories in

electric

properly-equipped

lecture-rooms that their existence has been taken for granted. I

have great pleasure

skilful assistance

out the

which

I

in

acknowledging the

have received

experiments from

Mr.

H.

T.

Lecture Demonstrator, and from Mr.

in carrying

J.

Norris,

M. Smith,

one of the Students of the Chemical Department,

who

kindly volunteered his services

illustrate the lectures.

FINSBURY TECHNICAL COLLEGE, February, 1889.

in

helping to

CONTENTS. LECTURE

I.

I. Photo-physical and Photo-chemical Photo-chemistry of Iron Salts, 12 ; of Copper, 17; Mercury, 18 ; Uranium, 19; Gold and Platinum Salts,

Definition

of

Change, 20.

Subject,

7.

Photo-reduction of

and Oxidation,

Appendix

to

Chromates,

Lecture

I.,

Dissociation,

24

;

30.

LECTURE v

21.

25.

II.

Photo- chemistry of the Silver Compounds, 35. Historical, 36. The Chloride ; Loss of Chlorine on exposure to Light, 38.

Composition of Darkened Product, 39. Analogy of Silver Haloids with Cuprous, Mercurous, and Thallous Haloids, 45.

The Bromide and

Iodide, 57.

the Photo-salts, 58.

Appendix

to Lecture II., 64.

Carey Lea's Experiments on

CONTENTS.

Theory of the Action of

LECTURE

III.

Sensitizers,

71.

The Colour Change

of

Haloids not a Quantitative Indication of their Relative Sensitiveness to Light, 74. Foundation of Modern

the

Silver

Photography, 82. Wedgwood, Davy, and Fox Talbot, 82. Joseph Nicephore Niepce, and Daguerre, 85. The Collodion

Wet

Collodion

Plate Process, 92.

Appendix

Emulsion Photography,

Collodion Emulsions, 105.

103.

The

IV.

113.

of

Sensitiveness

of

Gelatino-bromide Plates,

Recent Applications of Modern Dry

Appendix

Gelatine

Process of

Different Ripening, 121. Physical Aggregation of Silver Bromide, 127.

Emulsions, States

Plates, 95.

to Lecture III., 100.

LECTURE

Extreme

Dry

to

Lecture IV., 139.

LECTURE

V.

General notions of Development, 142.

Image, 146.

131.

Plates, 132.

The

The Latent

or Invisible

action of Sensitizers in different Processes,

150. Theory of Developers and Restrainers, 153. Development by Vapours, 156. Acid Development by Ferrous Sul-

Alkaline Development by Ammonium Pyrogallate, and by Ferrous Oxalate, 166. phate, 158.

Appendix

to Lecture V.,

177.

CONTENTS.

LECTURE

VI.

Rapidity of Emulsion Plates partly due to Developer, 134. Ferrous Oxalate Development, 186. Collateral Phenomena of Develop-

The Developer a Quantitative Microchemical Test, Development of Chemical Reduction Product and of

ment, 189. 189.

Pressure Marks,

Relative

191.

Sensitiveness

Haloids in Different Processes, 192.

Image due the

Image

one,

to Reversed

Chemical Change,

or Solarisation, 208.

of the

Silver

Relapse of the Invisible 197.

Reversal of

The Phenomenon a Recurrent

Theory of the Process, and the Function of the

214.

Sensitizer, 219.

Appendix

to Lecture VI., 231.

LECTURE Composition of \Vhite Light, 236. Intensity

and Photographic

Objects, 246.

VII.

The Spectrum,

Intensity, 243.

238.

Visual

Colours of Natural

Graphic RepreAction of the Spectrum on the Concurrent Reversal in Mixtures, 259.

Spectrum Photography, 250.

sentation of Intensity, 252. Silver Haloids, 253.

Reversal in the Spectrum under various Conditions, 267. Solutions of Oxidizing Substances and other

Reversal in

Reversing Agents, 272.

Appendix

to Lecture VII., 285.

LECTURE

VIII.

and Absorption Relationship between Photochemical Decomposition of Light, 288. Draper's Law, 290 Effect of Molecular AggrePhotogation upon Sensitiveness of Silver Haloids, 293. graphy of Red and Infra-red Rays, 294.

Physical Analogy

CONTENTS. between Emission Spectra and Photographed Spectra, 300.

f I

^photochemical Action probably Intramolecular, 302. Effects^ of Dyes on Local Sensitiveness, 305. Orthochromatic PhotoI

Appendix

to Lecture VIII., 327.

LECTURE J^ost-developmental Processes, 330.

/

^/

Photochromy, 321.

graph y, 309.

Weakening of the

fication, 333.

IX.

Fixing Agents, 331.

Intensi-

Silver Image, 339.

Printing

Albumenized Paper Process, 342. Emulsion Reduction Products of Organic Silver Compounds, Image Transference, 349. Toning of Silver Prints with

Processes, 342. Prints, 345.

348.

Gold, 352. Process,

359.

Theory of the Operation, 353. The Platinotype Chromatized Gelatine and Pigment Printing

Processes, 364.

Appendix

to Lecture IX., 369.

ERRATUM. The formulae assigned to the soluble and insoluble silver sodium The thiosulphate have been erroneously transformed on p. 332. soluble salt has the formula Ag2 Na 4 (S 2 3 )3 and the equation should

O

be:

2AgBr + 3 Na 2 S 2 O 3 = Ag 2 Na4(S 2 O 3 8 + 2NaBr The insoluble salt is AgNaS 2 O 3 A similar correction must be made on p. 358. )

.

THE CHEMISTRY OF

PHOTOGRAPHY.

THE CHEMISTRY OF PHOTOGRAPHY LECTURE Definition of Subject.

Change

I.

Photo-physical and Photo-chemical

Photo-chemistry of Iron

Salts

Mercury, Uranium, Gold, and Platinum reduction of Chromates.

THE

of

Copper,

Photo

-

and Oxidation.

Photography, which

of

subject

Dissociation

;

Salts.

has

been

chosen for this session's course of special lectures, appeals to us both as an art and as a science.

The

of

applications

photography

to

the

pro-

duction of pictures, either landscapes or portraits,

and

its

uses

lithographer

in

the service of the engraver or

may

be

branches of technical the subject

e

it

is

regarded art.

With

not proposed

to

as

so

this

aspect of

deal

many on the

B

THE CHEMISTRY OF PHOTOGRAPHY.

2

present occasion.

It

more with the

is

photography that

principles underlying

engage your attention during in

the

first

place these

scientific

wish to

I

because

this course,

principles

our

concern

Chemical Department as constituting a branch of technical chemistry, I

am

disposed

and because

to

believe

in the

that 'the

art

in

is

This

danger of outstripping the science. state of affairs

next place

is

a

which we often witness when a

discovery of immediate practical utility

is

made.

There are many branches of industry which arc

now

suffering in this country because they have

been allowed to go on from the time of their foundation without any attention being paid to the scientific principles underlying them.

It

has

been found by a long course of experience that certain

results

way and

that

can be obtained

mode

in

of procedure

a

has

stereotyped into a kind of article of

such cases no trouble the

particular

desired results science.

mode

is

of

particular

become

faith.

taken to ascertain operation

leads

to

In

why the

the art has here outstripped the

NECESSITY FOR INVESTIGA TION. But

we

if

thus allow a useful discovery to de-

we

generate into a purely empirical industry,

gave us the discovery are

and not only

;

taken

scientific

competitors

investigate

hands

our

of

out

who have no

who have taken

and

formula

why and

the

moment wish you is

graphy

an

there

has

suffered

no

be

in

forgotten

that

subject

countrymen. state

the art going

condition to

some

of

have

been

When

of affairs

Neither

devotees.

in

the

I

which there

danger,

imbue you with that

subject

de-

must

be

it

advances

made by our own

therefore

with

discovery,

this

greatest

allude is

beyond the science

fraught

a

experimenting of

velopment by the haphazard unscientific

dif-

photo-

scientific

its

to

for

that

English

doubt that

considerably

of the

do not

I

understand

to

more

by

trouble

wherefore

particularly

can

but

the

but

this,

stereotyped

the

ferent steps in their processes.

in

first

running into the danger of seeing the

industry

its

are

science which

neglecting our duty to that

we

3

spirit

I

of

a as

to

that

chance of being

simply

a

wish

investigation

B 2

THE CHEMISTRY OF PHOTOGRAPHY.

4

which

the

is

and soul of advancement

life'

every department of

we

in

this

not

may

country

order that

in

knowledge,

short

fall

in

our

in

the future development of the

contributions to subject.

On

considering the the

in

adopted

mode

of treatment to be

course

present

culty of selection presented

the

usual It

itself.

diffi-

obvious

is

a

wide

field

photography but

little

information of value

that

such

in

be given

dismissed

even in paths

as

which

beyond

our

in

used

of

well

lectures

by can

work,

present

many

lenses

the

proper

subject

form

the

the

and

inviting

Thus

optics

the

for

object,

of

of

etc.,

the

theory various

and

arrangement

subjects

but

scope,

directions.

of

once

at

offers

it

its

subject

may be

aspects

of

on

the

narrow

by photographers, the

construction

Illumination

very

side,

opening

kinds

has

only

scientific

its

apparatus

and

not

artistic

purely

offered

somewhat

to

restricted

is

For

limits.

that

a short series of lectures unless the

in

treatment

as

might

course

of

photography

a

a

DEFINITION OF SUBJECT. branch into which

on

this

occasion.

of photography in

several

make a

is

ways,

it

5

not proposed to

is

Then

the

again

enter

chemistry

capable of being dealt with

which we

between

have

to

choice.

The photographer in the course of his practice has necessarily to make use of various chemicals which he generally procures ready prepared and of a purity guaranteed sufficient for his purpose. these days,

when photography has been

by the introduction of dry is

baggage of the

addition

camera to

the

the requirements of the

tourist,

amateur can be met

so simplified

plates that the

becoming quite a common

In

at

every stage, and

the

practical manipulative details are thus reduced to

Both professionals and

the extreme of simplicity.

amateurs would however derive benefit from a

knowledge of the chemistry of the materials which they so constantly employ

that

is

to say from a

knowledge of the methods of preparation, the characteristic

properties

and

modes of valuation of these words,

reactions,

materials.

we may have a branch of the

and

the

In other

science dealing

THE CHEMISTRY OF PHOTOGRAPHY.

6

with the chemistry of photographic materials

But inasmuch as

kind of materia photographica.

although of undoubted

this sort of information,

not absolutely essential to successful work

is

value,

photography, and since those

in

find

in

it

a

many

who

require

of the published manuals,

can more profitably occupy both

my own

I

it

can

think

I

time and

yours by dealing with the subject from a somewhat higher point of view.

The all

future

advancement of photography, as

other branches

proper

recognition It will

principles.

make

clear to

you

of

science,

depends upon a

and application of be

my

of

scientific

endeavour therefore to

in the course of these lectures

our present position with respect to the chemistry of photographic processes, so that you

may

see on

the one hand what principles are involved in these processes, realize

what

how much we have

yet

to learn

may

and

in

directions further advance seems necessary.

From that

and on the other hand that you

this

it is

no part of

in practical

of plan

declaration

my

scheme to

photography

;

it

will

be seen

offer instruction

the details of processes

PHOTO-PHYSICAL CHANGES. will

only be given

you

to understand the principles underlying them.

I

have decided upon

may add it

in GO far as

mode

this

much

after

necessary to enable

of treatment

consideration

I

because, as

seems to me, our knowledge of the chemistry

of photographic reactions obscurity,

be

to

and a wide

is still

field

before

explored

much

involved in

of investigation has yet the

be

can

subject

established on a truly scientific basis.

The

we

action of light with which

concerned

in

photography

action resulting

compounds

in

is

a

are

chiefly

photo-chemical

the decomposition of certain

usually salts of silver

and the sub-

sequent completion of the reaction by the plication of appropriate chemicals.

By

ap-

a photo-

chemical action must be understood a chemical

change produced by the action of essential

to

bear in

mind

that

we

It

light.

are

is

mainly

concerned with actions of this kind because the influence of light does not always induce changes

of

a

purely

chemical

nature.

We may

physical changes, in matter produced it

is

sometimes

difficult to

by

light,

have

and

decide in any particular

8

THE CHEMISTRY OF PHOTOGRAPHY.

case

how much

and how much

may

of the effect

is

due to physical

That

to chemical change.

light

induce purely physical change will be evident

on reference to the numerous recorded instances, a few of which It

light

is

well

may

known

be here mentioned.

under the influence of

that

such elements as sulphur and phosphorus

gradually become transformed into their so-called "

"

allotropic

modifications,

ordinary

phosphorus

becoming red or amorphous and a strong solution of sulphur in carbon disulphide slowly depositing the insoluble modification

when exposed

light in a sealed tube [A].

So

salts,

to sun-

also certain metallic

such as the crystalline chloride or iodide of

silver, nickel

sulphate and zinc selenate, experience

a change in crystalline form under tha influence of light.

These are a few examples of photo-physical changes requiring more completion.

produce selenium,

an

In

some

or

less

time

instances light

instantaneous

change.

for

is

their

known

Crystalline

which under ordinary circumstances

a very bad

conductor of

electricity,

to

is

becomes a

PHOTO-DECOMPOSITION AND COMBINATION. much

better conductor

conductivity again

when

light falls

upon

the dark

falling off in

it,

;

9

the this

property of selenium, discovered by Willoughby

Smith in 1873, has been

utilised in Bell's

photophone.

All these instances are illustrations of physical

changes

induced

by

the

light

chemical

position of the substances referred to altered: white

and red phosphorus

is

and

modifications of sulphur, and the

same

the

are

no way

are. chemically

soluble

so

identical,

in

com-

insoluble is

true of

the different crystalline forms assumed by a salt

such as nickel sulphate or zinc selenate.

Regarded produce

both

although

it

any

a

as

is

chemical

agent

and

combination

light

may

decomposition,

often very difficult to say whether

particular

effect

other or of both

is

the result of one or the

modes of

action.

Consider for

instance one of the most familiar cases, the action

of light upon a mixture of chlorine and hydrogen.

These gases, as

is

well known, react explosively

under the influence of a strong

hydrogen chloride

[B].

This

as a case of combination, but

is

light

to

form

usually spoken of

it is

highly probable

THE CHEMISTRY OF PHOTOGRAPHY.

io

that a decomposition of the molecule of chlorine

precedes

its

The reason

union with the hydrogen.

for believing in

such preliminary decomposition

is

that the rays which are most active in bringing

about the reaction are the blue and those

rays

which

absorbed

are

violet,

i.e.

just

chlorine. 1

by

Without attempting therefore to draw any hard

and

fast line

bination,

let

between us pass

decomposition

on to the consideration of

some of

the photo-chemical

likely to

be of

advisable

to

reactions

which are

interest, either directly or indirectly,

connection with

in

and com-

our

approach

present

subject.

photography

from

It

is

this

broad point of view because we have now become' so accustomed to associate photographic processes

with the

salts

of silver that there

forgetting the fact that other

is

a danger of

compounds

are also

possessed of photo-susceptibility.

As examples by

of reactions promoted or caused

light let us first consider

1

some of the more

According to some researches recently published by Pringsheim (Ann. Phys. Chem. Ser. 2, vol. xxxii. p. 384), water vapour is essential to the reaction.

u

PHOTO-CHEMICAL REACTIONS. which

familiar cases

may be

said almost to obtrude

themselves under the notice of chemists in

exposed to water

of

habit

the

keeping

under

known

It is well

light.

the

on

reagents

of

influence

who

are

shelves

that chlorine

gradually

light

becomes acid from the formation of hydrochloric acid, whilst

oxygen

to

this

compare

is

liberated.

It

is

instructive

change with the same reaction

brought about by heat.

By

passing steam and

chlorine through a red hot tube hydrogen chloride

and oxygen are formed equation

[C]

according

is

brown from the is

view because resolution its

liberation

of interest

or

it

but

acid

also

and becomes

of iodine.

This

re-

from our present point of not

a

dissociation

constituents,

oxygen of the

is

.

of hydriodic

solution

aqueous

gradually decomposed by light

action

the

:

2C1 2 +2H 2 O=4HC1 + O 2

An

to

a

case

simple of a

compound

into

which

the

reaction

air takes a part

of the

in

:

4 HI+0 2 =2H 2 0+2l 2

.

THE CHEMISTRY OF PHOTOGRAPHY.

12

The proof of occur when air air

this is

that the change does not

is

In

excluded.

the presence of

an aqueous solution of hydrogen iodide under-

goes the same decomposition

more

in

the dark, only

slowly, so that in this case light acts as an

accelerator of a chemical change which takes place less rapidly in the dark.

The

upon metallic compounds

action of light

such as the oxides and

salts

is,

speaking

terms, a reducing action, so that

remember

that

show the most are

the

metallic

it

compounds

Among

which

photo-chemical changes

striking

very generally those

oxidation.

general

important to

is

which are capable of

being reduced from a higher to of

in

the

a

lower state

metallic

compounds

which chemical students are

in the habit

of re-

garding as typical of those capable of forming two such

classes, the salts of iron will at

themselves.

It is well

known

once suggest

that ferric salts in

the presence of reducing agents lose

some of

their

oxygen or other negative constituent and become converted

into

ferrous

This

compounds.

duction can also be effected by

light.

It

re-

must be

PHOTO-CHEMISTRY OF IRON COMPOUNDS.

13

understood however that the photo-chemical change in the

can only take place

presence of oxidizable

matter, so that the reduction of the ferric salt and

the oxidation of the associated substance proceed simultaneously.

The

photo-sensitiveness of ferric

long been known, having been

examined by graphy was

Sir

in

John

its

compounds has

first

Herschel, to

systematically

whom

photo-

early days indebted for

many

important contributions.

As

chemical change

be instructive to enter

somewhat

it

will

a

type

of photo-

further into these reactions on the pre-

Let us

sent occasion.

considerations

Reducing

start

from purely chemical

:

agents

such

as

nascent

hydrogen,

sulphurous acid, &c., immediately reduced

compounds [D] of light

ferric

either in the presence or absence

:

+ SO + 2H O = 2FeSO 4 + 2H SO 2

2

2

Here

the

4

Ferrous sulphate.

Ferric sulphate.

reducing

becomes oxidized

agent,

sulphur

dioxide,

into sulphuric acid at the ex-

THE CHEMISTRY OF PHOTOGRAPHY.

H

pense of the oxygen of the light

falls

salt

say

because

Now when

ferric salt.

upon an aqueous solution of a

ferric

no reduction takes

place,

ferric chloride

under

circumstances

these

the

water does not act as a reducing agent the

easily

;

but

in

organic compounds which are

of

presence

solvent

which

and

oxidized,

absorb

therefore

oxygen or other negative elements,

or,

in

other

words, which act as reducing agents, ferrous chloride

Thus an

formed.

is

of ferric chloride

ethereal or alcoholic solution

reduced by light

is

[E].

In the

case of alcohol the change might be represented

thus

:

Fe 2 Cl 6

+ C H6 2

= Fe 2 Cl 4 + C 2 H 4 O +

Alcohol.

Ferric

Ferrous

2HC1.

Aldehyde,

chloride.

chloride.

If therefore

we have a

ferric salt in

contact with

something that will act as a reducing agent, we can insure the formation of a ferrous salt on exposure to light.

You must

because

the whole

action

is

involved

substance that does

notice the expression will secret of the therein. act,

We

act,

photo-chemical

do not want a

such as sulphurous acid,

PHOTO-CHEMISTRY OF IRON COMPOUNDS.

15

but a compound ready to act when, under the stimulus of light, the substance with which associated

decomposed

is

it

is

we must have a system

:

ready to undergo rearrangement, but waiting the application of

some external energy

to set

it off.

In order to utilize the photo-reducibility of a

we may

for the production of a picture

ferric salt

spread the

salt

over a surface of paper because

being an organic substance, acts as the re-

this,

Of

ducing agent. thing

if

we

course

it

comes to the same

use an organic ferric

salt.

By

exposing

such a sheet of ferric-coated paper to the electric light for a short

time under a design cut out in

black paper, the portion exposed to the influence of

light,

ferrous

i.e.

salt,

the

and

because of the

becomes delineated

design,

thus lighter

becomes colour of

visible,

faintly

the

compared with the yellow

ferric

contrast can of course be

made more

in

latter

compound.

as

The

striking

by

washing over the exposed paper with a solution of

some substance which forms a coloured compound with one salt and not with the other. chemists

know

Thus

all

that potassium ferricyanide gives

THE CHEMISTRY OF PHOTOGRAPHY.

16

only a dark colouration and no precipitate with ferric

salts,

whilst

with ferrous salts

it

forms a

blue precipitate of ferrous ferricyanide (Turnbull's" blue)

:-

3 Fe 2 Cl 4

-!-

2K 6 Fe 2 Cy 12 = 2Fe3 (Fe 2 Cy12) +

By washing

I2KC1.

the exposed surface with a solution

of ferricyanide the pattern therefore becomes revealed

or,

as photographers

would

say, developed in

Turnbull's blue [F].

The photo-reduction

of the ferric salt

is

some-

times accompanied by the visible evolution of a

This

product of oxidation. solution of ferric oxalate

dioxide and deposits

:

Fe 2 (C 2

The

the case with a

which under the influence

of light gives off carbon ferrous oxalate

is

4) 3

= Fe

2

(C 2

4) 2

+ 2CO 2

.

organic acid of the ferric salt here plays

the part of the reducing agent, the reaction being sufficiently definite to

enable

it

to be

employed as

a method of chemical photometry.

This behaviour of the iron

salts

under the

in-

COPPER SALTS.

17

fluence of light will serve as a useful introduction

We

to the photo-chemistry of other metallic salts.

may

look upon these reactions as typical cases of a salt from a higher

of the photo-reduction

to a lower state of oxidation in the presence of

The development by

organic reducing agents.

means of

ferricyanide

may from

our present point

of view be put into the background as being only of

The one

secondary importance.

conveyed by the action

reducing

illustrations

of

always mean a removal negative elements

by the

reducing

chlorine

when

may

is

not

that the

necessarily

of oxygen, but

also

agent,

chosen

does

light

great lesson

other

be removed and fixed

such,

ferric chloride

example, as

for

oxidizes alcohol or

ether under the influence of light.

Without

in

any way attempting a complete

treatment of the photo-chemistry of all the metallic salts

which are influenced by

interest to consider here

light, it

some of the

will

better

be of

known

instances.

In the case of copper

becomes reduced

it

to cuprous

is

known

that cupric

compounds, such C

for

1

THE CHEMISTRY OF PHOTOGRAPHY.

8

example

an alcoholic or ethereal solution of

as

cupric chloride, which becomes reduced to cuprous chloride under the influence of light. is

The

action

here analogous to the photo-reduction of ferric

and requires no

salts

to

respect shall

special explanation.

reduction

the

of

With

cuprous chloride

I

have to say more hereafter.

Mercuric chloride in aqueous solution

by

decomposed

manner

bright

the

in

light

is

slowly

following

:

4HgCl 2

-f

2H 2 O = 2Hg

As oxygen

is

2

Cl 2

+ 4HC1 + O 2

Mercurous

Mercuric chloride.

.

chloride.

here liberated you will readily

understand that in accordance with the principles previously laid

down

the reaction would be pro-

moted by the presence of some oxygen-absorbing, i.e.

oxidizable substance.

It

has accordingly been

found that the addition of organic substances, such

and

other

acids, sugar, &c. hastens the decomposition.

Thus

as

formic,

oxalic,

succinic,

a mixture of mercuric

oxalate under

the

tartaric,

chloride

influence

of

and ammonium light

gives off

MERCURY AND URANIUM carbon dioxide and chloride [G]

rapidly

deposits

19

mercurous

:

2HgCl 2 + (NH 4 ) 2 C.,0 4 = Hg,Cl 2 + This

SALTS.

decomposition,

change undergone by

the

like

:

.

corresponding proceeds at

ferric oxalate,

and has been

a measurable rate

2NH 4 C1 + 2CO

for the

utilized

quantitative determination of the intensity of light.

Other

mercuric

such

salts,

sulphate, carbonate,

the

as

chromate, &c.,

nitrate,

when spread

over the surface of a sheet of paper and exposed to light, also undergo reduction accompanied

by a

change of colour [H]. Uranic

salts in contact

with organic substances,,

such as alcohol, ether, glycerol, paper, &c., behave, like the ferric

compounds and undergo reduction

on exposure to

doubt have

result

not

chemical

light.

from

been side

Uranous compounds no

this reduction,

sufficiently

to

enable

but the reactions

studied

us

to

represent

changes satisfactorily by equations. borne

in

mind

from

It

their

the

must be

that the so-called uranic salts are

really salts of the

oxygenated radical

UO

2,

C 2

uranyl,

THE CHEMISTRY OF PHOTOGRAPHY.

20

so that in the photo-reduction this radical

with

its

Thus,

oxygen.

the

in

case

may

part

of uranyl

sulphate dissolved in alcohol and exposed to light,

uranous sulphate

is

deposited

:

2UO SO = U(SO 4) + UO + O 4

2

2

The oxygen

is

2

2 (absorbed).

of course not liberated, but

is

used up in oxidizing the alcohol, probably with the formation of acetic acid. It

must

suffice for the present to

the sensitiveness of the uranic salts

mention that is

sufficient to

enable these compounds to be used for photographic purposes

Good

[I].

illustrations

are furnished

by the

of auric chloride, substances

is

of photo-chemical reduction salts of gold.

AuCl 3

reduced

,

Thus a

in contact

first

to

solution

with organic

aurous chloride,

AuCl, and then to metallic gold.

The photo-

graphic importance of the easy reducibility of gold salts will

be seen subsequently when we come to

consider the chemistry of the process of toning.

The photo-reduction

of the platinum salts takes

place readily in the presence of organic substances,

CHROMIUM COMPOUNDS. especially

when as

reduction,

21

the platinous salts are used. of

case

the

in

The

compounds,

gold

proceeds to the extreme, metallic platinum being deposited in an extremely fine state of division.

The two

classes of platinum salts are typified

platinic chloride, PtCl 4

and

,

platinous

PtCl 2 both of which form double

salts

,

chlorides of the alkali metals

PtCl 4 ,2KCl

=K

2

PtCl 6

,

known

are

those

among

to chemists as

are easily reducible

= K 2 PtCl 4

.

Potassium chloroplatinite.

Potassium chloroplatinate.

The chromates

chloride,

with the

:

PtCl 2 2KC1

.

by

salts,

well

powerful oxidizers, which

by

in

light

the presence of

organic or other oxidizable substances and which are of great

importance

in

The

photography.

general character of the change which takes place

when a chromate

is

reduced

may

be exemplified

by the well-known reduction of potassium dichromate by means of sulphuric acid and alcohol :

K

2

Cr,O 7

+ 4H SO + 2

Potassium dichromate.

Cr2 (S0 4) 3 Chromium

2

3C 2 H 6 O = Alcohol.

+ K S0 +

sulphate.

4

4

3C 2 H 4 O Aldehyde.

+ ;H

2

O.

THE CHEMISTRY OF PHOTOGRAPHY.

22

It

is

be observed

to

dichromate, which oxide,

is

in

this reaction that the

a salt of the acid chromic

CrO 3 becomes reduced ,

oxide Cr2 O 3

to a salt of the basic

[J].

This reaction

will serve to

illustrate

eral nature of the photo-reduction.

potassium or

the gen-

A solution

ammonium dichromate

of

spread over

paper and exposed to light becomes brown on the

exposed portions [K]. size)

(or its contained

here plays the part of the reducing agent,

just in the

same manner that

of ferric chloride. as

The paper

sugar, glycerol,

it

does in the case

Other organic substances, such albumen, gums, gelatine, &c.,

exert a similar influence.

No

free acid is neces-

sary in these cases of photo-reduction

;

it

appears

indeed probable that the oxygen of the dichromate is

used up

in

forming acid products of oxidation

from the organic substances, so that the alkali

would be thus neutralized. for is

example

among

it

In the case of gelatine

has been proved that formic acid

the products of

its

photo-oxidation by

The dichromates moreover

are themselves acid

dichromate.

CHROMIUM COMPOUNDS. salts,

23

containing an excess of acid oxide, and

has been shown that the

it

step in their photo-

first

reduction in the presence of an organic substance is

in

many

instances, if not generally, the

chromate, Cr 2 O 3 is

,

CrO 3

Cr3 O 6 which compound ,

capable of further reduction

4 Cr 3

The is

or

6

= 6Cr 2

action of light

chromium

:

+ 3 O2

3

.

upon chromatized gelatine

the foundation of several important printing pro-

cesses.

By

this action not

reduced, but the gelatine

rendered insoluble. insoluble

compound

chromium

It

only at

is

is

the dichromate

same time

the

seems probable that the

thus formed contains oxide of

as an essential constituent.

interest here to

remark that

It will

in this case

be of

we have

another reaction in which light acts as an accelerator of chemical change,

gelatine

since

becomes gradually insoluble

chromatized in the dark>

especially in a moist atmosphere.

We have

now

considered a sufficient

number of

more

clearly the

instances to enable us to realize

THE CHEMISTRY OF PHOTOGRAPHY.

24

general

when

nature of the change which takes place

light acts

upon reducible

of oxidizable substances.

salts in the

either a liberation

of oxygen or of

negative element, such as chlorine

we must expand our

Although

in the cases given

essential to the reaction, all

to say

it

i.e.

a*s

deoxida-

an oxygen or

a reducing agent,

must not be supposed

photo-chemical changes are of this charac-

It will

ter.

is

ideas of photo-reduction so

chlorine-absorbing substance,

that

some other

that

as to include dehalogenization as well

is

be well

mind that photo-reduction may mean

to bear in

tion.

presence

It will in future

serve to broaden our views

if

we now

consider a few other typical illustrations. In

some instances

light, like heat,

can produce

changes of the nature of pure dissociation, such for

example

as

when mercurous oxide becomes

converted into mercuric oxide and mercury, or

mercurous chloride into the mercuric metal [L]

salt

and the

:

Hg = HgO + Hg Hg Cl = HgCl + Hg 2

2

So

2

also certain metallic

2

oxides, such

as those

PHOTO-DISSOCIATION AND OXIDATION. of gold and residue

silver,

give

containing

the

All

light.

such

oft"

25

oxygen and leave a

metal

on

exposure

decompositions as

these

to

may

be regarded as cases of photo-dissociation. 1

Under tion

and

the influence of light the rate of oxida-

by atmospheric oxygen it

is

is

often accelerated,

probable that this photo-oxidation plays

an important part

in

many

photographic phenom-

ena which we shall have to deal with hereafter.

may remind you

I

of one illustration of this kind

of action given in the early part of this lecture, viz.,

the oxidation

an

of

aqueous

solution

of

hydriodic acid with the liberation of iodine (see p.

In

11).

in a

manner potassium

similar

stable

although alkaline

a

when

dry,

moist

becomes

gradually

and discoloured when exposed

iodide,

~to

chemical oxidation by atmospheric oxygen

4KI 1

%

+ 2H O + 2

Gaseous hydrogen iodide

sure for a

summer month

found to be decomposed.

O, is

- 4KOH +

2 :

2 1.,

decomposed by light ; after expo80 per cent, of the gas was

to daylight

G. Lemoine, Ann. Chim. Phys.

vol. xii. p. 145. 2

light

state, the change being due to photo-

Vidan, Pharm. Jaurn. Trans. 3rd

ser. vol. v. p. 383.

ser. 5,

THE CHEMISTRY OF PHOTOGRAPHY.

26

In

case

this

the

potassium iodide

may

itself

play the part of the reducing agent, since

known

is

it

that this salt combines with two atoms of

iodine to form the unstable triiodide,

KI 3

.

Other examples of oxidation are furnished by lead compounds, moist litharge

becoming gradually

deeper coloured and a thin film of lead sulphide

becoming bleached on exposure

So

[M].

far as the

and

to light

air

chemistry of these changes

concerned the reactions are simple enough

is

:

6PbO + O 2 Pb 3 O 4 + Oo - 3PbO 2 In the case of litharge the lead

and the

final

is

red

product the brown peroxide.

In

first

product

the case of the sulphide the sulphate

is

formed

:

PbS + 2O 2 = PbSO 4 Certain

resinous

when exposed in and air become other

oils

and

bitumenous

substances

thin films to the action of light

insoluble

which

dissolve

in

hydrocarbons and

them

freely

before

BLEACHING ACTION OF LIGHT. The

such exposure. to

action since

photo-oxidation,

air

here probably due is

historically interesting as

the

in

first

This

essential.

oxidation

kind of photo-chemical

particular

employed

is

27

is

having been the method

successful attempt to fix a

photographic image by Joseph Nicephore Niepce

about the year 1824.

This inventor

who, with

Daguerre, must be considered as the founder of coated a metallic plate with bitumen

photography

some hours

and exposed

for

with a

The image was

lens.

by immersion

in a

afterwards developed

in oil of lavender,

which dissolved the

portions of the film unacted upon

a picture in insoluble bitumen. of

this,

the

first

camera provided

by

light

The

and

left

discovery

truly photographic process, laid

the foundation of a printing process which

is

still

in use.

Among

other instances of the photo-chemical

oxidation of carbon the well certain

Here

by

compounds may be mentioned

known bleaching

natural and

for instance are

the action

action of light

artificial

colouring

upon

matters.

examples of designs formed

of light upon

papers dyed with

THE CHEMISTRY OF PHOTOGRAPHY.

28

and

eosin

methylene

fading on

to

exposure

sunlight

The

ten

for

typical cases of photo-chemical action

have now been considered illustrative of a special

or

termed

be

may be

regarded as

photo-chemistry, in

and

of

action

effected

many by

other

light

way have been graphic

directly

kinds

if

of

which

upon

have been tempted to dwell

There

chemical I

should

they could in any

connected with photo-

Thus the decomposition

processes.

this

sense here dealt

the

with must be looked upon as an offshoot. of course

which

branch of our science which

photographic chemistry

are

being

amount

minutes [N].

fifteen

may

former

enough to show a perceptible

sensitive

of

the

blue,

of

carbon dioxide by the green leaves of plants, the photo-decomposition of the vapours of organic

compounds such etc.,

of

investigated liquids

like

as

amyl

part

attacked

isopropyl iodide,

by Tyndall, the polymerisation vinyl

markable influence of precise

nitrite,

bromide, light

in

and

the

re-

determining the

of the molecule of a hydrocarbon

by chlorine or bromine

recently

in-

OTHER PHOTO-CHEMICAL CHANGES. vestigated

by

Schramm,

1

are

all

types

29

of

photo-chemical action which would lead us too far

astray

if

followed

up

in

the present course

of lectures. 1

Ber. Deutsch. Chem. Gesell. 1885, vol.

1,272; Ibid. 1886, vol. xix. p. 212.

xviii.

pp. 350, 606, and

APPENDIX TO LECTURE

A

(p.

hibited

8).

Two

sealed

glass

I.

tubes were

ex-

both containing a portion of the same

saturated solution of sulphur in carbon disulphide. One tube had been exposed to light for a week or

two and contained a deposit of

crystalline sulphur

incrusting the side of the glass, while the other, which had been kept in the dark, was perfectly clear.

B

(p.

mental

9).

For

details

concerning the experi-

explosion of hydrogen and chlorine under the influence of light see Roscoe illustration of the

and Schorlemmer's Treatise p.

011

Chemistry, vol.

i.

125.

C

(p.

1

1).

Chlorine

is

containing boiling water

passed through a flask

and the mixed gas and

water vapour passed though a porcelain strongly heated in a gas combustion furnace. escaping gas

is

tube

The

received over dilute caustic soda

APPENDIX TO LECTURE

I.

solution to absorb excess of chlorine, to be

D

oxygen by 13).

(p.

divided

into

side

by

solution

portions,

or

an

each,

iron

salt

chloride

ferric

to

aqueous

one

is

which

of

of

solution

The two beakers

ferricyanide

into

of

a

are placed

on white paper and a few drops of

side

potassium

A two

added.

is

sulphite

and shown

the usual tests.

acid

sulphurous

31

the

solution

allowed to

beaker

the

fall

reduced

containing giving a blue precipitate and the unreduced solution a brown colouration.

E

14).

(p.

A

solution

of

ferric

chloride

in

alcohol

(methylated spirit answers the purpose) divided into two portions in two large test tubes, one of which is placed on the table is

away

from the influence of light and the other in a beaker of water (to keep it cool) and exposed for five or ten minutes to the electric beam from an arc

lamp concentrated by means of a lens. After exposure the two tubes are placed side by side and ferricyanide

added to each, when the solution

in the

exposed tube gives a deep blue precipitate and the other the usual brown colouration.

F

(p.

1

6).

Paper

is

coated with a 10 per cent,

solution of ammonio-ferric citrate by floating on the liquid for a few minutes and then

to dry in a dark room.

allowing it After exposure for about

THE CHEMISTRY OF PHOTOGRAPHY.

32

ten minutes to the electric light under a design cut

out in black paper, the sheet is withdrawn and developed by brushing a solution of potassium ferricyanide over the surface.

G

(p.

The mixture

19).

of mercuric

and ammonium oxalate used oxalate photometer" can

experiment.

No.

I

The

5

c.c.

as follows

ammonium

this

in

employed

made

is

4 grams crystallized IOO

No. 2

solution

be

chloride

Eder's " mercury-

in

:

oxalate.

distilled water.

grams mercuric chloride. IOO c.c. distilled water.

Before use two volumes of No.

I

are added to

one volume of No. 2 and the clear solution placed in a flask in the electric beam. The separation of the shining white crystalline mercurous salt soon

commences, and

forms a

very

striking

lecture

demonstration.

H

(p.

19).

Dark mercurous oxide prepared by

precipitating mercurous nitrate with caustic potash is collected on a filter, washed with water, and

A

allowed to dry in the air in the dark. test tube is filled with the and dry powder exposed to light for

some days, when the

becomes

ochreous

owing

side towards the light

to

the

formation

mercuric oxide, the other side remaining

of

dark.

APPENDIX. This tube was exhibited and also old specimens of various mercurial preparations which had been kept for a long time in a cupboard with glass doors, the sides of the bottles towards the light showing in many cases discoloration. Signs of decomposition were particularly noticeable in the case of a specimen of calomel. I

Paper solution of uranium (p.

20).

is

floated

on a 15

nitrate in distilled

per.

cent

water for

about ten minutes and then allowed to dry in the The paper is exposed under a design for dark. minutes to the electric light and then developed with ferricyanide, as in the case of the It saves time if ammonio-ferrous citrate paper.

about

fifteen

the paper

given a preliminary exposure before

is

the lecture. J

22).

(p.

A

hot solution

of potassium di-

chromate of & distinct orange colour is placed in a beam of electric light in a flask and alcohol

dropped

in

till

by transmitted

the colour changes to green (red The oxidizing action of light).

chromic oxide was also shown by the familiar experiment of dropping alcohol on to the solid crystals,

to take

K

when the heat produced causes the

alcohol

fire.

A

design was exhibited produced by a sheet of paper coated with potassium exposing (p.

22).

D

THE CHEMISTRY OF PHOTOGRAPHY.

34

dichromate under a pattern cut out portions are brown.

in

black paper.

The exposed

L

(p.

The specimens

24).

referred to in exp.

H

illustrate these reactions.

M

A

design can be printed on paper coated with lead sulphide by floating on a solution (p. 26).

of the acetate and then exposing to sulphuretted hydrogen till a brown surface is produced and

drying to

in

the dark.

bright

light

the

After some days' exposure

exposed

portions

become

lighter.

N (p. 28). Papers were coloured by floating on a solution of eosin ('25 grm. per litre) and methylene blue ('25 grm. per litre) and then exposed under a design in the usual way. In the course of about a week the exposed portions have faded

sufficiently to render the design

spicuous, and

after three

con-

weeks the blue and red

patterns appear on almost a white background. Many other coal tar colouring matters, such as

the methyl violets, iodine green, susceptible of photo-oxidation.

etc.,

are equally

LECTURE

II.

i

Photo-chemistry of the Silver Compounds.

The

Chloride

;

loss of Chlorine

Historical.

on exposure

to Light.

Darkened Product.

Analogy of Silver Haloids with Cuprous, Mercurous, and Thallous Haloids. The Bromide and Iodide. Carey Lea's Experiments Composition of

on the Photo-salts.

AMONG silver

the photo-reducible metallic salts those of

occupy such an important place

in all

photo-

graphic processes that their consideration has been deferred in order that they might receive their due

share

of attention

in

Not

the present lecture.

only are these salts of great practical importance at

the

present

time,

but

they are historically

interesting as having been the

compounds

in

which

a change of colour on exposure to light was

observed and recorded.

I

regret

much

D

first

that the 2

THE CHEMISTRY OF PHOTOGRAPHY.

36

scope of the present lectures will not admit of

lengthy historical digressions, and

it

must

suffice

here to mention that the darkening of the skin

when touched by Albertus

silver

nitrate

was known

The

in the thirteenth century.

Magnus

darkening of native

to

silver chloride (horn silver)

on

removal from the mine was described by Fabricius century, and in the seventeenth

in the sixteenth

century Glauber and Robert Boyle speak of the discoloration of silver

compounds, although none

of these earlier writers appear to have suspected the darkening

that light.

The

was due to

the

action

of

distinct statement as to the dark-

first

ening of a silver compound being the result of the influence of light J.

was made by a German physician,

H. Schulze, who

in

1727 observed that when a

solution of silver in nitric_acid

was poured on

to

chalk the precipitate darkened on the side exposed to light,

and he

clearly proved

that this effect was produced

by

by experiment light

and not

1 by heat

All the salts of silver, both inorganic and organic, 1

Eder's

Handbuch der Photographie,

1884, Part L, p.

2.

DARKENING OF SILVER CHLORIDE. are

more or

less

decomposed by

and sometimes only substances. nitrate

is

in

37

especially

light,

the presence of oxidizable

Thus an aqueous

not 'acted upon

solution of silver

light unless organic

by

matter or some other form of reducing agent present.

But although

offer for consideration

photo-chemical action,

the various silver salts

many it

instructive

importance

in

photography,

bromide, and iodide, g^n^ral

The finally

dpsig^Hon

known

of

cases of

be better to limit our

will

attention to those which are of -tical

is

paramount pracviz.

:

the chloride,

to chemists under the

the"spvpr h^lnjHg^/'

fact that silver chloride

becomes

violet

and

brownish violet on exposure to light has

been known since the middle of the eighteenth century, the earliest statement with respect to the precipitated chloride (as distinguished from native "

horn

Baptist

silver ")

being attributed to Prof. Johann

Beccarius of

Turin.

This property, so

familiar to all students of chemistry,

is,

I

need

hardly remind you, utilized at the present time in

our silver printing processes.

The

cases of photo-

chemical decomposition which have been considered

THE CHEMISTRY OF PHOTOGRAPHY.

38

in the last lecture will

you

for

the

first

we must now

chemical change does the

of silver chloride indicate

discoloration

a chemical change, or if

sure have prepared

questions which

What

put to ourselves.

am

I

is

it

?

purely physical

?

Is

it

Is

it,

chemical, a process -of dissociation, of double

decomposition, or of oxidation

?

chemists to consider

esting to us as

be

inter-

how

these

It will

questions have been answered. It

has been held by some authorities that the

change

is

a physical one

;

that violet silver chloride

has the same chemical composition as the unaltered

But

compound. once, because chlorine on

this is

it

view

may

be dismissed at

certain that the chloride loses

exposure to

light,

a fact

Scheele so far back as the year 1777.

shown by numerous analyses chloride chloride.

is

poorer

This

in

may be

that the change

is

It

that the

chlorine

known

to

has been

darkened

than the white

regarded as a direct proof

a chemical one, and there are

moreover several methods by which the liberation of chlorine, or at any rate some chlorine containing gas, can

be directly demonstrated [A]. Then we

LOSS OF CHLORINE

BY

have indirect proof that chlorine fact that chlorine absorbents,

CHLORIDE. is

39

given off in the

such as silver

nitrate,

stannous chloride, or organic substances, accelerate the photo-chemical decomposition, thus reminding us of the action of alcohol

under the influence of

light.

On

chloride

ferric

upon

the other

hand

oxidizing agents which tend to prevent the absorption of chlorine, such as mercuric or stannic chloride, ferric salts,

or chlorine

decomposition,

or, if

prevent

it

itself,

retard the photo-

present in sufficient quantity,

altogether [B].

Taking

it

as established that the discoloration

of silver chloride on exposure to light

is

a photo-

chemical and not a photo-physical action,

we have

next to push our inquiries farther and to ask what chemical difference exists between the violet and white chloride.

Beyond the

chloride contains less chlorine

that our chemical It

may be

knowledge

of service

if I

fact that it

the violet

must be confessed

is still

very deficient.

here point out exactly

what we know and what we do not know with respect to this fundamental photo-chemical position.

You will

find

it

decom-

generally stated in books

THE CHEMISTRY OF PHOTOGRAPHY.

40

that the change

is

due to the formation of a sub-

chloride of -silver according to the equation

4 AgCl

= 2Ag

Now, whatever may be

Cl

+

C1 2

.

the plausibility of this

must not be forgotten that the ex-

it

reaction,

2

:

istence of the

subchloride

is

only inferred from

the analogy of silver with the metals of the copper

group, and

is

not the result of the analysis of the

Amongst the work published by chemists who have studied these com-

pure compound. the various

can find no satisfactory proof that the

pounds

I

subsalt,

Ag

2 Cl,

has ever been isolated

form as to establish

The

in

such a

chemical individuality.

its

notion that the subchloride

is

the product of

the photo-decomposition of the normal chloride

was

first

suggested

reiterated in

why

1814 by Fischer, and was

1834 by Wetzlar, and has since

ceived general see

in

acceptance.

this suggestion

the books as though

chemical truth.

I

be admitted that

it

It

is

not

has found

difficult

its

way

to

into

expressed an established

have already said that silver

re-

chloride loses

it

must

chlorine

SUPPOSED SILVER SUBCHLORIDE. on exposure

to light

that only one of

that the salt

must be reduced to the metallic

the

in

has been assumed

it

two things could happen,

On

or to a sub-salt.

have

hence

;

41

first

viz.

state,

assumption we should

this

mixture of metal

instance a

with unaltered chloride, and in the second case a

Now

mixture of chloride and subchloride.

known

it is

well

that the darkening of silver chloride takes

place under nitric acid, and

it

has also been found

that this acid does not dissolve out

any appreci-

able quantity of silver from the darkened chloride,

so that

it

has been concluded that the latter does

not contain any free metal.

reasoning

known

as exclusion

By it

that process of

has therefore been

assumed that the other alternative holds good that the darkening

is

due to the formation of sub-

chloride.

What

has

modern

chemistry

to

respect to the existence of the subchloride

an d priori point of view stated, the

we

with

say ?

From

have, as already

analogy suggested by the relationship

of silver to copper and

metals form subchlorides.

mercury, both of which

But we

all

know

that

THE CHEMISTRY OF PHOTOGRAPHY.

42

analogy, although at times a very useful guide,

apt to lead us into

to a

much

in the first

safer principle,

work has done of

trusted too im-

difficulties if

Let us turn

plicitly.

is

place therefore

and see what laboratory

late years

towards the solution

of the problem. It

must be confessed that the

direction have been

somewhat unsettling

concerns the sub-salts of

ment

in

always

results in

One

silver.

this

as far as

strong argu-

favour of the existence of these salts has

been

the

suboxide,

supposed

Ag

4

O,

obtained in 1839 by Wohler by reducing the citrate

a current of hydrogen at iooC.

in

2 1 by Muthmann, by Friedheim, and

investigations

by Bailey and Fowler to

me

to

But recent

at

Owens College 3 appear

render the existence of Worrier's, sub-

oxide extremely doubtful, thereby making

all

the

statements about the subchloride being formed by 1

Ber. Deut. Chem. Gescll. 1887, p. 983.

2

Ibid.

1887,

2554 and

p.

Fillitz (Zeitschrift fur

1 888,

p.

307.

See also papers by

Analyt. Chemie. vol. xxi. pp. 27 and 496), by

Spencer Newbury (Amer. Chem. Journ. vol. viii. p. 196) and by der Pfordten (Ber. Deut. Chem. Gesell. 1887, pp. 1458 and 3375

v.

;

and 1888, 3

p. 2288).

Jcurn. C/icm,

Soc. Trans. 1887, p. 416.

SUPPOSED SILVER SUBCHLORIDE. the action of hydrochloric acid upon

43

sub-

this

oxide equally doubtful. 1

Such being the outcome of recent work upon

we may now come back

the suboxide,

main

and

question

the

to

ask whether the darkened

chloride has ever been submitted to such an ex-

amination at the hands of chemists as to have furnished

the

subchloride

of the existence of

evidence

distinct

therein.

In

answer

to

must be admitted that the evidence thus tained

goes

it

far ob-

no farther than to show that the

darkened chloride contains unaltered chloride. cases

this

less chlorine

than the

This result has in nearly

all

been arrived at by determining one con-

stituent only, either the silver or the chlorine,

and

then representing the loss of chlorine as corre-

sponding to the formation of so much subchloride.

By

this

means

different experimenters

different

estimates of the

formed.

Carey Lea

1

The

for

have given

amount of subchloride

example

v.

Bibra

p.

741),

substance obtained by this means has according to

the formula

Ag 4 Cl 3

(Ber.

Deut.

Ghent.

when

states that

GeselL

1875,

but the above quoted investigations tend to show that mixture and not a definite chemical compound.

it

is

a

THE CHEMISTRY OF PHOTOGRAPHY.

44

chloride

silver

days one per

exposed under water

is

cent,

for

five

reduced to subchloride, and

is

Riche asserts that after exposure

for a

year and

a half the formula of the reduced salt corresponds to

Ag3 Cl

2

.

But

methods

such

that

sideration

become evident on con-

will

it

first

its

give

of the

the existence of this salt

assumed and the

buted to

never

to the formation

satisfactory evidence as

subchloride, because

can

is

loss of chlorine then attri-

as clear a case of circular

production

reasoning as one could wish to meet with.

So lief in

far

then

we

get no justification for the be-

the formation of silver subchloride

The

photo-reduction of the white chloride. duct, whatever to

be

it

identified

may as

characteristics with

be,

a the

by the pro-

not definite enough

is

sub-salt

by

its

same sharpness

chemical that

we

can distinguish between other chlorides and subchlorides, such, for

example

as mercuric

curous or cupric and cuprous chlorides. action silver

of solvents which are chloride, such

as

known

to

and mer-

By

the

dissolve

sodium thiosulphate, po-

tassium cyanide, or ammonia, the darkened chloride

ANALOGY WITH OTHER always leaves a residue of metallic again

we

SALTS.

45

But here

silver.

get no real information as to the presence

of a sub-salt, because the existence of the latter

is

assumed and the action of the solvents represented by such equations as these

:

= AgNaSA + NaCl + Ag = + 2KCN AgK(CN) 2 + KC1 + Ag = Cl + 2 Ag. + 3NH [2AgCl + 3 NH 3] 2Ag 2 3 Ag.2 Cl Ag2Cl

+ Na 2 S.

2

3

But these equations seem,

to

say the least of

them, somewhat improbable, because chloride were such an unstable

it

is

darkens under less rapidly,

Now

let

it

is

hardly

could be formed in the presence of

it

such a powerful oxidizer as theless

as to

compound

be decomposed by these solvents likely that

the sub-

if

known

well

nitric

that

nitric acid and,

acid. silver

Neverchloride

although somewhat

even under the strongest acid.

us consider the arguments from analogy

The

and see what value

really attaches to them.

nearest analogues

of the silver haloids are the

corresponding thallium.

salts

Thus

the

of

mercury,

insolubility

copper,

of

and

mercurous,

THE CHEMISTRY OF PHOTOGRAPHY.

46

cuprous, and thallous chlorides reminds us of silver that

so

chloride,

must lead us

analogy shows anything

if

to believe that silver

it

the analogue

is

of copper, mercury, and thallium in the cuprous, mercurous, and thallous chemists

all

by

sometimes

but when

;

it

asserted, as

is

rendered

is

existence of cuprous

or

speaking not analogous

mercurous chloride, the

to.

Silver

understand

terms

so

reality

comparable

that

what

with

strictly

is

chemists pre-

meant by these

white chloride

the

is

copper and mercury,

but to cuprosum and mercurosum will

we

by the

probable

analogy has been over-strained.

sent

admitted

is

find in books, that the existence of a

subchloride

silver

This

states.

these

only

other

is

in

chlorides.

But no chlorides of copper, mercury, or thallium are

known containing

less chlorine

than the cu-

prous, mercurous, or thallous chlorides,

there

is

no warrant

and thus

for believing in a silver chloride

containing less chlorine than the white chloride in other

attempt silver

words, the analogy breaks

to

compare the darkened

with the

lower chlorides

down

if

we

chloride

of

of these

other

ANALOGY WITH OTHER metals, but

is

other subchlorides. this,

If

limit the

silver chloride

and the

we compare

the four metals

we

the only admissible standpoint,

find that the

47

we

if

perfectly legitimate

comparison to the white

from

SALTS.

shall

analogy not only does not favour the

view that the darkened product of photo-decomposition

subchloride, but that

is silver

strong arguments against this view.

many

In the

first

place, the fact that the product of

at

highly- coloured

a

is

photo-reduction is

variance with the

curous chloride resolved

lecture,

mercuric

as

is,

chloride

and

chloride

compound

all

the other

pointed out in the last

I

by the and

that

fact

In the next place mer-

subchlorides are white.

lower

furnishes

it

action

metal

of

and

light

not

into

Then again

chlorine.

into

a the

lower chlorides of mercury, copper, and thallium are perfectly definite salts easily prepared

methods

;

but

presence of it

is

silver is

they

cannot

be

by chemical

formed

oxidizing agents, and that

impossible

to

believe

that

the

compound, which forms under

a subchloride.

in is

the

\vhy

darkened

nitric acid,

THE CHEMISTRY OF PHOTOGRAPHY.

43

It is

hoped that the time which

to be

devoted

the

to

to

proportion

be

not

of silver will

its

told

lectures

things

one

this

salt

out

of

upon

as

but

the

photo-

really

a type

chloride

at starting

you

have

is

concerns our present subject.

reaction as far as I

looked

importance,

of the

decomposition

of

consideration

I

that the

was to enable you

to see

how much we have

of these

object

to

yet

among

other

learn

con-

cerning the chemistry of photographic processes,

and now face

to

the

at

we

outset

find

ourselves

with a fundamental reaction which

face

cannot be

very

satisfactorily

From one

explained.

point of view, however, this

is

perhaps rather a

fortunate circumstance, because you will perceive that there in

is

yet

this direction.

room

Then

for

further

again,

you once ade-

if

quately realize the fact that the

4AgCl = 2Ag 2 Cl+Cl 2 does not rest upon an

mental

basis,

to receive

the

you true

will

absolutely

be

the

investigation

equation

:

,

secure

better

explanation when

experi-

prepared it

comes,

SUPPOSED SUBCHLORIDE.

49

without that severe shock to the feelings which is

generally produced

honoured notions.

graphy

so

is

by the upsetting of time-

Moreover, as

modern photo-

dependent on the photo-

largely

decomposition of the silver haloids, extract

information

the

all

we had

better

we can from

the

chloride before proceeding to the other salts.

To resume

the

argument,

I

will

remind you

then that the ground has been to some extent cleared

by our having

that silver

on

chloride to

exposure

may

undoubtedly, loses

the

subchloride,

a

there

that

is

no

But whatever

Ag2 Cl.

be the chemical composition of this product

of photo-decomposition, it

but

light,

chlorine

evidence that the darkened product

satisfactory

contains

arrived at the conclusion

reduction

we may

product, because

provisionally call

contains less

it

chlorine than the unaltered chloride

;

and

expressing myself dogmatically with

to avoid

respect

to

the formula of a substance which has never been

propose throughout these lectures to

isolated,

I

speak of

this

and of the analogous compounds

simply as reduction products.

E

THE CHEMISTRY OF PHOTOGRAPHY.

50

Now

with reference to the photo-decomposition

of silver chloride,

ordinary

is

it

circumstances

well

known

that

is

of strong reducing (chlorine the reaction

mean

a

is

to say that the

one,

of

product

by which

I

compound does not go on

continuously losing chlorine duction

agents

absorbing)

diminishing

absence

the

in

under

that

a

till

definite

darkened

re-

is

composition

left,

but the amount of decomposition becomes smaller

and smaller and the end result

a

large

small

amount

of

of reduction

investigation such

small and

a

much

that if

a

The quantity

ist.

its

it

is

larger

limit,

so

product

that

difficult

of

chloride

that

one

reduction

has for

plus [C].

made the

product

a It

the

chemis

so

associated or combined with such

amount of unchanged

chloride,

isolation has baffled all ingenuity.

the chloride

get the

unaltered

circumstance

very

a

always a mixture consisting of

is

percentage

this

is

finally reaches

is

exposed

maximum amount

Even

in thin films so as

to

of photo-decomposition,

the difficulty of separating the reduction product

from the unaltered chloride without decomposing

OXYGEN IN DARKENED CHLORIDE ? the former

a serious obstacle to the practical

is

solution of the problem.

But,

may now

it

be asked,

the darkening of the

if

srjuJfrjLV

chloride silver

what be

ot due to the liberation of metallic

is

nor to the formation of a sub-chloride, to chemical

attributed

change

stage to inquire

forward

rejected,

is

change

obvious

sufficiently

may

it

the

discoloration at

necessary

this

what other views have been put Passing over the theory

in explanation.

the

that

be

will

It

?

must

physical, which,

purely

has

reasons,

be considered

in

been

already the

for

first

place

whether the chloride simply undergoes dissociation

under the influence of

light.

The

apparent

absence of free metallic silver in the darkened product existence

and

the

of a

is

therefore

really

It

may

whether

composed only

or whether

it

of evidence

subchloride,

highly improbable. place

want

may

elements of water

render

be asked

the

the

next

in the

product

of silver and chlorine,

essential

must be confessed that

to

such a view

darkened

not contain as

as

in

this

oxygen or the constituents.

direction

E 2

It

there

THE CHEMISTRY OF PHOTOGRAPHY,

52

certainly a great

is

It

investigation.

erates

the

make

the

this

known

is

moisture

that

but

photo-decomposition,

known whether moisture

want of further experimental

in the

is

change

statement

a

not

air

and

arrested.

altogether

with

is

it

absence of

total

accel-

knowledge of

full

made

the numerous experiments which have been

knows how

to test this point, but every chemist difficult it is to

with respect to times

substances

difficulty

of moisture

I

is

it

gases

we have

that

enormous

get

to

of course

allude

gaseous

amount of evidence

that

or both are essential to

of silver chloride, and I

it

within

the

in

and

recent the

last traces

the

to

There

explosions.

pure,

appreciate

of removing

obtained by Prof. H. B. Dixon

on

really

only

learnt

I

results

his researches is

moisture

a

certain

or

oxygen

the photo-decomposition is

for

reason

this

have directed attention to the want

that

of further

1

investigation. 1

According to Abney dry silver chloride sealed up in a Sprengel vacuum does not darken when exposed to light for months. The presence of mercury promotes photo-decomposition. in Photography, 1882, p. 19.

Recent Advances

ABSORPTION OF OXYGEN. That oxygen the

was

chloride

the late Robert

first

announced

distinctly

Hunt 1

,

and

darkening of

the

to

essential

is

am

I

53

by

of the

glad

present opportunity of recalling attention

the

to

work of an investigator who has been so recently removed the

from among

preparation

of the

frequently had occasion

of

this

author,

and

amount of

the large

I

to

on

best described in his "

The

refer to

have

struck with

performed experi-

left

the

on

present

own words

subject

by another very easy experiment.

closed at one end, in

its

was

action

of

1

exposed

sunshine,

frequently

shaken,

in

is

com-

proved

Some

pure

bent

tube

a

and the other end immersed

a bottle of distilled water.

chloride

is

:

bination with the decomposing chloride,

was arranged

The

record.

absorption of oxygen, or rather

chloride of silver

have

I

the writings

been

carefully

bearing

lectures

present

mental work which he has

experiment

connection with

In

us.

for

during for

In

this

state

many days which time

the

to

the

it

was

the purpose of exposing

Researches en Light> 2nd

eel.

1854, p. So.

THE CHEMISTRY OF PHOTOGRAPHY.

54

the whole

of the

tube,

and

silver

on

it

prove

air

this

modern chemical

of

of

1

had taken

criticize

some absorption

that

evident

quite

we

thus ap-

of oxygen

substitution

the

of

under the agency of solar radiation.

atmospheric If

the

of chloride

precipitate

addition of the nitrate,

the

for chlorine

was

a

gave

appearing to

It

As

influence.

its

darkened the water rose into

chloride

the

to

powder

place."

experiment

in

the

knowledge,

it

must

light

be

admitted that the proof that the oxygen which disappears

is

absorbed by the darkening chloride

not perfectly conclusive, because

is

that

the

nascent

chlorine

in

it

the

is

possible

presence

water vapour and under the influence

form some oxide

may solves

in

the

water. 2

of

chlorine

But

in

of

of

light

which

dis-

of

this

spite

A similar experiment is described in The Art of Photography by Dr. Halleur. English translation, 1854, p. 76. 2 It is sometimes asserted that hypochlorous acid is formed 1

<

when

silver chloride

is

decomposed by

light

in the

water, but the equation usually given, apart from

of the existence of the

degree improbable.

4AgCl +

unknown

subchloride,

This equation

HO = 2

is

presence of

its

in

assumption the

is:

2Ag 2 Cl + HC1O + HC1,

highest

OX YGEN POSSIBL Y ESSENTIAL. the experiment

objection

one and

we

necessary

for

under

the

element is

is

supplied

reaction

is

suggestive

a hint that oxygen

get

the

of

darkening

action

of

taken

from

At

light,

but

the

air,

the

chloride

whether

this

or whether

not at present manifest. either

known

to

the free state or in the

in

chemists

that

HC10 + HC1 = The

and hydro-

hypochlorous

earliest record I

H2

+

p. 107)

:

CLj.

can find as to hypochlorous acid being formed

in the report of the British Association

Rep. 1859,

it

the

in

chloric acids mutually react with the liberation of chlorine

is

be

may

by water vapour taking part

If oxygen,

It is well

very

well worthy of further study.

is

rate

any

a

is

55

wherein

taining silver nitrate) in

it is

which

Committee of 1859 (B.A.

stated that the clear solution (con-

silver chloride

had become darkened

gave a further precipitate of chloride on the addition of sulphurous acid, this being considered as evidence of the presence of an oxide of

have repeated

chlorine in the solution.

I

confirm the statement.

It

is

this

experiment, but cannot

true that sulphurous acid produces

a white precipitate, but this dissolves up completely on warming with dilute nitric acid and therefore contains no chloride : not the slightest turbidity

which had been

was observed

in contact

in

a solution

with silver chloride

thus

for

treated

more than

a month, and exposed with frequent agitation to daylight and sunlight during that time.

The same white

precipitate

on adding sulphurous acid to a solution of is no doubt silver sulphite.

silver

is

produced

nitrate,

and

THE CHEMISTRY OF PHOTOGRAPHY. form of water vapour,

of silver

decomposition

for

the

of

the

chemical

contain

some It

oxychloride.

we

future

solution

constitution

of

the

of

the

This view has been suggested more

photosalt.

once,

rigorous

evident

venture to think

I

look

problem

than

that

direction

in this

is

it

chloride,

the nature of an

compound of

must

essential to the photo-

darkened product must

that the

is

is

but

at

rests

it

present

without

state

that

undergoing investigation by

Dr.

may

I

experimental proof.

the

subject

W.

R. Hodgkinson, and although his results are

is

not yet completed, guilty

tains

Ag

4

shall

I

of a breach of confidence

authority that

his

believe

I

the

an oxychloride

0(V

or

of

if

darkened the

I

not

be

state

on

product

probable

con-

formula,

:

/Ag.AgCl

\

\Ag.AgCl

If

we 1

we appeal certainly do

to analogy

get

in

warrant

Dr. Hodgkinson has since informed

this result.

a

me

legitimate

for that

way

believing

in

he has confirmed

OTHER SILVER HALOIDS. the probability of is

analogy

the

exposure to

first

becoming the

the

cuprous

and then

violet

change being

become

but

light,

in

all

to

of

photo-chemistry

work

our

simplify

in

when moist

salt

brownish black,

due to the

now been chloride

silver

dealing

given

with

will

other

the

The remarks which have been made

haloids.

with respect to the supposed with equal iodide

on

[D].

consideration which has

the

discoloured

probability

formation of an oxychloride

The

lowest

the

in

Both thallous and cuprous

state of chlorination.

are white

of copper

subsalts

and thallium, which are already

chloride

The

formula.

forgoing

by the

supplied

57

that

force is

to say

under the

influence

sub-

we have no evidence

Ag 2Br

such compounds as

apply

and

subbromide

the

to

subchloride

of

or

light.

Ag

2

I

formed

are

Silver

that

bromide

darkens on exposure to light and loses bromine, the product finally

becoming of a greyish

violet,

but never as dark as the reduction product of the chloride.

presence

As

in the

of bromine

case of the

absorbents such

latter

as

the

silver

THE CHEMISTRY OF PHOTOGRAPHY.

58

nitrate,

stannous chloride, or organic compounds, photo-decomposition, while such

accelerates

the

compounds

as mercuric, ferric, or stannic chloride

In the

retard the decomposition.

iodide the discoloration

and

the bromide, unless

by

does

not

take

some iodine absorbent

apparently unchanged by of silver

nitrate

or

light,

other

is

The

present.

nitrate

silver

is

absorbent

iodine

it

The darkened

1

more readily decomposed by

acid than the corresponding

all

but in the presence

acquires a greenish grey colour.

product

at

place

is

pure iodide free from excess of

than with

less

is

light

of silver

case

nitric

compound from

the

bromide.

Before leaving the photo-chemistry of the silver haloids

attention

must be

that time will not permit to the

1

According to Abney

(

under these circumstances

secondary reaction.

We

I

regret

do much more

to

salts

recently

Treatise on Photography, 5th ed. p. 24

also Hunt's Researches on Light, off

me

forms of these

coloured

and

called

;

2nd

ed.

98),

p.

but this

oxygen

doubtless

is

have observed that both

and bromide when exposed

to sunlight

give off bubbles of gas, especially

under

when

is

due

;

given to

a

silver chloride

silver nitrate solution

the solution

is

warmed.

COLOURED SILVER HALOIDS.

59

Lea and described by him

obtained by Carey

under the names of photochloride, photobromide.

and

photoiodide.

of

details

Without

1

the

these coloured haloids,

by

produced

methods

various

may

I

of

preparing

state that they are

such as would tend

reactions

give rise to the formation of subsalts these

to

haloids.

The

be partially

that

is

converted

may

silver

suitable

the

a

of

common

of

Thus the photo-

acid.

be obtained of a red colour and

nitrate

silver salt

sufficient

ammonia

chloride.

To

the

in

slight

to

I

formation

its

for

add

excess,

dissolve

ammoniacal

Amcr. Journ. of Science

1887.

from treat-

by

be shown as a lecture experiment. solution

1

reducing

haloid

with sufficient rapidity to enable to

in

should

salt

after being freed

into

ment with the necessary chloride

a

reduced by

agents, and the product, impurities,

(supposing

concerned, expressed

principle

the briefest terms, first

to

admixture with the normal

in

exist)

the

into

entering

May, June,

up

a solution

and

then

the

silver

solution

July,

Taking

ferrous

and November,

THE CHEMISTRY OF PHOTOGRAPHY.

60

sulphate solution

then added and the

allowed to subside,

precipitate

poured

is

the

black

clear

liquor

and the precipitate washed two

off,

The

three

times by decantation.

made

acid with dilute sulphuric acid

cipitate again

liquor

or

now

is

and the pre-

On

washed by decantation.

boiling

the latter with dilute nitric acid, washing as before,

and then boiling with

we

get

reddish

this

dilute

hydrochloric acid,

compound

accordance

in

with Carey Lea's statement [E].

had

have

I

some of the other experiments repeated before the lecture and the results are before you [F],

These coloured "

photosalts" identical

which the author

because he considers

with

the

products

contain

decomposition,

normal

haloids,

haloids,

and

less

the

them

calls

to

be

formed by photothan

halogen

reasons

the

adduced

in

support of the view that the latent photographic

image

is

formed of such photosalts appear to be

perfectly

valid.

however

to

significance

respect

to

The

enable at

the

discovery us

to

present, but I

chemical

is

too

estimate

may

its

recent real

observe with

composition

of

these

PHOTOSALTS.

this

mining

method adopted

the

photosalts that

former

part

of

to say

that

the

assumed

to

is

composition

lecture.

this

been

precisely in

urged this

By

deter-

for

to

open

have

objections which

those

6r

mean

I

deficiency of halogen has

formation

the

represent

of

a

been

a cor-

responding amount of subsalt without adequate

p roo f

may

it

be

due

really

the existence

to

an oxyhaloid compound as an essential con-

of

stituent of the coloured salt.

There which

is

not

lake

worked

chemical with

product

".

further consideration,

little

that the combination "

these recent researches

the suggestion

definite

reduction

a

in

worthy of a

is

and that are

one point

is

It

is

is

at

the

unaltered

of

the

haloid,

but

more of the character of to

chemical

certain precipitates such as

all

who have

analysis

barium sulphate,

and chromic hydroxides, alumina, property of bringing

photosalts

compounds

known

well

practically

that

&c.,

that ferric

have the

down with them a small

quantity of the salts contained in the solution

from which they are precipitated.

The

soluble

THE CHEMISTRY OF PHOTOGRAPHY.

62

thus

salts

form

chemical

definite

precipitates,

but

the

made

by

use

practical

considered

that

the

in

are

very

shows

that

preparation

known

of those coloured compounds

the

This property

exists.

of

to

with

they

washing

some kind of combination is

be

compounds

fact

remove

to

difficult

cannot

retained

as "lakes,"

which consist of some organic colouring matter precipitated

In

the

in

of

presence

alumina

[G].

accordance with this view of the nature of

the photosalts

been

has

it

found

that

silver

possesses the property of retaining

chloride also

small quantities of other metallic soluble chlorides

such as

mercuric,

ferric,

manganese, cobalt, and easily

on

present

mention

that

point

metallic

the

nickel,

washed out of the

the

and

chlorides

which

it

will

silver

form some kind of physical

not

are

As

precipitate.

of

bearing

be of interest to can

be

made

combination

to

with

magnesia, the properties of the metal being so disguised

that

whole of the tion.

The

mercury

silver,

fails

to

extract

the

even on prolonged amalgama-

compound

in

question

is

simply

PHOTOSALTS. formed by precipitating

silver

63

nitrate

with mag-

the precipitate, washing, drying,

nesia, collecting

and strongly igniting the mixed oxides of and magnesium. 1 that

From

the photosalts

may

these results

it

silver

follows

consist of such physical

combinations of unaltered haloid with the reduc-

and

tion product,

information this

is

in the

absence of more precise

the view

of their constitution

which most strongly commends

itself to

of photographic chemists. 1

Friedheim, Ber. 1888,

p. 316.

the notice

APPENDIX TO LECTURE

A

(p.

38

).

The

II.

following experiments illustrate No. i. Silver chloride is

the point referred to

:

prepared by precipitation in the usual way, taking care to use a slight excess of silver nitrate so as to

no

have

free

soluble

After

chloride.

being *

dark thoroughly washed by decantation (in a room ) the chloride is brought into the lecture room in the flask in which it was prepared and '

shaken up with of

water to which a few drops solution have been added.

distilled

sulphurous acid of the clear liquid

Some

is

filtered off into a test

tube and the flask then placed in the electric beam and kept there with occasional agitation for five minutes, filtered

another obtained.

by

side,

when a second quantity of the liquid is The flask is exposed as before for

off.

five

minutes and then

The

a

third

filtrate

three filtrates are then placed side

acidulated with

nitric

acid,

and

silver

APPENDIX. nitrate

added

second gives a

to each slight,

the

:

first

and the

65

remains

clear, the

third a considerable

liberation of showing the gradual chlorine from the chloride which at the same time turbidity,

becomes darkened.

The

of exposing moist silver chloride to light in one limb of a bent tube, the open end of which dips into a vessel of distilled

No,

water,

2.

was

exposed

old experiment

also

for

The

shown.

about

week

a

apparatus figured below

The water from in

had been in

previously

the

-

:

FIG.

chlorine

chloride

i.

the beaker on being tested for way gave a distinct .pre-

the usual

cipitate.

No.

3.

Some

moist silver chloride

is

placed in F

THE CHEMISTRY OF PHOTOGRAPHY.

66

the bottom of a tube and a strip of paper sensitized with potassium iodide and starch suspended above

mouth of the tube being closed by a well cork or india-rubber plug. The apparatus fitting is shown in the figure the

it,

:

FIG.

2.

On for

placing the arrangement in the electric beam about ten minutes the starch paper becomes

intensely blue.

This gives a most striking lecture

demonstration of the liberation of chlorine or of

some

chlorine

containing gas by the action

of

light.

B

The

(p. 39).

made use of

:

following

illustrations

were

APPENDIX. No.

i.

67

Three tubes were exhibited

in

which

silver

chloride (prepared by precipitating silver nitrate with excess of hydrochloric acid and washing by

decantation) had been sealed up, (a) moist in air (&) dried and in dry air (c) dried and in chlorine.

;

;

These tubes had been exposed to

light

some days

before the lecture, the chloride in (a) being very dark, that in (#) less discoloured, and that in (c)

had remained white. No.

Freshly prepared and washed silver was placed with water in two test tubes, one of which a few drops of mercuric chloride 2.

chloride to

solution were added.

On

exposing the two tubes

together to the electric light the chloride in the one containing the mercuric salt remains white, while that in the other rapidly darkens.

C

The increase of decomposition of with the time of exposure was shown by placing a sheet of paper coated with the chloride 50).

(p.

silver chloride

beam

light, and covering up means of a black paper portions by screen after exposures of five, ten, and fifteen

in

the

of electric

successive

minutes.

D

(P-

chloride

57)-

may

The photo-decomposition of cuprous be shown and makes a most striking

lecture demonstration.

The two

following experi-

ments can be performed before the audience No. i. Cuprous chloride is prepared by boiling :

F 2

THE CHEMISTRY OF PHOTOGRAPHY.

63

cupric oxide and scraps of copper

hydrochloric acid

till

foil

with strong

the solution becomes clear.

After being allowed to settle for a few minutes the clear solution is poured into cold water contained

an upright glass cylindrical vessel (gas jar), and the white crystalline cuprous salt immediately settles down to the bottom. The clear liquid is in

poured off and the precipitate washed once or twice with distilled water by decantation, an operation

which takes but

little

time owing to the rapidity

with which the heavy crystalline salt subsides to the bottom. The cylinder is then placed in the electric light when the cuprous chloride towards the light becomes almost black in 5 10 minutes, the side away from the light remaining

beam of

white.

No. clean

A

polished copper plate with a perfectly surface is immersed in a strong neutral

2.

solution

of cupric

chloride (prepared

by

boiling

hydrochloric acid with cupric oxide in excess)

a uniform

This in

is

film

of a greyish colour

is

till

obtained.

best done in an ordinary photographic dish,

which the solution can be rocked to and

over the

surface

of

the

plate.

In

about

fro five

minutes the plate is removed, washed with water and drained on blotting paper and then exposed, the surface being slightly moist, under a design cut out in black paper, to strong electric light for at

APPENDIX.

On

least ten minutes.

69

removing the black paper

the design

appears distinctly photographed, the exposed portions being darker than the protected This experiment has been adapted for portions. lecture

from

purposes

Priwoznik

(Ding.

Poly.

Journ. Vol. ccxxi., p. 38).

E way 8

(p. 60).

The experiment was shown

described

10 minutes for

F

the text.

in

its

A

(p. 60).

It

requires

completion. of the

specimen

rich

in the

about

purple

photobromide was exhibited, prepared according to Carey Lea's directions which are here subjoined :

3

grams

nitrate are

silver

dissolved

in

100

c.c.

water and ammonia added so as just to redissolve

The ammoniacal

the precipitate. is

of

100

poured into ferrous

silver

solution

water containing 6 grams

c.c.

sulphate in solution. To the mixed 2 grams of pure caustic soda

add

solutions

dissolved in 25 short time

of water, allow to stand for a

c.c.

and then add 2\ grams of potassium

bromide dissolved

in

a

little

water.

Make

the

mixture acid with dilute sulphuric acid, wash the residue thoroughly

by decantation, and warm

(it is

safer not to boil) with dilute nitric acid (1*36 sp. gr.

acid with

G

5

(p. 62).

strikingly

bulk of water). The formation of a lake

times

its

shown by taking a

a hot solution of

ammonium

can be

large flask containing alizarate (prepared

by

70

THE CHEMISTRY OF PHOTOGRAPHY.

adding commercial alizarine paste to the water and then a few drops of ammonia), and then adding to the hot solution, which is of a deep purple colour, first

some sodium carbonate

solution of alum.

solution

The coloured

lake

and then a is

at

once

precipitated and on pouring the contents of the flask on to a filter the solution comes through quite colourless.

LECTURE Theory of the Action of

III.

Sensitizers.

The Colour Change Indication of

of the Silver Haloids not a Quantitative their

Relative

Sensitiveness to

Modern Photography. Talbot.

The

Joseph

Collodion

Wedgwood,

Foundation of

I>avy,

Niepce and

Nice'phore

Wet

Light.

Plate

Process.

Fox

and

Daguerre.

Collodion

Dry

Plates.

IN the

last lecture

photo-chemistry

we devoted

of

the

silver

ourselves to the

haloids

without

special reference to their photographic utility,

without

taking

into

consideration

any

and

other

circumstance than that they are decomposed by the action of light with the liberation of a portion of their halogen. this

It

appeared incidentally that

photo-decomposition was accelerated by the

presence of halogen absorbents and that in the

THE CHEMISTRY OF PHOTOGRAPHY. of

case

the

iodide

some

such

was

substance

absolutely essential in order to render the product

We have

of decomposition visible. a

little

more

closely into the theoretical bearing of

this principle in order that

we may approach

practical part of our subject with

We have

next to inquire

in fact arrived at the

the

due preparation.

border-land between

photo-chemistry and photographic chemistry, and it

will

be advisable to commence with a com-

parative study of the three haloids.

From or in

the fact that halogen absorbents facilitate

some

cases are absolutely indispensable for

the photo-decomposition of a silver haloid,

we may

deduce a general principle which can be expressed in abstract

terms and which

will

be useful for

we say that a substance is composed of the elements A and B we simply mean that the compound A B is in a state of chemical further progress.

equilibrium

If

that

it

stable

is

If

from external sources either

in the

some

librium tends to

become

ordinary

energy be supplied

conditions of temperature.

electricity, or, in

under

form of heat or

instances, light, the

upset, that

is

equi-

to say the

THEORY OF SENSITJZERS.

73

compound tends to become resolved into A + B. Now some compounds have the equilibrium of their internal chemical forces more readily upset by one form of energy, and others by some different kind of energy. Thus the silver haloids which are so readily

decomposed by

Why

under the influence of heat. the case

are

light

why some compounds

very stable

this

should be

should be sus-

ceptible to ihe influence of one form of energy

others to another form of energy

molecular physics into which

The compounds which

is

and

a question of^

we cannot now

enter.

the photographer has to

deal with are of course those which are especially susceptible to the action of light.

The

first

principle

of

point in connection with the general

which

I

am now

considerable

importance,

circumstance that

nothing of

its

about to develop

it is

if

only

from

the

.having an important bearing on

The

point

is

are liable to estimate the action of light entirely

change which

one

apt to .be overlooked, to say

photographic operations.

compound

is

it

by

the

amount of

undergoes.

In

that

we

upon a

visible colour

consequence of

THE CHEMISTRY OF PHOTOGRAPHY.

74

we may over-estimate

this

the sensitiveness of

compounds and under-estimate

we may

fail

compound

is

to

altogether

acted upon

shows no change

by

that of others, or

simply because

optical accident, to bear in is

mind

after

it

is

a highly

only a chemico-

all,

and we must be careful therefore

amount of colour change

that the

no absolute measure of the amount of decomposi-

tion

that

different

is,

of the relative sensitiveness of the

compounds which may be compared with

respect to the action of light

Supposing then we

upon them.

have

a

compound

AB

capable of being resolved by light into A-f-B,

may the

compound A C with

sitiveness of

more

reason

AB

is

;

capable of forming

increased,. or the

AB

of the substance

of interest to bear in

mind

facilitates

words the sen-

in other

sensitive to light than

we speak

C

A or B C with B

the photo-decomposition

It is

it

be regarded as a general principle that the

presence of another substance

is

a

But the circumstance

that the product of photo-decomposition is,

that

recognize

light

in colour.

coloured substance

some

system

AB+C

alone, for

C

which

as a sensitizer.

that the sensitizer

THEORY OF SENSITIZERS. may

75

be a simple or compound body and that

may be either

in

any

state of physical aggregation,

gaseous,

examples

many

a

paper acts as

:

such as silver

salts,

To

or solid.

liquid,

and potassium dichromate

it

i.e.

give a few

sensitizer

towards

nitrate, ferric chloride, ;

alcohol

and ether act

as sensitizers towards such salts as the ferric, auric,

and

As an example

platinic chlorides.

sensitizer sition

of a gaseous

hydrogen accelerates the photo-decompo-

of silver chloride. 1

The

foregoing remarks will suffice to function

the. true

point to which

I

make

clear

of a sensitizer, and the next

wish to

call

your attention

an

is

extension of the general principle which has some

photographic

A B-f C

the. system

AB

importance. is

The

not only greater than that of

alone, but .its sensitiveness

affinity of

point

C

more

for

fully

A

of

sensitiveness

or B.

is

Let

by an appeal

a function of the

me

illustrate

to a special

this

case.

Silver chloride in the presence of silver nitrate

more

Is

sensitive to light than silver chloride alone

because silver nitrate 1

is

a chlorine absorbent.

Hunt's Researches on Light, 2nd ed.

p. 78.

On

THE CHEMIS TR Y OF PHO TOGRAPHY.

76

the other hand, as

I

stannic or mercuric

decomposition of

showed

the last lecture,

in

the

retards

chloride

silver chloride

photo-

because these

salts

not only have no affinity for chlorine, but they tend to prevent the liberation of that element.

compare

silver chloride

moistened with

nitrate solution, (2) with water,

tion of

stannic chloride,

system AgCl + AgNO 3

and

is

more

system

with a soluthat the

sensitive than the latter

AgCl + SnCl 4

,

chlorine readily enters into reaction with less

readily with

H

2

we

(i) silver

we should say

system AgCl-f H 2 O, and that the sensitive than the

(3)

If

O, and not at

is

more

because

AgNO

3,

with SnCl 4

all

[A].

Bearing these considerations

now

in

mind we

shall

find ourselves in a better position to deal with

the comparative action silver haloids.

If

of light upon

we expose

the three

three strips of paper

coated respectively with the chloride, bromide, and iodide of silver,

we

shall

find

.that

with

equal

exposure the chloride darkens considerably, the

bromide

slightly,

and the iodide not

Experiences of this kind formerly gave

at

all

rise to

[B].

the

ORDER OF DARKENING. impression that the chloride

bromide and the

light than the tive

is

77

more

sensitive to

latter

more

The papers used

than the iodide.

sensi-

in

this

experiment have not been specially sensitized they have been prepared

in

each case with an

excess of soluble alkaline haloid so as to insure the

absence of free silver

paper and moisture, question

now

be interpreted

nitrate.

The same

sensitizer,

The

present in each case.

is

before us

is

How

:

are the results to

?

In working out the answer to this question AVC shall

be enabled to apply the general principles

which have already been

laid

down.

You

are

prepared for the possibility that the amount of discoloration

may

not be a measure of the relative

sensitiveness of the three haloids,

and you are aware

that the sensitiveness of the salts depends also

the nature of the sensitizer.

suggests itself therefore

is

sensitiveness of the

silver

varied

by a

at

sensitizers.

pleasure It will

into an abstract

The

first

whether the

relative

could

not be

haloids

proper

selection

be instructive to put

form

:

upon

idea that

this

of idea

THE CHEMISTRY OF PHOTOGRAPHY.

78

A B+C

The system

more or

is

C

light according to the affinity

If

constituents, say B.

D

compound,

same

sensitive

sensitizer C, the

DE+C

sensitiveness of the system

compared with that of

one of the

for

we have another

E, and use the

be

may

A B-hC, because

fairly

we have

then only to consider the relative stability of

and the

and

DE

and the

relative affinities of

same substance

E

is

equal,

AB

compounds

then

and

the

DE

C

sensitiveness

measured

is

AB

B and E

If the affinity of

C.

to

less sensitive

for

for

of

B

the

by the

amount of decomposition which they undergo with the same amount of exposure to light On the other hand

if

AB

and

D E

are

sitive, their relative sensitiveness

by

selecting

different this

degrees

case

systems

some

other

of affinity

we have

for

equally

might be varied

sensitizer for

sen-

F

B and

consideration

:

AB + C AB + F

(2)

DE+C

(3)

DE + F ......

(4)

(i)

having E.

the

In four

COMPARATIVE SENTIVENESS OF HALOIDS. If the affinity of

F

C

for B, then (i) will

C

If the affinity of for

B,

B

is

greater than that of

be more sensitive than

for

B

is

It is quite

the changes on

all

gather that even

D E, of

F be

will

unnecessary to go on ringing

the systems, but you will readily if

we had two compounds

which the

stable than the second,

first

AB

was absolutely more

we might

equalize or even

reverse their relative sensitiveness sensitizer

(2).

than that of

less

then the order of sensitiveness

reversed.

and

for

79

selecting a

by

which had a much greater

affinity for

B

than for E.

Now we may

descend once

more from

the

general to the special and consider a particular In order not to complicate matters too

instance.

much we

will limit ourselves to

only, say the sensitizer

bromide and

two of the haloids

iodide.

If

we had

a

which entered into reaction with bromine

more readily than with iodine we should bromide more sensitive than the iodide

;

find the

on the

if

we had a substance capable of fixing more readily than bromine we should find

other hand iodine

the

order

of

sensitiveness

reversed.

I

may

THE CHEMISTRY OF PHOTOGRAPHY.

8o

mention incidentally that

this state

of affairs

actually seen in photographic processes.

mean as

to

may

I

is

do not

imply that the causes are quite so simple

appear from the reasoning, but there

doubt that the main principle

Thus

indicated.

is

of the

is

no

nature

has been found that in some

it

processes the iodide

is

more

than the

sensitive

bromide, while in other processes the bromide has the advantage. In considering the comparative action of light

upon the three

silver haloids

that no absolute

term tizer

"

and

therefore evident

meaning can be attached

sensitiveness

used,

it is

"

we

unless

when

even

to the

specify the scnsithis

done

is

our

ignorance of the degree of affinity between the

halogen and the sensitizer often renders

it

very

is

the

best

If

we take

-

difficult

adapted

to for

predict which

sensitizer

any particular

haloid.

three strips of paper coated with the three haloids,

paint on each a stripe of the solution

and expose

the action of light,

all

we

same

three for the

shall find the

sensitizing

same time

to

same order of

darkening as with the unsensitized papers

the

DARKENING OF THE SILVER HALOIDS. chloride darkens

most and the iodide

only effect of the sensitizer

is

to

discoloration in all three cases [C].

least.

From such an

decomposition.

the

to

the degree

relative instability of the three haloids is

The

accelerate the

experiment we gain no information as

of darkening

Si

no measure of the amount of There

may be

just

much

as

more of the bromide decomposed than there

is

or

of

the chloride, only the oxychloride, or whatever the

product of decomposition

may

be,

more

being

highly coloured than the oxybromide, makes the greater show,

and the same comparison can be

made between

the bromide and iodide.

to

make

perfectly

the

comparison

fair,

we must

amount of

the

haloids

not only disregard

discoloration but

have the same

the

the sensitizer must

affinity for the three halogens, or

different sensitizers

must be chosen, so that the

halogen-absorbing power cases.

between

In order

is

the

same

Such conditions would be very

realize in practice, but the

in all

three

difficult to

conclusion which

may

be drawn from this discussion of the comparative action of light

upon the three haloids

is

that for

G

THE CHEMISTRY OF PHOTOGRAPHY.

82

photographic purposes the order of sensitiveness bears no relationship to the

and may be If

in fact quite the reverse

we apply

that there

iodide

most

is

order of darkening

physical considerations

the least stable and

stable, the heats of

Ag, Cl Ag, Br Ag,

I

we

shall see

every reason for believing that the

is

haloids being

of this order.

the

chloride

the

formation of the three

:

.

.

.

.

.

.

29,380

.

.

.

22,700

.

.

.

13,800

gram

But although we are now able

units.

to look

upon the

degree of photo-discoloration of a silver haloid as a character quite distinct from

photographic sense,

it

its

sensitiveness in the

can be readily understood that

the earliest efforts were directed to that

compound

which undergoes the greatest perceptible change on exposure to

light.

This brings us to the beginning

of the art of photography in the year 1802,

when

Thomas Wedgwood and Humphry Davy published

an account of a process

for

copying pictures

WEDGWOOD, DAVY, AND FOX TALBOT. painted on glass or profiles cast by a strong

by means of paper or

83

light,

leather impregnated with a

solution of silver nitrate or coated with the chloride.

The

delineations thus produced were of course but

transitory because, on exposure to light, the un-

darkened portions of the design also darkened and the

picture

agent had

adds

" :

gradually

at that time

Nothing but a method of preventing the

by exposure

to the

day it is

made by Fox Talbot

in 1839.

to the

this

is

being

wanting to

elegant"

step in the direction indicated

Royal Society

from

delineation

render this process as useful as

A

fixing

been discovered and Davy

unshaded parts of the coloured

No

disappeared.

by Davy was

In a communication

experimenter described a

process for coating paper with silver chloride,

and

for obtaining impressions of objects placed in con-

tact with the prepared surface

The advance upon

by exposure

the experiments of

and Davy was however but

efficiency

Wedgwood

slight because the fixing

agent used was very imperfect in fixing agent

to light.

its

action.

was a solution of common

This

salt,

the

depending upon the solvent action of

G

2

THE CHEMISTRY OF PHOTOGRAPHY,

34

sodium chloride upon those portions of the chloride unacted

necessary

to

upon by

say that

It is

light [D].

these

earliest

silver

hardly

Talbotypes

were of no permanent value owing to the imperfect removal of the unaltered the process has

because

we have

importance

this discovery the first pro-

in

its

modern development we

as a silver print.

be of interest to

It will

add that the fixing agent now sodium

nevertheless

;

a certain historical

duction of what in

know

silver salt

in

general use,

was known to Sir John

thiosulphate,

Herschel as far back as 1819, and the application of this salt was described to the

Fox

in

in

1839

Royal Society, read shortly

a paper

after that

by

Talbot.

Although to give will

by hirn

it is

no part of

my

scheme to attempt

you a complete history of photography,

be advantageous at

historical order,

and

matter of duty to

who have

in so

name

doing

it

keep to the

becomes only a

the different investigators

helped to develop the art to

state of utility.

Davy and

this stage to

of

The

Fox

it

processes of

its

present

Wedgwood and

Talbot, with which

we have

JOSEPH NICEPHORE NIEPCE.

85

hitherto been dealing, were all based on the visible

discoloration of a silver salt under the influence of

and although such prepared surfaces were

light,

well adapted for the production of prints they were

not sufficiently sensitive to record the image formed

by a

lens

that

is

the camera picture with

gradations of light and shade.

The

efforts

fixation of this

camera image, and

in

were directed to the discovery of some sub-

former lecture

I

to fix

first

known

This investigator made his

in

1829, after fifteen years'

upon various substances. this date

work

Five years previous to

another French experimenter, the painter

Daguerre, had in

who was

a camera picture on a sensitive

surface of bitumen.

process

In a

called attention to the experiments

of Joseph Nicephore Niepce of Chalons,

and

view

all their

stance sufficiently sensitive for the purpose.

the

its

great desi-

deratum which the early experimenters had

was the

all

commenced work upon photography,

1829 he and Niepce entered into partner-

ship and prosecuted their experiments conjointly.

Niepce died

in 1833,

and Daguerre continued the

experiments upon iodized

silver plates,

which had

86

THE CHEMISTRY OF PHOTOGRAPHY.

been commenced during his partnership, the out-

come of these as the

researches being the process

known

Daguerreotype, which was announced

in

1839, a year of memorable importance in the history

of photography because this was the application of a silver

compound

first

practical

to the fixation

of a picture formed by a lens in the camera.

The system of Daguerre has now passed out of the domain of practical photography, but we shall see subsequently that this old process to teach us.

method

is

From our

still

has

much

present point of view the

of the greatest importance, because

it

brings us into contact with a kind of photo-chemical action which may, at different order to

first sight,

any of the actions with which we

have hitherto been concerned. revealed

by

this

appear of quite a

The

great principle

discovery was that the photo-

reduction of silver iodide could take place without

any

visible discoloration of the

invisible

surface

that an

image could be formed by a comparatively

short exposure, the picture only

when submitted velopment.

becoming

visible

to a subsequent operation of de-

All the photographic processes which

THE DAGUERREOTYPE.

87

have been invented since the time of Daguerre

from

take their departure

this

discovery of the

formation of an invisible image and

its

development

by appropriate treatment.

now propose

I

to pass briefly in review the

more

important of the methods which have been or are of practical

still

not with the object of

utility

giving detailed instruction in the working of the

numerous processes, perly

done

in

a

for this

of

course

could not lectures,

be pro-

and

the

can be obtained from the

necessary information

various works dealing with practical photography. It is

my

object rather to give a sufficiently distinct

general idea of the nature of the operations per-

formed to enable the chemical principles underlying

them

to be understood as far as permitted

present state of

knowledge.

necessarily be brief,

and

deal with the processes i.e.

it

will

up

by our

This review must perhaps be best to

to the

first

the formation of the invisible image.

stage only,

The

sub-

sequent operations of development, intensification, fixing, &c., will

because

be better dealt with separately

we can make

their study a comparative

88

THE CHEMISTRY OF PHOTOGRAPHY.

one, a

method of treatment which

will the better

enable us to extract any general principles that

may

underlie them.

Commencing with

the Daguerreotype,

sensitive surface prepared

we have

a

by exposing a highly

polished plate of silver (or silvered copper) to the

vapour of iodine and bromine alternately, so as to obtain a film of the bromo-iodide of silver.

proper degree of sensitiveness

is

judged by the

colour of the surface, and this requires

much ex-

In the original process iodine alone

perience.

The

was

used, but with pure silver iodide an exposure of from fifteen to

twenty minutes was necessary, so that

portraiture

was hardly practicable

of the process.

It

in the early

was found by Goddard

in

days 1840

that the sensitiveness

was considerably increased

by the use of bromine

in

conjunction with iodine,

and a good Daguerreotype plate can now be prepared of about the same degree of sensitiveness as a wet collodion plate.

The

early efforts

been alluded

to.

of

Fox Talbot have

already

Shortly after the introduction of

the Daguerreotype this experimenter

made known

THE CALOTYPE PROCESS. an improvement upon his former process,

which

in

the sensitized paper was coated with silver iodide instead of the chloride, and a mixture of gallic acid

and

siver nitrate

was used

as a developer.

It

may

be mentioned incidentally that the discovery of this J.

developer

iodide acid,

ing

be due to

to

The complete method

B. Reade.

a picture

said

is

of obtaining

by exposing paper coated with

and

Rev.

the

sensitized with silver nitrate

silver

and

gallic

developing with the same materials and

by sodium

Talbot

in

Calotype.

thiosulphate,

fix-

was patented by Fox

1841, the process being

known

as the

Improvements were subsequently made

by many workers

the sensitiveness of the paper

;

was increased by the use of a mixture of bromide and iodide modification

instead

of silver iodide alone,

being due

to

this

Blanquart-Evrard of

Lille.

The advantages

of these paper processes over

the Daguerreotype were that in the picture

was more

artistic,

surface instead of on a

first

place the

being on a dull paper

polished metallic mirror

which had to be viewed at a particular angle, and

THE CHEMISTRY OF PHOTOGRAPHY.

9o

next place the paper photographs could be

in the

used as negatives, from which a large number of positive prints could be obtained, the Daguerreo-

type

plate

One drawback

purpose. the

of course

being

want of

gularities in

unsuited

to these processes

and the

definition

for

frequent

this

was irre-

the prints, due to the coarseness cf

the paper fibre and the consequent inequalities in the transparency of the paper negative. this,

Blanquart-Evrard proposed

the paper negative by coating

and

in

in

1

To

obviate

848 to improve with albumen,

it

1849 Le Gray employed gelatine for the

same purpose. troduced the negatives,

This

last

method

and by

this

of

photography from

waxing the Calotype

who

Through the kindness has been associated with

its earliest

days,

show you some waxed paper prints obtained from them,

able historical

in-

means rendered them more

transparent and uniform.

of Mr. John Spiller,

experimenter also

interest,

I

am

enabled to

negatives,

and the

which are of consider-

being the work of

Fox

Talbot himself.

The

defects in the paper negatives

which have

THE NIEPCEOTYPE. just

91

been alluded to led as a natural result to

attempts to obtain negatives on a surface of greater uniformity and transparency, a glass plate being of course the

most

suitable

for

this

purpose.

It

appears that a successful photograph on a film of silver chloride deposited

was taken as

far

on glass (by subsidence)

back

as

1843

Sir

by

John

Herschel, but the difficulty of obtaining films this

method was too great

become generally

by

to enable the process to

practicable.

The

first

introduction of glass plate photography

successful

due to

is

Niepce de

St. Victor,

who about

the year 1847 tried various substances

nephew of the older Niepce,

for coating the plate, the first of these

paste which was soon replaced the whites

of eggs.

Niepceotype after

its

The

by albumen from

process,

discoverer,

being starch

was

known

as the

carried out

by

coating the plate with albumen previously mixed

with a solution of potassium iodide and, when dry, the film was sensitized

by immersion

nitrate bath, a layer of silver iodide

substratum being thus formed.

in

a silver

on an albumen

The

prints

from

these albumenized glass negatives were a consider-

92

TtiE

CHEMISTRY OF PHOTOGRAPHY.

able improvement

upon those obtained from Calo-

type waxed paper negatives. to

mention that

in

It will

1849 Blanquart-Evrard showed

that these plates could also be so that

we have

here the

Numerous

photography.

be of interest

employed when dry,

first

hint at dry plate

modifications and

im-

provements have been made from time to time

in

the original process of Niepce, but these do not

new

bring us into contact with any essentially principles.

The next advance

in

photography brings us to

the use of collodion as a substratum for the silver

The employment

haloid instead of albumen. this material in

was

first

suggested by Gustav

1850 and was made known

by Scott Archer

in

1851.

of

le

Gray

in a practicable

form

To attempt

anything

approaching a complete description of this process

would necessitate more than an

entire lecture

would moreover take us beyond our present of treatment. here that

till

It is

and

limits

no doubt well known to

all

within the last few years the collodion

process was the only one which could claim to be in general use

by photographers.

I

will

endeavour

THE COLLODION PROCESS. to compress a concise account of the

93

main

features

of the process into the shortest possible space.

Collodion

is

by

prepared

products obtained

nitrated

from

in the

is

which

the

is

product

C 12 H 14 O 10 (O.NO 2) 6 and

of the

is

is

By

extreme

cellulose hexanitrate,

insoluble in alcohol-ether,

the lower nitrates only being dissolved solvents.

Gun-

was discovered by Schonbein

This compound ,

and

nitric

appropriate strength.

nitration of cellulose, in 1846.

a

form of cleansed cotton wool and

of

sulphuric acids

in

cellulose is

by means of a mixture of

nitrated

cotton,

cellulose

The

mixture of alcohol and ether.

employed

certain

dissolving

by

these

stopping the nitration short of the

production of gun-cotton, which can be done by

employing a mixture of with a

and

little

nitric

and sulphuric acids

water, the trinitrate,

tetranitrate,

C 12 H 17 O 10(O.NO 2)3

C 12 H 16O 10 (O.NO 2)4

,

are formed and

these possess the necessary solubility, constituting

what

is

known

as collodion pyroxylin.

In addition

to this difference in solubility, gun-cotton

is,

as

you

know, much more inflammable and burns with greater velocity than collodion pyroxylin [E].

It

94

THE CHEMISTRY OF PHOTOGRAPHY.

will

be

well

mention here

to

passing that

in

nor the soluble pyroxylin are

neither gun-cotton

nitro-derivatives in the sense

known

to chemists,

but are organic nitrates of the type of ethyl nitrate,

C 2 H 5 .O.NO 2 is

,

it

desirable to mention this

on

works

many

formulae

C 3 H 5 (O.NO 2 ) 3

trinitrate,

glyceryl

thought

or the so-called nitro-glycerin which

because

in

the

chemistry

photographic

are written

have

I

.

though the pyroxylins

as

were true nitro-derivatives.

When

collodion

of a perfectly clean ether

poured on to the surface

is

evaporate rapidly and leave a beautifully

uniform

of

film

transparent

order to pre-

In

pare the photographically sensitive collodion

"

is

salted

iodide

soluble

the two.

or

bromide,

the

immersion

or

The

monium

in

"

bromide

salts

a

with the

iodide

plate

bath

of

and

are

nitrate

bromide or iodide

in

of

am-

cadmium

sensitized

being silver

some

it

mixture

a

usually employed

cadmium coated

or

the

surface,

dissolving in

by

which

pyroxylin

adheres firmly to the glass.

reacts,

and

glass plate, the alcohol

the

by

which film,

COLLODION DR Y PLA TES. giving a layer of

silver

double decomposition

95

bromide or iodide by

:

NHJ + AgNO = Agl + NH NO 4

3

3

CdBr2 +2AgNO 3 = 2AgBr + Cd(NOa) a

The

plate

is

exposed

in

a moist state, that

is

to

say, covered with a film of adhering silver nitrate solution.

It is

mind because that

is

important to bear this condition

it is

this film of silver nitrate solution

essential as a sensitizer

having no sensitizing

action

are sufficiently well

We

;

the collodion

absorbing

halogen

power,

itself,

no

has

[F].

The inconveniences of

it.

in

the wet collodion process

known

to

all

who have worked

need only consider the cumbrous equip-

ment required

for field

work and the

difficulties

of

carrying about and working in hot climates with a

solution

containing

ether.

Soon

after

the

introduction of the collodion wet plate process the first

attempt to prepare collodion dry plates was

made by Taupenot

in

1855,

and

this process

was

ultimately developed through various stages into a

THE CHEMISTRY OF PHOTOGRAPHY.

96

method

useful

really

Russell.

It

must

1861

in

by the

here to point

suffice

principles concerned in this

most

Col.

late

out the

advance

distinct

photographic methods.

in

The

of dry

object

furnish a sensitive

which can be kept

film

required for use, and which for

without

exposure

to

is

photography

plate

is

any time ready

at

to

having

till

the

undergo

operations of coating and sensitizing immediately before

placing

difficulty

in

which had

the

The

camera.

was the tendency of the collodion off the

the

plate

when

dry,

to

film

peel

when submitted

or for

to

dry

plate

develop-

holds

the

requisite

firmly to the glass

ment.

be met

Some substratum which

operations

ment.

practical

in the first place to

is

therefore

In practice the plate

is

albumen or gelatine or with

the first

a

first

film

require-

coated with

film

of

india-

rubber by pouring a solution of the latter substance in benzene or chloroform over the surface of the glass and allowing the rate.

When

the

substratum

coated with salted collodion

is

solvent

to

evapo-

dry the plate

and

sensitized

is

by

COLLODION DR Y PLA TES. immersion

Up

way.

in

a silver nitrate

bath

97

the operations

to this point

usual

the

in

be

may

regarded as leading to the production of a wet collodion

with

plate

a

film

supported

on

a

substratum of albumen, gelatine, or india-rubber.

But now the plate undergoes further treatment and a

little

explanation

make

will

clear

the

principles concerned in the subsequent operation.

were allowed to dry

If the plate

after

being

taken out of the bath, the silver nitrate in the adherent film would crystallize as in

so

would disintegrate or

doing,

dried and,

it

freckle

the

surface of the silver haloid so that the photographic efficiency

destroyed.

of

the

latter

would

The excess of silver

be

completely

nitrate

is

therefore

removed by washing and, from what has already been said

in connection

with the wet plate col-

lodion process, you will readily understand that in

washing out the excess of

silver

nitrate

we

practically destroy the sensitiveness of the plate.

The

next

surface

operation

of the

film

therefore

is

to

coat

the

with some substance which

will restore the sensitiveness

that

is

with some II

THE CHEMISTRY OF PHOTOGRAPHY.

98

which

substance

plays

function

these

and

are in

after

known

In all

"preservatives"

this

part inasmuch

surface

sensitive

contact with the

the

silver

as

technically

as they preserve the

manner of a

the

Such substances as

drying.

one sense they play

mediate

of

part

wet process and which retains

nitrate film in the this

the

much

air,

from imthe

after

varnish.

early days

of dry plate photography

kinds of substances were tried as preservatives,

and. on looking back of

history

the

art

into

we

this

chapter a

find

collection

formulae which are amusingly grotesque.

only remind you of the tea, beer processes. gallic acid,

these

in

and tannin, the

the

substances

sensitizing

action

first

latter

preparation

gum and

are

being of interest

of dry plates.

organic

sensitizers,

All their

being due to their faculty of

to explain that the preservative

of

need

used successfully by Col.

absorbing the free halogen [G].

surface

coffee,

I

of

Other preservatives are albumen,

as the substance

Russell

of the

the plate after the

It is

remains only applied to the

removal

of the

PRESERVATIVES. silver

nitrate, the

99

excess of preservative solution

being drained off and the plate being then allowed to

dry.

Preservatives

amount of moisture results

as

in the

first

a

far

sensitizer

sensitizer

is

as

which

necessarily

regards

likely to

be

is

next

the in

give

sensitiveness,

place water itself

and, in

a

retain

more

to

slight

the

best

because

some extent

place,

a

moist

perfect contact

with the silver haloid of the sensitive

film.

H

2

APPENDIX TO LECTURE

III.

*

A

(p.

trated

The

points referred to were illustaking a sheet of paper coated with silver

76).

by and painting on

two

chloride

it

solution of silver nitrate,

and

stannic chloride.

On

(2)

stripes, (i)

with a

with a solution of

exposing to the

electric light

few minutes the ground-colour darkens, the stripe (i) becomes darker than the ground-colour and the stripe (2) remains white. The paper is for a

prepared with excess of sodium chloride so as to contain no free silver nitrate.

B

(p.

76).

The papers coated with

were fastened on a board and

same time

C

all

the haloids

exposed

for the

to the electric light.

The

experiment was repeated after a stripe of a solution of silver nitrate had been painted on each of the three strips. The excess of (p.

Si).

last

silver nitrate solution

blotting paper.

should be removed by clean

APPENDIX.

D

The

84).

(p.

solution of

I0 1

solubility of silver chloride in

sodium chloride

is

easily

shown by

a

boil-

ing some freshly precipitated chloride with excess of strong brine and pouring the clear solution into cold water when the dissolved chloride is reprecipitated.

E

The

93)-

(p-

difference Jn the combustibility

of gun-cotton and collodion pyroxylin can be shown by placing a loose piece of each on the

top of a

heap of rather fine-grained gun-

little

powder. The gun-cotton on

ignition simply scatters

the powder, while the slower burning pyroxylin explodes the latter. This experiment requires some dexterity.

F

-(p.

A

95).

collodion

and

the usual

way

glass plate

is

coated with salted

sensitized in a silver nitrate bath in in

"

non-actinic

"

light.

On

removal

from the bath one half of the plate is immersed for a few seconds in a solution of potassium iodide and, after

washing

slightly, a brief

exposure

is

given to

On developing by iron (electric) light. developer the portion of the plate from which the excess of silver nitrate had been removed

diffused

by immersion

in

the

potassium

iodide

remains

white, while the other half rapidly darkens. The action of an organic sensitizer (p. 98).

G

can be shown by coating and sensitizing a wet " collodion plate in " non-actinic light in the usual

102

way.

THE CHEMISTRY OF PHOTOGRAPHY. After removal from the bath the plate is in a stream of water till free from silver

washed

of water drained off on blotting paper, and a design painted on the surface of the plate with a brush moistened with a solution of nitrate, the excess

gallic acid

thickened with a

little

gum.

The

plate

then exposed to the electric light for a few iron or seconds, washed, and developed (by

is

ferrous oxalate developer)

when

the design

ground.

the usual

manner, appears darker than the backin

LECTURE Emulsion

Collodion

Photography.

The Process

Emulsions.

IV.

Emulsions.

Gelatine

Different States

of Ripening.

Extreme

of Physical Aggregation of Silver Bromide. Sensitiveness

Applications of

THE

Modern Dry

photographic

in the last

Emulsion.

of Gelatino-Bromide

lecture

processes

belong

now

the historical

to

is

of

the

highest

interest

a thread of

them which enables

us to follow their development in a is

reviewed

briefly

portion of the subject, but there

continuity running through

Recent

Plates.

for

manner which

those

who

take

pleasure in studying the evolution of scientific discovery.

It

would lead

me

too far astray to attempt

such treatment on the present occasion and

must commence

at once with the latest

we

methods

of dry plate photography, the legitimate offspring

104

THE CHEMISTRY OF PHOTOGRAPHY,

of the collodion dry plate processes last described.

The

collodion

of

and

transport

the ad-

all

as

far

regards

manipulation, but

and,

till

manu-

their

was properly understood, were apparently

facture

their

in

capricious

point of view

we know what it

is

by the

liberated

action

development

subject to which

we

shall

efforts

in

the

introduction

of

1862 by Col. Russell, a

have occasion to return. left

much

of sensitiveness.

The

at their best these

to be desired

the

dry plates was con-

the

by in

of

wet plate process.

of collodion

facilitated

But even

so-called

possessed by a sensitizer such as

The working

alkaline

lack of sensitiveness

means that the

silver nitrate film of the

siderably

chemical

a

have not the same power of ab-

sorbing the halogen light as

From

action.

in this case

preservatives

next

as

of

in sensitiveness

they failed

means

them

of

expected

vantages facility

dry plates possessed

way

dry plates

of photographic experimenters were

directed to the production of silver haloids in an

extremely

fine state of division

viscid sensitizing

suspended

medium which

in

some

could be spread

COLLODION EMULSION.

105

over the surface of the plate and there allowed

The development of

to

dry.

us

down

idea

this

brings

to the photography of the present,

i.e.

to emulsion photography.

As find

with so

many

other useful inventions,

germ of

that the

existence long before

this

it

discovery

bore

came

As

fruit.

far

we into

back

as 1853 Gaudin proposed to use the silver haloids in

the form of emulsions, and

of

details

his

1861

in

Three

experiments.

he gave

years

later

emulsion process was

made

known by Bolton and Sayce, who used

silver

the

successful

first

bromide emulsified collodio-chloride

was

described

now attempt of

paration

which

I

felt

in

collodion

and

;

emulsion for printing

by Wharton

1865 a

in

purposes I

Simpson.

must

to give a brief account of the pre-

these it

emulsions, and

the

remarks

incumbent to make when dealing

with the wet collodion process must be repeated here with

tenfold

meaning

;

for

if

could occupy an entire lecture for

that

its

process

description,

the emulsion processes, owing to their multiplicity

and the

immense

amount of

practical

detail

THE CHEMISTRY OF PHOTOGRAPHY.

106

connected with their working, would demand a

whole course of realize

time that

this

by

you must

But

lectures.

am

I

fully

not dealing with

photographic manipulation, and

I

shall

be obliged

branch of the subject only

to treat of this

in

a

very general way. s~~\

To

a

prepare

emulsion

collodion

the

silver

r*

must be precipitated

haloid self.?

them salt

it-

There are of course several ways of doing

but

this,

the collodion

in

the

all is

one general

principle

and the haloid

that the silver nitrate

with which

it

reacts should

underlying

be brought to-

gether in the presence of a solvent in which the collodion

is

Alcohol

itself soluble.

which meets these requirements, and in

this

liquid

some haloid

solving therein and solution,

salt

salt the

we can then add an

is

the solvent

if AVC

dissolve

capable of dis-

collodion with this alcoholic

solution

of silver nitrate and thus precipitate the silver haloid in the required state of subdivision in the collodion, wherein to

it

remains

the viscosity of the

haloids

suspended

medium.

Of

owing

the silver

the bromide appears to lend itself most

COLLODIO-BROMIDE EMULSION. readily to this process of emulsifying,

and

107

collodio-

bromide emulsions are therefore the more generally

As

prepared.

cadmium

or

the soluble bromide that of zinc

is

usually employed, the

necessary

quantity of this salt dissolved in alcohol being

added to the collodion and then the alcoholic soluof silver nitrate

tion

is

gradually poured

in,

the

mixture being briskly agitated after each addition.

When

the whole of the silver nitrate has

been

added

the

for

some time our

occupy needless

ducted

So

in

cerned

to

ripen,

attention

allowed to stand

is

a

process which will later

again

rationale of the process

is

is

simple

at least as far as the chemistry

but the emulsion

use at this stage.

for a sensitizer in

is

in the

emulsion prepared

It

evident

that

is

con-

by no means ready

Consider for a

moment

to the necessity

the wet collodion process, and

us ask whether there

is

It

red or "non-actinic" light [A].

what has been said with regard

let

on.

say that these operations are con-

far the

enough

for

to

emulsion

the

is

any

in the

sensitizer present

manner

described.

only substance

capable

THE CHEMISTRY OF PHOTOGRAPHY.

io8

of performing this function

and

the silver nitrate,

is

might be imagined that

it

of this

salt

if

the quantities

and of the zinc bromide were so

adjusted as to leave a slight excess of the nitrate

over and above the quantity indicated

by the

equation

ZnBr 2 + 2 AgNO 3 = 2 AgBr + Zn(NO 3) 2

the

all

conditions

But

would be met.

for

necessary

impracticable, and a

this little

plan

is

,

sensitiveness

unfortunately

consideration will

make

clear the reason.

The

emulsion, at the stage where

contains

addition

in

the pyroxylin,

to

a

soluble

and

salts

would

would thus

crystallize

disintegrate

photographic surface. fit

for use

it

is

order to remove

it,

haloid and

silver

by double decomposition.

were coated with

plate

left

excess of silver nitrate and

the

the zinc nitrate formed If

the

we

To

therefore

this

as

mixture

the

and

film dried

destroy

render the

salts,

the

emulsion

washed with water

the soluble

the

and

this

in is

COLLODIO-BROMIDE EMULSION. done either on the plate the emulsion

is

washed

itself after

in mass.

It

109

coating, or

sometimes

is

customary to speak of these two kinds of emulsions as

"

washed

"

and

misleading because latter

"

unwashed," but

after coating

in

bulk after preparation.

to

describe the

emulsion

and the former

would be better

It

as

is

both washed, the

they are

on the plate

this

either film-washed

or mass-washed.

The

preparation of a collodion emulsion

by film-washing

plate

a comparatively simple opera-

is

After the plate has been coated with the

tion.

emulsion the latter

is

allowed to set and then,

before the salts have time to crystallize, the plate is

all

immersed

in

water

diffused out.

It

till

these soluble salts have

might be expected that by

thus washing out the soluble salts the sensitiveness of

owing is

by

the

film

would

be

seriously

impaired

to the removal of free silver nitrate.

no doubt that the sensitiveness this

treatment,

destroyed because,

but in

it

making

is

There

is

diminished

not

altogether

the emulsion, the

quantities are so adjusted as to .have

an excess

i

THE CHEMISTRY OF PHOTOGRAPHY.

io

of

silver

when

and

nitrate,

precipitated under these conditions

more

sensitive than

an

of

excess

of

when formed the

trace

an excess of that

salt

same way other

of silver is

emulsion-coated plate the sensitiveness

is

in

coat

the

After the

preservative before

some

authorities

plate

with

in

the

it

indicated,

case

suffi-

customary

some is

organic

stated

of

the sensitiveness

preservative.

In

is

silver

not increased by the

other words, the retained trace

a better sensitizer than

the

playing no part

this

of silver nitrate

is

preservative, the

latter

kind of emulsion beyond the film

by

collodio-

bromide emulsion prepared with excess of nitrate,

of the

washing

It is

drying, but

that,

the

traces

manner

to

when

found to be

therefore

washed

The

in

just

retains

cient for photographic purposes. to

presence

nitrate

present,

chlorides.

always

no doubt due

is

that silver chloride

soluble

in the

is

bromide under these

different sensitiveness of the

the retention of a

is

it

bromide.

soluble

two modes of preparation

bromide

silver

protection

from atmospheric influence.

in

of the

RIPENING OF THE EMULSION.

When

bromide has been

silver

collodion

manner

the

in

takes

it

maximun

its

in

a

degree of

period of so called "ripening"

sensitiveness, the

varying from

emulsified

described

time to acquire

certain

in

8 to 24 hours

the

to

according

soluble bromide used as a precipitant, the quality

of the collodion, &c. too

loses

it

long

bromide

particles

the

If

in

emulsion

the

sensitiveness,

become denser

is

and

kept silver

tend

to

out of the medium, and the photographic

settle

is

efficiency

Now

destroyed.

behaviour

this

the silver haloid in an emulsion

is

of

of particular

importance to us at the present moment, because this

is

state

the

hint

first

of aggregation

influence

upon

we

get

may

critical

of molecular aggregation

have

shall

have

a

that

of

much more

the to

as

a

soon

as

possible

after

it

bromide.

ha

haloid.

silver

do

with

this

will readily

outcome

practical

property the stock of emulsion as

determining

point in the state

ripening process hereafter, but you

gather

physical

the sensitiveness of the

There seems to be a

We

the

that

must

of

be

ripened,

this

used

and

1

THE CHEMISTR Y OF PHOTOGRAPHY.

12

this

one

is

of

the

of

disadvantages

film-

'

washing.

To

prepare an emulsion by mass-washing the haloid

silver

is

the

in

precipitated

collodion

as before, using either an excess of silver nitrate or an

excess

of

emulsion

the character of the

being allowed to ripen are

evaporated or

gelatinous

the

distilled

emulsion

under

remain

to

water

are thus

sion

is

dry

in

freed

the

according

After

required.

and

ether

and

the

semi-

off,

by being allowed

which

The

is

frequently

soluble crystalline

removed, and the washed

from water by being air

to

alcohol

washed

poured off and renewed. salts

haloid

soluble

spontaneously,

emul-

allowed

to

by heating

"on

a water-bath, or by being washed with alcohol.

The emulsion may

at

this

be

stage

regarded

as consisting practically of a dried silver

pyroxylin that

is

necessary

amount in

the

dry.

jelly,

and to prepare is

it

for

haloid

use

all

to redissolve in the requisite

of

alcohol

and

ether,

usual

way,

apply

the

The advantage

of

coat

the

preservative

mass-washing

is

plate

and that

GEL A TINE EMULSION. a large quantity of emulsion can at

one

deterioration

The more

and

operation

emulsion than

historical

because,

as

you

displaced

by

gelatine

transcend them

with

the

because cally

are

in

was the

the

interest

have

they

which I

greatly

have dealt

some

in

been

detail

emulsion process practi-

we have been enabled

to

although there are neces-

points which

many

practical

emulsions

now

a good general idea of the prin-

it

A

to pass over.

under

of

aware,

first

ciple of emulsification, sarily

without

are

processes

emulsion

introduced, and

get out of

prepared

kept

sensitiveness.

collodion

it

be

3

required for use.

till

collodion

of

may be

1 1

I

collodion

most

have been obliged

emulsion plate, even

favourable

circumstances,

can

seldom be made as sensitive as a collodion wet plate,

whereas

have

from ten to one hundred

sitiveness

The

a

gelatine

emulsion

plate

of a collodio-bromide emulsion

reasons

for

this

increased

will

but we have

consider the process

first

to

plate.

sensitiveness

of gelatine emulsion

appear

may

times the sen-

in

due course,

I

itself.

THE CHEMISTRY OF PHOTOGRAPHY.

ii4

The

use of gelatine as a vehicle was suggested

by Gaudin

in

1853

but the development of the

;

modern process dates from the year R.

Dr.

L.

Maddox made known

of the

details

the

practical

method of preparing a

gelatino-

Of

bromide emulsion. but

little

the chemistry of gelatine

known.

at present

is

when

1871,

It is

a nitrogen-

animal substance extracted by boiling from

ous

bone

cartilage,

skin.

It

connective

sinew,

and

tissue,

appears to be resolvable into two com-

pounds, one derived from the unhardened cartilage

and

called

from the hardened

The of

cartilage

the

other

known

substance constitutes the

latter

the

and

chondrin,

purer

forms

of

gelatine

as

derived glutin.

chief

used

by

part

the

photographer. It will

be advisable to give here a short account

of those properties of gelatine which are connected

with

its

emulsion.

use as a vehicle for the

When

allowed

to

silver

remain

in

haloid

contact

with cold water, gelatine swells and absorbs a considerable

quantity of the

the substance

gradually

latter

forms

a

;

if

warmed

solution,

the

GELATINE AS A VEHICLE.

on cooling to a gelatinous mass

latter solidifying

more or

firm

less

a

in

Eder

used.

is

available

gelatine sions is

confers

it

;

the

dissolved

the

for

the

water

necessary

degree

upon

when C.

It

renders

of emul-

preparation

keeping the finely divided

for

which

property of gelatinising

this

jelly

cooled to 20

is

that

states

good specimen should give a firm

a four per cent, solution

to

according

consistency

of water

the quantity

115

which

in

of viscosity haloid

silver

without

undergoing

decomposition

changes

soon

to

ordinary

temperatures

begin

take

and

chemical

;

even

place

more

still

at

rapidly

The time when decomposition depends much upon the quality of

on heating.

an

gelatine,

but

which

usually slightly acid

is

alkaline

ten

alkaline

of

and

days

the

four

to

in

Moist gelatine cannot be long kept

suspension.

in

it

average

to

begins

or

so.

start

production

high

of

at

evolve

Inferior

with,

quality first,

becomes after

which

may show

ammonia

the

sample,

ammonia

kinds,

sets

are

indications

three

after

days. I

2

or

THE CHEMISTRY OF PHOTOGRAPHY.

n6

As

a point of practical importance

mentioned

be

that

the

gelatine seriously impairs

its utility for its

should of

decomposition

making, not only because is

it

the

emulsion-

power of "setting"

diminished, but also because the products

of

decomposition have a tendency to reduce the

its

silver light,

haloid

i.e.

directly,

without

and thus to give an emulsion which "fogs"

With

on development.

to

respect

the

various

processes for preparing gelatine emulsions refer

to

exposure

you

I

must

works devoted

for practical details to the

to photographic manipulation. 1

I

propose to give

only a sufficiently concise account to enable you

understand the general method of their pre-

to

drawing comparisons between

paration,

gelatine

and collodion emulsions whenever we can learn

any point of importance treatment. familiar,

such

The bromide emulsion

is

mode the

of

most

although for certain purposes the gelatino-

chloride 1

from

emulsion

is

preferred.

The former

will

See for instance Abney's Photography with Emulsions, 1885, or

Eder's Ausfiihrliches

Handbuch der Photographie,

th.

iii.,

"Die

Photographic mit Bromsilber-Gelatine und Chlorsilber-Gelatine," Halle, 1886.

GELATINO-BROMIDE EMULSION. however be

our present

for

typical

sufficiently

117

purpose.

order to prepare an emulsion

In

with

haloid

we may

gelatine,

___

of a

either

silver

mix

the

-

with the gelatine solution

silver nitrate solution

and add the soluble

haloid, or

we may mix

latter

solution

with

silver

nitrate,

ln either case the

same

the

silver

the

is

making a

collodion emulsion.

(sometimes mixed soluble is

haloid

one point

from

a

in

which

no excess of

little

which

mind, and that is

in

the

therein

by

as

it

iodide)

is

in

is

it

that

is

is

the

There

[B],

gelatino-bromide

emulsion

of potassium bromide

the

Potassium bromide

generally employed

collodion

to bear in

with

the

is

precipitated

same manner

in

add

result

emulsified

is

agitation

just

and

gelatine

haloid

and

solution

gelatine

the

the

differs

essential

the quantity

so adjusted as to leave

silver nitrate.

It will

be instructive

to consider the chemical reasons for this difference

between the two emulsions. It

is

a well-known fact that organic colloidal

substances,

such as gelatine and albumen, form

THE CHEMISTRY OF PHOTOGRAPHY.

US

some kind of combination

with crystalline

chemical compounds

An

silver nitrate.

do not say

I

will

experiment

water to which a

we

If

point perfectly well.

salts

like

illustrate this

dissolve

silver

little

definite

gelatine

nitrate

is

in

added,

a certain amount of the salt combines with the

and forms what may be called techni-

gelatine cally

a

"

stable

sufficiently

the

silver

little

this

gelatino-nitrate,"

to

the

disguise

We may

salt.

compound being presence

add

instance

for

potassium bromide or chloride

or

of a

hydro-

chloric acid to such a solution without producing

any

precipitate as long as the reagents

in

excess [C].

if

there

is

a

the whole of

unpleasant

heat

with

oxide of containing

developer

deficiency

the

this

last

the

silver

and

of

or

not

de-

the

photographic

by

light

or

of

metallic

silver

or

some organic compound

which

fogs

is

compound has very

decomposed

formation

silver

potassium bromide

from

is

it

;

of

"gelatino-nitrate"

properties

view

point of

not

In consequence of this property,

Now

composed.

are

the

acts

plate

directly

on

[D].

This

the

of

COMPARISON OF EMULSIONS. means that

course

nitrate

readily understand

and you

latter,

therefore

19

reduced by

is

when heated with the

gelatine will

silver

1

why

it

so

is

important to insure the complete decomposition "

"

of the

gelatino-nitrate

by having a

sufficient

quantity of potassium bromide. In

you silver

a collodion

know

emulsion, on

we may have an excess and this does no harm if it

that

nitrate,

washed out afterwards. there

may

the other hand,

But even

in

this

veloper and

what

is

case

be formed certain organic compounds

containing silver which act directly on

cautions

of

are

these

fog

the

taken.

unless

plate It

compounds

is

special

pre-

exactly known

not

are,

the de-

but

during

the

nitration of cellulose small quantities of oxidized

products are formed which are not removed by

washing, impurities

and in

which the

remain

pyroxylin.

as

contaminating

It

is

these

products which form compounds with the of the nitrate, and in

making a

with excess of the latter salt

add a small quantity of

nitric

bysilver

collodion emulsion it

is

necessary to

acid in order to

THE CHEMISTRY OF PHOTOGRAPHY,

120

decompose these

silver

This

compounds.

precaution Avhich has to be taken, and the reason

now

is

emulsion performs

acid in a collodion

function

work,

hope

a

nitric

similar

the excess of potassium bromide in

to

a gelatine emulsion. its

I

The

clear.

sufficiently

the

is

The

acid, after

of course removed

is

it

has done

by the washing

process.

remembered

be

will

It

collodion

emulsions

when

precipitated

first

that

the

silver

not in

is

property sion

When silver

the

state,

and

is

sensi-

time to reach

ripening.

The same

shown by a gelatino-bromide emulmore in

precipitated

bromide the

much

a

in

first

division,

of

period

bromide

most

its

tive condition, but takes a certain this

with

connection

in

is

size

in

of

striking

the

emulsion

an extremely the

great extent determined

particles

by the

manner.

fine

the

state

being to

of a

state of dilution

or concentration of the solutions of gelatine and silver nitrate

and of the potassium bromide.

size of these finest particles has

The

been determined

by Eder from micrometric measurements

to

be

THE RIPENING PROCESS.

121

from *ooo8 to -0015 of a millimeter in diameter. In

this

but

stage a gelatine-bromide emulsion offers

emulsion

advantage over a collodion

little

Now

from the point of view of sensitiveness. in

was observed by Bennett that

it

1878

gelatine emulsion

a

week

at a

sensitiveness

is

moderate temperature (32

became

increased.

the temperature to 60

C.,

many days

in

ripening

then

the ripening of

by

increasing

a few hours

same

the

producing

the

C.)

was

It

the emulsion could be accelerated

as

a

allowed to stand for about

found by Stuart Wortley that

temperature

if

the

at

this

sensitiveness

cold.

In

1879

Mansfield carried this principle a stage further

and showed that by keeping the emulsion the temperature of boiling water for less

an hour, the could

be

point

in

maximum

reached.

the

history

at

than

of practical sensitiveness

This

is

of our

the

culminating

modern dry-plate

processes.

After the precipitation of the silver bromide in the gelatine solution the

next step

paration of the emulsion

to ripen

is

in the preit,

and

this

THE CHEMISTRY OF PHOTOGRAPHY.

122

done

is

by placing the

vessel

the

containing

gelatino-bromide in a bath of boiling water and it

keeping

there with

occasional

The proportions of gelatine

thirty or forty minutes.

and water given

in

for

agitation

the published formulae, have

been selected by experience as being the best for

precipitating the

silver

bromide In

state of molecular aggregation as

such

a

that

to insure

the ripening takes place in the shortest possible time.

The

reason

of

this

will

subse-

appear

quently.

When

the operation of ripening

the emulsion

cooled

is

the right consistency, strength

by

dilution

gelatine solution.

we have

and,

if

not

made up

is

is

completed already of

to the proper

with a further quantity of

It is

obvious that at this stage

a jelly containing potassium nitrate and

excess of potassium bromide in addition to the silver

bromide.

been explained sions

be

For reasons which have already in

the soluble

removed

connection with collodion emul-

and

before

allowed to dry on

crystallizable

the

emulsion

the plates.

salts

film

There

must

can is

also

be

a

COMPLE TION OF EMULSION. why no

special chemical reason

sium bromide should be

which we later

be

shall

on

therefore washed, being

means

The

in

warm

plates

There

is

gelatine

fascination haloid. is its

have been

salts

the solution

is

is

dis-

redissolved

and the

filtered,

can then be coated with a uniform film

and allowed

a

the soluble

water,

of canvas)

and then treated with

The washed emulsion

solved out.

is

reduced by mechanical

first

to a fine state of division till

emulsion,

emulsion

cold

through the meshes

(forcing

cold water

the

a better position to appreciate

course.

this

in

in

23

excess of potas-

in

left

1

to dry.

one operation

in the process

emulsion which I

possesses

certainly a very remarkable investigation, utility,

results in the

researches

of

a

certain

allude to the ripening of the silver

The gradual change

practical

of making

apart

from

cannot but

in

molecular state

phenomenon, and the

lead

domain of molecular investigators

question to

of

important

physics.

The

who have occupied

themselves with this subject, and especially those of

Van Monckhoven,

Eder, Abney, and Vogel,

THE CHEMISTRY OF PHOTOGRAPHY.

124

tend to show that the change

The

one.

by and

increase in sensitiveness

is

accompanied

probably the result of an actual increase

is

the size

in

a purely physical

is

of

the

of

particles

The micrometric measurements shown that the most

sensitive

bromide.

silver

Eder

of

have

form of emulsion

contains particles of from "003 to '004 of a milli-

meter for

If the

diameter.

in

ripening

continued

is

too long a period the particles

go on

still

growing, but chemical action at length begins to take place.

be heated

In

for too

bromide

silver

if

long a time with the gelatine

becomes reduced and gives

it

solution,

other- words,

rise

to

products which are directly acted upon by the

and

developer reason

why

bromide

in

thus is

it

the

produce

necessary to

first

place

aggregation as to insure of ripeness with the

Not only increase

but

the

changes

in

is

the

the

in

the

precipitate

the

such a

state

maximum

the

minimum amount

of

growth

the optical

bromide

the is

degree

by an

particles,

associated

properties

of

of heating.

ripening accompanied

size

molecular

in

This

is

fog.

with

which enable

CONDITIONS OF SENSITIVENESS.

125

the operator to ascertain whether the emulsion

first

bromide

precipitated

reflected

yellowish white

and reddish by transmitted

light

this

the

particles

layer of

and

is

the insensitive, unripened

is

grow the

the emulsion

violet

light

in

red

richer

by a blue

other

words, the larger particles absorb more of red rays and allow

more of the blue and

rays to pass through. is

When

ripe the

grey or greyish violet by transmitted

the silver bromide appears greenish light.

This

haloid,

and

is is

the

most

sensitive

the condition

by

;

As

in

In

rays.

by

light

condition.

transmitted

becomes

and poorer

The

of sensitiveness.

of the proper degree

is

the

violet

emulsion light,

and

reflected

form of the

sought

in

the pre-

paration of the emulsion.

The changes undergone by are, as

acter in

I

the silver bromide

have already stated, of a physical char-

and are not accompanied by any change

chemical composition.

solved

out of

a

If the gelatine

ripened emulsion

the

is

dis-

residual

bromide has the normal composition corresponding to

the

formula AgBr.

Moreover, the emulsion,

THE CHEMISTRY OF PHOTOGRAPHY.

126

if

well

prepared, unless

developer

not

is

acted

previously

upon

It

is

most

light,

attained.

is

evident that the bromide in the

therefore

emulsion

sensitive

to

exposed

even when the extreme of sensitiveness

the

by

not

is

same chemical condition

as

it

is

the

in

present after

photo-

But we know that over ripening

decomposition.

causes a decomposition of the bromide and enables it

to

be acted upon directly by the developer

without previous

in

exposure

longed heating of the

other words pro-

silver haloid in the

presence

of gelatine produces a similar effect to a short

exposure to

light.

In an

we have overstepped

over-ripened emulsion

the boundary of

physical

change and have entered the region of chemical change.

This molecular transformation

silver salt

of

the

from a small-particled, insensitive state

to a larger-particled, sensitive condition, thus in-

troduces a

new

factor

photo-instability of a

into

the question of the

chemical compound.

The

physical state of aggregation must be taken into consideration

as

well

as

those

conditions which have been

other

shown

to

chemical

have

a

MOLECULAR AGGREGATION OF HALOIDS.

127

determining influence upon the 'sensitiveness

of

photo-decomposable compounds.

The

fact that the chloride

and bromide of

silver

are capable of existing in different molecular states

has long been known, detailed accounts of the modifications

various

Stas

in

having been

This

1874.

published by

eminent investigator even

observed that by the prolonged boiling of silver

bromide with water, the

salt

acquired an increased

sensitiveness analogous to that

With

the process of ripening.

in

in

growth during will

size

of

be necessary, and

some

respect to the

bromide

the silver

this last process

which takes place

particles

further explanation

this is the

most favourable

opportunity for considering the subject. Silver

bromide

is

practically insoluble in water,

bromide

and

gelatine in hot, aqueous solution enables a

per-

but

the

presence

of

potassium

ceptible quantity of the silver salt to be dissolved up.

Let

me

illustrate this

periment shown lecture.

If

bromide and

we

in

an

take

gelatine

by

reversing an ex-

earlier part of the present

a in

solution

of potassium

water and add

silver

128

THE CHEMISTRY OF PHOTOGRAPHY.

nitrate

solution,

we must put

produce a precipitate when gelatine

latter to

when the same amount of

present than

sium bromide

is

words

other

soluble

of

excess

in

especially in the

of

bromide

silver

potassium

potas-

gelatine is

[E].

perceptibly

and

bromide

Now

presence of gelatine.

the emulsion undergoing ripening

is

same quantity

dissolved in the

of water without the addition In

more of the

in

we have

in

a hot

bromide

solution of gelatine containing potassium

the conditions most favourable for dissolving up

some of the

silver

There are reasons

bromide.

believing that the growth of the particles this

solvent action

of the

and which continually deposits the bromide

of

silver

this

it

is

There gelatine

is

yet

is

to be ripened

another

bromide,

salt

on

latter.

indispensable that an

potassium bromide should be present emulsion which

is

the

already present, thus

causing the increase in the size of the this reason

due to

menstruum, which

always saturated with dissolved

particles

is

for

by

For

excess of

in a gelatine

heat.

method of ripening a

emulsion to which

attention

must be

RIPENING B Y AMMONIA. called before It

we can

heat in

as

acquire

ways

its

ammonia

causing the

this

not

now

enter but for which

As

practical treatises.

and

details of

must

I

bromide to

silver

process,

numerous formulae into the

same

acts in the

There are

sensitive condition.

of working

29

leave emulsion photography-

has been found that

way

1

different

there

which

refer

I

can-

to the

you

a point of history

are

should

it

be mentioned that the ammonia process of ripening has been

hoven and by Eder and to manipulation

it

potassium bromide the

emulsion usual silver

by

mixture be

may

way, and haloid

respect

will suffice to state that in the

ammoniacal solution of

ripen

With

Pizzighelli.

preparation of such an emulsion,

of

Van Monck-

developed chiefly by

at first

then

brought

we may add an

silver nitrate to in

a

aqueous gelatine and gentle

partially

the

up

a solution

heat

or

;

ripened in the of

sensitiveness to

the

the desired

the

stage

the addition of ammonia, and continuing the

digestion perature.

for

a

The

short

time

ripening

ammonia has been

at

faculty

a

lower

possessed

successfully applied

in

K

tem-

by cold

THE CHEMISTRY OF PHOTOGRAPHY.

130

emulsifying processes,

such as those of Cowan

and Henderson.

We is

must pause here and ask ourselves why

that

ammonia

acts as a ripening agent.

immediate cause silver all

bromide

who

in

is

it

The

the solubility of

doubtless

ammonia, a property familiar to

are acquainted with the characters of the

silver haloids.

On

tion with water, or heat, the dissolved

ammoniacal

diluting the

on driving bromide

is

off the

solu-

ammonia by

reprecipitated [F].

An

action of this kind no doubt goes on during

the

ammonia

bromide

ripening process, the growth of the

particles

taking

place

by the gradual

deposition of bromide from the solution on to the particles already in suspension, just in

manner

by is

as in the alternative

digestion.

the

same

method of ripening

That the increase of

sensitiveness

due to a physical and not to a chemical change,

may be

proved by arguments similar to those

which have been made use of alternative

process

in dealing

of ripening,

with the

and there

is

no

occasion to repeat them here. I

have dealt at some length with the ripening

CAUSE OF SENSITIVENESS. but

process,

it

131

must not be concluded that the

extreme sensitiveness of a gelatine emulsion

due solely to the molecular

of the silver

state

There are other conditions which have to

haloid.

we can

be taken into consideration before

member

mercy of the preservative

Now

action.

vehicle itself

a

is also

is

in

a

will

re-

a bromine absorbent

sensitizer^

for sensitiz-

emulsion

gelatine

the

the gelatine

and because of the intimate

contact

which

gelatine

and the imbedded

everywhere

the sensitizing action in the

take

washed collodion emulsion we

that in a

are at the

ing

You

of this part of the subject.

leave

is

is

exists

between the

silver haloid

particles,

capable of being exerted

most perfect conceivable manner short of

the vehicle being actually in a state of solution.

Then

again, the perfect

imbedding of the

silver

haloid particles in the gelatine enables us to use

developers of great strength, and thus to

make

the

most out of the impression produced on exposure

;

but this part of the subject will be better understood after

velopment.

we have The

dealt with the theory of de-

co-operation of

all

these condi-

K

2

THE CHEMISTRY OF PHOTOGRAPHY.

132

tions,

both physical and chemical, has resulted

sitiveness,

has in

of

of a mixture which, for sen-

the production

in

towards

hitherto

transcends anything that

light,

been

and has

discovered,

placed

modern photographers a

the hands of

methods which,

for

and

rapidity

series

conveni-

ence of manipulation, must surpass the wildest

hopes

ever

of the

art.

The to

entertained

by the

early

pioneers

practical applications of gelatine emulsions

photographic

purposes

known and need

are

not be dealt

with at

in the present course of lectures.

sensitiveness of such films

well

sufficiently

length

The extreme

enables us to secure

instantaneous impressions of moving objects such as breaking waves, yachts at sea, express trains,

animals in motion or birds

in

flight,

other wonderful effects with which familiarized

of

late

years.

and

all

the

we have been

The problems con-

nected with the locomotion of animals and the flight

of birds have been

recently

considerably

advanced towards solution by the use of cameras with

revolving slides,

capable

of

taking

many

MODERN METHODS.

1

1 pictures in the course of a second of time.

easy manipulation of the emulsion enables

it

33

The to be

spread uniformly over the surface of paper, so that the operator can take into the field a very large

amount of sensitive

surface having but

and occupying but a small space sensitive film as

One

grapher to take no

on one continuous

less

pictures can be developed at

and the

after exposure,

stripped off

photo-

than four dozen negatives

of sensitized paper. 2

roll

in the

of the latest

in this direction enables the

developments

weight

such an area of

would have been impossible

days of glass plate negatives.

image

little

These

any convenient time

film with

and transferred

its

developed

to a thin trans-

parent sheet of gelatine so as to form a light negative which

is

practically

unbreakable, and

from which any number of prints can be obtained the usual way.

in

In

its

application to other branches of science

modern photography has already proved 1

See Nature, February

2

Reference

is

here

the courtesy of the

to be of

16, 1888.

made

to the

Company

Eastman Stripping Film.

By

a demonstration of the process was

given at the termination of the lecture.

134

THE CHEMISTRY OF PHOTOGRAPHY.

the very highest value, and holds out

prospects plies to

the future.

for

emulsion bids

still

greater

This particularly ap-

which science the sensitive

in

astronomy,

1

fair to largely

supersede the older

and more wearisome methods of eye observation. Almost from the dawn of its discovery by Daguerre, attempts were for

graphy

securing

The

bodies.

pictures

collodion

cessfully applied

made

by De

to utilize photo-

of

the

heavenly

wet plate was la

Rue

in

suc-

first

1852 to the

production of lunar photographs, and through the kindness

of Mr. Joseph

Beck

I

am

enabled to

show you on the screen some of Mr. De original pictures.

wards

brought

perfection

and the

in

la

Rue's

Lunar photograph}' was

after-

a

to

is

degree

of

America by Mr. L. M. Rutherfurd

late Dr.

Henry Draper.

of the moon's surface plates

considerable

now being

by

The mapping

the aid of gelatine dry

carried

on at Paris by the

brothers Henry. 1

One

of the latest applications of highly sensitive plates

Professor Oettingen, of Dorpat,

who

is

by

has secured impressions of

the flash produced by the combustion of hydrogen and oxygen in a closed vessel showing the intermittent character of the explosion.

SOLAR PHOTOGRAPHY. With is

culty

photography the

solar

to

respect

one of brightness

135

;

diffi-

to the excessive

owing

brilliancy of the sun, special flashing shutters have

to be

The

employed

sun's disc

in order to avoid over exposure.

is

however capable of being photo-

graphically registered with

all

its

rapidly changing

details of structure with

an exposure according to

M. Janssen, our most

successful

branch, of Ttnnnnr

f

The

a second.

pictures produced at

worker

Meudon by

years.

The camera was

splendid sun

this last observer

have been familiar to astronomers ten

in this

for

first

the last

successfully

applied to the registration of a total solar eclipse

by De has

la

Rue and

since

taken

Secchi in 1860, and no eclipse place

which

in

the

photography has not been called into

Whenever

visible the

sun

is

made

aid

of

requisition.

to give a daily

impression of himself on photographic plates in several

that

observatories

the

both here and abroad, so

appearance and position of the spots

which are so frequently seen automatically recorded.

Astronomer-Royal

I

By

on

his surface are

the courtesy of the

have here for your inspection

THE CHEMISTRY OF PHOTOGRAPHY.

136

some specimens of the Greenwich sun photographs. It

evident that the wet

is

many

presents

work.

collodion for

disadvantages

Among

drawbacks

its

mention the necessity

for coating

process

astronomical

need

I

and

only

sensitizing

each plate immediately before use, and the limited time of exposure admitted by such a moist

film.

this last reason the process is not at all

nebulae,

which require prolonged exposure.

This difficulty

of course overcome

is

of gelatine emulsion plates which can

be exposed sitive

for hours.

By means

by the use if

necessary

of highly sen-

dry plates the stars can now be photo-

graphed of

adapted

photography of faint objects, such as stars

for the

and

For

in

their

myriads

it

is

only a question

mechanical perfection of adjustment

driving rotation,

gear which

compensates

and length of exposure.

and nebulae too

faint

to

in

the

for the earth's

Distant suns

be seen by

human eye

with the best of instruments, and so remote that their light

may

take years to reach the earth, can

be made to record themselves faithfully on that

STELLAR PHO TOGRAPHY. surface which retina

The

has been well

which never

in

followed

in

up

1876,

secured

One

workers, Mr. A. A.

of our most success-

Common, whose

photograph of the great nebula in

1883

is

so

known

well

of

by observers too

countries

all

impressions

The work has been

plates.

numerous to mention. ful

of gelatine emulsion

use

on dry

stellar spectra

as "the

photography was made by Dr.

for astronomical

Huggins, who,

described

37

forgets."

successful

first

1

to

in

magnificent

Orion taken

astronomers, has

been good enough to forward some specimens of his

achievements

for

exhibition.

The

success

which has attended the use of gelatine dry plates resulted

last

year

(1886)

in

the meeting of an

International Congress in Paris at the instigation

of Mr. David

Gill,

Her Majesty's Astronomer

at

the Cape, for the purpose of discussing a scheme for a joint

astro-photographic survey.

the heaven

in

down

to the fourteenth

are to be photographed 1

The

best results obtained

plates in 1864

showed

stars

by the

All the stars

magnitude

l

different nations

by Rutherfurd with wet collodion

down

to the ninth magnitude.

138

THE CHEMISTRY OF PHOTOGRAPHY.

taking part in the work. will

photographs

by

A series of

this

about 10,000

means be secured

in

the course of five years on which will be recorded

some twenty tial

chart

tribute

to

million stars

which, the

material

when completed,

utility

of

for

a celes-

will

be

a

modern photographic

methods such as has never before been paid by one branch of science to another.

APPENDIX TO LECTURE

A

(p.

107).

The

IV.

principle of emulsification can

Two be illustrated by the following experiment are filled about 100 each c.c., up flasks, containing :

to the

same

level,

one with a 6 per cent, solution of

zinc bromide in water,

and the other with collodion

with the same quantity of zinc bromide. more satisfactory to weigh out the materials

salted It is

and make the solutions

in

the lecture room.

A

quantity of silver nitrate equivalent to the whole quantity of zinc bromide in the two flasks is

then dissolved in alcohol with a

little

water,

and

the solution divided into two equal parts, one of which is added to the aqueous zinc bromide solu-

and the other to the salted collodion. The same amount of silver bromide is of course formed in each flask, and on briskly agitating, the bromide formed in the aqueous solution soon becomes granular and settles to the bottom on standing,

tion,

while that formed in the collodion emulsifies.

THE CHEMISTRY OF PHOTOGRAPHY.

HO

B

(p. 117).

The

emulsification of silver bromide

easily shown by the following experiment Soak 20 grams of gelatine in a little water for half an hour and then dissolve completely, by

in gelatine

is

:

the aid of heat, in a larger quantity of water and add 12 grams of potassium bromide to the solution,

which should be made up to 150 ized gelatine solution,

when

c.c.

This brom-

cool has poured into

it

gradually with occasional agitation a solution of

operation

nitrate

silver

grams

15

is

in

150

best conducted in

c.c.

The

water.

a glass flask, the before the lecture.

two solutions being prepared It makes the experiment more instructive flask

is

if

a

prepared with a solution of 12

companion grams of potassium bromide in 150 c.c. water, and a solution of 15 grams silver nitrate in 150 c.c. gradually poured into this with brisk The silver bromide precipitated in the agitation. latter case becomes granular and soon subsides,

water

is

while that

precipitated

in

the gelatine solution

emulsifies.

C the

(p.

1 1

8).

Into two beakers or flasks introduce

same bulk of

(i) distilled water, 'and (2)

per cent, solution of gelatine. the

same quantity of

Add

silver nitrate solution,

very small quantity of this

salt

a 2

to each flask

only a

being necessary

just enough to give a distinct precipitate when potassium bromide is added to the aqueous solution.

APPENDIX. From

burette allow

a

a

141

weak

solution

(exact

strength immaterial) of potassium bromide to run into each flask. It will be found that (i) gives an

immediate

turbidity, while (2) will require

more of the bromide

much

solution before a precipitate

forms.

D in

(p. 1

1

8).

Some

inferior gelatine

is

dissolved

water and a few drops of silver nitrate solution The flask is heated in a water bath till the

added.

This experiment serves

solution begins to darken. to

show the reducing action of the

the silver

E

gelatine

upon

salt.

A

strong solution of potassium bromide (about 20 per cent.) is divided into two equal portions, to one of which is added about one128).

(p.

fourth

its

volume of a

5

per cent,

solution

of

gelatine, and to the other the same quantity of water. The two solutions of course contain the same quantity of potassium bromide per c.c. A

weak

solution of silver nitrate

beaker (from a burette).

is

It will

now

run into each

be found that the

solution containing the gelatine requires a

much

larger quantity of silver nitrate to produce precipitate than the purely aqueous solution.

F

(p.

bromide is

1

30).

a

A clear ammoniacal solution of silver

divided into two portions, one of which boiled and the other diluted with water. In both is

cases the dissolved bromide

is

precipitated.

LECTURE General Notions of Development. Image.

The Action of

V.

The Latent

or Invisible

Sensitizers in Different Processes.

Theory of Developers and Restrainers. Development Acid Development by Ferrous Sulphate.

by Vapours.

Alkaline Development by

Ammonium

Pyrogallate and

by Ferrous Oxalate.

THE

processes which have

may be

regarded as so

film of silver haloid of

now been

many ways more or

considered

of obtaining a

less sensitiveness

on which the picture formed by a lens can be

made

to impress itself in an

exposure for a suitable period. this invisible or latent

know

invisible

form on

In order to

image reveal

itself

make

you

all

that certain substances called "developers"

have to be applied to the

development

must

next

film,

and

engage

this process of

our

attention.

ACTION OF DEVELOPER. now

143

There

is

effects

hardly any phenomenon more striking than

even

in this

age of startling

scientific

the gradual appearance of a picture, with delicate gradations

all

its

of light and shade, on what

was apparently a homogeneous

surface,

on the

application of the developing solution.

In general terms a developing agent

may

be

described as a substance which exerts an action

upon those portions of a photo-sensitive compound which have been exposed to which

it

exerts upon the unexposed portions.

this difference of action

ence

in

light different to that

colour,

is

If

associated with a differ-

while the colour of

the photo-

reduction product does not differ to any striking

extent from that of the unreduced compound,

may have a ment.

strong contrast brought out on develop-

This point

is

ments shown

in

uranic

It will

salt

we

salts.

the

well illustrated first

by the

experi-

lecture with ferric

be remembered that a

becomes reduced to the ferrous

state

and ferric

on ex-

posure, but since the latter does not differ very strikingly in colour from the ferric salt, a ferrous

design on a ferric background

is

hardly perceptible

THE CHEMISTRY OF PHOTOGRAPHY,

144

at

a

The same remark

distance.

design on

uranous

a

uranic

to

applies

On

background.

washing the exposed surfaces with a solution

ferrous design

becomes developed

the

of

corresponding

colour of the design

is

in blue

and

owing to the forma-

the uranous design in brown, tion

of

know what happens.

potassium ferricyanide you

The

a

The

ferricyanides.

in these cases

an accident

contingent upon the fact that ferrous ferricyanide is

blue and the uranous ferricyanide brown.

By we may impart a different The salts of gold and silver

using another developer colour to the design. for

example, are reduced by uranous compounds

but not by uranic compounds, so that

if

we wash

the surface of the exposed uranium print with gold chloride solution, the uranous design only

upon and

is

"

"

developed

in finely divided

is

acted

reduced

gold which produces a picture of a purple colour. Similarly

if

we

uranous design deposition nitrate

by

of

use a solution of silver nitrate the is

developed

metallic

the

in grey,

silver

uranous

owing to the

reduced

compound

from [A].

the

The

chemistry of such processes of development as

POSITIVE these

AND NEGATIVE IMAGES

comparatively simple, and

is

may

145

be repre-

sented by the well-known reduction of gold and silver salts

4

by

ferrous salts

:

+ 6AgN0 3 =2Fe (S0 4)3 +Fe 2(N0 3)6 -h3Ag

There

2

is

2

another point brought out by the ex-

amples chosen to

illustrate the general principle

of photographic development which

tageously be dealt with now.

may

Not only

advanis

the

colour of the developed image dependent on the

nature of the developer, but the production of a positive or negative

is

also in a sense accidental.

Thus supposing a pattern black paper and a

under the

latter,

to be cut out in a sheet of

ferric

coated surface exposed

the ferrous design thus impressed

might be regarded as a negative because

it

appears

lighter than the background of unaltered

compound.

But on washing with ferricyanide the

faint negative

positive,

ferric

image becomes converted into a blue

because

it

happens that the colour given

by the photo-reduction product with the

ferricyan-

THE CHEMISTRY OF PHOTOGRAPHY.

146

ide

is

Now

much more if

intense than the ground colour.

instead of using a ferricyanide

we use

as a

developer some salt which gives a darker colour

with the

than with the ferrous compound,

ferric

such for instance as a ferrocyanide, matters are reversed, positive

From

and we get a negative

on development

instead

of

a

[B].

these preliminary considerations you will

be enabled to gain a general idea of the function of a developer, and at the same time the ground will

have been cleared by the establishment of the

principle that the developed

may

image

be positive

or negative according to the nature of the photo-

reduction product on the one hand and

developer on the other hand.

development

in the abstract,

We

and so

of the

must consider far as

present concerned, the fact of a

we

picture

are at

being

positive or negative in any particular case must be

regarded as accidental to the process with which

we

are dealing.

The next

point that

claims

attention

nature of that invisible image which sensitive film,

and which

is

is

revealed

is

the

formed on the

by the process

THE INVISIBLE IMAGE. of development.

As

147

we had

long as

compounds which underwent a

do with

to

visible

change in

colour under the influence of light, no such mystery as that involved in the formation of an invisible

picture obtruded itself

we were

in the region of

we had only to the

upon our notice

as long as

:

photo-chemistry proper,

due

to regard the change in colour as

production

of

some coloured product of

But as soon as we came to consider

reduction.

the photographic processes

parently

new

presented

itself.

kind

of

'

this

themselves,

ap-

action

photo-chemical

In discussing this subject

it

will

be necessary to limit ourselves to those compounds universally viz.

employed

the silver haloids.

for

photographic purposes,

What

is

the nature of the

image which exists on the surface of a sensitive film after

exposure

any relationship

in the

camera

?

has this image

to the visible product of reduction

resulting from the action of light

upon the

haloids in the presence of sensitizers

In answer to these questions

it is

admitted that the invisible image

silver

?

now is

generally

of the same

chemical composition as the darkened products of

L

2

THE CHEMISTRY OF PHOTOGRAPHY.

148

that

photo-decomposition

it

differs

only in degree and not in kind.

good

from the

There are

latter

many

reasons for this belief, although these

The

necessarily of an indirect kind.

are

difficulties

of

investigating this subject are evidently great,

owing

amount of the

silver

to

extremely minute

the

haloid which undergoes pjioto-decpmposition__on the

a

surface_of

what

in

is

itself

nothing more than

If the investigation of the photo-reduction

film.

products of the silver haloids

exposed

in

large

quantities to the action of light has hitherto baffled

ingenuity,

of

understand that the

readily

demonstration of the composition of the

direct

latent

will

you

image

our

beyond the reach

at present utterly

is

most

refined

chemical

methods.

reasons which have led to the belief that invisible

image

as the darkened

is

built

haloid

The this

up of the same material are

the substances

that

which accelerate or retard the formation of the latter

act

in

a

precisely

similar

respect to the latent picture. silver nitrate which, facilitates the

as

A

we have

manner with substance like

seen, so greatly

decomposition of the haloid acts also

ACTION OF SENSITIZERS.

149

as a sensitizer in all photographic processes

:

sub-

stances like mercuric, stannic or ferric salts, which are

inimical

enormously

the liberation

to

diminish

photographic film

the

free

sensitiveness

halogen, of

of a similar nature

and there

if I

is

much more

only had time to submit

photo-chemistry and photographic chemistry.

We

regard the invisible image as being formed of

an oxyhaloid such a fate

it

enables us to bridge over the gap between

you

may

the

[C].

This kind of evidence

to

of

salt

of

salt

of silver or of a combination of

with unaltered haloid (photosalt).

the

infinitesimal

quantity

of

The

liberated

halogen will depend entirely upon the nature of the sensitizer, but the

maximum that

surface

with the

is,

it is

evident that in order to get

a

photographic

to get an invisible

image formed

sensitiveness

minimum

of exposure,

some halogen absorbent distinctly formulated It will

of

by

we must have

present, a necessity first

Prof.

H. W. Vogel of Berlin.

be instructive to consider the action of the

sensitizer

processes.

in

the

more

important

photographic

THE CHEMISTRY OF PHOTOGRAPHY.

ISO

In the Daguerreotype iodide

and

is

it is

difficult at first sight to

can be.

silver

itself

silver

plays

silver,

imagine what the

generally supposed that

It is

the liberated iodine,

this

and

experiment made known

A

of

a film

on a substratum of

supported

sensitizer

the

plate

by absorbing

part

this

proved by an

is

1866

in

Poitevin.

by

film of collodion containing silver iodide

fectly

free

from

excess of

silver

pressed on to the surface of a

exposed to

light.

The

plate

nitrate,

per-

was

plate

and

thus coated

was

silver

found to be far more sensitive than a glass plate coated with the same collodion, clearly showing

But

that the metallic silver acted as a sensitizer.

the sensitizing action in this case

due on

is

not entirely

to the silver, because a film of pure silver iodide

glass,

prepared by iodizing a silver mirror de-

posited on glass, also gives a latent developable

image on exposure.

The

action

feeble under these conditions, in strong light

but the result silver

is is

is

certainly very

and a long exposure

required to produce

important because

iodide can act to

it

some extent

much

effect,

shows that as

its

own

A CTJON OF SENSITIZERS. This property of

sensitizer.

silver

1

known

to exist between silver

of the alkalies. tri-iodide

Potassium

KI 3 and ,

for

1

iodide might

have been anticipated from the analogy which well

5

is

and the metals

example forms the

the existence of this

compound

prepares us for the belief that silver iodide

may

be capable of combining with a further quantity of iodine.

In a former lecture

I

stated that the Daguerreo-

and we can now

type plate had

much

extract a

further information from the facts

which

little

to teach us,

have just been considered.

emphasize very clearly the laid is

down

These

previously

principle

that the sensitiveness of

facts

any system

a function both of the photo-instability of the

compound and of

the affinity of the sensitizer for

one of the constituents of the compound.

Consider

the sensitiveness of

plate

a

Daguerreotype

compared with that of a on

In the

glass.

first

film of pure silver iodide

case

we have

Agl-f-Ag, and

in

Agl + AgI,

the relatively strong

metallic

i.e.

silver

for

the

as

second case

the system the system affinity

the liberated iodine as

of

com-

THE CHEMISTRY OF PHOTOGRAPHY.

152

pared with

the

for iodine, this

feeble

of silver

affinity

iodide

difference in sensitizing efficiency

determining the superior sensitiveness of the

first

system. In the Talbotype process the sensitizer

know,

and

silver nitrate in

alone.

The

sensitizing action of this last salt

In

halogen.

instance, the reaction

following equation

61

you

process silver nitrate

due to the readiness with which liberated

as

conjunction with gallic acid,

the wet collodion

in

is,

is

the

case

it

is

absorbs the

of iodine,

usually represented

for

by the

:-

+ 6AgNO + sH O = 5 Agl + AgIO + 6HNO 3

3

2

The products according silver iodide

to this

and iodate and

3

.

scheme would be

nitric acid, the free

iodine of course resulting from the photo-decom1 position of silver iodide.

In the dry plate and emulsion processes either 1

This requires corroboration.

when exposed

to

light

in the

strongly acid solution, but the

has not yet been determined.

It is

known

that silver chloride

presence of silver nitrate gives a

amount of

free nitric acid

formed

ORGANIC SENSITIZERS.

153

the preservative, as in the case of tannin, or the vehicle

The

sensitizer.

case of gelatine,

in the

as

itself,

nature of

precise

formed when these organic

upon by the

a practical detail

will

is

products are acted

sensitizers

liberated halogens

the

unknown, but

immaterial from our present point of view.

this is

As

the

is

I

may mention

absorb over 20 per cent of

bromine

bromine

from

ordinary

atmospheric

water

in

that gelatine

weight of

its

four days at

temperatures, the product

consisting of brominated

compounds of unknown

constitution.

So much

now

let

invisible

us

for the

chemical action of sensitizers

return

to

the consideration

image armed with

conditions

essential

to

its

this

of the

knowledge of the

That the

formation.

image formed by the short exposure of a sensitive film of silver haloid should

be invisible

appear surprising, but the wonder just call to

the

mind how

reduction

product

purely superficial haloid

will

may

at first

vanish

if

we

infinitesimal in quantity

formed.

The

the surface layer of

action

is is

reduced

must be of such extreme thinness as

to

154

THE CHEMISTRY OF PHOTOGRAPHY.

parable with the actual

Nor must

themselves.

reduction chloride,

product, is

a diameter com-

section

transverse

present in

of

size it

molecules

the

be forgotten that the

except

the

in

by no means highly

of the

case

For

coloured.

can hardly be expected that the

these reasons

it

undeveloped

picture

should

be

anything

but

invisible.

We

must therefore regard the image formed

by the camera

lens as being painted in reduction

product on the surface of the sensitive

film,

the

varying intensity of action corresponding to the lights

and shades being represented by varying

amount of reduction lights

we have

position

the

the

number of

decomposed,

feebler lights fewer molecules are in

the deep shadows there

at

all.

The

the strongest

maximum amount

greatest

molecules are there

In

product.

is

of decom-

silver

while

haloid in

the

decomposed, and

no decomposition

actual state of affairs can be repre-

sented diagrammatically by taking a simple case.

Imagine three squares of paper on an orange, "non-actinic wall," one square

(A)

being black,

REDUCTION PRODUCT IN FILM. other

the

and the

white,

(c)

intermediate shade

film

;

s,

substratum.

A, B, and

If these squares

(B)

of an

:

FIG.

/

third

155

C

3.

are seen in elevation

;

/"and s

in section.

were photographed the black

would produce the smallest amount

-of

decom-

position, the white the largest amount of decomposition,

and

the intermediate

shade an

mediate amount of decomposition. of reduction product formed

shown

in

1

The amount

scale.

The quantity

by each square

the section of the film,

an exaggerated

relative

of silver haloid decomposed

amount

is

of course on

1

is strictly

not directly proportional to the intensity of the light in

The

inter-

speaking all cases.

varies in different films, but this consideration

THE CHEMISTRY OF PHOTOGRAPHY.

156

Such being the of the film

to

state of affairs

on the surface

which the developer

we may now proceed

to

is

consider the

applied,

methods

of development, selecting for this purpose types of developers which have been or are in actual use

by photographers.

Commencing with

Daguerreotype the latent picture aware, developed in this process

mercury

vapour.

This

mode

as

is,

the

you are

by exposure of

to

development

stands alone as a type of the action of vapours

upon the

invisible

image.

It

is

stated that the

process was discovered accidentally

who put one visible

on

it

by Daguerre,

of his exposed plates with nothing

away

in

a cupboard, and the next

morning found a picture developed,

this

result

being ultimately traced to a vessel of mercury

on one of the shelves. In a case of this kind development

due to can

the

circumstance

that

is

evidently

mercury vapour

form a compound with the product of

duction and not with the unaltered haloid. may be above.

neglected in a coarse illustration such as that

made

re-

The use of

DEVELOPMENT BY VAPOUR. amount of

mercury

is

deposited

157

proportional

amount of reduction product formed on

to the

different is

picture

parts of the plate, built up.

and

in this

way

the

sometimes asserted that

It is

the reduction product attracts the mercury vapour,

but this hardly expresses the facts of the case.

We

must consider the plate over a

warm

vessel of

mercury as being subjected to a constant bom-

bardment of mercury molecules over the whole

Where

surface.

the

molecules

meet

where they

product there they remain attached

meet unaltered haloid they the

reduction

product

reduction

fly off again,

alone

so that retains

ultimately

The

a layer of molecules of varying thickness. action

might be roughly represented by taking

a sheet of glass on which a design had been painted in gum, and blowing sand on to the surface. off,

Where

where

it

the sand

met gum

met it

glass

would be

it

would

retained,

the design would be thus " developed particles

[D].

As

far

as

"

in

fall

and sand

the physical action

concerned this explanation must at present

is

suffice,

but the chemical nature of the compound formed

THE CHEMISTRY OF PHOTOGRAPHY.

158

by mercury with

the

reduction

present unknown.

It is

not even

definite

any

compound

possible that the action

is

at

known whether

formed at

is

is

product

all

it

;

is

purely physical, the re-

" duction product being capable of being wetted,"

so

to

by the condensed mercury, while

speak,

the unaltered haloid it

be

may

In this connection

not.

mentioned that

the

Daguerreotype

capable of being developed by water

is

picture

is

vapour, although of course only in a transitory

be excused

may

I

way.

for

sizing

how much we have

oldest

known photographic

both to

The

its

once again empha-

yet to learn from this process with respect

physical and chemical principles.

iron developer

the wet collodion

so generally

process will

employed

serve

of the next class of developers.

in

a type

as

The

solution

contains as essentials ferrous sulphate and acetic acid,

some alcohol being added

to

insure

the

uniform flow of the solution over the surface of the plate. this

Let us

first

consider the action

developer from a chemical

Ferrous

salts

which were

first

point

used

of

of view. in

photo-

IRON A CID DE VELOPER. graphy by Robert Hunt agents

in

1844

which readily reduce

silver

1

are

reducing

nitrate,

have practically no reducing action on the haloids

under ordinary conditions.

59

but

silver

we add

If

ferrous sulphate to a solution of silver nitrate

we

get a precipitate of metallic silver [E], formed in

accordance with the reaction referred to a short

time ago

:

6FeS0 4 + 6AgN0 3 = 2Fe2(SO4)3 + Fe2(NO3)6 + 3 Ag2

Now image

the is

collodion

covered with the

film

and

of reduced

over the plate. disaster

with

invisible

its

sensitizing

film

of

we poured on to this a solution sulphate, we should of course have a

silver nitrate,

of ferrous

plate

if

silver

The

immediately deposited acetic

acid

by checking the reduction

nitrate, so that the free silver is

prevents of.

all

this

the silver

gradually deposited

on the reduction product, the amount of deposit being,

broadly

amount of is

built

up

speaking,

this product. in

proportional In this

metallic silver.

way

to

the

the picture

THE CHEMISTRY OF PHOTOGRAPHY.

160

This

developer

of

is

some of the

a

course

details

the

iron

mere outline sketch,

of which

Let us

deavour to supply.

of

action

explanation of the

must now en-

I

first

consider what

can be learnt from the use of acetic acid, which retards the precipitation of silver from the solutions of

its salts

the

although acid.

metal

action of reducing agents,

itself

is

insoluble

in

the

Substances which exert this retarding action

are called

or less

of the

by the

restrainers^

and these act with more

completeness according

to

the

stability

compound which they form with

If the restrainer

silver.

forms a very stable silver com-

pound we may have any degree of retardation

up to the silver.

total prevention of the precipitation of

The

organic salts of silver are less stable

than the inorganic

salts,

and the organic acids are

accordingly milder restrainers than acids.

Even

the

mineral

indifferent organic substances, such

as sugar, glycerol, or gelatine, act as restrainers virtue

by

of the viscosity which they confer upon

the developing solution, or because of a tendency to

form

feeble

compounds with the

"

nascent

"

GROWTH OF SILVER silver.

of as

"

DEPOSIT.

161

Such substances are sometimes

spoken

physical restrainers

them from

acids or

"

order to distinguish

in

compounds which are capable

of combining chemically with the

may

silver,

and which

therefore be regarded as chemical restrainers

[F].

The next siderable

point to be considered

one of con-

at the root of the

lies

it

importance

is

explanation of the development of the invisible

image

in silver.

attaches

itself

Important as

is

Why

in the

that the nascent silver

only to the

reduction

product

cannot be satisfactorily

it

present state of knowledge.

generally stated that the reduced haloid, invisible image, attracts the nascent silver

developing solution.

?

of pho-

this question to the theory

tographic development,

answered

is it

But

this

It is z>.,

the

from the

statement must be

carried further before the theory of the developer

can be considered in any

way

complete.

It is

ob-

vious that the reduction product attracts the silver,

otherwise the picture would not be developed at

To

say therefore that the silver

carry the explanation

very

is

far

all

attracted does not it

amounts

M

in

THE CHEMISTRY OF PHOTOGRAPHY.

162

reality to little

All

more than a restatement of the

who have used

;

know

that

cumulative in

its

the picture gradually gains in intensity

by

other developers

like all

action

the iron developer

fact.

is

it

the accumulation of fresh silver on the deposit

formed.

Now

reduced

silver

the question

is,

first

does the deposit of

begin to form at once upon the

reduction product, or

the latter in the

is

first

place

superficially reduced to the metallic state by the

developer

?

In other words,

can the reduction

product be reduced further by ferrous sulphate

These are the questions which difficult to

answer, and

I

am

it

is

?

now very

glad of the present

opportunity of pointing out a line of investigation

which might easily be taken up by any one of you

who

has the requisite chemical knowledge.

It

is

generally believed that no direct reduction of the invisible in this

image by the ferrous sulphate takes place

kind of development, 1 but that the nascent

silver is

from the very beginning of the process

deposited on the reduction product only. 1

Some experiments made

for

me by

What-

Mr. T. H. Norris in the

laboratory of the Finsbury Technical College confirm this belief.

DEVELOPMENT B Y ACCRETION. ever

may

be the truth with respect to the

action, there

is

no doubt that the

when once formed continues

to

feed

it.

there

any

is

163

to

silver

initial

silver deposit

grow as long as

mode

This

of

deposition will remind you of electrolytic action or

Evidence

of crystalline growth.

not wanting

is

that the growth of the silver deposit

may

of electro-deposition, the film of silver

be a case

formed

first

constituting one element of a micro-galvanic couple

by the developing

excited

solution.

shown by LermontofT that with nitrate

solutions of silver

and ferrous sulphate and a

silver a galvanic circuit

growth of metallic

can be formed, the nitrate

silver

The development means of the

strip of metallic

and depositing a

becoming reduced

has been

It

on the

crystalline

silver strip [G].

of the photographic image

ferrous

sulphate

developer

by

thus

resolves itself ultimately into a process of accretion.

Given a

faint silver picture to start with,

and the

metallic deposit will go on gaining in thickness as

long as there

is

nascent silver to feed

We

it.

can

imitate these conditions artificially without calling in the aid

of light at

all.

Here

for instance is

M

2

a

THE CHEMISTRY OF PHOTOGRAPHY.

i64

sheet of paper coated with silver chloride on which stripes of metallic silver

The

reduction.

have been produced by

stripes are at present faint, but

them over with a

on

solution

of ferrous

sulphate containing acetic acid and a

little silver

brushing

nitrate,

they become almost black, owing to the

further deposition of silver

upon them [H].

conditions in this experiment are

same

The

much about

the

as in an ordinary photographic plate under-

We have

going development.

a surface composed

partly of unaltered haloid and partly of metallic silver,

the nascent silver from the developer being

deposited on the metallic stripes only and none on the unreduced haloid. It will

tions

be readily gathered from these explana-

that

the

developers

typified

by the

iron

developer build up a visible picture by depositing

reduced reduced the

silver

on the

invisible image.

silver is supplied

haloid

is

collodion plate

easily

by

That the

the nitrate and not

shown by exposing a

by

\vet

and then washing out the excess of

silver nitrate before

applying the developer.

these circumstances no picture

is

Under

developed at

all,

DE VELOPMENT B Y MERCUR V.

165

or at most only a very faint one, probably the result

of the retained trace of silver nitrate which difficult to

On

remove by washing.

so

is

adding a few

drops of silver nitrate to the developer such a

washed plate can be developed as usual

mixture of substances containing what a

called

developer

when the

requisite conditions, electro-

All that

chemical or otherwise, are set up. necessary

is

and not the unaltered

capable of having requisite

is

a reducing agent capable of reducing

nitrate

silver

may be

deposit will act as a

silver

potential

Any

[I].

by a

its

haloid,

and

reducing power modified

restrainer.

It is

if

quite unnecessary

here in dealing only with general principles to the different kinds of developers which

specify

all

act

the

in

way

indicated.

As

illustrating

the

general theory an

experiment of Carey Lea's

highly instructive.

An

after

and

exposure is

nitrate is

is

is

iodized collodion wet plate

washed

free

from

silver nitrate,

then treated with a solution of mercurous

and pyrogallol when the

developed

in

mercury.

invisible

The analogy

developer with the iron developer

is

image

of

this

sufficiently

1

THE CHEMISTRY OF PHOTOGRAPHY.

66

the solution contains a potential deposit

obvious

of mercury, and the image

is

accordingly developed

in this metal instead of in silver.

Another important group of developers may be typified

by the

alkaline developer so familiar to

who have worked with word or two development which was

was used

as

may

first

to

A

gelatine emulsion plates.

the history of this kind of

not be out of place.

Gallic acid,

obtained pure by Scheele

in

1786,

as a developer in the early days of the

Talbotype

process.

The

sublimate

crystalline

obtained by heating gallic acid was

first

as a distinct substance (pyrogallic acid)

connot

all

in

1831,

and

its

identified

by Bra-

use as a photographic

developer was suggested in 1851 by Liebig and Regnault. as

I

It

was the

late

Colonel Russell

stated in a former lecture,

first

who

made known

in

1862 the advantages of an ammoniacal solution of pyrogallol for the development of dry tannin plates,

so that

when

gelatine emulsion plates

use there was a developer for them

came

into

already to

hand. In considering the principles of alkaline develop-

ALKALINE DEVELOPMENT. thing to be borne in mind

ment the

first

have no

free silver nitrate

collodion

167

process.

That

on the plate as

is

one reason

we

that

is

in the

the

why

ordinary iron developer cannot be used with dry Pyrogallol

plates.

is,

most of you know, a

as

hydroxybenzene derived from tive distillation,

gallic acid

tri-

by destruc-

carbon dioxide being given off as

one of the products of decomposition

:

C 6 H 2(HO)3 .COOH = C 6 H 3 (HO) 3 Gallic acid.

An

alkaline

Pyrogallol.

of pyrogallol

solution

is

a most

powerful reducing agent, greedily absorbing oxygen

from the

air,

Not only

is

and rapidly reducing the

silver nitrate

salts

of

silver.

reduced by such a solution,

but even the silver haloids are reduced, so that

we

have to deal with a substance of which the reducing

power transcends that of

ferrous sulphate

this reason the alkaline pyrogallate

alone

*t requires restraining, or

reduction over the whole plate fog.

It

;

[J].

For

cannot be used

we should have in

other words, a

has been found that a soluble bromide, such

as that of potassium, exerts the necessary restrain-

THE CHEMIS TR Y OF PHO TO GRAPH V.

1 68

that the developer actually used

ling influence, so

consists of an -ammoniacal solution of pyrogallol

mixed with potassium bromide. restrainer can

Here

and

action of the

be illustrated by an experiment.

a sheet of paper coated with silver bromide,

is

if

The

we

paint on

its

a stripe of

surface

am-

moniacal pyrogallol we get a dark stripe of reduced silver.

Now

on adding some potassium bromide to

the developer and again painting a stripe you see

we

get a

much

fainter impression

action of the bromide has been

the reducing

weakened by the

restrainer [K].

So much

Interpretation. is

now

for the facts

The

let

us consider their

point that chiefly concerns us

the retarding action of the potassium bromide.

It is

generally believed that the restraining power

of this substance

double

salt

is

bromide

i.e.

tendency to form a

itself.

less easily reducible

the well-known

fact

is

salt

than the

That such a doubk

potassium bromide exists

bromide

its

with silver bromide, this double

being more stable, silver

due to

silver

rendered probable by

of the solubility of silver

in a solution of

potassium bromide

[L].

COURSE OF THE PROCESS. The

restrainer

169

the alkaline developer acts

in

therefore in a different

way

to the acetic acid of

the iron developer, although the final result

The potassium bromide mixed with

same. alkaline

of

deposition

over the

all

on the

haloid

silver

prevents the

on

the

pyrogallate checks the reduction of the

unaltered

silver

the

is

a

of

film

The

latter.

and thus

plate,

iron

reduced

developer,

no tendency to reduce

the other hand, has

the unaltered haloid, but only the small quantity

of silver nitrate which

and

too

the

checked vents

rapid

by the

the

of

the

ciently clear

of

acetic

haloid.

alkaline to

the process

present on the plate,

reduction

deposition

the unaltered

is

of this

which

acid,

of

a

The

developer

of

film

principles will

nitrate

thus

is

pre-

silver

on

of action

now be

suffi-

enable us to follow the course of development

somewhat more

deeply.

Supposing we begin with an exposed gelatinebromide plate

;

a product which

unchanged

the invisible is

more

image

is

formed of

easily reducible than the

haloid, so that the first action of the

1

70

THE CHEMJS TR Y OF PHO TOGRAPHY.

developer

to reduce the reduction product,

is

Now

the invisible picture, to the metallic state. the alkaline developer differs from

the

i.e.

iron

de-

veloper in having no potential deposit of silver dissolved in it

it.

I

say no potential deposit, but

would be more correct

only a trace of dissolved

to say that silver,

it

contains

because the am-

moniacal pyrogallate does dissolve a trace of

silver

haloid out of the film, and this becomes reduced

and adds to the density of the image. appear therefore at

first

sight

It

might

that the alkaline

developer simply acts by reducing the invisible

image to the metallic picture being increased

state, the intensity

by the minute amount of

resulting from the

silver

of the

reduction

of the silver

bromide dissolved by the ammoniacal developing solution.

may

This explanation however, simple as

it

appear, will be found inadequate on further

examination.

In the

first

place,

it is

evident that the

amount of reduction product which forms the visible

image

metallic pictures.

silver

is

so small that

would

equivalent in

but the

give

In the next place,

its

it

in-

faintest

of

can easily be shown

DENSITY OF SILVER DEPOSIT. that the

the film

amount of

silver

bromide dissolved out of

by the ammoniacal

more than a

and

trace,

is

solution

picture.

same strength

as

is

nothing

fully-developed

ammonia of about

of

to

insufficient

quite

account for the density of the If a solution

171

the

in the alkaline de-

that used

veloper be poured repeatedly over the surface of

an unexposed gelatino-bromide found that the merest trace of

The

present in the solution.

plate, silver

it

be

will

bromide

is

density of the de-

posit in the completely developed picture clearly

shows therefore that the metallic image from

some other

The answer

Whence comes

source.

additional supply of silver

is

fed this

?

to this last question

must now be

dealt with before the theory of alkaline develop-

ment can be considered metallic

in

any way complete.

image produced by the

developer, which image silver of the reduction

is

first

The

action of the

in fact the equivalent in

product formed by the action

of light, comes into existence in the presence of the

unaltered bromide in which

have therefore what

may

it

is

imbedded.

We

be regarded as nascent

THE CHEMISTRY OF PHOTOGRAPHY.

172

silver in contact

of

silver

where is

wetted

being

particles

solution.

with silver bromide, the two kinds

Under

by the developing

these circumstances the unaltered

bromide becomes reduced it is

in contact

by this means

in

those places

with the nascent

that the additional silver

which reinforces the

It

silver. is

film first produced,

supplied

and gives

density to the image.

In the case of alkaline de-

velopment the picture

is

plied chiefly from the

unchanged

built

up from

silver sup-

silver haloid

not from the bromide generally,

the emulsion itself

but only from those portions which are

mediate contact with the the

first

in

acid

In

raised in relief

im-

image resulting from

is

Thus the image,

supplied by growth

one case the

silver picture

is

upon the surface of the collodion

in the other case the

The

in

up by accretion from above

development,

from below.

film.

silver

action of the developer.

instead of being built as

in

image

black, white,

is

;

sunk into the gelatine

and intermediate squares

of paper used to illustrate the formation of the latent

image

graphed

and

(see p. 155)

developed

would

give,

when photo-

by the two

methods,

COMPARISON OF METHODS. results

173

which might be thus represented diagram -

matically

:

E

A

P

Acid development.

Alkaline development. FIG.

In proof of this

ment the

silver

it is

image

4-

found that

is

condition.

On

after develop-

dissolved off the plate

nitric acid, the surface of

ferrous sulphate

if

the film developed

simply restored to

is

the

its

by

by

original

other hand, the picture de-

veloped by alkaline development, being imbedded in the film instead

leaves

its

after the

of being raised upon

its

surface,

negative impression sunk into the emulsion

removal of the

this beautiful

silver

by the

acid.

For

demonstration we are indebted to the

late Colonel Russell (1865).

174

THE CHEMISTRY OF PHOTOGRAPHY.

Although the foregoing explanation of the process of alkaline

and

is

development

probably in the main correct, there

point which

is

now

under notice

I

generally received

is

bring

it

made

not quite clearly for the

out,

is

one

and

I

same reason that

have dwelt upon other obscure points

in the course

of these lectures, viz. in order to incite investigation.

The

part of the process to which

ducing action of the nascent

I

refer

silver

is

the re-

representing

the invisible image upon the silver bromide with

which

it

is

In

in contact.

ducing action

is

decomposition.

the

all

probability this re-

result of micro-electrolytic

All the conditions can in fact be

reproduced by pressing particles of

silver into a film

of gelatino-bromide emulsion and developing with

an appropriate developer without any exposure to light

at

all.

Each

metallic

particle acts in this

case as one element of a galvanic couple, and re-

duces the bromide with which result

it is

in contact,

the

being a corresponding number of dark spots

dotted over the plate, which appear distinctly on

brushing off the silver particles and fixing with thiosulphate in the usual

way

[M].

BUILDING UP OF SILVER IMAGE.

175

There are one or two other proofs that building up of the

velopment been

which

described,

first

place

experiment by in

it

exposed

few minutes I

our

in

concluding.

In

Abney, who took a dry the

and

camera,

plate,

before

then,

one half of the

collodio-bromide emulsion

oped as

occupy

remind you of a striking

will

coated

developing;

such as have

well

may

de-

alkaline

in

image

the result of actions

is

attention for a

the

silver

the

with

plate

and afterwards devel-

The image on

coated

half

was much denser than that on the uncoated

half,

usual.

clearly indicating that

from

the

film

of

silver

the

had been supplied emulsion

collodion

as from the bromide in the film plate.

The developing

well

as

of the

original

solution in this case soaks

through the film of collodion emulsion before

The

can reach the invisible image.

undergoes

image

is

the

feed

reduction.

between it

On

with

and

reduction

the

two

silver

separating

the

latter

then

the

silver

and below, being

fed both from above

sandwiched

which

usual

it

films,

by

both

of

micro-galvanic

films

an

image

THE CHEMISTRY OF PHOTOGRAPHY.

176

is

found

on

collodio-bromide

the

was not exposed,

Then

plate [N].

well

as

there

is

which

film,

as

on the

original

an

equally

striking

experiment due to Dr. Eder, who coated a plate with a great thickness of gelatino-bromide emulsion and,

that

taken

the

after

exposing and developing,

reduction

of the

place under the

found

bromide had

silver

illuminated

portions

of

the film throughout the whole thickness of the In this case the

emulsion.

course fed entirely

bromide beneath

silver

is

of

the reduction of the silver

by

and as the

it,

picture

light

could not

have exerted any influence on the lower portion of a layer of emulsion

made

so thick as to

which was

be

opaque,

intentionally

the

reduction

which commenced at the surface must have extended downwards by the same process as that

by which

it

spreads both upwards and

downwards

in the

experiment of Abney's previously described.

Such

results as these are strongly suggestive, if not

actually demonstrative of electrolytic action.

APPENDIX TO LECTURE

A Exp.

An

144).

(p.

Lecture

I,

I,

V.

ordinary uranium 33) after

p.

one of which

print

exposure

is

(see

cut

developed with potassium ferricyanide, the other with silver nitrate, and the third with gold chloride solution. After into three strips,

strips are pieced together so that

development the the design

is

appears tricoloured, brown, grey and

purple.

B

146). " Pizzighelli's

follows

1.

Paper (well sized)

(p.

"

solution

Cyanotype

coated with

is

prepared

:

20 grams

gum

arabic in 100

c.c.

water.

2.

50

ammonio-ferric citrate in 100

3-

5

ferric

c.c.

No.

i

.

.

No. 2

No.

3

.

.-

.

water.

chloride in 100 .

.

.

c.c.

.20

c.c.

8

c.c.

water.

5 c.c.

N

as

THE CHEMISTRY OF PHOTOGRAPHY.

178

The mixed

solutions

if

too thick can be cleared

with a

A

little dilute hydrochloric acid. sheet of paper thus prepared and allowed to

exposed under a design cut out in black paper to the electric light for about 10 minutes, and then cut into two strips, one of which is developed is

dry

by being brushed over with a solution of potassium ferrocyanide and the other with a solution of the ferricyanide (about

15

per cent, solutions).

two

The show

strips are then pieced together so as to the developed pattern half positive and half negative.

may

The

developed with ferrocyanide require clearing by immersion in dilute hydroportion

chloric acid.

C

(p.

painted on chloride, electric

it

and

A

has a design with a solution of ferric or mercuric

149).

is

light.

gelatine

plate

then given a brief exposure to the The plate is washed and then

developed with ferrous oxalate developer, when the design comes out on a dark background, showing the retarding action of the ferric or mercuric salt.

D

(p.

scribed

157). in

The experiment

the text,

stripes

is

shown

ported vertically while a sand blast its

E

de-

being painted with

thick gum-water on a sheet of glass which

to

as

is

is

sup-

directed on

surface. (p.

159).

The

experiment

is

shown

as

APPENDIX TO LECTURE

V.

179

described in the text, a solution of ferrous sulphate being added first to a solution of silver nitrate to

show the formation of a precipitate, and then a stripe of the same solution painted on a sheet of paper moistened with silver nitrate, when a dark patch at once appears. For comparison a stripe of the solution is also painted on a sheet of paper coated with a silver haloid and free from excess of silver nitrate.

F

(p.

and

The

161).

physical,

ment

:

action of restrainers, chemical

shown by the following experi-

is

Equal quantities of a dilute solution of, (about one per cent.) are placed in four

silver nitrate

beakers.

To

(i) is

added about one-fourth

its

bulk

of a five per cent, solution of gelatine to (2) is added the same quantity of glacial acetic acid to ;

;

added the same quantity of dilute sulphuric (3) acid and to (4) is added the same quantity of is

;

distilled water.

The

four beakers thus contain the

same bulk of a solution of silver nitrate and each contains the same quantity of silver. A dilute solution of ferrous sulphate

is

then run into each

beaker from a burette, the same volume of solution being added to each beaker. The precipitation of reduced silver occurs at once in (4), less

rapidly in (i) and

G made

(p.

163).

(2),

and

least rapidly in (3).

Lermontoff's

experiment can

into a lecture demonstration

be

by dividing a

N

2

THE CHEMISTRY OF PHOTOGRAPHY.

180

two partitions by means of a vertical brown paper septum, which must be cemented watertight to each side and to the bottom of the cell. The latter is then filled up in one partition with a two per cent, solution of silver glass cell with parallel sides into

nitrate,

and to the same

level in the other partition

with a cold saturated solution of ferrous sulphate. The two solutions are connected by means of a bent silver wire which dips about half way to the On projecting cell in each partition. an image of the apparatus on to the screen, the

bottom of the

crystalline

growth of

silver is seen

on the end of the

With

wire in the silver nitrate solution.

a horizon-

lantern the arrangement of the cell

tal

must of

course be modified.

H

(p.

The paper

164).

silver chloride in the Usual

striped

with

weak

a

is

coated

first

way

and,

of

solution

when

with dry,

is

ammoniacal

As pyrogallate containing some sodium sulphite. soon as the reduced stripes are distinctly visible the paper

is

the text. in

washed, and then developed as described in The whole experiment can be performed

a few minutes

if

the chloride-coated paper is It is better to moisten the

previously prepared. surface of the paper with silver nitrate after

solution

washing and before development, instead of

putting silver nitrate into the developer. I

(p.

165).

A

glass

plate

is

ruled

with

a

APPENDIX TO LECTURE

181

V.

diamond down its centre and then coated with collodion and sensitized in the silver bath in the usual red light.

in

way

When

sensitized the plate

is

given a brief exposure to the electric light and then broken in half. One half is washed thoroughly in

water and then both this and the unwashed half are simultaneously developed with the iron developer, the washed portion of the plate after this treatment

being

much

lighter than the other.

The reducing power of ammoniacal was shown by the following experipyrogallol ments J (p. 167).

:

An

ammoniacal solution of silver nitrate was mixed with ammoniacal pyrogallol, an immediate 1.

precipitation of silver taking place. 2.

Sheets of paper coated with chloride, bromide,

and iodide of silver were painted with stripes of the same solution of ammoniacal pyrogallol. A dark stripe of

reduced

silver

most strongly on the

appears in

three cases

all

chloride, least strongly

;

on the

iodide.

This

last

experiment serves well to

illustrate the

relative reducibility of the three haloids.

must be

K

(p.

free 1

from excess of

68).

The papers

silver nitrate.

The experiment

is

performed as

described in the text.

L in

(p.

1

68).

The

potassium bromide

solubility is

easily

of silver bromide

shown by

boiling

THE CHEMISTRY OF PHOTOGRAPHY.

182

some

freshly prepared silver

bromide with a strong

bromide and pouring the a beaker of cold water. clear solution into solution of potassium

M

(p.

A

174).

gelatine dry plate

some

with water and

moistened

is

dust (prepared by

silver

with ferrous sulphate) The metallic particles sprinkled over the surface. are pressed on to the film by passing a glass roller silver

reducing

nitrate

A

over the surface of the plate. solution

oxalate the

(ammonium is

developer)

action

has

strong developing pyrogallate or the ferrous then applied, and when

been allowed to

sufficient

time

particles

brushed

the

plate off,

is

and

go on the

washed, a

fixing

for

a

silver

solution

(thiosulphate) poured over the surface. After being fixed and washed the plate can be put into a lantern

and its image projected on to the screen, when the darkened appearance of the portion which was under the

silver dust is seen to greater

makes the

more

advantage.

a design striking screen and the latter is paper applied to the surface of the plate before the silver dust is sprinkled on the design then appears in the It

is

illustration

if

stencilled out in a

:

last stage as

a dark granulation of the

film.

It

is

perhaps hardly necessary to say that the experiment

must be conducted

in red light

up to the application

of the fixing solution.

N

(p.

176).

Abney's experiment can be shown

APPENDIX TO LECTURE as a lecture demonstration

V.

by exposing a

183

gelatine

dry plate to diffused white light for a few seconds,

and

allowing a streak of a previously prepared coliodio-bromide emulsion to flow over The streak of the surface of the gelatine film.

then

allowed to dry and then the developed in the usual way (by ammoniacal

collodion emulsion plate

is

pyrogallol oxalate).

is

and potassium bromide or by ferrous striped portion of the plate comes

The

out darker than the background.

LECTURE

VI.

Rapidity of Emulsion Plates partly due to Developer

Ferrous

Oxalate Development

chemical

Test

Quantitative Micro-

Development of Chemical Reduction

Product and of Pressure Marks of the Silver Haloids in of the Invisible

Phenomena

Collateral

The Developer a

of Development

Relative Sensitiveness

Different Processes

Relapse

Image due to Reversed Chemical Change

Reversal of the Image or Solarisation The Phenomenon a Recurrent one Theory of the Process and the Function of the Sensitizer.

THE

distinction

between acid and

velopment which was dwelt upon lecture,

two

and the

kinds

of developers,

enable

the

why

reasons

cesses

in

which

was

us to appreciate

our

the

delast

mode^ of action of the

different

plained, will

alkaline

modern

also

more

emulsion

have such an enormous advantage

exfully

proin the

DEVELOPMENT OF GELATINE PLATES.

185

of rapidity over the old wet-plate methods.

way The

sensitiveness of the emulsion

has, as

itself

has already been pointed out, a great deal to do

with this rapidity, but a certain share must also In the wet-plate

be credited to the developer. silver

nitrate

present as a sensitizer.

is

If

we

applied an alkaline developer to such a plate, the free silver nitrate

results of a

for

would be at once reduced with

most disastrous nature to the picture

this reason

alkaline

development cannot be

In a gelatine emulsion plate on the other

used.

hand,, as there

danger of

no

is

fog

free silver nitrate,

from

this

we

being imbedded a

as

reducing

enough metallic

there

is

And

much

not only

use a stronger developer for this reason,

but also because

acts

we run no

and a

source

stronger developer can be used.

can

;

of the in

physical solution

silver

haloid

particles

a substance, gelatine, which restrainer,

which

we can apply a

would

be

strong

to reduce the silver haloid directly to the state

were

it

Thus

not so protected.

a gain at both ends of the process

have a more sensitive film to

start with,

we

and we

i86

THE CHEMISTRY OF PHOTOGRAPHY.

can use a strong developer after exposure. are the reasons

now

These

instantaneous photography

why

is

possible to every amateur.

It is

unnecessary here to specify

all

the different

kinds of developers which have been

or which

act in accordance with the

are in use.

They

principles of

one or the other of the typical kinds

all

Of

of development.

the numerous reducing sub-

stances which have been proposed as substitutes for

ammonium

I

pyrogallate,

need only mention

the pyrogallates of potassium and sodium (with or

without the addition of a sulphite,) hydroquinone (quinol),

and

pyrocatechol, resorcinol, hydroxylamine,

certain

ferrous

organic

are very largely used, a

oxalate

by Carey Lea in mind that it

1877

acts

alkaline

developers

devote a

little

Ferrous insoluble

;

class

developer introduced

and the

to this,

same way

generally,

we may

bearing as the fairly

consideration.

oxalate in

in

latter

good type of the

being the ferrous in

The

salts.

cold

is

a

yellowish

water but

a solution of potassium

readily

oxalate,

salt

almost

soluble

owing

to

in

the

FERROUS OXALATE DEVELOPER.

solid form, has

K

Fe(C 2 O 4 ) 2

2

as

the

2H 2 O

,

preparation tains

double

of a

formation

K

formula

2

Fe(C 2 O 4) 2 ,H 2 O or to

according

active

this

salt

mode

its

mix

obviously to

and potassium oxalate tions, as

of

solutions

potassio-ferrous

ferrous

it

is

sulphate

the necessary propor-

in

pointed out by Eder in 1879

FeSO 4 + 2K 2 C 2 O 4 =

of

solution con-

and the simplest way of preparing

oxalate,

the

in

which,

The developing

[A].

its

oxalate

187

K

2

Fe(C 2 OJ 2

:

-f

In order to restrain the strong reducing power

of the ferrous

salt

to the developer,

its

potassium bromide function being the

the ferrous oxalate developer the

ferrous

stance,

by

oxalate, which

is

is

it

in

action of

simple enough

;

the reducing sub-

becomes oxidized to the

the bromine which

added

same as

The chemical

the alkaline pyrogallate.

is

ferric condition

withdraws from the

re-

duction product, the latter being further reduced to

metallic

silver

while

bromide, except where

it

the is

unaltered

in contact

silver

with the

1

THE CHEMISTRY OF PHOTOGRAPHY,

88

nascent

not attacked owing to the pre-

silver, is

sence of the restrainers

bromide

(potassium

The

the solution and gelatine in the film).

between

the

oxalate and the

ferrous

product thus resolves

bromine upon ferrous oxalate potassium oxalate

3Br 2

2

2

reduction

of

action

the presence of

2

4

=-

3Fe 2 (C 2 O 4) 3

+ 6KBr.

addition of sodium thiosulphate, suggested

by Capt. Abney this developer, salt.

in

action

:

+ 6FeC O 4 + 3K C O

The

the

into

itself

in

in

1880, increases the activity of

probably by reducing the

There has been some discussion as

ferric

to the

chemical action of this accelerator and the equa-

manuals are generally

tions given in the current

erroneous.

The

is

thiosulphate

most probably

oxidized to tetrathionate, as in the equation

Fe 2 (C 2 O 4 ) 3

+ 2Na

Ferric oxalate.

2FeC 2 O 4 Ferrous oxalate.

Sodium

2

S 2O s

=

thiosulphate.

+ Na C O + Na 2

Sodium

2

4

oxalate.

:

2

S 4O6

Sodium

.

tetrathionate.

CHEMICAL RED UCTION PROD UCT. To

the

developers of this

class

ferrous citro-oxalate developer of

1

89

belongs the

Abney which

is

a mixture of potassio-ferrous oxalate and citrate

with excess of potassium

citrate.

Before leaving the theory of the action of photographic developers, there are certain collateral phe-

nomena more

or less connected with this theory

which must next claim our attention. in the first place present the action in

another light

there

is

me

Let

of a developer

nothing like looking

all

round an explanation when we wish to assimilate it

A photographic

thoroughly.

developer

may be

regarded as a microchemical test for the presence of traces of reduced silver haloid too minute to be In the cases which have been

detected by the eye.

considered, the infinitesimal trace of reduction pro-

duct forming the latent image

by the

action of light,

the analogy a

of course produced

and the developer,

little further, is

but also a quantitative

is

test,

to

push

not only a qualitative,

showing the quantity of

reduction product formed on different parts of the

exposed deposited.

film If

by the varying amount of the

reduction

silver

product could be

THE CHEMISTRY OF PHOTOGRAPHY.

190

formed on a surface of

by a

unaltered haloid

purely chemical method without calling in the aid of light at

developer would reveal the

the

all,

presence of this product although nothing might

be visible before

its

application.

Any reducing agent capable of reducing the silver haloids to the metallic state may be supposed to give as a

first

product the same compound as that which

constitutes the invisible image, provided the reducis

ing agent

applied in the presence of an excess

Thus the

of the silver haloid. phites or an

alkaline hypophos-

alkaline solution of certain sugars,

such as glucose or lactose, produce an invisible

image on a surface of is

silver chloride,

capable of development in the ordinary

[B].

Such

illustrations in

support of

invisible

is

similar

image

coloured product formed

upon the will is

way

have been recently adduced

by Carey Lea

it

which image

silver haloid.

in

the view that the

composition to the

by the

action of light

Whether

this

argument

hold good or not in the light of future work, at

reducing

any

rate

action

an interesting of

certain

fact that,

by the

chemical substances

PRESSURE MARKS DEVELOPABLE. upon the

a compound can be formed

silver haloids,

which behaves towards developers

same way

191

as the product formed

in precisely the

by

the action of

light.

Not only can a developable image be produced by chemical means without the action of light, but that the same result can be it has been found simply by mechanical

produced fact

was

for

iodized

made known

first

collodion

by Eder

firmed

Warnerke

and

for

pressure.

This

in

1866 by Carey Lea

plates,

and has been con-

bromized collodion, and by

Abney

for

emulsion

gelatine

The experiment is easily shown. It is only necessary to make marks with the rounded plates.

end of a glass rod on a gelatino-bromide

and

then

develop

exposure to developed as

light.

in

The

one

with

of

usual

way

plate,

without

pressure marks can be

though they had been formed by

the action of light [C]. as

the

the

This must be considered

obscure

photography.

means obvious, and

phenomena

The explanation it

is

have brought the subject

for

this

under

connected is

by

reason that notice,

no I

because

THE CHEMISTRY OF PHOTOGRAPHY.

192

I

do not want you to go away with the

pression

our

that

complete

as

research.

Moreover

offer

which

to

may

room

have

I

or

which

vestigations

no

leave

may have

a

or

change

can

compounds by It

pressure.

that

such

haloids

made

undergo

not

is

a

induced

of

amount

minute

of

as

mixtures to

great

therefore

the

sensitizers,

of

late

show that

improbable

compounds

the presence

in

them

submitting

unstable

in

be

In-

fruitful.

years by Spring and others, seem to

chemical

to

suggestion

been

so

further

for

be

not

are

theories

photographic

im-

silver

should

decomposition

by strong mechanical pressure, the decomposition being so infinitesimal

as

to

be

revealed

only

on application of that most sensitive of microchemical

There

tests, is

the photographic developer.

another point in connection with the

use of the silver haloids for photographic purposes

which we are now is,

in

a position to deal with

;

that

the relative sensitiveness of the three haloids in

different processes.

you

know

that

From the

the thermochemical data

order of sensitiveness,

as

ORDER OF REDUCIBILITY. measured by the

stability of the three

193

compounds,

might be expected to be iodide, bromide, chloride.

The is

order of darkening

the reverse, but this

by photo-decomposition

is

an accident connected

with the colour of the reduction products. order of reducibility of the three

compounds

the same as the order of darkening,

The

bromide, iodide [D]. reducibility

is

little

consider

to

way

these

who

problems,

the reduction of a silver haloid

regarded as the action between the halogen

and hydrogen.

Now

three haloid hydracids

the heat of formation of the is

in

H,C1 H,Br

gram

units

-}

4-

-

H,I

It is

to those

explanation will clear up this point.

In a general

may be

chloride,

the reverse of the order of stability

unaccustomed

but a

viz.

is

fact that the order of

may appear somewhat anomalous are

The

:

22,coo 8,440

6,040

not surprising therefore that silver chloride

should be the most ready to part with to reducing agents.

The

its

chlorine

practical application of

O

THE CHEMISTRY OF PHOTOGRAPHY.

194

these considerations

seen

is

the

in

comparative

facility

with which the haloids can be developed

by the

alkaline

of dry plate

method, when used

The

films.

the form

in

iodide requires a stronger

developer than the bromide, and the latter a stronger

A

developer than the chloride.

developer which

is

just strong

enough to develop a bromide plate

perfection,

might fog a chloride plate owing to

the

easier

of

reducibility

the

latter

These remarks of course apply only developers

which

the

haloids

I

silver

need

hardly

have

a

directly

remind

to reduce the silver

;

an

you,

haloids,

alkaline

reduce

to

acid

developer,

no

has

haloid.

to

tendency

to

tendency

but acts by reduc-

ing the silver nitrate present as a sensitizer. In actual photographic

order of sensitiveness of the haloids maintained, a fact for which

prepared when

we

plicating influences

the

processes

we

consider the

introduced

is

normal

not always

shall

be quite

numerous com-

by

the sensitizer,

the vehicle, or the state of molecular aggregation

of the haloid. sensitiveness

is

In the wet collodion process the

normal,

i.e.

the iodide

is

the most

ORDER OF SENSITIVENESS.

195

sensitive

and the chloride the

gelatine

emulsion processes the bromide

more so than the

most

sensitive

when

the latter

;

is

even

submitted to the ripening pro-

The

sensitiveness.

the

is

iodide,

which confers upon the bromide

cess

In

least sensitive.

order

for

extreme

its

emulsion

gelatine

appears to be bromide, chloride, iodide.

Such

facts as these are at present

very

difficult

to interpret, not only because of the complexity

of the conditions which determine sensitiveness,

but also because of our ignorance of the weight to

be

attached

bromide in

may

gelatine

to

each

condition.

emulsion

may

than

it is

AgBr the

+

because

be

a

the

strictly

of

is

a

In other words

absorbent

an iodine absorbent, so that the system gelatine

system

do

vehicle

bromine

better

Agl

may

+

comparing the haloids

we

the

be more sensitive than the iodide

better sensitizer for the former.

gelatine

Thus

not

know

comparable as

molecular

be more sensitive

gelatine. in the

whether

Then

than

again

in

state of emulsion,

the

salts

are

far as regards their

state

aggregation

we have

no means

O

2

THE CHEMISTRY OF PHOTOGRAPHY.

196

of

whether

knowing

till

this

the

is

complexity

the

same

known we

is

state

of

the two

cases,

and

unable to form

any

in

are

molecular

opinion as to the effect of the molecular condition

upon the

me when

follow

will

dark

as

emulsion

Ag2 Br

Ag3Br3

2,

the in

is

or

,

Chemists

say that we are

I

whether

to

gelatine

of the haloid.

sensitiveness

silver

the

Br n

Agn

,

bromide

condition

and

in

the

in in

a

AgBr,

comparing

such an emulsion with a silver iodide emulsion

we may of

a

action

in reality

different

quite

of

be comparing two molecules

the

consideration

;

Furthermore, the

order.

developer

must

be

is

to

susceptible

a weaker de-

veloper and thus has the advantage

if

into

the bromide, being more reducible

than the iodide,

point of view.

taken

from

this

This discussion will be instructive,

only as showing

how complex

are the

phenomena

with which the photographic chemist has to deal,

and how much has yet

to be learnt before

some

of the most familiar facts can be properly understood.

Closely

connected

with

the

phenomena of

RELAPSE OF INVISIBLE IMAGE. and

development

which

of

the

may now

our attention.

the

general

the

is

action,

photographic destruction

with

197

of

theory

or

disappearance

invisible

image,

a

subject

conveniently claim a share of It

is

a familiar fact to

all

who

have had experience of photographic processes, that the latent indefinite

time.

image cannot be preserved

The

during which the

period

image can be retained

in a

in different processes

from a few hours

;

an

for

developable state varies in the case

of Daguerreotype to two years or more in the case

With

of a good gelatine plate.

the lapse of time

the developable image gradually fades out in processes,

and

we have now

to

consider

all

the

explanation of this phenomenon.

Looking

at the invisible

image as a photo-reduc-

product we shall be prepared to admit that

tion

such a compound, like

all

susceptible of oxidation [E]. this it

reduced products,

is

In accordance with

view of the composition of the invisible image,

is

found that the latter can be destroyed by

oxidizing agents, using this term in a wide sense.

This

fact, I

need hardly stop to point out, gives

193

THE CHEMISTRY OF PHOTOGRAPHY.

additional support to the view that the invisible

image

is

product and

a chemical

physical modification of the haloid. that the

most

reduced haloid

that the

a mere

It is

obvious

regenerating the

direct

way

that

to say of restoring the lost

halogen to the film of the halogen

of

not

is is

itself.

to apply a further quantity

Of

course

not essential

it is

same halogen should be

applied, because

you can readily understand that there may be a series of

chemically analogous reduction products

oxyhaloids or whatever they

one halogen

may

may

be

which

Thus the

another.

replace

in

photo-reduction product of silver chloride would

be destroyed by bromine with the formation of a mixture of

silver chloride

halogens are

among

and bromide, so that the

the most effective agents for

destroying the invisible image.

exposed Daguerreotype plate

If for instance is

an

submitted to the

vapour of bromine or iodine, no picture can be obtained on development the

work done by

precisely silver

the halogen has undone

light.

analogous to that

chloride

is

exposed

The phenomenon is which occurs when to

light

in

a closed

DESTRUCTION OF INVISIBLE IMAGE. vessel.

You have

chlorine

is

seen in a former lecture that

evolved

when

haloid

this

sealed

up

becomes

If the chloride

coloured by photo-decomposition. is

199

a tube the halogen cannot escape,

in

and when the tube

is

removed from the influence

of light the action slowly becomes reversed

the

;

liberated chlorine reacts with the reduction product

and regenerates

darkened by

latter slightly

A

silver chloride.

light

tube of the

and put away

during the night will be found to have become white again by the morning.

such as

nitric acid, the

Oxidizing agents

chromates, permanganates,

metallic perhaloids, &c., are equally effective

in

destroying the invisible image [F].

The next

step

is

to connect these facts with the

spontaneous relapse of the invisible image

a task

which, in the absence of precise knowledge con-

cerning the chemical composition of the reduction product, form. are

but

is

by no means an easy one

Since

known

oxidizing

to

natural

of the latter

or

to

halogenizing agents

destroy the invisible image,

to is

per-

it

is

suppose that the disappearance

due to gradual oxidation or to

THE CHEMISTRY OF PHOTOGRAPHY.

200

There can be but

rehalogenization.

that

this

although

it

is

difficult in

doubt

true in a general way,

is

explanation

little

many

cases to follow the

exact course of the chemical changes which occur.

For

this reason

it

impossible to group

is

all

the

causes of relapse in the different processes under

one

chemical

general

principle

reaction,

be sketched

may

in

but

the

The

outline.

duction of a silver haloid by light

is

broad

not, as

I

re-

need

hardly remind you, a simple case of dissociation the reduction takes place at the expense of the

oxidation or halogenization of the sensitizer, whatever the latter is

may

meant when

I

be.

Chemists

say

that

directly, but

water

may

may

know what

halogenization

oxidation are often synonymous.

halogen for instance

will

The

and

liberated

not attack the sensitizer

take part in the reaction,

the halogen decomposing the latter and liberating

the oxygen which, in

the

nascent condition,

absorbed by or oxidizes the sensitizer C1 2 4-

H O + sensitizer ?

2

is

:

H Cl + (sensitizer oxidized).

In other cases again the halogen

may

be directly

REVERSAL OF CHEMICAL ACTION.

201

absorbed by the sensitizer, forming substitution or additive

of

Whichever of these modes

products.

action

takes place,

photographic process film

reduction

Now when is

clear that in every

is

it

we have formed and

product

oxidation

the stimulus of light

is

may

same

product.

withdrawn there the outward

no further liberation of halogen

tension, if I

in the

use such an expression, of the

halogen ceases, and we have reduced product in contact with oxidized product, an unstable system

capable of reacting chemically.

Such a reaction

would take place but slowly, partly owing to the solid

condition

of

the reacting

substances

and

partly depending on the stability of the oxidized

product, but as the reaction goes on the reduced

product would gradually become oxidized, while the

oxidized product would at

become reduced restored to

its

the

film

would

original condition

ture

would fade away.

that

we have what may be

same time

the

;

be

gradually

the latent pic-

In such cases

it

is

clear

considered as a reversed

chemical change.

The explanation which

is

here submitted

is

no

THE CHEMISTRY OF PHOTOGRAPHY.

202

doubt true "

be

in the abstract, but

in the air

"

deal with

me

Let

practical side only.

some of the

iodine than

in

free

may be

photography from

its

endeavour to

illustrate

principles

To

laid

begin

the Daguerreotype plate.

we have

a reduced product poorer

the original silver iodide

amount of

certain

may

it

are

typical processes.

with the simplest case

After exposure

afraid

who

by applying the

the explanation to

am

as far as concerns those

accustomed to

down

I

that halogen.

The

by a

iodine set

regarded as combined, wholly or partly,

with the excess of silver iodide, a possibility for

which you have been prepared

We

151).

(p.

thus

have

in a

reduction

contact with iodized silver iodide. is

product

But the

in

latter

a comparatively feeble compound, and as soon as

the plate it

former lecture

is

withdrawn from the influence of

begins to give

runs the

in the film

down film

its

The chemical

product.

duced

back

is

obliterated.

in the

tension which

by the action of dark

re-iodized

light

iodine to the reduction is

thus pro-

light gradually

the de-iodized portion of

and the

invisible

image

is

FADING OF IMAGE IN COLLODION PROCESS. Consider process.

by the

in

The

203

the next place the wet collodion liberated

iodine

is

here absorbed

(silver nitrate), possibly in

sensitizer

ac-

cordance with the equation:

+ 6AgNO 3 +3H 2O=5AgI+AgIO 3 In the absence of the external stimulus which, as long as

it

acts

is

tending to set free iodine,

it

not improbable that the mixture of oxidizing

is

substances in contact with the reduced

product

gradually react in the reverse manner, liberating iodine z>.

and rehalogenizing the reduced

destroying

the

invisible

product,

image according to

the reaction:

5AgI + AgI0 3 +

6HN0 = 3

6AgNO 3 + 3H 2 O + Whether

this

represents

61

(absorbed).

the actual course of

events or not must be decided

by

future work,

but you will readily understand that the reduction

product could not remain

for

any great length

of time in contact with nitric acid and an iodate

THE CHEMISTRY OF PHOTOGRAPHY,

204

without becoming oxidized. that

alone

acid

nitric

is

is

indeed

capable of

From

the latent image [G]. it

It

known

destroying

these considerations

follows that a wet collodion plate should retain

its

longer after exposure

impression

were washed of

free

sistence of the

It

the film

from the secondary products

photo-decomposition.

to be the case.

if

is

been

has

This

found

obvious also that the per-

image would be prolonged

the

if

formation of highly oxidized products were pre-

vented ab

initio.

The

truth

of

this

has been verified by Professor H.

W.

deduction

Vogel,

who

exposed an iodized collodion plate under sodium sulphite,

a

reducing

prevents the acid

agent

which

effectually

formation of an iodate or of nitric

:

Na 2 SO 8 +H 2 O = Na 2 SO 4 + 2HI The for is

a

invisible

much

image

is

retained

by such a

longer time than when

plate

silver nitrate

the sensitizer. In

cases

such as have been

where the chemical action

of

now the

dealt with, sensitizer

is

FADING OF IMAGE IN DRY PLATES. obvious, or where

of

degree

205

can be conjectured with some

it

cause of

the

probability,

the disap-

pearance of the latent image can be represented chemically with more

when we come

or

less

But

plausibility.

to deal with sensitizers like tannin

or gelatine in the dry plate processes, the explanation

is

much more

difficult

owing to our ignorance

of the nature of the halogenized products, and

we can

at

present

only advance

Thus the halogenized in the

absence of

gelatine for

light,

and

by analogy. example may

contact with the

in

reduced silver haloid, gradually rehalogenize the generally believed

latter.

It

spheric

oxygen

is

(or

also

atmo-

that

ozone) plays a part in

obliteration of the invisible

the

in fact it

has

been proved by experiment that ozone acts

like

other

The

in

agents

oxidizing

destruction of the

image

this

image may

sensitizer

and so

may

;

that

is

to say

oxidize the halogenized

liberate the halogen,

which then

The

longevity

attacks

the

of

impression on

the

such a case be

in

due to indirect rehalogenization the oxygen or ozone

respect.

reduction

product.

dry gelatine plates

may

THE CHEMISTRY OF PHOTOGRAPHY.

206

be due to the being its

in the solid state

can part but slowly with

halogen, while at the same time the action of

the air it

fact that the halogenized sensitizer

is

for the

same reason

retarded.

Nor must

be forgotten that the particles of the reduction

product are actually imbedded

in the vehicle in-

stead of forming a surface film as in the collodion

wet

plate.

Among

the

reagents which destroy the

latent

image and of which the mode of action

is

no means

Nitric

acid

clear,

are

the

mineral

acids.

by

no doubt acts simply as an oxidizer, but the

haloid hydracids cannot act directly in this way. It

is

extremely probable that there

case of these acids a film-reaction in acid

is

side

with reduction product, and

decomposed by being

in

which the

contact on one

with atmospheric oxygen, as shown ing scheme

the

in

is

on the in

other

the follow-

:

-Hcn HBr (HI

Reduction product + 2 \

V

+O =

I

(

Silver

-{

(

Chloride

Bromide Iodide

)

V J

+

HO

DESTRUCTIVE AGENTS. If for the is

product

moment we assume

207

that the reduction

actually an oxyhaloid, the mineral acids

might be supposed to act directly upon such a

compound

in

accordance with the equations

:

Ag4 OI 2 + 2HNOy=2AgI + 2AgNO 3 + H 2 O Ag4 OBr2 + 2H I=2AgI + 2AgBr + H 2 O But

hold

explanation does not appear to

this

good because

carries with

it

it

the

implication

that the silver haloids should not undergo photo-

decomposition when immersed

and

reduction

the

that

mineral acids,

in

products when

should be decomposable by such acids.

ment disproves

haloid

bromic

is

deduction

bromide can

or

chloride

this

suspended

acid,

in

be

;

formed Experi-

darkened

silver

formed when

hydrochloric

and the darkened

the

or hydro-

product

is

not

bleached by immersion in these acids, even when the latter

are

of

sufficient

the latent image.

no action in

in the

It

strength to destroy

appears that the acids have

cold on the reduction

bulk when this product

tact with the air, as

is

is

the case

product

excluded from con-

when

it is

suspended

THE CHEMISTRY OF PHOTOGRAPHY.

208

the

in

acid

thin

film

action,

acids

the

and that that

probable

the

reduction

spread over a plate in the form of a

is

product

But when

solution.

in the process.

is

a destructive

readily exert

the reason

why

oxygen

atmospheric

seems so

it

takes

part

1

The explanation of

the destruction

or

spon-

taneous disappearance of the latent image which

has been

advance

submitted will enable us to

a step towards the consideration of another phen-

omenon which has long been a puzzle I

photographers

allude to the reversal or solar-

isation of the image.

from the experimental as

we proceed

ledge

of

the

in

Let us approach

this subject

side, interpreting the results

the light of our present

conditions

of the

struction

to practical

the

necessary for

reduction product.

know-

we

If

de-

take

a sheet of paper coated with silver chloride and

darken of 1

may

it

by exposure

reduction It is

to light,

product which

we have

can

of

a surface

course

be

not improbable that the halogenized sensitizer (or vehicle)

give back

its

halogen when acted upon by mineral acids in

the presence of the reduction product. these acids

may

The

destructive action of

thus be due to indirect rehalogenization.

RE YERSIXG A GENTS.

209

bleached by any substance which acts as a re-

This experiment

halogenizing agent [H]. viously nothing

more than a

of that which occurs

when

treated with similar

is

this lecture)

Now

visible

demonstration

the invisible product

reagents (See exp.

F

of

and requires no further explanation.

was observed by Lassaigne

it

ob-

is

in

1839,

and

about the same time by Sir John Herschel, that a solution of potassium iodide also bleaches the

darkened product of photo-decomposition under the influence of light, a fact which was investigated others if

we bear

by Robert Hunt, Fox Talbot and This result

[I].

in

further

mind the

is

also easily

explicable

fact alluded to in a

lecture (p. 25), that potassium iodide

is

former

susceptible

of photo-oxidation by atmospheric oxygen with the

liberation

Herschel's in

of

In

iodine.

Lassaigne's

and

experiment we have potassium iodide

contact with a reducing agent on one side and

with atmospheric oxygen on the other conditions

are

favourable

of the potassium iodide sensitive

salt

and

the

for

;

all

the

the decomposition the photo-

the latter

is

reduction

product

acts

THE CHEMISTRY OF PHOTOGRAPHY.

210

towards

as a

it

sensitizer,

halogenized

and

and chloride

act in a similar

because these

thereby becoming re-

salts are

more

way but more

of greatest

point

connection with these facts

slowly,

stable in the presence

of light and oxygen than the iodide

The

bromide

Potassium

bleached.

[J].

importance is

us

to

the strict

in

analogy

which obtains between the bleaching or reversing action of the alkaline haloids and the destruction

of the reduction product, visible or invisible, by

The only

dif-

the latter act at once upon

the

oxidizing or halogenizing agents. ference

is

reduction

that

product

in

act only under the stimulus of light acids,

c.

the former

the dark, while l ;

the mineral

are in the one case chemical reagents,

while the alkaline haloids are in the other case 1 It has been observed that potassium iodide to some extent bleaches darkened silver chloride paper even in the dark, but

this

preliminary action

probably due to the formation of a

is

certain quantity of the lighter coloured iodo-reduction product

double

decomposition.

reaction

would be

Assuming the

oxyhaloid

formula

by the

:

Ag4 OGl 2 + 2KI = Ag4 OI 2 +

2KC1.

This would be analogous to the well-known decomposition of silver chloride

by potassium

iodide.

RE VERSAL BY POTA SSIUM photochemical

example

A

IODIDE.

Consider a

reagents.

2

1 1

concrete

:

sheet of paper coated with a silver haloid,

say the chloride,

is

exposed to

for

light

a few

seconds and a stripe of hydriodic acid painted

on

On

it.

washing and developing, the portion

acted upon by the acid

remains white, because

the reduction product has been rehalogenized in

accordance with principles with which familiar. is

paper a

In

next place a similar sheet of

the

darkened by exposure, then painted with

stripe of

exposed

you are

to

potassium iodide solution and again This

light.

schel experiment,

is

the

Lassaigne-Her-

and you know what happens

the portion of the paper painted with the potas-

sium

iodide in the presence

of light behaves exactly

the hydriodic acid in the absence final result, so

duct, I

is

the

The potassium

becomes bleached.

iodide

far as

same

need hardly

in

of

light,

like

and the

concerns the reduction pro-

both

cases.

stop to point out the practical

application of this bleaching action of potassium

iodide

;

but you will readily understand that P 2

if

a

1

THE CHEMIS TR Y OF PHO TO GRAPH Y.

2

sheet of darkened silver chloride paper

coated

is

with a solution of potassium iodide and exposed

under a design, we

have a print

shall

in

which those

portions of the surface protected from the light re-

main dark while the exposed portions become bleached.

from

A

process for obtaining positive prints

based

has been

&c.

engravings,

on

this

principle [K].

From

the

the alkaline

bered

and

which

haloids,

the

upon

depend

destruction itself

plate

it

be remem-

must

of

co-operation

we

led

of the latent image on

the

without

the

are

extraneous

aid

familiar under the

name

of " solarisation," because

an

effect

ally

in

the presence of strong sunlight.

first

describe

is

plate

gains

is

in

of a

This phenomenon has long been

reversing agent.

it

light

naturally

atmospheric oxygen, the

to

of such reversing agents as

action

produced by over-exposure, especi-

the

facts

in

exposed to strong intensity

then begins to

up

fall off.

landscape the sky

outline. light,

me

If a sensitive

the latent image

to a

certain

Thus

in

may come

Let

point

and

an over-exposed

out light instead

CONDITIONS OF SOLARISA TION. of dark

the negative,

in

we should have

a reversal of natural

process

is

more

solarised

by

of white.

this

defect,

tendency This

over-exposure.

being the result of too strong an illumination

upon

certain

of the plate,

portions

apparent that the high lights

by

and

be said that the

may

it

printing

effect

sensitive the plate the greater the

become

effect

from

free

altogether

but speaking generally

to

on

that

sky would appear black instead

the

No

so

213

will

be

may

picture

means become reversed and give an un-

this

natural

of light

distribution

The

print.

through

the the

and shade

enabled

of Mr.

kindness negative

am

I

was

seen that the trees have

John

taken

years ago on a wet collodion

well

is

appearance

general

the silver print which

whom

a

in

it

a

in

seen

Spiller,

great

plate.

in

exhibit

to

It

the

by

many

will

be

come out almost white

against a dark sky in the positive.

The

conditions leading to solarisation, and the

peculiarities in the

phenomenon

studied

observers,

by

many

itself,

and

have been

especially

by

Abney, but beyond the establishment of the very

THE CHEMISTRY OF PHOTOGRAPHY.

214

important fact that atmospheric oxidation essential factor in its production,

facts

and

point,

I

has led in

In

an

must be con-

it

fessed that our theoretical notions are

what vague.

is

some-

still

attempting to co-ordinate the

them from a chemical stand-

to explain

am prompted by the same motive me to dwell upon other obscure

photographic theories,

suggestions which

the

viz.

which points

hope that the

venture to put forward

I

may

serve as a stimulus to further experimental inquiry.

The phenomenon

of

been

has

solarisation

observed by Bennett, Janssen, and others to of a recurrent character.

has

observer

phases in

solar

upon

experiments

of the

dry plates

In

course

the

the

to

light

process

of

spontaneous

;

that

is

to

agent.

dition

in

which

is

it

reversing

appears that the

succeeded by a neutral con-

the

plate

is

on development and shows no pression (ist neutral

reversal

say reversal by the

these observations

ordinary negative

last

successive

film itself without the use of a special

From

of his

photography, this

brought

be

stage).

uniformly sign

This

dark

of an

last

stage

imis

NA TURE OF SOLARISA TION. image coming out

the

followed by reversal,

The

a positive on development. followed

by another

whole plate light field

The

is

on

ordinary

negative,

each

other

light,

next

uniformly

neutral stage).

by

a

second

and the phenomena succeed in

it

same order

the

effects

and

(2nd

is

as

which the a

gives

succeeded

is

again

These are the strong

and

development

condition

reversal

neutral stage, in

solarised

last

215

is

as before.

of continued exposure to

estimated that the second

negative stage requires for

its

production a light

of 100,000 times the intensity of that required for the formation of the

There are other

first

facts

negative image [L].

connected with solarisation

which must be taken into account before presented what

in

termed a complete long been

known

legal case.

I

have

phraseology would be In the

first

place

it

has

that the preliminary exposure of

a sensitive plate to diffused white light greatly facilitates

subsequent reversal.

Then

the strength

of the developer has to be considered.

A strong

developer acts in the same

way

as over-exposure or

too great an intensity of

light,

so that a solarised

216

THE CHEMISTRY OF PHOTOGRAPHY.

plate might give an ordinary negative with a

weak

developer, and a reversed negative (positive) with a

Then again

strong developer.

the

through

chiefly

it

has been found, of Abney,

researches

that

oxidizing agents facilitate reversal, while reducing

agents retard

it

or prevent

it

altogether.

Oxidizing

agents and preliminary exposure act in the same

way.

A

washed with a chromate, perman-

plate

ganate, &c.,

is

washed with a

easily reversed, while a similar plate nitrite or sulphite is

not susceptible

of solarisation with ordinary exposures.

must be borne reversal

mind

in

go on invisibly

that the in

Lastly,

it

phenomena of

the film because the

amount of reduction product formed never reaches Eder

a visible quantity.

bromide plate to

may

states that a gelatino-

be exposed for a

sufficient

time

produce strong solarisation on development with

ferrous oxalate, without a trace

being apparent.

now

to

be

Such are the

dealt

with

from

of discoloration facts

a

which have

chemical point

of view.

At

the outset

we

are as before confronted with

our ignorance on two points, the chemical com-

DIRECT POSITIVE PICTURES. position of the reduction product

217

and of the halo-

genized sensitizer, so that any theory of solarisation

can at present be expressed only in very general terms.

Now

reversing

the

idea to be grasped

first

like

agents

potassium

upon the

the

manner previously explained

fresh

+ 2H O + O

in

2

product,

iodide

for

i.e.

1859 that

will re-

I

-.;

4KOH +

2l 2

:

,

it

is

we take an

Potassium

can be used as an

artificial

Poitevin showed as long ago as

possible to get a positive on glass

instead of a negative

by means of

this substance.

iodized collodion plate, sensitize

and then expose

it,

to diffuse light for a few seconds,

get a film of reduction product on the surface

of unaltered haloid. to

2

the latent image.

instance

solarising agent.

we

and

in

a precisely similar manner upon the in-

visible

If

which

your memories by repeating the equation

4KI act

iodide,

that

product of photo-reduction

acts

visible

is

remove

potassium the camera

silver

nitrate,

iodide for

If the plate

or

then washed

coated with a film of

solution,

three

is

and then exposed four times

in

the period

THE CHEMISTRY OF PHOTOGRAPHY.

218

necessary to get a good negative, the most strongly illuminated portions of the

become

film

rehalo-

genized, while the deep shadows remain as unaltered

reduction product, and the intermediate shades get

On

partially rehalogenized.

come out

lights therefore

development the high

white, the

shadows dark,

and the intermediate shades of an intermediate

we

other words

in

This

negative.

tint

get a positive instead of a is

experiment

for

the invisible

image what the Lassaigne-Herschel experiment product of photo- reduction [M].

for the visible

The

principle

Poitevin's this

that

:

revealed

image.

it

just

amounts

agent It

is

is

able to act as a

by rehalogenizing the

for this

reason

that

an emulsion, because ness of the latter. this

salt

a former lecture

it

if

left

in

diminishes the sensitive-

That

is

why

by washing, (p.

invisible

an excess of

potassium bromide acts detrimentally

remove

to

under the joint influence of

and atmospheric oxygen,

reversing

experiment of

this

any photochemical reagent capable of

liberating a halogen light

by

a very simple one

is

is

123).

as

it

is

essential to

pointed out in

REVERSING ACTION OF SENSITIZER. In

ordinary

special

of

cases

solarisation,

photochemical reagent

added

is

219

no

where

to the film,

there must be something present which plays the part of such a substance.

we must

sensitizer that

tion of the

believe, to the

I

is,

look for the whole explana-

How the

phenomenon.

will of course

It

sensitizer acts

depend upon the particular process

the chemical reactions which take place

may

present quite impossible to follow out, but

be

fairly asserted that

when the

genized up to a certain point

promoted by

and taking place even

We 1

in

may

it

begins to react reaction

being

oxygen and moisture,

the presence of light.

thus connect solarisation

The

be at

sensitizer is halo-

with the reduction product, the the presence of

it

;

l

with those other

observation of Sabatier, that a collodion wet plate becomes

if, towards the end of development, daylight is suddenly admitted to the room, relates to cases of what may be called

reversed

pseudo-solarisation,

the present

which do not come under consideration

The

category.

was given by Seely

in

1860.

that if the surface of a plate

in

explanation of this kind of reversal

is

It

depends simply upon the

fact

strongly illuminated during develop-

ment, the portions of the picture already developed, being represented by a more or less opaque film of metallic silver, act as a screen and prevent the further action of light on the portions of the film beneath them, whereas the undeveloped portions of the

THE CHEMISTRY OF PHOTOGRAPHY.

220

phenomena which have tion

led us

up

to

its

considera-

with the destruction of the invisible image by

;

oxidizing or halogenizing agents, with the destruction

of this image

in

the

dark by the haloid

hydracids plus atmospheric oxygen, and with the

spontaneous relapse of the latent picture in the

dark by the action of the halogenized sensitizer itself,

more or

less

Let us see how

when applied

aided by the oxygen of the

air.

explanation will carry us

far this

to special cases.

Imagine a collodion wet plate exposed to the continued action of strong

The

light.

silver iodide

goes on losing iodine, while the sensitizer nitrate) concurrently absorbs the

(silver

halogen with the

formation of oxidized products, probably iodate and nitric acid.

The amount

silvei

of these oxidized

products at length becomes so great that they begin to react with the reduction product

the action

surface are susceptible of photo-reduction to an extent proportional to the intensity of the light

observation referred

which

falls

the light which

upon them. is

If,

thus admitted

as in the is

stronger than that by which the latent picture was first formed, it is obvious that the undeveloped portions of the surface will come out darker

than the

first

to,

developed portions,

i.e.

the picture will be reversed.

RECURRENT SOLARISA TION. begins to reverse

work done by the undone.

and a

itself,

first

lighter

than

shorter exposure.

We

if

we develop \ve

the plate

it

developed after a

have arrived at the

first

the system,

Reduction product still

the

Supposing the action to continue

neutral stage.

we have

amount of

action of light becomes

If at this point

would be

certain

22 1

+ AgIO 3 + HNO 3 + O,,

undergoing chemical change, the reduction

product becomes more and more halogenized and oxidized versed.

the whole action of the light

till

At

this point

true reversal,

re-

the stage of

and the plate on development would But

give no result.

same

we have reached

is

state as

when

we have normal

affairs

have now arrived at the

the experiment was started

;

haloid under a film of silver nitrate

again, this sensitizing salt resulting from the re-

duction by the reduction product of the oxidized

products

first

formed, and the cycle recommences.

After a further exposure

we should

get a negative

of the second order on development, and so on.

Consider again the solarisation of a gelatine-

222

THE CHEMISTRY OF PHOTOGRAPHY.

bromide

The

plate.

more brominated

becomes more and

gelatine

till it

begins to rehalogenize the

reduction product; then the latter becomes more or less

converted into normal bromide, state

original

of

affairs

operations begin to repeat themselves. process

is

the the

The whole

nothing more than an accelerated cyclical

repetition of that

which occurs

the spontaneous

in

disappearance of the latent image

So

and

restored

is

till

far the

explanation

may

in the dark.

be considered to

hold good in a general way, but there are other points which have

to

with the theory.

Perhaps the most obscure of

these

is

be brought into harmony

the part played

by oxygen and by

oxi-

dizing agents, but the obscurity here arises from

our

ignorance

of

chemical

the

halogenized sensitizer, and

good reason

for dwelling

this,

upon

nature

by the way,

it.

It

may

of

the

is

one

be safely

asserted that oxidizing agents act inimically to the liberation of halogen.

I

may remind you

of

some

of our earliest experiments in this direction. sheet of paper coated with a silver haloid

exposed to

light

A and

remains unacted upon at those

FUNCTION OF OXIDIZING AGENTS. portions of

with

its

oxidizing

surface which have been

such

solutions,

as

223

painted

mercuric

or

stannic chloride, potassium dichromate, &c., (Exp.

A

Now

Lect. III. p. 100).

any substance which

is

inimical to the liberation of halogen cannot act as a sensitizer,

and

if in

contact with a substance which

can act in this capacity action

of the latter

absorbing capacity.

it

will lessen the sensitizing

by diminishing

Thus a

its

halogen

sensitive plate in

an

oxidizing solution, or washed over with such a is

solution,

easily solarised, because the sensitizer,

being in contact with the oxidizing agent, can hold a less charge of halogen its

it

parts

more

easily with

halogen to the reduction product, and so more

readily rehalogenizes the latter.

Whether the oxygen of the oxidizer comes absorbed by the sensitizer, and

actually bereplaces the

halogen which the latter has passed back to the

re-

duction product, must depend upon the nature of the sensitizer.

Evidence

is

not wanting that such oxi-

dation does actually occur. if

a gelatino-bromide plate

image

is

Thus Abney states is

exposed

till

that

a visible

apparent, the portions of the film corre-

THE CHEMISTRY OF PHOTOGRAPH}'.

224

spending to the image do not swell up when the

immersed

plate

is

well

known

water. 1

in

to be

Such a property

conferred

upon

plicated in

by the

There

sideration which tends

however another con-

is

show

to

that

absorbed by the sensitizer

actually

oxygen during

process of reversal on dry plates, and that

is

is

the the

the intervals which separate the succes-

fact that

sive

com-

is

brominated gelatine acts

fact that

a similar way.

by

gelatine

oxidizing agents, but this interpretation

is

phases are not uniform but of continuously If the

increasing length.

process were simply a

case of alternate bromination and debromination

of the sensitizer, the intervals might be expected to

be uniform.

The

fact that the

points to the using zer

:

it

up of something

in the sensiti-

indicates the exhaustion of the halogen-

absorbing power

of

the

source of such

extraneous

oxygen

intervals lengthen

of

the

oxidizing

latter,

and

the

cnfeeblement solution

or

only is

of

the the

atmosphere.

These considerations 1

will

enable us to strip the

Treatise on Photography, 5th ed. p. 309.

COURSE OF CHEMICAL CHANGES. phenomenon of spontaneous its

We

mystery.

say gelatino-bromide

imbedded bathed on

in a its

exposed to

up

to

some of

reversal of

must look upon a

225

sensitive plate

as a film of silver bromide

bromine-absorbing substance, and

by atmospheric oxygen. When

surface

light the vehicle

becomes brominated

degree of saturation

a certain

complex

;

bromo-derivatives, or additive compounds, or oxi-

dized products are formed, and these at length begin to react with the reduction product aided

the external oxygen.

The

may be

process

regarded

as analogous to the destruction of the latent

by the haloid hydracids, and indeed

it

probable that some hydrobromic acid

by

not im-

is

is

image

actually

formed by the bromination of the gelatine, and takes part in the process of reversal.

events

will

scheme

:

AgBr +

Gelatine

be

understood

from

The the

+ O

course of following

First stage.

gelatine

+

oxidized products

+ HBr {Brominated Debrominated )

^j

V-

+ O

First negative.

(

HoO

\

First neutral

stage and reversal.

THE CHEMISTRY OF PHOTOGRAPHY.

226

From

these explanations the other facts

con-

nected with solarisation follow as necessary consequences. plate

It

obvious that a highly sensitive

is

would halogenize

than a

less sensitive plate

occurs

more readily

We

sensitizer

its

that

;

most

in the

is

more rapidly

why

solarisation

sensitive processes.

must be prepared to admit that a vehicle

capable of becoming highly charged with halo-

genized products might act without the aid of

oxygen, and

it

has accordingly been found that a

gelatinp-bromide

plate

prolonged exposure

be

can

reversed

in a neutral liquid

which prevents access of

air.

1

by

(benzene)

Then again we

see

that a substance capable of neutralising the effect

of external oxidation, and of preventing the accu-

mulation of too large a quantity of halogenized products in the sensitizer, must act antagonistically to reversal.

on the

film.

Such

So

is

the action of reducing agents

also

process which partly

it

is

now

evident that any

halogenizes

the

sensitizer

1

"If a plate be exposed in benzene, however (a liquid which does not permeate through the gelatine), the phenomena are still existent."

Abney's Treatise on Photography, 5th ed.

p. 309.

FUNCTION OF THE DEVELOPER. must the

accelerate

the

of

rate

saturation point,

i.e.

because

reversal,

halogenized sensitizer

partially

227

nearer

is

of halogenization

the point

necessary for the reversal of the reaction to set

This

why a preliminary exposure of

is

its

in.

the plate is

so favourable to solarisation.

The

last

attention

is

phase of the subject demanding our the part played

have to follow out plate as

upon a

action

its

When

film.

exposed

action

its

compared with

by the developer

solarised

upon a normally takes

reversal

we

place,

the

portions of the film where the greatest action has

been exerted,

i.e.

the strongest lights, consist of the

most highly halogenized (and oxidized) portions of

At

the sensitizer or vehicle.

the

same

time,

owing

to the length of exposure, the silver haloid over the

whole surface of the plate posed.

Now

is

to

some extent decom-

the general result of halogenizing or

oxidizing such a vehicle as collodion or gelatine is

to harden

solutions

;

it,

and make

the greater the

the less pervious does

it

it

less

pervious to aqueous

amount of halogenization

become

[N].

When

fore the developing solution is applied, the

Q

there-

amount 2

THE CHEMISTRY OF PHOTOGRAPHY.

228

of silver deposited or reduced on any part of the film

is

determined by the readiness with which this

solution can get at the reduction product

the sensitizer is

where

most highly halogenized not only

is

the developer less able to attack the reduction

product, but there

may

be

in

some cases

of the latter present owing to

by the

On

reversing action.

portions

of

the

illumination are

film

the image

is

partial destruction

the other hand, the

which have

;

these portions

reversed on development.

proportional to the

deposit

;

in is

less

come out

solarised portions remain

development the density of the

formed

received

more pervious and respond sooner

to the developing action

dark, while the

its

actually less

light

;

In ordinary

silver deposit

is

amount of reduction product

a solarised film the density of the stiver inversely proportional to the degree of

halogenization of the sensitizer.

This

is

doubtless

the chief function of the developer, but the action is

further

complicated by the element of time

having to be taken into account.

During the process of development the

solarised

portions must be regarded as also undergoing reduc-

FUNCTION OF THE DEVELOPER. tion (or accretion) to

some extent

is

entirely a

With a strong de-

case of relative rates of action.

veloper the action upon

it is

229

the unreversed portions

exerted at once, because these are the most

A

pervious to the solution.

weak

developer, on

the other hand, cannot attack the relatively small

amount of reduction product on the unreversed portions, but as effect at all

vious

it

solarised

it

must act longer

to produce

has time to penetrate the parts

of

the

film,

and

any

less per-

to reach

amount of reduction product which they contain. Hence a weak developer

the relatively larger

give a negative, and a strong developer a

may

positive

This,

I

on development of an over-exposed

may

remind you,

characteristics of the

is

plate.

one of the well-known

phenomenon with which we

have been dealing. I

have been tempted to dwell at some length

upon these

residual

phenomena, both because of

the promising fields of investigation which they offer to the scientific student,

importance action.

in the

and because of

their

general theory of photographic

There has been too

great

a

tendency

230

THE CHEMISTRY OF PHOTOGRAPHY.

hitherto to limit attention to the sensitive silver haloid, as

future

though

we must

with which

it

A

The

two correlated

silver haloid

factors

and the

sensitizer

which cannot be con-

any of our photographic

processes.

ray of light falling upon a sensitive plate

like the

which

is

In

associated as playing an equally

is

sidered apart in

in chief.

look upon the sensitizer or vehicle

important part. are

were the agent

this

is

motive power driving a dynamo-machine feeding a storage

cell.

When

of the latter has reached a certain point

the charge

it is

capable

of reversing the motion of the system, and of converting the

dynamo

into a motor.

plays the part of such a storage

becomes charged,

i.e.

The cell

down and

reversal takes place.

when

;

a

halogenized, to

amount, the chemical energy stored up to run

sensitizer

in

it

certain it

begins

APPENDIX TO LECTURE

A

187).

(p.

A

VI.

solution of potassium oxalate

is

added gradually to a solution of ferrous sulphate in order to show the precipitation of ferrous oxalate

and

its

B

(p.

solution in excess of potassium oxalate. An alkaline solution of milk-sugar 190).

painted in stripes on the surface of a sheet of and after being allowed to act

is

silver chloride paper,

for

a few minutes

(at the

ordinary temperature of

the room) the paper is well washed, and then brushed over with a ferrous oxalate developing solution.

The

stripes

which have been reduced by the milk-

sugar, although before

development undistinguish-

able from the rest of the surface,

come out

intensely

dark.

C

(p.

191).

The experiment

scribed in the text.

is performed as deCare must be taken not to tear

After developing and fixing, the pressure marks can be projected on the screen by means

the film.

of the lantern.

232

THE CHEMISTRY OF PHOTOGRAPHY.

D E

I

(p-

93)-

Repetition of Exp.

J, 2,

Lect. V.

197). Exp. E, Lect. I. is repeated, the of the solution exposed to light being

(p.

portion

divided into two parts before the addition of the There are thus three vessels, (i) ferricyanide.

containing unexposed solution, (2) and (3) conTo one of the latter taining exposed solution.

some chlorine water

added, and

is

The

cyanide to all three.

then

ferri-

chlorinated and the un-

exposed solutions give the same result, showing undone the work done by the

that the chlorine has light.

F

199).

(p.

bromide nitrite,

is

A sheet

of paper coated with silver solution of sodium

sensitized with a

or with silver nitrate, and exposed to the

electric light

till it

The paper

shows

just

faint signs of

darken-

then well washed, and three or four stripes painted on it with various oxidizing solutions, such as bromine water, potassium diing.

is

chromate, mercuric chloride, potassium permanganAfter the reagents have been

ate (acidified), &c.

allowed to act for a few minutes

(in

red light) the

again well washed, and then developed with ferrous oxalate developer. The stripes come out

paper

light

is

on a dark ground, showing the destruction of

the reduction product .

G

(p. 204).

A plate

by the oxidizing is

agents.

coated with iodized collo-

dion and sensitized in the silver nitrate bath in red

APPENDIX TO LECTURE

VI.

233

A

few seconds' exposure to way. white (electric) light is then given, after which one half of the plate is immersed in nitric light in the usual

diffuse

acid (one part 1-4 sp. gr. acid and two parts water), then withdrawn and washed, re-dipped in the silver

and developed by

iron developer.

The

portion of the plate acted upon by the nitric acid remains undeveloped, showing the destruction of the inbath,

product of photo-reduction. sheet of paper coated with silver (p. 209).

visible

H

A

chloride

is

exposed

to

diffuse

white

light

till

distinctly darkened, and then painted with stripes of chlorine water and a solution of iodine in

potassium iodide in order to show the bleaching action of these substances. I

(p.

The darkened

209).

surface of the paper

painted with another experiment solution of iodide and exposed to potassium stripe white light till bleached. This experiment serves

used

in the last

well to

show the

is

similarity in the action of the

potassium iodide under the influence of light,

and

of the chlorine water and iodine solution in the

absence of

light.

A

sheet of paper coated with silver chloride and darkened as before is painted with J (p. 210).

stripes of solutions (about 5 per cent) of

chloride,

bromide and

strong white light

till

iodide,

potassium

and then exposed to

the iodized stripe appears

THE CHEMIS TR Y OF PHO TOGRAPH Y.

234

yellowish

white.

lighter than the

The bromide

ground colour,

is

the iodide, while the chloride

bleached at

K paper

has hardly

A sheet of darkened silver chloride

sensitized with a

is

stripe

all.

212).

(p.

stripe, although then darker than

5

per cent solution of in a moist state

and exposed

potassium iodide, under a design stencilled out

On removing

screen.

out light on a dark little acid to the

a black

in

paper

the screen the design comes ground. The addition of a

potassium

iodide

solution

accelerates the action.

L by

Bennett's experiment can be shown placing a sliding screen of black paper over a (p.

215).

gelatino-bromide plate, and giving different stripes of the surface exposures to the electric light for periods varying from two or three seconds to ten or fifteen minutes. On development by ferrous oxalate the darkness of the stripes increases up to a certain It is impossible exposure, and then falls off again. to name the precise period of exposure necessary

to

show the

different

effects of reversal, as

plates.

An

this varies in

instantaneous exposure of the plate to diffuse light

the whole surface of

hastens the experiment.

M

(p.

218).

An

iodized

sensitized in the silver bath,

collodion

exposed

plate

for a

is

second

or two to diffuse white light, and then washed with

APPENDIX TO LECTURE

VI.

235

One half of the plate is then red light). solution of potassium iodide cent per dipped nitric acid), and the whole with dilute (acidified surface again exposed to white light for about a water

(in

in a 5

minute.

On

washing, re-dipping in silver nitrate,

and developing with the iron developer, the portion of the plate which has been dipped in the potassium iodide solution

comes out much

lighter than the

other portion.

N

A

design is painted with weak bromine water on the surface of a gelatino-bromide (p.

227).

plate (in red light).

After remaining

in

contact

with the film for a minute or two the plate is well washed, and then exposed to diffuse white light for a

few seconds.

On

development by ferrous

oxalate the design comes out

background.

lighter

than the

LECTURE

The Spectrum.

Composition of White Light. tensity

and Photographic

Objects.

Intensity.

Spectrum Photography.

Visual In-

Colours of Natural

Graphic Representa-

Action of the Spectrum on the Silver

tion of Intensity.

Haloids.

VII.

Concurrent Reversal in Mixtures.

Reversal in

Spectrum under various Conditions. Reversal in Solutions of Oxidizing Substances and other Reversing

the

Agents.

WE

have thus

considered

far

photographic pro-

cesses as originating in the photo-decomposition of

the silver haloids without taking into account the particular constituent of the light that produces the

chemical change.

It is

doubtless well

that ordinary white light

of colours, and

it

is

it

made up

of a

to

all

number

a familiar fact that such light

can be decomposed into allowing

is

known

its

component colours by

to pass through a prism of

some dense

DECOMPOSITION OF WHITE LIGHT.

237

transparent substance, such as glass or quartz, or

by causing

it

to be reflected from or transmitted

through a surface on which are ruled a number of very

fine parallel lines. is

decomposition persion

;

In the case of a prism the

produced by refraction and

dis-

the different coloured rays being unequally

refracted, are separated

by the dense medium of the

The

prism through which they pass.

violet

and

blue rays are the most refrangible and the orange

and red are the

least refrangible, while the green

and yellow are refracted to an intermediate extent and thus occupy a position intermediate between the violet and blue on the one side and the orange

and red on the

other.

The decomposition of light by means of a ruled " surface, known technically as a grating," is due to a process called diffraction.

too far into

physical

It

would take me

considerations to

attempt

here to treat of the theories of dispersion and diffraction refer

you

;

to

for this portion of the subject

works on physics.

explanations will suffice to

A

make

I

must

few preliminary

clear those points

which are essential to the complete understanding

238

THE CHEMISTRY OF PHOTOGRAPHY.

of the practical bearing of these familiar facts upon

modern photographic

The

rainbow-tinted band of colours produced

passing a

known

processes.

beam

of white light through a prism

and

as the spectrum,

blue, green, yellow, orange,

and

viz. violet, indigo,

red.

of the intensely glowing carbon

lamp

you see that

is it

now

The spectrum of

points

in the

way

consists of

From what

of explanation,

that the position of

spectrum colour.

more

is

It

a continuous

band

in

in the

has been already said will

it

be understood

particular colour in the

any

a measure of the refrangibility of that will

materially assist us in gaining a

precise idea of the true significance of that

spectrum with which we

shall

in the present lecture, if

the physical nature of

I

have so much to do

give a brief sketch of

light.

Those among you who have studied physics

know

the

before you on the screen, and

which the colours graduate into each other order named.

is

customary to

is

it

recognize seven spectral colours,

electric

by

that light

is

an undulatory movement

highly elastic medium,

known

will in

a

as the ether, which

PHYSICAL NATURE OF COLOUR. all

pervades

239

and penetrates between the

space

When

particles of matter.

the ether

thrown

is

into waves by a sufficiently rapid series of vibra-

wave-movement

the

tions,

is

in

propagated

all

directions from the source of the disturbance as a

centre with a velocity of about 186,000 miles per

second.

the oscillations are below a certain

If

frequency they are

;

human

to affect the

of being

capable

instruments

fail

detected

eye, but

by appropriate

these longer-period waves are

as radiant heat.

On

the other hand

known

we may have

ethereal undulations which are too rapid to affect

the eye

;

these also can be detected

methods well known to Colour

is

physicists.

the visual impression corresponding to

oscillation-frequency.

waves of one

If

we had

a series of ether-

definite rate of oscillation

comes to the same only,

thing,

we should have

one

as you have seen, in

is

or,

what

definite wave-length

a pure monochromatic light

of a certain definite refrangibility.

which

by appropriate

not pure

;

it

is

physical language means

White

light,

polychromatic, that

it

is

a

jumble of ether- waves of different periods and

THE CHEMISTRY OF PHOTOGRAPHY.

240

When

wave-lengths.

a thin slice of such light

analysed by a prism, the spectrum which ceive

is

we

is

per-

the result of the sorting out of the waves

of different lengths and their classification according to wave-length or oscillation-frequency. shortest

The

and most refrangible waves produce upon

the eye the sensation of violet, while the longer

and

less refrangible

of red. well

The

known

waves produce the impression

actual lengths of the ether-waves are to physicists, but

concern ourselves with

now

that the wave-length

is

increases from the violet towards

the red, while

the oscillation-frequency diminishes as the spectrum in the

same

direction.

we

formation which about, because

contingent

it is

it

is

upon

1

The

visible

in the red to line is

H

in

mind

possesses

mode

kind of spectrum that

most familiar to photographic

must be borne

its

investigators.

extreme violet

0-0007606 and of the

is

It

that the extent occupied

spectrum extends from about the Fraunhofer line in the

of

know something

desirable to this

traverse

1

The spectrum formed by a prism certain characters

we need

that

all

;

the wave-length of the

H line 0-0003971

of a millimeter.

A A

REFRA CTION AND DIFFRA CTION SPECTRA. by the

different colours in such a

24 1

spectrum does

not actually represent the relative proportions of these colours present

the white light

in

proportions are necessarily falsified

owing to the unequal

The

violet

frangible,

and

are

blue

rays being the

than the

and spread less

The

orange, and red rays.

by the prism

refrangibility of the colours.

separated

greater extent

these

;

more out

to

re-

a

refrangible yellow,

colours are thus

out at the violet end and squeezed

up

drawn at

the

red end, so that the space between two points at

the more other

refrangible

end separated from each

interval (expressed in

by any given

lengths) would be greater than

senting the

same

the

wave-

space repre-

interval at the red

end of the

spectrum. If

we wish

colour,

and

to observe the true distribution of

we must appeal

this is

to the diffraction spectrum,

accordingly called the normal spectrum.

For most photographic purposes the

refraction

spectrum answers

it is

to

remember

that

all it

requirements, but

well

has the defects indicated.

Then again you must look upon

the extent of the

R

THE CHEMISTRY OF PHOTOGRAPHY.

242

spectrum as a physiological accident de-

visible

pending upon the construction of our eyes, and not as indicating

a

real

ethereal undulations at either

the

to

end of the band of

Both beyond the

visible colours.

limit

physical

and below

violet

the red there are ether-waves incapable of affect-

human

ing

vision

may

already said,

and the

them

to

alter their

fall

can be

this

in

such a

visible

by

way

as to

Substances

property of changing the wave-

length of an ethereal undulation fluorescent [A].

have

substances

certain

upon

wave-length

made

bring them within our visual capacity.

which possess

I

be regarded as radiant heat,

ultra-violet rays

allowing

which

the infra-red waves, as

;

These

ultra-violet

are

said to be

waves have no

special designation, simply because

we have no

specialized organ of sense for perceiving them, but

we

soon learn that from a photographic point

shall

of view they are of the highest importance. In considering instances of photo-decomposition it is

essential that

we should know something more

definite than that a

posed by

light.

compound

If our

is

knowledge

simply decomis

to be

com-

VISUAL AND PHOTOGRAPHIC INTENSITY. we should know what

plete

of the light tion.

is

most

Looking

light,

The

particular constituent

effective in causing

decomposi-

at the visible spectrum of white

such as that of the electric light which

on the screen, the the

maximum

first

thing that strikes us

of visual intensity

is

is is

now that

in the yellow.

and most obvious question that

first

arises is

whether that colour which appears the most tense to the eye

ments made

in-

the most active in promoting

is

photo-decomposition as photographic

243

is

visual intensity the

intensity

The

?

in this direction

same

earliest experi-

take us back to the

year 1777, when Scheele allowed the solar spectrum to

fall

on a sheet of paper coated with

and thus found

that

the

rapidly in the violet rays.

salt

silver chloride,

darkened

most

This fundamental ex-

periment can be easily repeated by throwing our electric

light

spectrum on to a sheet of

You

chloride paper [B].

become perceptibly luminous effect

silver

see the violet end has

discoloured, while the strongly

yellow has

not produced

any

photographic intensity does not

coincide

with visual intensity.

R

visible

2

244

THE CHEMISTRY OF PHOTOGRAPHY.

The has

line of investigation

been

physicists,

opened up by Scheele

by numerous chemists and

followed

among whom

I

need only mention the

names of Herschel, Becquerel,

W.

J.

Fox

Draper,

Talbot, and Robert Hunt, to convince you that the

problem has been regarded as one of no inconsider-

more recent

In

able importance.

photographic investigator self

with the

has

felt

every

who has concerned him-

development of the subject

scientific

necessary to study the action of the

it

spectrum upon the sensitive

times

silver haloids

As the

compounds.

final

or other photo-

outcome of

the work which has hitherto been done,

it

all

may be

taken as fully established that visual intensity and

photochemical activity are fact that

no way

The

related.

yellow and orange appear brighter to us

than violet accident.

in

and

blue

is

another

In the majority of

be photo-sensitive, of the spectrum decomposition.

is

it

physiological

compounds known

happens that the

violet

to

end

the most effective in promoting

Could we look upon nature with a

photographic eye,

we

should

experience

strange reversals of colour impression

;

a

some

wood

in

SUPPOSED CHEMICAL FORCE. springtime carpeted with violets and cinths, for instance,

spectacle

than

wild

would appear a more

meadow

a

245

with

ablaze

hya-

brilliant

golden

buttercups.

That the

violet

and even the

ultra-violet rays

are the most effective in the majority of cases fact that

also

for

salts, for

is

is

a

true not only for the silver haloids, but

the

and mercuric

uranic, cupric,

ferric,

the chromates, and for the salts of gold

and platinum.

In view of this circumstance

it

is

not surprising that the more refrangible end should

have been formerly regarded as the special

chemical force.

physics

we

find the

In

the

region

of

older works

on

spectrum divided into the three

regions of heat, light, and actinism

or chemical

With the advancement of knowledge this the notion that a division has broken down

force.

;

chemical force resided

in the violet rays

was due

simply to the circumstance that the most familiar

compounds, and especially the

more that

sensitive to these rays.

many

frangible

salts

It is

of

now

silver,

well

were

known

substances are sensitive to the less re-

radiations,

and even the

silver haloids

246

THE CHEMISTRY OF PHOTOGRAPHY. we

themselves can, as

presently, have

see

shall

their sensitiveness so modified as to

be decomposed

by the red end of the spectrum.

The importance of knowing

the specific action of

the spectral colours on the silver haloids

be apparent at ever,

that the various

camera screen

If

first sight.

in

may

not

you consider, how-

objects

focussed

on the

order to be subsequently photo-

graphed are possessed of different colours, the necessity for this knowledge will It will

become obvious.

be readily understood that objects will im-

press themselves

upon an ordinary photographic

plate with an intensity proportional to the

amount

of violet and ultra-violet light which enters into the colour which they reflect or

composition of the scatter.

The

fact that reds

and yellows impress

themselves feebly, while violets and blues impress themselves strongly, experiences

of

the

is

one of the most familiar

practical

photographer.

So

also the varying photographic activity manifested

on different days according to the state of the atmosphere, the horizon, &c.,

is

height

well

of

known

the

sun

above the

to all workers with the

COLOURS OF NATURAL OBJECTS. The

camera.

different

247

on

necessary

exposures

amounts

different occasions represent the varying

of blue, violet, and ultra-violet rays, absorbed from the sun's light

by our own atmosphere under

its

different conditions of humidity, or according to

amount of suspended matter which

the It

it

contains.

can easily be understood also that the length

of atmosphere traversed by the solar rays depends altitude of the sun above the horizon,

upon the this

is

light

why

and

the photographic efficiency of day-

greater at noon than earlier or later in

is

the day.

From

the photographer's point of view

it

is

an

extremely fortunate circumstance that the colours of natural objects are never pure.

this I

mean

no colour would appear perfectly mono-

to say that

chromatic

By

we analysed its light and observed its The colour of any natural object is

if

spectrum.

always a mixture

some rays of the spectrum

are absorbed and others are reflected, so that the

colour which finally reaches the eye effect.

In addition to this

white light

is

is

a residual

a certain amount of

scattered from all surfaces except

THE CHEMISTRY OF PHOTOGRAPHY.

248

Were

dead black.

it

for the latter circum-

not

would present much

stance, a photographic picture

more

and shade than we

violent contrasts of light

actually witness.

The composite

of a

character

which

colour

appears simple to the eye can be well shown by

means of

certain artificial colouring matters.

light transmitted

solution of

"

through a glass

aniline blue

"

cell

containing a

appears on the screen

of a rich blue colour, and this solution of "

On

analysing

these

"

iodine

nothing but green.

apparently transmits

green

The

by passing them

colours

through a prism you see that red

is

present in

both cases, a fact which the eye utterly

fails

to

perceive before the prismatic analysis [C]. It will

be evident from the explanations which

have now been given colour in the spectrum

that is

the position

due to the length of the

corresponding ether-wave, so that physical measurement its

it

is

a true

For photographic pur-

necessary to have

positions of reference

we have

of colour independent of

subjective sensation.

poses

of any

some

some recognized

definite lines in the

FIXED LINES IN SOLAR SPECTRUM. spectrum which we

mentioning lines are

the

can

in the

found

about instead

talk

spectrum of sunlight and

The

are generally used for this purpose.

the solar spectrum to which

dark

lines

traversing the

physicist is

who

lines in

allude are narrow

I

spectrum at irregular

intervals in a direction perpendicular to

they are known

of

Such fixed

by name.

colours

249

its

length

;

as the Fraunhofer lines, after the

first

mapped them.

accurately

It

no part of the present subject to explain here

the origin of these dark lines state that they are

;

it

will suffice to

produced by the absorption of

the particular waves which they represent in posi-

by gaseous and vaporous substances

tion

sun's, as

well

as in

our

see

in

the

photograph

The

own atmosphere.

more important of these dark of

the

lines,

solar

the

in

which you spectrum

thrown on the screen, furnish the necessary positions of reference.

In order to study practically the photo-reducibility

of the silver haloids or other compounds

by

different colours of the spectrum, the older experi-

menters often made use of coloured glasses

;

but

250

THE CHEMISTRY OF PHOTOGRAPHY.

this

method

obviously unsatisfactory because

is

by such media

the light transmitted

monochromatic of procedure

is

A

scientific

mode

photograph the spectrum

itself,

[D].

to

seldom

is

more

far

a narrow slice of white light being allowed to pass

through a prism, and the spectrum being then

on

focussed

examined. sun,

the

The

the electric

magnesium, or in rays

of

remember

all

surface

photographic of

source or

oxyhydrogen

be

may

light

light,

be

to

the

burning

any other source of radiation rich refrangibilities.

in carrying

important to

It is

out such experiments that

the materials composing the lenses and prisms of

the spectrographic apparatus have a certain specific well known, for instance,

absorbing power.

It is

that glass absorbs

more or

light,

so that in very exact

less

of the ultra-violet

work we must

either

use a diffraction spectrum or have our lenses and

prisms of quartz, as

first

pointed out by Prof.

Stokes in his classical researches on fluorescence.

When

a spectrum

graphic

surface,

induced

in

is

the

allowed to

fall

on a photo-

amount of decomposition

the different regions

is

shown by the

CUR VES OF INTENSITY.

25 1

degree of darkening of the silver compound, instead of allowing the spectrum to act discoloration

be

developed

in

invisible impression

the usual way.

visible

may

produced, a short exposure

is

given and the

employed by

till

or,

Both these methods have been

The

different experimenters.

results

may be expressed by a graphical method which is now so generally adopted that we must understand

its

principle before

making

Supposing, for example,

further progress.

we wished

visual intensity of the spectral colours

of a curve. lines

We may take

drawn perpendicular

to

the

made

spectrum, the proportional to

the intensity of the colours as seen

more

For

accurately, as

by means

as ordinates a series of

height of these lines being

or,

show the

to

by the

measured photometrically. 1

practical purposes the strongest of the

hofer lines answer

all

eye,

requirements.

The

Fraun-

intensity

of the colour seen at the position corresponding to

each of the selected lines would be marked off as a point on each 1

line,

and the points then joined by a

See Abney and Festing's Bakerian Lecture for 1886.

Trans. Part

II. 1 886, p.

423.

Phil.

THE CHEMISTRY OF PHOTOGRAPHY.

252

curve which would show at a glance the distribu-

The

tion of visual intensity in the spectrum. is

shown

in the

ML

H

diagram

FIG.

a

such a diagram

maximum

little

D.

is

H the

in

it

also

that

near the the

A.

JfaL.

distribution

in

case

this

same method

is

be

in

in

violet.

of heat

that

in the yellow,

side of the line

A

line

extreme

is

the visible

sented by another curve, the

would

B

C

Yellow.

at once seen

is

more refrangible

seen

extends from

way

Green

of visual intensity

to the

It

about

D

F

Blue

VISUAL INTENSITY OF COLOURS OF SOLAR SPECTRUM.

5.

From the

:

G

Violet

result

spectrum red

the In

might

maximum

to

a

similar

be

repre-

of which

the infra-red.

The

applicable to the representation

SPECTRUM PHOTOGRAPHY.

253

The amount

of decom-

of photographic intensity.

by a

position undergone

parts of the spectrum

is

silver haloid in different

shown by the degree

of

darkening on long exposure, or by the density of

the

silver

deposit with It

development.

is

and

short exposure

only necessary to

make

the

lengths of the ordinates (Fraunhofer lines) proportional to

the degree of darkening, and then

The

connect the abscissae by a curve as before.

curve then shows at a glance the distribution of

photochemical

spectrum

We

efficiency

in

every part

for the particular salt

are now,

I

upon the

The

in

spectrum

action of the pure spectrum

vehicles has

investigators, especially

Abney

work

silver haloids in various conditions

in different

Herschel,

employed.

venture to think, in a position

to follow the results of recent

photography.

of the

been studied by

by

J.

W.

and

many

Draper, Hunt,

and Becquerel, and most recently by with

improved

appliances,

and

with

greater care in eliminating the effects of impurities

than has hitherto been bestowed upon this

branch of work.

I

shall of course

be unable to

THE CHEMISTRY OF PHOTOGRAPHY.

254

do more than give you a general notion of the

The

results obtained. is

question to be asked

first

whether the different haloids have the same

maxima

of sensitiveness, and this can be answered

by experiment

way by

in

a

somewhat rough and ready

allowing our spectrum to

fall

on three

of paper coated with the respective

slips

haloids

and, after a sufficient exposure, developing in the

usual

way

[E].

chloride the

or

a

trum blue, trifle

little

;

The

shows that with the

result

maximum beyond the

is

limit of the visible spec-

with the bromide the

maximum

and with the iodide also

more towards the

notice also that the

about

in the violet, just

in the

but a

in the blue,

violet

You

end.

bromide spectrum

than either of the others

is

that

it

is

will

longer

extends more

towards the red, a fact of which we shall see the practical significance hereafter.

The experiment shown

is

necessarily a

hasty

one, and gives only a crude idea of the

action of the spectrum

In

upon the

more exact work the

local

different haloids.

greatest

precautions

have to be taken to insure trustworthy

results

PHOTOGRAPHED SPECTRA. the effect due to

with the

haloid,

every substance

whether as

stituent of the film, or as

an

255

associated

con-

essential

an accidental impurity,

must be studied individually before we

arrive

the true photographic spectrum of the haloid. is^

at It

no doubt the neglect of these precautions which

has led former workers to give different positions

maxima

the

to

The

haloid.

especially

felt

of decomposition

action

of

the

same

makes

itself

for

impurities

with prolonged exposure, so that

the spectrum printed up to the stage of visibility

never so satisfactory as that formed by a short

is

exposure and subsequent development. result of all the

up

recent experiments

in the following

M

L

diagram

is

final

summed

:

D

II

The

C

B A

IT.

M. FIG. I.

Chloride

6.

;

II.

SPECTRUM PHOTOGRAPHED ON THE SILVER HALOIDS. Bromide

;

III. Iodide.

The

dotted curves show the extension

towards the red by longer exposure.

THE CHEMISTRY OF PHOTOGRAPHY.

256

The

chief point of interest in connection with

these

curves

is

that

What

the silver haloid molecule.

we

molecule

We

molecule.

cannot

of these curves

pure

haloids

same

position.

itself,

;

and

maxima

The same

inherent in

maximum

are

of

are

sensitizer,

;

any

are observed,

by development

;

neither

provided

does

the

these

do

by

the state of

vehicle

not

the

obtained,

molecular aggregation of the haloid makes difference

that

in

always

curves

when the necessary precautions direct printing or

resides

careful experiments with the

by

the

one par-

in

The cause

is

shift the

property

curves show

most susceptible

is

ticular region of the spectrum.

the

that

The

shall learn subsequently.

that each haloid

in

physical

they indicate a fundamental property of

reality

is

a

they represent

nor

form

no the

actual

chemical compounds with each other or with the silver

haloid.

spectrum or

may

The

length of the photographed

be increased by longer exposure

we may encroach more red, or we may even so

by other methods

and more

into

the

modify the haloid as to get a new

maximum

PHOTOGRAPHED SPECTRA.

257

the less refrangible region, but the position of

in

maximum

the

blue

the

in

or

violet

not

is

thereby affected.

we

Interpreting these results practically

has the advantage for light which

silver chloride

and

rich in violet

is

see that

ultra-violet rays, while the

bromide and iodide have the advantage It

rays.

is

be

not to

photographed

understood that

these

any measure of

us

give

spectra

for blue

absolute sensitiveness, because, as you are

from

aware also

influenced

is

But when white

conditions.

or the rays from a coloured object

sensitive

which

is

plate,

acted

spectral colours

way

this

lectures,

by many other

light

a

former

is

it

fall

upon by the greatest range of must have the advantage

of sensitiveness.

You

will

sums up on the surface of the emitted

A 1

by the

haloid which

on

evident that that haloid

in the

understand this

you consider that the image thrown by a

if

now

object is

whose

affected

lens

film all the rays

image

by the

forms. 1

it

violet, blue,

Excepting the small percentage absorbed by the material of

the lens.

S

THE CHEMISTRY OF PHOTOGRAPHY.

258

green,

and yellow rays during the same exposure

which

in the case of

another haloid would allow

only the violet rays to tages in the

way

that with the

must

act,

You have

of sensitiveness.

same exposure

advan-

offer

silver

bromide gives

a longer spectrum than the other haloids

means

that

green,

yellow,

and

iodide, arid there is

one of the

make

haloid

this

is

there

present

is

is

a

so

or

property

sensitive

highly

in

is

the action

mixtures of the haloids.

an excess of some good sensitizer

capable of rapidly absorbing the liber-

maximum due

is

impressed

resultant

condition

is

due to the compounding of the

not

efficient sensitizer

observed.

to each spec-

the photographed spectrum

But

spectra of the separate haloids.

is

for

combine to

point to be considered

ated halogens, the

trum

this

that

processes.

of the spectrum upon If

chloride

causes which

many

;

sensitive

the

no doubt that

modern photographic

The next

than

red

bromide

silver

more

is

seen

fulfilled,

i.e.,

if

if

there

the above is

not

an

present, a very different effect

This

effect,

whatever

may

be

its

SPECTRA ON MIXED HALOIDS. true explanation,

we

who

discovery to Capt. Abney,

its

has worked out the essential conditions by

almost laborious nothing more

do

a very remarkable one, and

is

are indebted for

259

series

of experiments.

now than

a

give

summary of the conclusions of this The general nature of these results

I

can

very brief 1

investigator. will

be best

seen from the following curves, 2 selected as types

M

B A

L

T.

E.

nr.

w. FIG.

CONCURRENT REVERSALS

7.

from

a

large

illustration

Let 1

me

IN

MIXTURES OF SILVER HALOIDS.

number published

by Abney

in

of his work. first

explain

the

meaning of each

See chap. xxxv. of his Treatise on Photography, 5th

ed. p.

277

;

also Proc. Roy. Soc. vol. xxxiii. p. 164. 2

I

curves

am

indebted to Capt.

shown

in this

Abney

and other

for permission to

reproduce the

figures.

S 2

THE CHEMISTRY OF PHOTOGRAPHY.

260.

ciple

which they

reveal.

The

been selected so as to be

two

Thus

pairs.

In

I.

I.

and

II.

of bromide

upon mixtures lodion.

the

attempting to interpret

before

curve

prin-

four spectra have

comparable

fairly

represent the action

and iodide

in

a wet plate sensitized with silver

an acid or alkaline developer.

In

II.

the con-

were precisely the same, excepting that

ditions

the excess of silver nitrate was washed out. III. there

being developed In IV. the

no other

after a short

as

in

gelatine,

and the image

Now

before.

image

exposure as before.

sensitizer being present,

developed

the

silver nitrate, the

same haloids were used

compare the

:

The only in

In

was a mixture of iodide and chloride

on paper sensitized with

results

col-

was used, and the image developed by

nitrate

was

in

difference

former

there

between is

a

I.

and

II. is

sensitizer

present, while in the latter there

is

that

(AgNO 3)

nothing but

a mixture of bromide and iodide with practically

no

sensitizer.

have

You

see that in curve

been expected,

we have

the

I.,

as might

superposed

SPECTRA ON MIXED HALOIDS. spectra of

common maximum is

and

bromide

the

a totally different state of

G

near

there are

now two maxima,

H, and another

and

compounded

when

the

such

a

the

in

sensitizer

comparatively

feeble

(rather

Reduced

simply mean that

mixture of haloids green,

and

between

G is

and iodide curves for

gelatine

sensitizer

maximum

;

is

its

we have

this case

G, a

that

maximum

in

than

in

nearer the blue

and another minor

near H.

common

a minor one near

absent

is

may be neglected in before a minimum about

II.),

there

present, the curve

is

of the chloride

the blue-green

the

blue-green

action as

II.

difference exists between III.

When AgNO 3

and IV.

in

has become a minimum, and

The same

F.

But

affairs

maximum

with their

iodide

about G.

261

in

the violet,

to practical terms these facts in the is

absence of sensitizers a

more

sensitive to the blue-

less sensitive to the violet rays

of the

spectrum than either haloid separately.

We may

have next to consider how these

be interpreted.

The

results

case of a mixture of

haloids with excess of sensitizer

is

quite normal,

262

THE CHEMISTRY OF PHOTOGRAPHY.

and requires

no

special

markable destruction

and the

of

green when the sensitizer

about

G

there

first

about

G

into the blue-

absent, are the chief

is

sight appear to be that

a reversing process going on,

is

owing to the rehalogenization of the

sensitizer,

accordance with the principles discussed in the

in

This explanation

last lecture.

by Abney, is

re-

The most obvious ex-

comment.

planation would at

the

;

sensitiveness

maximum

shifting of the

facts requiring

explanation

for the

is,

however, rejected

very good reason that the effect

not analogous to ordinary solarisation, because

the

same

result

exposure.

is

obtained even with a very short

But although ordinary reversal

missible under these circumstances,

probable that

we have

phenomenon due

to

to the

is

it

is

not im-

do with an analogous

same chemical

cause.

In order not to complicate the argument

be well to

restrict attention to

only, say the bromide

and

greatest

is

about

The

iodide.

G

;

it

will

one pair of haloids

the spectrum where the greatest position goes on

inad-

region of

amount of decom-

it

amount of bromine and

is

here that the

iodine would be

CONCURRENT RE VERSAL. If the

liberated in a given time.

263

removal of the

halogens keeps pace with their liberation, as

is

the case in the presence of a sensitizer, the largest

amount of reduction product

we have

region

is

maximum

the

development as appears

in

formed

of intensity on

curve

I.

halogens are not speedily removed

But

if

is

more or

less

the

no active

if

present, their rapid accumulation

sensitizer

rays

in this

must

undo the work done by the blue

not only because of the actual quantity of

;

these halogens as

compared with the amount of

reduction product, but also because of the fact that bromine and iodine

ously would iodine,

form

a

when

liberated simultane-

compound, a

bromide of

which would probably be more active as a

rehalogenizer than halogen alone.

Chemists hava

long been familiar with the fact that a mixture of halogens

is

often

more

reactions than the halogens

So

far

image

to is

it

will

in

chemical

by themselves.

then the explanation

forward, and similar

effective

is

perfectly straight-

be seen that the process

that which

occurs

when

is

the latent

destroyed by a halogen, only the latter

-

THE CHEMISTRY OF PHOTOGRAPHY.

264

supplied (and probably in a more active form)

is

by

the haloid in the film itself instead of from

The

an external source.

effect is thus

analogous

to ordinary reversal with the part played

halogenized sensitizer

The element

out.

left

by the of

time does not come in because the rehalogenization goes on with the action of light ab tion of sensitiveness about

what

I

The

will

venture to

initio

G may

the reduc-

:

be attributed to

call concurrent reversal.

increase of sensitiveness in the

attributed

by Abney

green

to the formation of a

is

new

molecule, a bromo-iodide of silver of the formula

AgBr,AgI, or

Ag

2

This compound

BrI.

is

sup-

posed to be formed by the action of the bromine liberated from the latter

bromide upon the

being almost insensitive in this part of the

spectrum, while the bromide colour, as will be seen

is

acted upon by this

by comparing curves

III. of Fig. 6.

The bromo-iodide

believed to be

more

in

iodide, the

II.

thus formed

sensitive for the green,

support of this view

it

and

and

has been found that

a surface of washed silver bromide

is

is

if

darkened by

exposure to light and then brushed over with a

DO UBLE

SIL VER HALOIDS.

solution of iodine, a

compound

265

obtained which

is

towards the spectrum in precisely the

behaves

same way

The

as the mixture.

from these

conclusions drawn

but chemists

facts are perfectly sound,

are not very willing to admit the existence of

molecules without adequate proof.

you therefore that there

I

new

remind

will

a rare bromo-iodide of

is

occur as a native mineral in Chili,

silver said to

and there are a whole

series of

chloro-bromides

containing various proportions of metal, chlorine,

and bromine, of which the Chilian mineral em-

Ag3 BrCl 2

bolite,

,

is

But the most conclusive Abney's

light,

to

the

Under Fig.

7,

spectrum

furnished

of

surfaces

by

silver

iodine,

the

and then exposed

presence

of

sensitizers.

these circumstances curves similar to IV., are

illustration is

by

in

is

known.

best

darkened by exposure to

chloride,

rehalogenized

proof

with

experiments

bromide or

the

perhaps

is

obtained

[F].

The

point

of

this

that the sensitiveness to blue-green

shown even

in the

presence of a sensitizer

;

if

the silver haloids were present as a mere mixture,

you know that under

these

circumstances

we

266

THE CHEMISTRY OF PHOTOGRAPHY.

should have had a resultant spectrum as in curves I.

and

III.

With

respect to the chemistry of this last exis

it

periment

difficult to

which are not more or

offer

any suggestions

less speculative,

owing to

our ignorance of the true composition of the reduction product.

But

be useful to consider

may

it

what might happen on the supposition that product the

an oxyhaloid.

is

formula

sub-haloid

old

It is

this

pretty certain that will

have

to

be

abandoned, and the sooner we familiarize ourselves with other possible ways of regarding these actions, the better will

Thus,

we suppose

if

by the action of oxybromide,

duct by iodine

in

be

upon ,

2

I

2

future progress.

compound formed

silver

bromide

is

the

the destruction of this pro-

alcoholic

represented by the equation

Ag4 OBr +

for

that the

light

Ag4 OBr 2

it

re-

solution

might be

:

+ C 2 H 6 O = 2Ag2 BrI + C 2 H 4 O + H 2 O

In the absence of alcohol the

meets a different

fate

it

may

oxygen of course escape in the free

REVERSAL IN SPECTRUM. state or

it

may form

of the iodine.

oxidized products with some

The bromo-iodide may

give as a

mixed oxyhaloid,

Ag4 OBrI.

product a

reduction

267

Such are the possible processes which may go on

when the spectrum bromide and

on a mixture of the

falls

iodide, the iodine

which rehalogenizes

the oxybromide being in this case supplied

decomposition of the

silver iodide

by the

by the

liberated

bromine.

From

the action of the spectrum upon the silver

haloids with short exposure,

is

it

only a natural

step to the action of the spectrum with prolonged

exposure

duce this

:

the exposure

if

reversal, in

answer

in the

long enough to pro-

what part of the spectrum does

phenomenon occur

shown

is

?

Two

of Abney's curves,

accompanying diagram

this question for silver

(p. 268), will

iodide

and

for

a

mixture of bromide and iodide. In No.

I.

silver nitrate

No.

II.

pure

silver

iodide was sensitized

and developed

after long

exposure

in

a mixture of three molecules of the iodide

and one molecule of the bromide was exposed the

by :

presence

of

excess

of

silver

nitrate

in

(wet

268

THE CHEMISTRY OF PHOTOGRAPHY.

collodion plate), and then developed or alkaline method.

the

that

fact

iodide

curve

Both these curves point

region

of reversal

of

decomposition

maximum

ordinary

The

the

by the acid

is

making

is

now

D

G

FIG.

ORDINARY REVERSAL

8.

Pure Agl

+ AgN0 3

;

II.

3 AgI

IN

;

the true

elevation on

From

was discussed

is

A

It is

.

evident

maximum reversed,

is

only

and the

the less refrangible side simply re-

presents a small product.

maximum

B

C

SPECTRUM.

nearer the green had been produced.

apparent

in

maximum

+ AgBr + AgN0 3

however, that this shifting of the

:

depression,

appear as though a new

it

ML

I.

a

G.

maximum

the position where the curve shows a

thus

near

particularly instructive

with short exposure, there

the

at

is

to

residue of unreversed reduction

the theory of solarisation

in the last lecture,

it is

these curves express results which

which

obvious that

might almost

PHO TO- OX1DA TION IN SPECTR UM. have been is

halogen

anticipated

most

269

where the evolution of

:

rapid, rehalogenization (reversal)

might be expected to occur the soonest.

It

be regarded as a general rule that when a haloid

may

silver

given a very prolonged exposure to the

is

in the

spectrum begins

in

reaches

a

that

presence of a sensitizer, reversal region

maximum

We may

now

where

the

decomposition

with a short exposure.

pass on to the consideration of

the action of the spectrum upon the silver haloids

presence of

the

in

other

substances, both

This subject for a short will

It

true

will

reversing

agents and

oxidizing and

reducing.

be best approached by returning

time to purely photochemical actions.

be remembered that

it

was stated at the

very beginning of our subject that light in some cases promotes oxidation (Lecture

are

now

property putting

in

of

you

I.,

p.

25).

We

a better position to deal with this light,

in

and

I

possession

will

commence

of some

by

well-known

facts.

Although

in

most instances of photochemical

reduction, the violet

and blue rays are the most

THE CHEMISTRY OF PHOTOGRAPHY.

270

it

effective,

oxidation are the

that

appears

accelerated

is

most

cases where

many

in

by

the

light

red

rays

Thus the photo-oxidation

active.

of lead compounds, of mercurous oxide, of ferrous

and manganous compounds, of

more rapidly

sulphides, proceeds in

the violet rays.

that

I

because

arsenites in the

laying

we know

of

down

many

a

red

rays

photo-reduction

must

fully

general

law,

of photo-

instances

oxidation by the more refrangible

the

red than

must not be understood

It

am now

the other hand,

and of

rays, and,

we

shall

may

be

made

to

of

the

silver

salts.

on

learn subsequently that

bring

about

But

we

prepare ourselves for the possibility

of photo-oxidation

;

we must

learn to look

upon

a photo-sensitive surface as the seat of chemical

changes

in

which both reduction and oxidation

be taking

may result

place

simultaneously,

the

same

thing

as

on.

nature

of

the

compound

This amounts that

saying

product of photo-decomposition the

final

being determined by the relative rates at

which the two processes go to

the

is

the

final

determined by

with

which

the

BLEACHING BY POTASSIUM IODIDE. photo-sensitive

substance

by the nature of

as

is

this last substance.

from these general notions we

some concrete

to consider

In

the

last

lecture

iodide

potassium

was shown.

to

light,

of potassium iodide.

the

of

result

atmospheric

periment

and then

silver

of

chloride

was darkened bleached by a

in

the

this

By

oxygen.

of this

repeating

spectrum we

the

bleaching

light [G].

action

as

explained

oxidation

reversing process

the

rehalogenizing action

was

which constituent of the

that

action

reversing

the haloid

The

of the soluble iodide

in

may

exposure when painted with a solution

further

the

Starting

next proceed

be remembered that a sheet

of paper coated with

by exposure

well

instances.

upon darkened

It will

as

associated,

271

is

shall

being

salt

by

the

ex-

find

out

the most active

The

shows

result

takes place

in

the

same region of the spectrum where the maximum of

decomposition

reversing agent. as

in

ordinary

chloride

occurs

The

in

the

absence

action cannot be the

solarisation,

has already lost

its

because

the

of a

same silver

removable chlorine

THE CHEMISTRY OF PHOTOGRAPHY.

272

by preliminary exposure

iodide

the potassium

:

indeed be looked upon as playing the part

may

of the halogenized sensitizer, but the co-operation of

oxygen

The

experiment. the

condition

of

conditions

of

necessary

under

action

bleaching

of

a

is

result

the

a

is

good

from which we

principle

compound

photo-sensitive

iodide, the photographic surface

of

product

potassium

which

chloride.

silver

iodide

undergo

strongest at the

;

It

of

out

:

the

potassium

the reduction that

happens

those

most

oxidation

blue and violet rays is

one

is

is

the

illustration

set

here

is

its

compounds

rapidly in

the

hence the bleaching action

most refrangible end of the

spectrum. Investigations in the direction indicated last

by the

experiment have been made by Abney with

glass

plates

coated

in different vehicles.

with

The

the

plate

silver is

first

haloids

given

a

preliminary exposure, then immersed in a glass cell

containing the solution or gas to be tested,

and exposed developed.

to the action of the

The

spectrum and

results obtained are of

extreme

REVERSAL IN SOLUTIONS. and some of the curves are shown

interest

diagram on that

273

curve above

a

the

line

a curve below

while

action,

must be borne

It

p. 274.

normal

line

signifies

reversal, the local intensity of the reversal

indicated as before

mind

in

indicates

the

in the

being

by the height of the curve

at

different points. It is

by no means easy

in the present state of

knowledge to see the precise meaning of these

and we must be

curves,

satisfied

if

we can

get

but a glimpse into the photochemical processes

which they

reveal.

No.

I.

be called normal reversal

bromide

a

plate given

in

an ordinary gelatino-

prolonged

exposure

the spectrum, and about this there

You

mystery.

know

that

sensitive as far as the red if

enough

G

;

spectrum curve No.

gradually continues

it

I.

act

not

to

much

bromide

is

only gets exposure its

towards to

is

silver

the curve which has

creeps

what may

represents

maximum

the

(Fig.

red 6,

as

II.).

near the

This

simply shows therefore that while

halogenization of the sensitizer (gelatine)

on very rapidly

at the blue end,

it

is

is

going

going on

T

274

THE CHEMISTRY OF PHOTOGRAPHY.

R

G

D

A.

A?Br

in gelatine over-exposure.

IT.

AgBr in in

It.

W*

KBr

:

collodion

solution.

AgBr in gelatine in K^CraO? solution.

AgBr in gelatine in KaMngOg solution.

.

Agl in

in collodion solution.

KI

Agl in collodion in KgCrgC^ solution.

W.

Agl in

in collodion solu-

KgMn-jOg

tion.

in collodion in mineral acids.

\TJf* Agl

,

FIG.

9.

REVERSALS IN SPECTRUM IN PRESENCE OF REVERSING AGENTS.

JNTERPRETA TION OF CUR VES. more slowly the

at the red end, so that

blue end

constitute

has

what

received

I will call

red end has only received

by

27 5

the time

enough bromine

to

a reversing charge, the

enough to produce a

normal impression on development.

Such curves as

maxima,

are

most

the

satisfactorily, but

and VIII., which show two

II.

it

is

to

difficult

explain

worth while to make an

attempt to unravel their meaning,

if

only to get

suggestions on which to base further experiments.

The

first

of these

collodio-bromide to diffuse white

plate light,

per cent, solution of

exposing to

was a

obtained by giving

a

exposure

preliminary

then immersing in a five

potassium bromide

(acidified),

the spectrum and developing.

No.

VIII. represents the action of the spectrum upon a collodio iodide plate (after preliminary exposure)

when immersed

in

a

mineral acid.

It

added that collodio-iodide when immersed tion of potassium

similar to is

at b

into

y

No.

in

a solu-

bromide gives a curve somewhat

II.,

only the

and the second

the red.

may be

minimum

maximum

is

The curve showing

of reversal

shifted

more

the action of

T

2

THE CHEMISTRY OF PHOTOGRAPHY.

276

the spectrum on a silver bromide film in mineral acids

similar to

is

reversal

least

G

between

between

is

and

No. VIII., but the region of b

D

and

b.

In attempting to understand these is

bear

to

essential

We

results

it

mind the complexity of

in

under which the experiments are

the conditions

made.

of

instead

with a film which has had a

start

preliminary exposure and therefore containing a

mixture of unaltered haloid and reduction product, together

with

This

immersed

is

the

in

spectrum allowed to act upon are

favourable

haloid

is

to

sensitizes

halogenized

partially

reversal

and

solution

The

it.

because

conditions the

already partially dehalogenized. the

therefore

falls

spectrum

this

upon

the

silver

When surface,

the

rays which are most active in decomposing

the

substance

produce the

dissolved

maximum

the blue -end, in curves

due to

a

similar

when the spectrum of

darkened

silver

the

reversal II.,

action is

in

;

solution

the

will

maxima

at

V. and VIII., are thus to

that which occurs

allowed to

chloride

fall

on a sheet

paper coated with

INTERPRETATION OF CURVES.

These blue end maxima are

potassium iodide.

normal action reversed, the

fact the curves of

in

dotted

above

curves

showing the

277

the line

in

II.

of the ordinary

positions

and

V.

maxima

of decomposition.

The more

end of the spectrum are

reversals at the red

There

deal with.

difficult to

is

no doubt that

these represent the chemical destruction of the re-

duction product

most favourable to

tions are fact

oxidation, since oxidizing solu-

by

their production.

In

has been found that in reducing solutions,

it

such as potassium

no

gallol,

nitrite,

sodium

sulphite, or pyro-

reversal at all takes place.

Reversals in

potassium dichromate and potassium permanganate are

shown

in

curves

collodio-iodide

very that

way,

similar all

plate to

It

must be

understood in

the same

by preliminary exposure of the

then immersion to the spectrum.

reversal

VII.

is

to

in

the

The

less

and

solution

general effect

enormously

by the

A

hydroxyl gives a curve

these curves are produced

viz.

agents

in

and VII.

IV., VI.,

III.,

increase

refrangible

plate,

exposure

of oxidizing the

rays.

rate

of

Thus

in

THE CHEMISTRY OF PHOTOGRAPHY.

278

and

IV.

VII.

mixed one

it

the action

end and reversed that

seen that

is

normal

is

the red

at

the

result

a

the violet

at

This means

end.

of the

the latter region the destruction

in

is

reduction product has been going on to a great

extent by photo-oxidation, while

end

was

haloid

the

reduction

violet

undergoing

photo-

other words, reduction and oxidation

in

:

still

the

at

have been going on simultaneously at the two ends

of

the

spectrum.

Potassium

dichromate

causes reversal only at the red end as seen in III.

and VI.

probable that the specific absorp-

It is

something to do with

tion of the salt has

of

limitation

and

violet

appears

action,

end

blue

from

with

IV.

permanganate

one

other

given

and VI. exerts

end

end.

with

dichromate

the

absorption

and and

thus

action

i.e.

Potassium

VII.

violet

The dichromate its

This

spectrum.

of

the

off

by permanganate,

and the orange

the permanganate in

cuts

it

the

strong

but transmits the

green, at

of

a comparison

curves with those III.

because

this

in

blue red

at

agrees

upon

the

the rays

the

with less

ACTION OF SPECIFIC ABSORBENTS. refrangible

more

the

but acts as a

rays,

refrangible

rays

we have a

distinct

former this

in

image

we

reason

this

get no action in the blue with the

while

towards

screen

for

:

279

salt,

region

with the permanganate [H].

Now

although the

the violet and in

blue,

absence of either

the case of dichromate

light

of

the explanation there

offered,

or

direct is

action

all

in

reversed,

intelligible

in

which has just

the

been

an apparent mystery about the

is

proceedings at this end of the spectrum when

we have

do

to

with

that

permanganate

potassium

which requires clearing up.

allude to the fact

I

the violet and blue rays which are trans-

mitted by this solution produce or normal impression

which produces appears that

all

these

the

with

the

same

II.),

mineral acids (No. is

possible

direct

same exposure This

more remarkable when we consider

when

rays

through colourless solutions,

It

a

distinct reversal in the red.

very

bromide (No.

only

transmitted

such as potassium

potassium iodide (No. V.) and

VI II.),

that

the

cause strong reversal rays

in

question

are

THE CHEMISTRY OF PHOTOGRAPHY.

280

absorbed to some

extent

the permanganate

and

But

thus deadened. is

by

do not

I

through

passing

chemical

their

think

activity

that

this

the true explanation of the phenomenon.

seems more

we have

likely that

It

here a case of

retarded action in the violet and blue owing to the presence of the oxidizing

amply proved

in

It

salt.

has been

former lectures that substances

which tend to prevent the liberation of halogen retard the photo-decomposition of the silver haloids.

The to

action of the permanganate would thus

retard

be

the formation of reduction product in

that very region

where

the

in

absence of this

salt the

photo-decomposition would proceed most

briskly,

while at the red

end

there

would

be

going on photo-oxidation of the reduction product already

formed

The

subsequently. the time

a

final result

normal

distinct

formed by the reversed

by preliminary

violet

would be that by image

and blue

rays,

had a

been strong

image would be produced by the red

rays (Curves IV. and VII.). this

and

exposure

explanation

I

will

In accordance with

remind

you

that

an

REVERSAL IN MINERAL ACIDS. substance

oxidizing perfectly

VII.

to

result as

end

must

look

photo-oxidation

the permanganate

retarding and

normal

solution

is

curve

similar

very

the

upon

at the violet

due to retarded reduction

and

that

a

we

also

which

hydroxyl,

gives

colourless,

Here

like

281

red

at

the

is

very

end.

If

both

dilute,

oxidizing action are so enfeebled reversal

shown by the

as

occurs,

dotted curve in VII.

We

might thus go on

many more

such

curves

understand in

how

and

something

time to

these results

a general way.

VIII.) produced

almost tempts

this

these

enable

may

you to

be dealt with

The remarkable curve

by the

me

learn

But you have probably

from each one of them. seen sufficient by

and

discussing

action

(No.

of mineral acids

to digress further, but

I

will

content myself by pointing out that these acids, not being oxidizing agents (except nitric acid),

do not exert that retarding action

and

blue,

and

this

two maxima of

The

may

explain

reversal

so

in

the violet

why we

get the

strongly developed.

strong reversing action of potassium bromide

THE CHEMISTRY OF PHOTOGRAPHY.

282

shown

curve

in

must

salt

this

emulsion of

II.

washed

be

to

order

in

excess of

all

out

of

a

gelatine

the

insure

sensitiveness

greatest

why

explains

condition

Lecture

(see

IV.,

p.

123).

With regard in

to the chemical actions concerned

these reversals there

that

action

is

distinction

great

and

white light

of

spectrum

a

as

a

important to

is

it

the

some of which tend

regarded as the algebraical

of

the

White

light

undo the work of

to

we have an end

others

the

number of opera-

gives us the final result of a tions

between

action

agent.

reversing

remember

which

result

sum

may

be

of all the chemical

In the spectrum these effects

actions going on.

are differentiated and each set of rays produces its

own

reduction

particular

taking

complicated the

same

exert

place

in

by- concurrent place.

Thus

in

such

as

blue,

action

not at

the violet and

reducing

region

photois

photo-oxidation

and

their

the

that

one

permanganate

potassium transmit

so

action,

solutions

hydroxyl, these

which

rays

unchecked

can

except

PHOTO-REACTION OF REVERSING AGENTS.

action of the oxidizing substances

by the retarding upon the

why

This

silver haloid.

we

in these solutions

is

also another reason

get no normal reversal

the more refrangible end.

at

283

In the

red

rays

the photo-oxidation proceeds without check from

concurrent reduction, and thus gives strong

shown

reversal

in

the curves.

rise to

the

The

de-

struction of the reduction product in the spectrum is

thus

process that

simpler

in

white

phenomenon than

From

light.

would

immersed completely appear

such

play

reversals

that

in

made upon

the solutions,

in

oxygen

atmospheric

an

same

the

the circumstances

have been

these experiments

plates

not

a

important

the spectrum, even

part in

it

does these

in

the case of

reversing substances like the alkaline haloids or the haloid hydracids which are not oxidizing agents

under ordinary circumstances.

.

We

must

in fact

admit the possibility of direct interaction between the reduction product and these reversing agents

under the influence of particular the oxyhaloid formula

rays.

Adopting

we may expect such

actions as those typified

by the equations

:

re-

THE CHEMISTRY OF PHOTOGRAPHY.

Ag4 OBr + 2HBr - 4AgBr +H O 2

2

Ag4 OI + 2KI + H =4AgI + 2KOH Ag4 OBr + 2KI + H O = 2AgBr + 2 Agl + 2KOH 2

2

2

In the

by

such

2

as

salts

manganates, product

of direct oxidation by ozone or

case

the

may

and

dichromates

the

of

destruction

the

be due to indirect rehalogenization

by the oxidation of the halogenized as

explained

the

in

last

lecture,

or

sensitizer it

may

represented as a direct action of oxidation

2

per-

reduction

be

:

Ag4 OBr2 + 2O 3 = 4Ag2 O + 2Br2 + 2O 2 (first stage). 2Ag2 O + 2Br = 4 AgBr + O 2 (second stage). 2

The silver

nature of haloid

termine

the

the oxyhaloid.

is

the substance associated

particular

must

mode

with which the evidently

of destruction

de-

of

APPENDIX TO LECTURE

A

(p.

Shown by

242).

VII.

method of

the usual

placing a card painted with sulphate of quinine in the violet and ultra-violet region of the spectrum

a quartz prism is not available flint-glass must be used in order to get of the electric light. the best

B

If

effect.

A sheet

of paper coated with silver chloride and sensitized with silver nitrate is placed in the spectrum of the electric light as described in (p. 243),

the text.

During exposure the limits of the different

colours of the spectrum are marked out by pencil lines just below the spectrum seen on the paper. When the exposure has been sufficiently long to discoloration, the paper is withthen seen at once by the pencilled lines which region of the spectrum has produced

produce drawn.

visible

It is

the greatest

C

(p.

effect.

248).

The

solutions

matters are placed in sides

flat

of

the

glass cells

and interposed between the

colouring

with parallel

slit in

the lantern

THE CHEMISTRY OF PHOTOGRAPHY.

286

The aniline blue is the nozzle and the prism. " soluble blue of commerce dissolved in water and

"

acidified with a few drops of dilute sulphuric acid.

Instead of

"

malachite green," or almost any of the coal-tar greens can be used, as they all transmit a band in the red. The exact iodine green,

strengths of the solutions must be previously found trial.

by

D

The absorption-spectra of various (p. 250). coloured glasses were thrown on the screen by interposing the media between the slit and the prism in the usual way.

E

The papers

are

prepared in the ordinary way, care being taken to remove excess of silver nitrate by a final immersion in a solution (p.

254).

of the corresponding alkaline haloid. Three long and narrow strips are cut, washed with water till free

from soluble

solution of

salts,

sodium

strips are fastened

and then

sensitized with a

The

nitrite or sulphite.

on a board and exposed

three

for

one

or two minutes, the total width and length of the sensitive

papers being sufficient completely by the spectrum.

to

be

The

covered

strips

are

washed and developed by ferrous oxalate. The distribution of colour is shown (as in Experiment B.)

by

pencil lines

drawn beneath the spectrum during

exposure.

F

(p. 265).

Paper coated with

silver

bromide

is

APPENDIX TO LECTURE

VII.

287

exposed to the electric light till darkened, then brushed over with a weak alcoholic solution of

washed with water, sensitized by sodium nitrite, exposed to the spectrum for one or two minutes, then washed and developed by ferrous iodine,

The spectrum

oxalate.

as before

;

a

maximum

colours are pencilled out appears in the green on

development.

G

(p.

271).

Paper coated with

darkened by exposure to white over with a

silver chloride is

light,

then brushed

of potassium iodide and to the action of the spectrum (See Experiexposed ment I., Lecture VI., p. 233). The experiment can solution

be repeated with

silver

bromide, giving a short

exposure and developing with ferrous oxalate.

H of K in

279).

The

Cr 2 O 7 and

K

(p. 2

the usual way.

absorption-spectra of solutions Mn.2 O 8 are shown on the screen 2

LECTURE

VIII.

Photochemical Decomposition and Absorption of Light. Draper's Law. Effect of Mole-

Relationship

between

cular Aggregation

upon Sensitiveness of

Silver Haloids.

Photography of Red and Infra-Red Rays. Physical Analogy between Emission Spectra and Photographed Spectra. Effects

Photochemical Action probably intramolecular. Orthoof Dyes on Local Sensitiveness.

chromatic Photography.

Photochromy.

IN the present lecture we must tain

set out

fundamental physical conceptions.

compound absorbs some

If

from

cer-

a chemical

of the rays of the spectrum,

the energy of the ethereal undulations producing

those rays must be used up in doing

some kind of

work upon or within the molecules of the compound.

There

may

electricity, or there

be a production of heat or of

may be

chemical decomposition,

an equivalent quantity of the absorbed

light dis-

DECOMPOSITION AND ABSORPTION. These statements

appearing as such. evident to those

who

be

self-

are familiar with the teachings

modern physics

of

will

289

based

they are

upon the

universally received doctrine of the conservation of

At

energy.

the present

moment

chemical decomposition that

with photo-

is

it

we

alone con-

are

cerned.

The

inherent property of the molecule of a

silver haloid

which confers upon

to special rays

is its

This

is

faculty

it

sensitiveness

its

power of absorbing those

rays.

no doubt ultimately dependent

upon the mechanical structure of the molecule, but

is

it

beyond

my

province to deal here with

The proof

these questions of molecular physics.

of this statement does not rest however upon such

a priori mechanical conceptions, but

experimental demonstration.

It

is

capable of

has been shown

by many experimenters that whenever a beam of light

passes through a sufficient thickness

solution or gas in

which photochemical change

induced, the emergent light faculty

of a

of producing

is

is

deprived of the

any change

quantity of the same solution or gas.

in

a

further

The U

special

2 9o

THE CHEMISTRY OF PHOTOGRAPHY.

undulations which are effective for the particular

substance

are

out of the beam, and the

sifted

residual rays are thus

which

is

wanting

in that constituent

capable of affecting the substance being

This has been proved

experimented with. of

case of solutions

ferric

and mercuric

the

in

salts in

the presence of reducing agents as well as for other

mixtures [A].

It

thus be taken as a general

may

law that photochemical decomposition implies the absorption of some particular constituent of the light,

and as

this

was

first

by the American

distinctly enunciated in

physicist

J.

W.

Draper,

well to associate the principle with

and

will

be

discoverer

to speak in future of Draper's law. directly demonstrate the truth of

Attempts to this

its

it

1841

law

in the

pounds as the

case of such photo-sensitive

silver haloids are frustrated

opacity of these

salts,

com-

by the

but there are other reasons

for believing that the

law applies

These reasons

think be easily intelligible

will

I

after our treatment of

the last lecture.

in

these cases.

spectrum photography

in

If thin films of the silver haloids

in suitable vehicles are

examined by transmitted

DRAPER'S LAW. light, it will

are

291

be found that the absorption spectra

complementary to the photographic

Let us understand the

The maximum

full significance

absorption, as

analysis of the transmitted

spectra.

of this

shown by the

fact.

spectral

light, coincides with the

maximum of photo-decomposition as revealed by the This

developer after exposure to the spectrum.

means

that the waves which are the

photographically

are

precisely

absorbed by the haloid

most

those

effective

which are

other words Draper's

in

law of the relationship between absorption and photochemical decomposition has a direct bearing

upon

photographic

This

processes.

not

is

merely inductive leap, but a conclusion which

must now attempt to

a I

justify in detail.

Perhaps the best proof of the existence of such a relationship between photographic sensitiveness

and absorption which

is

furnished

by the

change

optical

associated with the change in the state of

molecular haloids

is

aggregation

during

the

undergone by the

process

of

subject has already been touched lecture (p. 124)

ripening.

upon

in

and we may now resume

U

silver

This

a former its

2

con-

THE CHEMISTR Y OF PHOTOGRAPHY.

292

with

sideration

bromide

silver

unripened state

be remembered that

red

will

it

appears reddish by transmitted

imply

?

What

transmitted.

is

You

in its

it

but as the particles grow by ripening

light,

less

Commencing with

advantage.

will of course

less

and

does this change

be prepared with the

answer that the larger particles absorb more of the red.

But

if

absorption means photochemical de-

composition > as Draper's law leads us to expect, the ripened bromide should be red,

and

this

the spectrum.

is

just

A

what

large

is

more

sensitive to

found on photographing

number of most

carefully

made experiments have been conducted by Dr. Eder and they all confirm

this conclusion.

Here are three

of Eder's curves which will speak for themselves

I

If

G

F

D

C

J?

:

A I.

K J2T,

FlG.

10.

- Si'ECTKA PHOTOGRAPHED ON DIFFERENT FORMS OF SlLVER BROMIDE

OPTICAL EFFECT OF RIPENING. No.

represents the action

I.

bromide and No.

II.

upon the unripened

the action upon the ripened

both being emulsified

haloid,

293

in

The

gelatine.

dotted curves show the effect of short exposure

and the continuous curves the No.

exposure.

III.

shows the

of longer

result

effect of ripening

almost up to the point of decomposition.

These curves are highly instructive taken

in

connection with the other purely chemical con-

which have been dealt with

siderations lectures.

It

normal

the

will be seen

maximum

materially shifted

now why

see

the

for

position

bromide

the effect of ripening

to extend the sensitiveness

We

that the

is

is

of not

simply

more towards the

the ripened bromide

affected

former

in

red.

the more

is

by a wider range of

sensitive

:

colours

than when in the unripened

This

the optical justification of the statement

is

it is

which was made

in

the last lecture

spectral

condition.

(p.

258) re-

specting the superiority of the bromide for rapid

photography.

The ments

general result of these and similar experiis

to

show that the

sensitiveness

of the

THE CHEMISTRY OF PHOTOGRAPHY.

294

silver haloids for particular rays

ent

state

their

upon

This

state

mode

of formation

in

is

largely depend-

aggregation.

physical

turn

dependent

upon the

of the

haloid, the

nature of

its

the vehicle in which

of

is

it

suspended, the state of

is

dilution of the reacting solutions, &c. priate

now

appro-

I

cannot

the details of which

treatment,

stop to explain, silver chloride

being obtained in

of molecular

the

to

it

illustrating

been found

has

that the highly sensitive

ripened

form

much more

difficult

to

bromide collodion

is

than

in

Abney

special precautions however, in

which region

appears is

1

sensitive

of

emulsion

means

blue to

far

the spectrum.

1

obtain in

in

in

collodion

it

is

possible

and

light,

the

infra-red

This special form of

confessedly difficult to prepare

is

the

has succeeded

by transmitted rays

of

taking certain

producing a form of the bromide

which

its

By

gelatine.

and

ripened

As

unripened forms of the bromide. the influence of the vehicle,

capable of

is

states

different

corresponding

aggregation

By

obtain

to

Phil. Trans. Roy. Soc. 1880, p. 653

;

;

by

photographs

Ibid. 1886, Part II. p. 457.

RED-SENSITIVE EMULSION.

295

limit of

of the spectrum far beyond the lowest visibility.

The

coarse-particled bromide

in this

is

form susceptible even to waves of such low frangibility as those emitted

The

water.

action

of

by a

the

vessel of boiling

below

shown

is

:

D

FIG.

this

spectrum upon

and the two modifications of the chloride in the curves figured

C

B

A

ii. SPECTRA PHOTOGRAPHED ON DIFFERENT FORMS OF SILVER CHLORIDE (I. and II.) AND ON THE RED-SENSITIVE FORM OF SILVER BROMIDE (HI-)-

No.

I.

the

represents

ordinary form

of the

chloride in collodion, the film appearing red

transmitted

light.

No.

emulsified in gelatine

red

No.

re-

;

this

II.

is

the

same haloid

and absorbing more of the

form appears grey by transmitted

III. is

by

light.

the photographed spectrum on the red-

296

THE CHEMISTRY OF PHOTOGRAPHY. form of the bromide

sensitive

in

collodion pre-

laid

before you in

pared by Abney.

The

which have been

results

such importance that no excuse

this lecture are of

need be made

for

attempting to get a

insight into the inner

little

further

mechanism of the processes

of absorption and decomposition which have been

The

revealed.

general history of the progress of

knowledge teaches us that the greatest advances are always

made when one branch

croaches upon another.

I

of science en-

might devote a whole

course of lectures to the illustration of this principle.

Consider for example the application of

electricity to the

decomposition of chemical com-

pounds, resulting in the discovery of the compound nature of the alkalies and the isolation of the

metals the

potassium

application

terpretation

to the law of

of

and

sodium

;

or

take

the

properties

of gases leading

Avogadro and the complete

tionizing of the

again

of physical principles to the in-

fundamental

revolu-

conceptions

con-

cerning the nature of chemical atoms and molecules.

Coming

nearer to our present subject,

remember

PROPERTY OF SPECIFIC ABSORPTION.

297

the influence of the introduction of optical methods into chemical

investigation

resulting in

covery of spectrum analysis with at

Looking

consequences.

its

the dis-

far-reaching

these curves showing

the effects of a change in the state of aggregation

upon the

sensitiveness of the silver haloids,

it

is

impossible to avoid coming to the conclusion that

we may have

phenomenon a point of contact

in this

between chemistry and physics which only awaits further

development

order to lead

in

to

results

of lasting importance to both branches of science.

That

inherent

property

molecule which causes

it

to

of

part of the

result of the

The

silver

undergo a

amount of decomposition per particular

a

haloid

maximum

unit of time in a

spectrum

is

no doubt

a

mechanical structure of the molecule.

rays are absorbed because the

molecule or

some constituent of the molecule has a period of vibration which of vibration that

nothing

synchronous with the period

of the ether-wave corresponding to colour.

particular as

is

of

yet

molecules, and

I

the

We

know

internal

absolutely

mechanism

must caution you that

I

of

am

THE CHEMISTRY OF PHOTOGRAPHY.

298

here entering upon speculative ground.

It

would

be rash therefore to attempt to draw any close

known

between

analogy

phenomena

physical

which are capable of being explained mechani-

and that which occurs

cally,

The only comparison

under the influence of light.

which

it

absorption to the

venture

to

safe

is

of light

in

a solid particle

in

these cases

is

analogous

of an air-wave corresponding

absorption

music by a stretched string

to a definite note in

or a

tuning-fork capable of emitting the

note

when thrown

into

case because

its

But even far,

and

if

this

pressed

instance that the

affairs,

to fly

but

corresponds

It

may

it

absorbed

it

off. is

lead

sometimes asserted

is

a

impulses

whole,

the

at length sufficient to cause

constituents

this

[B]..

too closely

molecule to swing as

becoming

string in

analogy does not take us very

astray altogether.

of

sound

period of vibration

with that of the air-wave

same

The

vibration.

tuning-fork can take up the

or

the

that

is

upon

This also

may

cause

us for

the

amplitude

one of the

be the state

possible

that

the

ABSORP TION B Y HALOGENS. absorbed

molecule

the

upon one constituent of

act

impulses

The

only.

lead to the conclusion silver

the

haloids

action

of the blue rays

we must have

to

But we have that

halogen absorbent present or

a

the decomposition

does not take

The photo-decomposition may

place

at

all.

thus be in reality

function of the halogen

a mechanical

is

we know

not the case

is

case of the

the

that in

would

view

former

cause photochemical dissociation. learnt that this

299

atoms

in

contact with a second substance with which they

can combine.

This view appears

probable when

we

haloids and

other photo-decomposable halogen

all

rays,

i.e.

sensitive

for

the blue and

very colours which are most

the

strongly absorbed by

Any

the

halogens themselves [C].

attempt to present a physical picture of

what occurs

in

a

silver

haloid

molecule during

the process of ripening would take us into

the more

consider that both the silver

compounds are most violet

all

the

region

of speculation.

We

still

further

know

for

a fact that the particles grow in size during this process, so that

the

red-absorbing particle must

THE CHEMISTRY OF PHOTOGRAPHY.

300

consist of a larger

aggregate of molecules than

the blue-absorbing

particle.

in

size

(and consequently in mass)

vibrating period

reach of

physical

at

is

affects

An

the

analogy might

employed with

if

the

of the

beyond

present

inquiry.

perhaps be useful here

we

increase

of any one constituent

system of molecules

If

But how

caution.

take a ray of light consisting of waves

of some two or three vibrating periods only, such for

instance

as

the

emitted

light

hydrogen, and examine

its

by glowing

we

spectrum,

see only

certain bright lines in the positions corresponding

These bright

to their respective refrangibilities.

spectral lines are the optical

expressions of the

mechanical structure of the hydrogen molecule they

indicate

that

the

molecule

is

capable of

vibrating in certain periods and thus of emitting

The width

waves of particular lengths. lines

will

of course

depend upon the width of

the slice of light which prism,

ix.

researches

is

passed

upon the width of the

spectroscope.

of

Now

of the

it

has been

Pliicker

and

through the slit

shown Hittorf

of

the

by the and

of

ANALOGY WITH EMISSION SPECTRA. Frankland and diameter

of

and

Lockyer

the

slit

width of these hydrogen

lines

that,

the

unaltered,

the

others,

remaining

goes on increasing

as the pressure of the glowing gas in other

I

increased

is

a real analogy

and the spreading of the

this discovery

photographic spectrum of silver bromide as particles will

become

show

maximum

maximum by

;

the pressure increases.

as

cannot but think that there

between

is

words, the discontinuous spectrum tends

become continuous

to

301

A

larger.

the

that

glance at the curves

position

of

the

normal This

remains unaltered throughout. is

its

the analogue of the position occupied

a bright line in the spectrum of a glowing gas,

and the

effect of

particle

is

an increase

in the size of the haloid

analogous to the effect produced upon

the spectrum of the gas

by

increase of pressure.

In both cases the spectrum widens out in such a

way to

as to

make

the discontinuous spectrum tend

become continuous

:

in

other words, to

special vibrations corresponding to the

of luminosity in

the

the

maximum

emission spectrum

or

of

absorption in the photographic spectrum, pressure

THE CHEMISTRY OF PHOTOGRAPHY.

302

in the

one case and increase of

case, causes

longer

may

shorter

the other

in

of vibrations of

the addition

and

size

periods

than

both

which

those

be regarded as the normal vibrations of the

gas or of the haloid.

Such considerations to

the belief which has already been expressed,

that as

it

is

not the molecule of the silver haloid

a whole

which

The

is

on

go

growing

photochemical action

a

otherwise distinct

we

of

appearance

show that

of this

this

major

the

more

the

size

to

most

seat

maximum

or

not the case

such

with

with

The

the

of

Were

find

associated

is

still

compound.

intra-molecular.

new maximum.

a the

curves

maximum

takes place about G, but

sensitiveness

minor sensitiveness or,

that

a

of decomposition to

in

special

remains while the

should expect

increase

displacement

is

the

seems to point

conclusion

the

to

distinctly

of

maximum

fact that the

particles

responsible for the

sensitiveness

photographic

it

as these lend countenance

there

is

added

a

for the less refrangible rays,

correctly speaking, the feeble sensitive-

ABSORPTION INTRA-MOLECULAR. ness for the less refrangible rays a practically workable pitch size

the

of

this point

add

that,

haloid

silver

home by

order

exposure

the

to

increase

To

particles.

in

bring

may

I

G

photograph requires

has found that

that

necessary,

while

times

thirty-five

The spectrum

an

than

greater is

A

about

region

times

twenty-five

for G.

Abney

the

impress

infra-red

exposure

by the

even with the red-sensitive form of the

to

required

exalted to

is

a practical illustration

collodio-bromide emulsion, in

303

this case

in

the

was

that of bright sunlight at noon.

The

modern photographic the

sensitiveness

orange,

then, so far

present position

and

red

processes,

for

such

be

can

as concerns

that

is

as

colours

with silver salts obtain a plate which

bilities.

Till

this

is

can be prepared which the

same order of

intensity as

representation

till

equally refrangi-

a plate

sensitive to colours in

is

no photographic picture faithful

is

and low

done, or rather

yellow,

we cannot

increased,

sensitive to rays of both high

although

of

will

we

perceive them,

give an absolutely

coloured

objects

the

THE CHEMISTRY OF PHOTOGRAPHY.

304

and

blues

violets

will

still

give

a

stronger

These more

impression than the other colours.

down

refrangible colours can of course be toned

by causing the

medium, such as red

it

of the ideal

is

plate

modern

of

given greater effect by this

lowered

perfection

investigations,

without

shall

actual visual

loss

is

of

intensities

in

be orthochromatic.

its

of

a plate which

sensitiveness

the

the aim

record

colours

Such a

of

plate

the

an

would

has already been pointed out

ordinary processes the silver haloid

all

records

It

is

to have

of

The

consequence.

which

object in their correct values.

that in

less

obvious that the total sensitiveness

is

state

but although the

glass,

are

refrangible rays

means,

through an absorbing

light to pass

colour impressions in quite a different

order to that in which they are recorded on the

human graphy

retina: the object of orthochromatic photois to

bring visual intensity

intensity into

The made

who

first

in

and pliotographic

harmony.

step in this much-desired direction

1873 by Professor H.

W.

was

Vogel, of Berlin,

discovered that the silver haloids could be

made

SENSITIZING A CTION OF sensitive to yellow

D YES.

305

and greenish-yellow by tinting

the films (collodion) with certain coal-tar dyes such as corallin, aldehyde green, cosin, cyanin

and many

This discovery formed the starting-point

others.

of a series of investigations which have been carried

on to the present time by various experimenters. I

cannot stop to recount the different steps

in the

progress of this discovery, but must limit myself to

a brief statement of the general results which have

been arrived

at.

The main

fact,

that

new maxima

of chemical activity can be obtained in the less refrangible regions of the spectrum

by the addition

of dyes to the film, has been confirmed by the work

of Becquerel, Waterhouse, Abney, Carey Lea, Edcr

and is

C.

H. Bothamley.

Now that

gelatine emulsion

so generally used in photography, the practical

aspect assumed

by the problem

is

the rendering of

such films orthochromatic by the addition of able colouring matters.

Immense numbers

suit-

of dyes

have been tried under various conditions, especially

by Dr. Eder of Vienna. pulation which film

may

it is

The only

point of mani-

necessary to explain

is

that the

be dyed by adding a solution of the colour-

X

THE CHEMISTRY OF PHOTOGRAPHY.

306

ing matter to the emulsion plate

may be

dipped

itself,

in the coloured solution

when dyed withdrawn and

dried.

colouring matter employed

is

ance, since too

little

or the coated dry-

The amount

of

also a point of import-

much both

or too

and

act detri-

mentally with respect to special sensitizing action.

According to some recent experiments of Abney's is

it

not even necessary to dye the film, but the same

effects are

produced by coating the sensitive surface

with a varnish containing the colouring matter.

The importance sufficiently obvious

of this

new departure

The

first

bearing of the discovery, these dyes

The

of Eder and

Eder's curves

amley

in the

number

1888, vol.

ii.

(I. to

Journ.

curves (XI. to ference

the course of the

to discuss the chemical

the precise way in which

the photographic spectrum*

following series of curves selected from the

results

1

influence

is

in

thing that must be real-

we can proceed

ized before

be

from the general considerations

which have been submitted present lecture.

will

XIV.)

Abney 1

show how the

X.) are taken from a paper by C. H. Both-

Soc.

Chem. 2nd.

for 1887, p. 425.

Abney's

are selected from a series given in the

of the Journal of the

p. 58.

will

Camera Club, March

Coni6th,

CURVES SHOWING ACTION OF DYES.

307

colouring matters affect the less refrangible end of

the spectrum.

NML

H

FIG. I.

FED CBA

C

12.

Violets; II. Greens; III. Iodine green; IV. Cyanin ; V. Eosin ; VI. Ammoniacal rose Bengal ; VII. Coerulein ; VIII. (_hrysaniline ; IX. Eosin on

XT I. AgCl; X. Eosin + Cyanin XI. Erythrosin on Agl + AgNO3 XIV. Cyanin on AgCl. ErythrosinonAgBr XIII. Erythrosin on AgCl Where the haloid is not specified AgBr has been used. ;

;

;

With mind

respect to these curves

that they

;

it

must be borne

show nothing more than the

X

in

relative

2

THE CHEMISTRY OF PHOTOGRAPHY.

308

of the dyed film for the different

sensitiveness

regions of the spectrum.

of

the absolute

maximum

new is

often

acquired

They

sensitiveness in

the

less

tell

The

of the plate. refrangible O

the expense

at

us nothing

region O

of a general

diminution of sensitiveness at the more refrangible end, so that a longer exposure has to be given to

bring out the special sensitizing action of the dye

:

in

other words the total sensitiveness of the plate

is

lowered.

Then again

for the colouring matters

it

must be observed that

used and under the partic-

ular conditions of the experiments, the

of action

still

is

in the

more

maximum

refrangible

region.

This means that such dyed plates, although tively

more

rela-

sensitive to green, yellow, orange, or red

than they were before, are sensitiveness for violet

still

and

possessed of greater

blue, so that in photo-

graphing a coloured object on one of these plates it

would

and

still

violet

be necessary to tone down the blue

by interposing an absorbing medium,

such as yellow

glass.

chromatic plates screen

is

it

Even with the

best of ortho-

seems that some such yellow

necessary before a coloured object can be

ORTHOCHROMATIC PHOTOGRAPHY. photographed with

all

the colours represented in

Another point

their true gradations of brightness.

which an inspection of these curves brings out the

309

same dye produces

different silver haloids.

is

that

different effects with the

This

will

be seen on com-

paring V. and IX. showing the action of eosin on the

bromide and chloride respectively, and XI.

XII. and XIII. showing the action oferythrosin on the iodide, bromide, and chloride.

In

practical applications orthochromatic pho-

its

tography distinct

is

most

differences

likely to be serviceable

where

of colour have to be repro-

duced, such as in the copying of paintings. will

be readily understood

also that

an

It

ortho-

chromatic plate would be valuable in cases where

we have to employ light,

which

is

artificial light, like

relatively

poor

in the

gas or candle

more

refrangi-

ble rays, because in such a plate the sensitiveness for the less refrangible rays is exalted

of the dye.

by the action

In continuing the experiments in this

direction, Mr. C.

H. Bothamley, of the Yorkshire

College, Leeds, has succeeded in preparing plates

which arc more sensitive to yellow and orange than

THE CHEMISTRY OF PHOTOGRAPHY.

3 io

and violet, and which thus approach the

to blue

ideal

coincidence between visual and photographic inten-

The

sity.

numerous

result of this recent

work

is,

colouring matters tested,

that of the

ammoniacal

erythrosin appears to be the most effective sensitizer for

the yellow, and

orange.

1

It will

ammoniacal cyanin

for the

be instructive to inspect the accom-

panying curves taken from Mr. Bothamley's paper

and showing the sensitizers

N

effects of

some of the most

upon gelatino-bromide emulsion

M

L

active :

ff

.FIG. 13. I.

Gelatino-bromide (undyed) II. Ammoniacal Eosin III. Ammoniacal Erythrosin; IV. Ammoniacal R^se Bengal; V. Ammoniacal Cyanin. ;

1

A

;

number of photographs taken by Mr. Bothamley were proThese showed in a very striking manner the

jected on the screen. difference

between the pictures of coloured

objects, such as flowers,

coloured papers, &c., taken on ordinary plates, and on plates dyed with ammoniacal erythrosin, both with and without the intervention of a yellow glass screen.

THEORY OF ORTHOCHROMATIC ACTION. It

generally admitted

is

by

311

photo-

practical

graphers that the seeds sown by Vogel in 1873 are value likely to develop into results of the highest to

the

art.

But although the

of

utility

ortho-

chromatic photography has been proved in

must be confessed

it

ways,

that,

of the discovery

scientific aspect

is

so

far

concerned,

are at present only on the threshold of an

There

region.

are

many

many as the

we

unknown in

discrepancies

the

very facts themselves, and different workers are by

no means concordant

in

their

From

results.

a

chemical point of view the data require purification not only figuratively, but also

There

literally.

is

no doubt that most of the discrepancies have arisen from

the use

of

commercial

colouring matters instead

of

specimens

of the

the pure chemical

compounds, and the action of the dyes has thus been complicated

in

unknown ways.

For

this

reason the results are hardly ripe for discussion as yet,

and

I

cannot do

much more than

lay before you

the existing views respecting the principles at work. If

we sum up

the evidence at present available

j

the one general outcome which appears to be of

THE CHEMISTRY OF PHOTOGRAPHY.

312

most significance

the

is,

that there

is

a distinct

connection between the specific absorption of the

dye and in

its

In order to find out

sensitizing action.

which part of the spectrum the sensitizing action

will

be exerted,

it

is

only necessary to examine

the absorption-spectrum of the

dyed

film

itself

where the dark absorption-band of the colouring matter appears, there will be a new

maximum

on

exposure to the spectrum and development [D], It

must however be borne

good only special

because

for

It

a

colouring

tize for the region

in

shows

matter it

means

no

by

absorption-spectrum, that

that this holds

matters which act as

colouring

sensitizers.

mind

in

a

where absorption takes place

other words, absorption

instance absorbs

sensitize

definite

will necessarily sensi-

may

or

may

associated with special sensitizing action. for

follows

the yellow,

a film for this colour.

but

not be

Indigo

does

Then

:

not

there

is

another point which must not be overlooked in stating

the

connection between absorption and

sensitizing action,

between

the

and that

is

that the coincidence

absorption-band

and

the

region

ABSORPTION AND LOCAL SENSITIZING. sensitized

is

when we examine the

only true

absorption of the dyed film

itself, Le.

gelatino-bromide emulsion

when

cidence

is

said

313

of the coloured

The

dry.

by Vogel and Eder not

coin-

to hold

good when the spectrum developed on a dyed gelatino-bromide film

absorption-spectrum alcoholic

solution,

is

of or

compared with the true the

in

a

gelatine

plain

The

containing no silver haloid.

the dye with the silver haloid

aqueous or

in

dye

is

film

association said

of

by these

observers to cause a slight shifting of the absorption-band, towards the

maximum

in the

red,

so

developed image

less refrangible side of the

the second

that is

a

little

absorption -band shown

by the spectrum of the colouring matter 1 containing no silver haloid.

in vehicles

But here again

must caution you against receiving these ments as

final,

I

state-

because other workers have failed to

obtain this shifting of the 1

on the

maximum.

This displacement of the band of absorption (and chemical towards the red is believed to be in accordance with Kundt's

activity)

law, which asserts that the absorption-band of a colouring matter

displaced towards the red as the refractive

which the substance

is

dissolved increases.

power

of the

medium

is

in

THE CHEMISTRY OF PHOTOGRAPHY. No

theory of the action of sensitizing colouring

can be as yet

matters upon the silver haloids

regarded as conclusively established. to be

There seems

no connection between the chemical con-

stitution

of

the

sensitizing action

;

neither

is

and

matters

colouring

there

their

any connection

between their fluorescent properties or their power of anomalous dispersion and their action as special

Two

sensitizers.

current,

distinct classes of hypotheses are

one physical

and

the

other

chemical.

According to the older view the vibrations absorbed

by

the dye are transferred to the silver salt and thus

render the latter sensitive in the region of absorption.

Some

such

idea as this

appears to have

influenced the course of experimental early days

of the discovery,

work

and the

in the

colouring

matters were in accordance with this view spoken of as " optical sensitizers."

This physical theory

has however been modified, chiefly through the researches of

Eder,

who

has

clusively that the silver haloid

the

shown most conis

actually dyed

colouring matter, the silver salt

compound of the nature of a

by

forming a

lake with the dycstuff.

THE OR Y OF SENSITIZING A CTION. Whether

these

silver haloid

compounds of the

31 5

with

the colouring matter are truly chemical, or whether

they are

"

molecular,"

t. physical, need not concern

us now, but Eder's experiments clearly prove their existence.

Starting from this

named supposes that the

fact,

the authority just

vibrations are absorbed

the coloured compound, whatever the latter

by

may

be, the energy of the absorbed vibrations being

chiefly spent in doing the

decomposition.

The same

work of photochemical vibrations absorbed

by

the dye alone, in the absence of the silver haloid, are chiefly

converted

into

heat

and

do

but

little

chemical work.

Against these physical hypotheses there to be urged

on a priori grounds, but

it

will

is

much

be safer

to confine ourselves to the results of experiment.

According to Abney the the

a*ction is

purely chemical,

colouring matter undergoing

photochemical

decomposition (by oxidation or otherwise) in the region of absorption and giving rise to products

which act as reducing agents upon the

The

silver haloid.

fact that the best sensitizing colouring matters

are just those which are

known

to be

most

easily

3i6

THE CHEMISTRY OF PHOTOGRAPHY.

affected

by

such as the

light,

group) and cyanin,

is

chemical hypothesis.

strongly

me

Let

phthaleins (eosin in favour of

the

remind you of the

bleaching action of light upon a sheet of paper dyed

with eosin, shown

Now

in the first lecture

(Exp.N,

p. 34).

the chief difficulty which chemists will expe-

rience in accepting this explanation stuff should

is,

that the dye-

by photochemical decomposition give

products which exert a reducing action upon

rise to

the silver haloid.

But

this

difficulty arises

from

our ignorance of the nature of the products formed

under these conditions.

must be remembered

It

that the molecules of these colouring matters are

extremely complex, as ing empirical active

will

formulae

be seen from the followa

of

few of the

most

:

C 20 H 6 Br 4 O 5 Ko C H a Cl 4 T 4 O 5 K 3 C 20 H G T 4 O 5 K 2

Eosin

Rose Bengal Erythrosin

Cyanin (quinoline blue)

It is

.

.

C^H^NoT.

not to be wondered at that

among

the pro-

ducts of decomposition of compounds having such

highly 'Complicated molecules as these dyes, there

should

be

Thus an

ABNEY'S EXPERIMENTS.

317

substances of a reducing

character.

reducing agent, formic acid, has

active

already been stated (page 22) to be

among the

ducts of the oxidation of gelatine

dichromate under the influence of production

of a

pro-

by potassium that the

light, so

reducing agent by the photo-

oxidation of a highly complex molecule

is

not

without a parallel.

There are other considerations which tell in favour of the chemical hypothesis, such for instance as the fact that the addition of

ammonia

increases the

special sensitizing action of the dyes, but

the experiments described by to prove

that

Abney appear

most conclusively that

we must

nomenon.

some of

it is

to

me

in this direction

look for the explanation of the phe-

The

nature of the evidence furnished by

these experiments will be best appreciated from

a brief description of a few typical cases

A

plain collodion film

dyed with eosin was ex-

posed to the action of the spectrum

band appeared

in the

:

;

a bleached

green in the position of the ab-

sorption-band seen in the spectrum of the light trans-

mitted through the

film.

.

The

plate

was then coated

THE CHEMISTRY OF PHOTOGRAPHY.

318

with a collodio-bromide emulsion, allowed to dry,

A

and then developed.

dark band appeared in the

by the bleached band before the

position occupied

On

application of the emulsion and development.

repeating the experiment with cyanin the same re-

was obtained, only the bleached band was

in the

orange and the dark band, after development,

in the

sult

same position.

Then again

other experiments were

conducted with collodio-chloride films tinted with cyanin and exposed to the spectrum

was visibly sis

we

in the orange.

the normal

of photoin

in the region of absorption,

the i.e.

But there was nothing of the kind

maximum

of decomposition

showed

the violet, but the orange band was bleached

as before.

orange

maxima

under these circumstances, one

and the other

itself in

the latter

on the physical hypothe-

should expect to find two

reduction violet

Now

printed.

till

With

short exposure both the violet and

maxima came

out dark on development,

showing that the decomposition of the dye orange had given

rise to

in

the

products which reduced

the silver haloid sufficiently to enable

dark band on development.

it

to give a

Similar experiments

ABNEY'S EXPERIMENTS.

319

were made with gelatine-chloride films with the

same

result,

and, what

is

more

still

found that the dye exerted

its

it

striking,

action

if

was

the plate

was coated with a plain collodion or varnish

film

coloured with cyanin, and then exposed to

the

spectrum and developed. collodion or varnish gelatino-chloride

maximum

Even when the dyed

was washed away from the

film

after exposure, the

orange

could be developed, showing that the

reducing product of the decomposition of the dye

had done

its

work.

Many

other experiments were

made with

the other haloids both with cyanin and

erythrosin,

and

viz.

that

it

is

all

leading to the

same

conclusion,

the action of the spectrum on the

colouring matter in the region of absorption, and

not upon the silver haloid to the second

maximum

itself,

in the

graphed on an orthochromatic that time will not permit

which gives

rise

spectrum photo-

plate.

I

regret

much

me to give a more extended

account of these interesting experiments, 1 but

I

have

1 The original description of the experiments will be found in the Conference number of the Journal of the Camera Club, March i6th,

1888.

320

THE CHEMISTRY OF PHOTOGRAPHY.

described a sufficient

number

that their general drift

is

to enable

to see

you

to disprove the physical

hypothesis and to substitute chemical action.

Before taking leave of this part of the subject

am tempted

to offer a suggestion

which

may

I

be of

The chemical

use in guiding future experiments.

hypothesis of orthochromatic action rests upon the fact that the best special sensitizers are the

Now

fugitive dyes.

there

is

reason for believing

that the bleaching of a colouring matter action of light If this

is

due

most

by the

to photochemical oxidation.

be the case atmospheric oxygen

be

may

essential for the production of orthochromatic effect,

and the formation of a second maximum less

refrangible

prevented

part

of the spectrum

by immersing the

reducing solution, or

in

some

which prevented access of this

dyed

in

the

might be

plate

in

a

inert liquid or gas

air.

Experiments of

kind might easily be made, and

in the green, yellow, or orange, did

if

the action

not take place

under these circumstances, the chemical hypothesis of orthochromatic photography would be raised to the rank of a proved theory, while on the other

COLOURS ON SILVER CHLORIDE. the second

if

hand,

maximum

still

321

appeared, this

hypothesis would not be disproved, because possible that the breaking

down of

it

is

the complex

molecule of the colouring matter by the action of light

might take place

and thus

in the

absence of oxygen,

also give rise to the formation of products

of a reducing character.

Before

we have

spectrum on the

finished with the action of the

have to

call atten-

tion to a special property of the chloride

which has

silver haloids,

I

long been known, and which

is

nection

of

with

problems

the

greatest

of interest in conall

photographic

the direct and permanent reproduction

of the camera picture in

its

natural colours.

me

state at once, in spite of occasional

or

sensational

newspaper

Let

rumours

paragraphs, that this

problem has never been solved.

The only approximative attempts have been rendered possible by

the

fact

certain

that

silver

chloride

is

capable,

under

conditions, of receiving an impression of

the spectrum

in

which the colours more or

approach the natural spectral colours.

Y

less

THE CHEMISTRY OF PHOTOGRAPHY.

322

The

first

observations in the direction indicated

were made by Seebeck

was more Sir

in

and the subject

1810,

closely investigated thirty years later

by

The experiments went

to

Herschel.

John

show that when a

of silver chloride on

surface

paper was exposed to white light violet,

till

it

became

the spectrum then impressed itself on the

chloride, after a

approximating to the have

vestigators different

in

lengthened exposure, natural

conditions,

and

Other

hues.

on

carried

the

have

work

of

photochromatic

in-

under

confirmed

general result that silver chloride can susceptible

colours

the

be made In

impressions.

1847-48 and 1855, E. Becquerel made known his

experiments with polished

silver plates,

which were

covered with a film of violet chloride by a brief

immersion solution,

films

in cupric chloride or other chlorinating

and then exposed to the spectrum

recorded the colours

Herschel's violet chloride

were

obtained

by

in

Niepce

such

same way

the

paper.

;

as

Similar results

de

St.

Victor

(1851-66), by Poitevin (1865), by Zencker (1868),

and quite recently by Carey Lea upon the red

PHOTOCHROMA TIC IMPRESSION. photochloride which has

A

former lecture.

been

alluded

modification

323

to

in

a

method

of the

which has lately been described by G. Staats, 1

me

enables

show you an approximation

to

serve

to

general nature of the results.

A

photochromatic the

illustrate

which

to

printing

will

copper plate has been silvered by electro-deposition,

and the polished surface covered with a

slate-

coloured film by a brief immersion in a solution

of ferric chloride.

been placed

in

Various coloured glasses have

contact with the film and the whole

exposed to sunshine. glasses

it

is

On removing

the coloured

seen that under each glass there

is

a colour approximating some\vhat to that of the glass is

the correspondence

is

not very perfect, but

sufficiently distinct to enable

each

glass

has

given

a

you to see that

differently

coloured

impression [E].

These experiments have not yet led

to

any

practical issue, because the colours cannot be fixed,

but from a chemical point of view the line of work

opened up by them promises to be of 1

interest.

Ber. Dcutsch. Chem. Gesell. 1887, p. 2322.

Y

2

THE CHEMISTRY OF PHOTOGRAPHY.

324

The photochromatic property apparently belongs to the reduction product, which we now have good reasons for believing to be an oxychloride. at

It

may

sight appear improbable that the coincidence

first

between the colours of the spectrum and the colours of the impressed film

although this think that

is

nothing more. is

difficult to

is

it

a

is

an

By

mere accident believe

I

but

;

venture to

accidental

coincidence and

mean

to say that there

this

I

no connection of a physical nature between

the vibration-period colours

assumed by the reduction product

connection of

compounds

spectrum, and

to

the

probably arises from the production

different

these

of the ether-waves and the

it

happens

compounds

those of

parts of the

in different

approach

that

the

colours

somewhat

in

of

shade

the corresponding spectral colours.

In the best specimens of these photochromatic spectra that certainly

I

have ever seen, the colours were

nothing more than approximations to

the pure spectrum spectra

the

colours,

and

some of the coloured

unaltered

ground

colour

even

effect

of

in

these

was due

the

film,

to in

PHOTOCHROMY ON PHOTOSALTS. regions where

some

particular

duced no action at

Of

coloured products

had

colour

pro-

all.

of these differently

nature

chemical

the

325

we know

absolutely nothing.

According to Carey Lea the purple forms of the photochloride do not give such good red

the

produce

forms. distinct

Violet,

and yellow has

Staats

and

red

as

glasses

photochromatic impressions upon

the red photochloride effect,

blue,

results

green produces but

;

capricious in

is

recently

its

little

action.

G.

made some experiments, 1

having for their object the isolation of the different coloured products, but after dissolving the colours off the silvered plate

by ammonia, no other product

than

red

Carey Lea's

means be obtained.

photosalt could

The

by any

coloured products are

destroyed by the action of the solvent, and

when heated on

colours to

By

Staats, converted

allowing thin

chloride solution

were

colours 1

for

a

the

the plate are, according

into the

silver

all

foil

red photochloride. to

float

few seconds

till

in

ferric

distinct

produced, this same experimenter

Ber. Deutsch. CJum. Gesdl. 1888, p. 2199.

326

THE CHEMISTRY OF PHOTOGRAPHY.

succeeded in obtaining films of a violet and red colour from which the excess of silver could be dissolved out

by

nitric acid, the films

retaining their original

colours.

thus treated

Until

analyses of these coloured products

it

we have useless

is

to attempt to speculate as to their chemical nature,

but a survey of

all

the facts available

improbable that we chlorides or altered

may have

it

in these cases

it

not

oxy-

compounds of oxychlorides with un-

chloride

containing varying

of water of hydration this

makes

would be

or

proportions

constitution.

Beyond

at present unsafe to venture in

attempting to deal with the chemical principles of

photochromy.

APPENDIX TO LECTURE

A

(p.

290).

The

VIII.

relationship between absorp-

and photochemical decomposition can be shown by taking two glass cells A B, C D, with

tion

parallel

sides, dividing

each

cell

into

two

parti-

septum and then strapping the two together side by side by means of india-

tions

by a

vertical

rubber bands.

mercuric chloride

(Exp. G,

p. 32)

in

A

B is filled with the and ammonium oxalate mixture

The

cell

one partition B, and with water A. The other cell is filled

in the other partition

with the mercuric oxalate mixture in both partiThe arrangement is shown in tions, C and D. plan below

:

JT2

FIG. 14.

The whole system beam, A

B

is then placed in the electric the light, and exposed towards being

THE CHEMISTRY OF PHOTOGRAPHY

323

the contents of

till

B become

distinctly opalescent.

On

disconnecting the cells and exhibiting the result it will be found that C has remained clear

B

D

has become opalescent, the rays traversing having been deprived of their decomposing

while

power by passing through the mercuric

solution.

D

The

having passed only through rays reaching are active towards the mixture. the water in

A

The

internal diameter of each cell should be at

least

an inch, so that the light

may

traverse this

thickness of solution in passing through the front Instead of the mercuric oxalate mixture, cell.

ammonio-ferric oxalate solution the contents of

C and

by means of potassium

B

(p.

notes

Two

298).

are

in

unison

D

may

be used and

tested after exposure

ferricyanide.

tuning-forks of which the are selected, one being

mounted on its sounding-box. The other fork is then struck and while vibrating is brought as near the fork on the sounding-box as is possible without actually touching. The latter soon takes up the vibrations and emits a note on its own account.

C

The

absorption spectra of chlorine, bromine, and iodine are shown in the usual way by placing flasks full of the gas or vapours in (p.

299).

front of the

slit

in the nozzle of the lantern

and

projecting the spectrum on the screen. E>

(p. 312).

The

absorption

spectra of

dyes

APPENDIX TO LECTURE

VIII.

329

magenta, cyanin, &c.) are shown by interposing gelatine films coloured with the substances in question in the path of the ray o light passing 'eosin,

from the lantern through the prisms of the spec-

trum apparatus. The special sensitizing action of a dye is shown by exposing a sheet of paper coated with silver bromide and dyed with a solution of eosin (i

of

the

gram per 1000 for

a few

c.c.)

to the action

minutes

spectrum developing with ferrous oxalate.

and

then

Two maxima

appear, one near G, and the other in the green. The limits of the spectral colours are marked out in pencil as

E

before during exposure.

(p. 323).

about

5

The

silver plate is

10 seconds in a

immersed

for

5 per cent, solution of

As soon as the surface has acquired uniform slaty colour the plate is withdrawn, washed with water, and dried on a blotting-pad. The coloured glasses are then laid on the surface ferric chloride.

a

and the whole exposed

to

coloured impressions are

formed.

found to be

much more

bright light

till

the

Sunlight was

effective than the electric

(arc) light, twenty or thirty minutes' exposure having been found necessary at the time the plate referred to was prepared. The plate can be used

over again by dissolving off the colours with dilute ammonia, washing, drying, and re-polishing with lixiviated chalk.

LECTURE Post-developmental Processes. cation

IX.

Fixing Agents.

Intensifi-

and Weakening of the Silver Image. Printing Albumenized Paper Process. Emulsion

Processes. Prints.

pounds.

Reduction Products of Organic

Image Transference.

with Gold.

Toning of

Theory of the Operation.

Silver

Com-

Silver Prints

The

Platinotype

Chromatized Gelatine and Pigment Printing

Process.

Processes.

who have worked

IT

is

at

photography that the developed

familiar to all

practically

image has to

be submitted to certain subsequent operations order to obtain a finished picture from

it.

in

The

chemistry of these post-developmental processes is

comparatively simple, and

this

concluding

The

first

after

lecture

thing to be

development

is

to

I

propose to devote

their

done with

to fix the

consideration.

the

image

negative ;

and

this

FIXING AGENTS. operation, chemically speaking,

331

nothing more

is

than the removal of the unaltered silver haloid

from the film by the use of a suitable solvent

which

will

take out the unchanged haloid without

The

attacking the silver picture.

now employed thiosulphate,

both

dissolving the

fixing agents

are potassium cyanide or sodium

these salts being capable of haloids

silver salts

in water.

in the case

the silver haloid

of their

virtue

which are extremely soluble

forming double Thus,

by

of potassium cyanide

dissolved out of the film in

is

accordance with such reactions as these

:

AgBr + 2KCN = AgK(CN) 2 + KBr Agl + 2KCN = AgK(CN) 2 + KI. The

sodium thiosulphate as a fixing

use of

agent was, as already stated, suggested by Sir

John

Herschel "

"

hyposulphite

in

1839.

The

old

name

has been abandoned by chemists

because a lower acid forming a series of

M' 2 SO 2 has been ,

berger,

of

since

described

salts,

by Schutzen-

and these are accordingly the true hypo-

sulphites.

It

would

be well

for

the

sake of

THE CHEMISTRY OF PHOTOGRAPHY.

33^

nomenclature

in

uniformity

would generally adopt

The double

the

sodium thiosulphate

necessary alteration. action

haloid

silver

of a

is

the

silver

:

'

The

photographers

formed by the

salt

thiosulphate upon

-fJNa 2 S 2 O 8

AgBr

this

if

solid crystalline

double

has the formula

salt

AgNaS O 3 H 22

>

It is well

is

that a strong solution of thio-

necessary in fixing

is

sulphate

known

too weak, instead

double

double

salt

salt

there

of the

of the formula

which would remain

extremely soluble

formed

is

another

Ag^Na^

in the

the solution

if

;

film

(S 2

insoluble

O

3

)^2H 2 O,

It

[A].

is

only

necessary to point out in dismissing the chemical action of fixing agents that the potassium

nide of

commerce

is

tendency to soften the gelatine

moreover

has

metallic silver, sity of the

a

cya-

always alkaline and has

slightly

This

film.

solvent

action

a

salt

upon

and thus tends to reduce the inten-

image

in the

medium

reasons sodium thiosulphate

is

tones

:

for these

safer as a fixing agent.

INTENSIFICATION OF IMAGE. sometimes

It

development and fixing cient density for

which

may

a

that

happens

negative after

found not to have

is

from

exposure or

insufficient

under development or other causes, as all photographers know,

I

shall

best

have to limit

my

is

remedied,

by a process of

There are many

fication.

suffi-

This defect,

printing purposes.

arise

333

intensi-

intensifiers in use,

and

remarks to a few of the

known substances employed

for this purpose.

In the old collodion process the image was often intensified

by the application of a

further quantity

of an acid ferrous sulphate or pyrogallol developer to

which a

just

little

before

use.

silver

nitrate

The

silver

enabled to go on increasing process as that It is

by which

image was

thus

density by the same

in

it

had been added

was

originally formed.

evident that this operation

is

nothing more

than a continued development. In

the

case

silver intensifier

tendency salts.

Of

of

of is

gelatine

films the

objectionable

gelatine

the various

to

use of

a

because of the

retain

intensifiers

soluble

now

in

silver

use,

mercuric chloride followed by the application of

334

THE CHEMISTRY OF PHOTOGRAPHY.

other substances tical

is

recommended by many

photographers, and

follow out the chemistry of this process.

negative

immersed

is

If

a

a solution of mercuric

in

chloride the dark image

prac-

be instructive to

will

it

becomes bleached, owing

to the reduction of the mercuric salt to mercurous

chloride

by the

metallic silver, as

by Scott Archer

in

1851 [B]

was pointed out

:

Ag2 + 2HgCl 2 = 2AgCl + Hg The image

is

.

mercurous chloride, both

being white and insoluble, but

not

sufficiently

enable the negative to be used for

to

After washing off the bleaching solu-

printing. tion,

Cl 2

thus transformed into a mixture

of silver chloride and

opaque

2

the white picture

is

therefore darkened

by

being washed over with some other solution which effects the

necessary change in colour.

using a dilute solution chloride chloride

out

dissolved

is

of

Thus, by

ammonia

the

and

mercurous

the

silver

transformed into the dark dimercurous-

ammonium

chloride [C]

:

Hg2 Cl + 2NH 3 = NH Hg 2

2

2

Cl

+

NH

4 C1.

INTENSIFICATION OF IMAGE. Sometimes ammonium sulphide

is

335

used, in which

case the mixture of silver and mercurous chlorides

transformed into a mixture of the sulphides.

is

Or

again, the bleached picture

may

be darkened

application of sodium sulphite [D], which

by the

reduces the mercurous chloride to mercury, or by

which

oxalate,

potassio-ferrous

both

reduces

the silver and mercurous haloids to the metallic state [EJ:

Hg

2

Cl 2

+ Na SO + H O = 2Hg + Na SO 3

2

2

2

4

+ 2HC1 2 AgCl

+ 2FeC 2 O 4 + K 2 C O 4 = Ag2 + Fe 2 (C O 4 2

2

)3

+ 2KC1

Hg

2

Cl 2

+ 2FeC

Such an

2

4

+ K C - 2Hg + Fe + 2KC1 2

intensifier

4

2

2

(C 2 O 4 ) 3

as potassio-ferrous oxalate

thus restores the image in silver and mercury, the total density

again

being obviously

treating

chloride

such

an

much

image

increased.

with

By

mercuric

and repeating the application of ferrous

oxalate, a further increase in density

so the process can

is

given,

and

be repeated any number of

THE CHEMISTRY OF PHOTOGRAPHY.

336

times in succession

Another

obtained.

many workers

for

intensifier

gelatine

of potassio-silver

formed

first

This solution

by mercuric

The

is

silver is

is

plates

nitrate

by adding

of

potassio-silver rise to

but the substance

containing also 1

and

redissolved.

applied to the image bleached

cyanide

is

upon

the formation of

a dark product which contains a large

stituent

the pre-

chloride [F].

action

of chlorine.

a solution

till

all

nearly

mercurous chloride gives

silver,

is

recommended by

cyanide, prepared

potassium cyanide to cipitate

the necessary opacity

till

amount of

a complex mixture

mercury, cyanogen, and a trace

Silver appears to be the chief conits

formation

ing to the reaction

may

take place accord-

:

Hg 2 Cl 2 + 2AgK(CN) = Ag 2 + 2Hg(CN) 2 + 2KCL 2

1

This statement

duct, obtained

is

based on a

series of analyses of the

by allowing a solution of

dark pro-

potassio-silver cyanide to

in contact with freshly precipitated and washed specimens of mercurous chloride for periods varying from seventeen hours to

remain

fourteen days.

J.

The experiments were made

at the

Finsbury Tech-

by two students of the Chemical Department Messrs. M. Smith and J. II. Coste.

nical College

INTENSIFIERS. Another

class of intensifies

is

337

by such

typified

as the ferricyanides of uranium

compounds

lead, the first of these

having been introduced by

Selle,

and the second by Eder and Toth.

silver

image

is

and

washed with a solution of a

If the ferri-

cyanide, the latter becomes reduced to ferrocyanide.

Thus

the case of lead ferricyanide

in

:

2Ag2 + 2Pb 3 Fe 2 (CN) 12 =Ag4 Fe(CN) 6 +3Pb 2 Fe(CN) 6 The

silver

image thus becomes converted into a

denser picture composed of a mixture or possibly of a

compound of Both these

silver.

to darken

the ferrocyanides of lead and salts

are white, and in order

them and render them opaque, the

plate,

after the intensifying solution has been thoroughly

washed

out,

is

treated with a solution of

ammonium

sulphide so as to convert the ferrocyanides into the

dark sulphides of the metals.

uranium

The

action of Selle's

intensifier is similar in principle,

only the

uranic ferrocyanide, being of a dark brown colour, at

once

intensifies the

picture without

the

sub-

sequent application of a darkening agent such as

ammonium

sulphide.

The

metallic ferricyanides

THE CHEMISTRY OF PHOTOGRAPHY.

333

used in these intensifiers are prepared by double

decomposition between a soluble

metal and potassium ferricyanide 3

Pb(N0 3 ) 2

-f

intensifiers

;

12

2

of the heavy

e.g.

K c Fe (CN) = Pb + 6KN0 3

The

salt

3

:

Fe 2 (CN) 12

.

most generally used having thus

served to illustrate the general chemical principles involved in the process,

much

it is

unnecessary to dwell

longer upon this part of the subject.

just point out that of the

numerous

possible

way

by

substituting

metal for the silver of the image. case,

for

chlorides

instance,

and

the

another is

the

pla-tinic

Ag2 + 2 AuCl 3 - Au^ + 6AgCl

and

selection

intensifier

auric

This

in

:

3

The

with

may

intensifiers

proposed by various operators, some act simplest

I

practical application of the

most suitable to any particular kind

of negative

is

a

matter of individual judgment

and experience, but

if I

attempted to deal with

WEAKENING AGENTS. this I

should be going beyond

my

339

present limits,

which, as you are fully aware, do not comprise

photographic manipulation.

The weakening

of a negative which

by over

exposure or over development has given too dense

an image,

another problem which the photo-

is

grapher sometimes has to deal with.

In this case

down by the The chemical

the silver deposit has to be thinned

use

of

an

appropriate

which

conditions

have

solvent.

to

be

met

are

simple

enough, although the practical application of the

method of

skill

is,

as in the case of intensification, a matter

and judgment.

employed

The substances

generally

are those which are capable of partially

converting the metallic silver into a salt which can

be

immediately

dissolved

weakening solutions

of

the

in use will serve to illustrate

the principle of action.

substances as

Two

away.

I

shall

speak of these

"weakening agents"

avoid the use of the term

"

in

order to

reducing agents," which

has a totally different chemical meaning. It is well

known

that a silver plate can be coated

with a film of chloride by immersion in a solution

340

THE CHEMISTRY OF PHOTOGRAPHY.

of ferric or cupric chloride reduces these salts [G]

:

words

in other

:

Ag + 2CuCl = 2 AgCl + Cu Cl Ag + Fe Cl 6 = 2 AgCl + Fe Cl 4 2

2

2

It

silver

2

2

2

2

would thus appear

.

wash the

to

sufficient

image with one of these chlorinating solutions so as to

cover the silver superficially with chloride,

and then dissolve

sodium

chloride with

Such a method of weakening would

thiosulphate.

no doubt be

off the

effective,

but

it

would be attended by

a serious practical difficulty, and that

is

that the

operator would not be able to ascertain to what

extent the chlorinating solution had acted

till

after

the removal of the silver chloride, and then he

might find too far.

late that the action

The weakening agents now

had gone too in

vogue have

therefore been devised with the object of enabling

the operator to see the course of the action of the solution, so as to stop the process at the desired stage.

The two weakening agents which

selected as types have been introduced

and Farmer

respectively,

I

have

by Eder

and they both depend

upon the reducing action of the

silver

of the image

WEAKENING AGENTS. upon a

ferric salt,

and the simultaneous removal of

the silver salt thus formed

Thus

341

by sodium

thiosulphate.

in Eder's process a solution of potassio-ferric

oxalate

is

ferric chloride to

prepared by adding

potassium oxalate, and this solution, immediately before use,

mixed with a

is

On washing

thiosulphate.

mixture the

silver

oxalate, which

thiosulphate

in

the

solution of

the plate with first

this

forms an

place

immediately removed

is

sodium

by

the

:

Ag2 + Fe 2(C 4)3 = Ag2 C 2 4 + 2FeC O 4 Ag C 2 O 4 + 2Na 2 S 2 O 3 = 2 AgNaS 2 O 3 + Na 2 C 2 O 4 2

2

2

Farmer's solution, which

weakening agent

.

generally used as a

is

for gelatine plates,

is

a mixture

of potassium ferricyanide and sodium thiosulphate.

The

silver in

this

superficially into

case

becomes converted

first

ferrocyanide,

and the

then dissolved by the thiosulphate

:

+ 2K 6 Fe 2 (CN) 12 = Ag4 Fe(CN)c +

Ag4 Fe(CN) 6 + 4Na

2

S2

3

latter

K

4

Fe(CN)

= 4AgNaS O 3

+ Na 4 Fe(CN) 6

2

.

is

THE CHEMISTRY OF PHOTOGRAPHY.

342

When

a finished negative has been produced, the

next operation

Of

to obtain a positive print from

is

the printing processes

now

albumenized paper process employed, and

therefore

is

familiar to you, so that

method.

this

printing

is

in use, the

ordinary

the most generally

no doubt the most

we may commence with

In order to prepare the paper for

it is first

coated with a solution of salted

latter substance

albumen, the

being obtained from

the whites of eggs, and the salt used being

nium is

chloride.

sensitized

When

floated for a short time on a

by being

Now

and

dry.

us

the chemical

prepared

in

this

ammo-

dry the albumenized paper

silver nitrate bath, after

is

it.

the

which first

it is

point

allowed to drain that

concerns

nature of the sensitive surface

The ammonium

way.

of course forms

chloride

silver

chloride

by double de-

composition, but the sensitiveness of the surface is

not

due

the albumen

to also

forms

silver

of the

nitrate

been

alluded

to

compound which

chloride

silver

(p. is

a

only,

because

compound with

the

a fact which has

already

The

of the

117).

nature

formed when a soluble

silver

SIL VER salt

ALB UMEN PRINTS.

343

added to a solution of albumen

is

unknown, but

this

mind the

bear in

is

quite

need not surprise us when we fact that

we know

practically

nothing of the chemical nature of albumen

itself.

The

(after

empirical formula generally assigned

Lieberkiihn)

C 72 H 112 SN 18 O 22

;

but there

is

is

really at

present no satisfactory evidence that the substances

found

animal and vegetable kingdom, and

in the

grouped by chemists under the general name of "

definite chemical

albumenoids," are

at

all.

methods of formula of the

to

who

Those

are

organic

a

of

will

having

complexity indicated

deduced from analytical

with

acquainted

analysis

compound

compounds

see

any

the

approach cannot be

above,

results with

that

the

any degree

reliability.

Whatever albumen,

it

may is

its

ble

compound with

this

substance little,

the

true

constitution

at present only essential that

should recognize

matters

be

is

of

you

property of forming an insolusilver nitrate

[H].

Whether

a salt or a molecular

but, like all

compound other organic silver com-

pounds, such as the "gelatino-nitrate" referred to

344

THE CHEMISTRY OF PHOTOGRAPHY.

in a

former lecture

(p.

nS),

light with the formation of a

product, and this

is

The

it

is

decomposed by

reddish-brown reduction

the property which

in the printing process ing.

1

utilized

which we are now consider-

sensitive surface in this process consists

therefore of a mixture of silver chloride tc

is

and

silver

albumenate," together with the excess of silver

nitrate.

The

negative

is

placed in close contact

with a sheet of this prepared paper, and all

know, exposed

as

you

the positive image

is

somewhat deeper shade than

is

to light

visibly printed to a

is,

till

With

actually required in the finished print.

re-

spect to the chemical composition of the positive print at this stage all that can be said

that

is

it

is

formed of a mixture of the photo-reduction products of silver chloride and silver silver nitrate

"albumenate."

The

no doubt acts as a sensitizer towards

the chloride, but

it is

improbable that

it

plays this

part towards the organic silver compound. 1

Certain organic bases belonging to the group of " anhydro" give gelatinous precipitates with ammoniacal silver nitrate

bases

which darken on exposure to light, and in their general appearance and properties are very suggestive of the silver compounds formed by albumen and gelatine. See a paper by the author and F. \Y. Streatfeild, Journ. Chem. Soc. Trans. 1887, p. 699.

-

PRINTING EMULSIONS.

345

Instead of using a printing surface in which the

and organic

silver chloride

formed together, as the silver haloid vehicle

silver

compound

are

the process just described,

in

may be emulsified in some

organic

and the paper or other printing surface Several of these emul-

coated with the emulsion.

Thus a

sion printing processes are in use.

collodio-

chloride

emulsion

was introduced by Wharton

Simpson

in

which

silver chloride is emulsified in

collodion,

and

an organic

citric acid

added to play the part of

A gelatine-chloride emulsion

sensitizer.

has been introduced by

Abney

of silver chloride and citrate tine

in

previously described salts.

which a mixture

emulsified in gela-

manner, and then washed as

the usual

of soluble

is

in

(p.

123) to

remove the excess

For the various modifications

and the modes of application of these printing emulsions

From

I

must

refer

you to the

a chemical standpoint

it is

practical treatises.

important to

re-

member

that the action of light

sions

allowed to continue up- to the stage of

is

visible decomposition,

mation of an

and

is

invisible image.

upon these emul-

not stopped at the for-

The

printed picture in

THE CHEMISTRY OF PHOTOGRAPHY.

346

these cases

all

is

therefore

composed of a complex

mixture of the reduction products of the

silver

haloid and of the organic silver compound.

There which

is

is

another method of obtaining silver prints

now very

frequently employed, and which

combines both printing and development on the

same

In this process, advantage

surface.

is

taken

of the extreme sensitiveness of gelatino-bromide emulsion, paper coated with this emulsion having

been recently introduced by the Eastman Company. It

would be superfluous

for

me

to say anything

further about the chemistry of this method, as the principles have been lectures.

amply

dealt -with in former

This bromide paper can be used

for contact-

printing from a negative in the usual way, only, instead of exposing up to the stage of visible decomposition, a short

exposure

is

given,

and the

image developed by ferrous oxalate

;

invisible

the print

is

then washed and fixed as in the ordinary gelatino-

bromide dry plate process. therefore in this case

and has the

in

The

positive picture

composed of

metallic silver,

consequence a very different

ordinary

albumenized

silver

is

"

tone

prints.

"

to

One

GELATINO-BROMlDE PAPER. advantage possessed by

this

347

bromide paper

is

that

the great sensitiveness of the emulsion for rays of

low refrangibility enables a print to be made by

means of gas it

light,

artificial

such as lamp light or

light,

and the same property of the

film enables

to be used for the production of enlarged prints.

It is

only necessary to put the negative into a kind

of magic-lantern, and throw

its

image, magnified by

the lens up to the desired scale, on to a sheet of the bromide paper and expose, develop, and fix as

With an ordinary

usual.

paraffin

lamp with a

metallic reflector behind it in the lantern, an ex-

posure of three minutes has been found sufficient to give an enlarged

negative area.

image from a quarter-plate

on a sheet of paper 12x10 inches

Some

in

of the results obtained by this method

of enlargement have been forwarded

man Company

for inspection.

these pictures

is

it

In looking upon

interesting to reflect

accumulated experience which possible for the

by the East-

has

upon the

rendered

modern photographer

it

to take a

negative in a very few seconds, and to obtain from it

an enlarged positive

print,

using

only such

THE CHEMISTRY OF PHOTOGRAPHY.

348

sources of light as would have been quite inadequate

any impression on the

to produce

employed

before

the

sensitive films

introduction

of

gelatine

emulsion.

Returning to the question of

composition

composed of

compound

image printed on a surface

the

chloride

silver

of

chemical

of the

and some organic

such as the

silver,

the most obscure point

albumenate,

the nature of the re-

is

duction product of the organic silver compound.

There

is

a wide

field

direction,

because

it

for

investigation

in

this

would appear from certain

experiments which have been made from time to time,

that

we may have

organic silver

reduction products

compounds which

the oxyhaloids or photosalts.

of

are analogous to

This

is

a very in-

teresting point in connection with the chemistry

of silver compounds, and

portant because

we know

it

is

all

the

more im-

absolutely nothing of the

nature of these organic silver reduction products.

The evidence which

justifies

the belief that these

products are the organic analogues of the haloid photosalts

may

be very briefly summarized.

IMAGE TRANSFERENCE. In the

first

349

place an albumenized glass plate

coated with silver albumenate by immersion in a

bath gives an image on exposure

silver nitrate

which can be developed by means of pyrogallol. This observation was made by Monckhoven

and

1863,

in

importance in connection with the

its

present subject consists in the fact that no silver haloid

is

present in the film, so that the image

is

formed entirely by the photo-decomposition of the

we

more

next

place

distinct evidence in the

same

direction from certain

experiments on what

called

albumenate.

In

the

is

"

get

image transference

"

which have been recently made by Carey Lea. In order to give a clear notion of

by

"

image transference," which

experiment

in

Pizzighelli

1881.

I

was

An

what

made

Eder

and

invisible

image

was

silver chloride,

that silver chloride

is

and

was treated

with a solution of potassium bromide.

this last salt

an

by

then, before development, the plate

known

meant

will first describe

formed on a plate coated with

well

is

Now

it

is

decomposed by

:

AgCl + KBr = AgBr+KCl.

350

THE CHEMISTRY OF PHOTOGRAPHY.

The

unaltered silver chloride

and not only

into bromide,

product (photochloride)

is

thus converted

this,

but the reduction

also

transformed into

is

the analogous photobromide

the image becomes

transferred from the chloride to the bromide,

the reduction

and

product of the latter can be de-

veloped as usual after the plate has been washed.

The experiment

is

not striking enough as a lecture

demonstration to warrant but chemically

it

its

repetition before you,

of great importance.

is

Sup-

posing the reduction products to be oxyhaloids the

change

be thus represented

may

:

Ag4 OCl + 2KBr - Ag4 OBr2 + 2

Now

Carey Lea has accomplished

silver

compounds what Eder and

done

for the

chloride.

He

2KC1.

for the organic

Pizzighelli

have

has found that such

salts as the tartrate, citrate, oxalate, benzoate, &c.,

are capable of receiving an invisible

image which

can be transformed into the corresponding haloid reduction product

by treatment with hydrochloric

or hydrobromic acid, ferred

can

be

and the image thus

developed

as

usual.

trans-

The

full

IMAGE TRANSFERENCE significance

351

of this observation will appear

repeat one of the experiments

paper coated with

A

[I].

silver tartrate is

about a minute to the

if

I

sheet of

exposed

for

In accordance

electric light.

with the general notions of photo-decomposition

we may now expect

to have a surface

On removing

reduction product of the tartrate. the paper and dilute

painting

hydrochloric

acid,

formed of the

a design

upon

we have

a

design

photochloride on a background of what

The

provisionally call phototartrate.

pound give

is

capable of development, but

with

it

I

latter it

in

may com-

does not

such a dense image as the photochloride,

so that on tearing the paper in half and developing

one portion with ferrous oxalate the design comes out black on a dark ground.

From

the other half

of the paper the phototartrate can be completely

removed by immersion

in dilute nitric acid,

does not attack the photochloride. ing

and developing

this

which

Now, on wash-

other half, the

comes out black on a white ground.

design

Such ex-

periments as this point to the formation of organic silver reduction

products, and

it

is

possible that

THE CHEMISTRY OF PHOTOGRAPHY.

352

" the darkened " gelatino-nitrate or albumenate of

silver this

consist of such

may

reason that

I

It is

compounds.

for

have directed attention to the

subject of image transference in connection with

the theory of silver printing.

Whatever may be the actual chemical composition

of

the

image

albumenized

this

is

image

This

toning."

a

surface

and

it

is

of

manner

the

in

reddish-brown

the

objectionable,

modified by a "

sensitized

paper

already described,

on

printed

colour

of

accordingly

subsequent operation known last process consists in

as

depositing

on the darkened portions of the print a finely precipitated

powder of reduced

gold

some

or

other metal, which changes the reddish colour of the

mixed reduction products

familiar in finished silver prints.

toning

being

is

now most

first

toning-bath

The auric

then

the tone so

In practice, gold-

generally in

washed and till

into

use, the

immersed

the desired shade

is

print

in

the

reached.

toning-bath contains a neutral solution of chloride,

AuCL,

3

as

its

active

constituent,

TONING OPERATION.

353

the salt being generally supplied in the form of the double

NaAuCl 4 ,2H 2 O, which

salt,

by evaporating a

double

crystallizing

and

neutral,

sodium chloro-aurate,

chloro-auric exists

chloride,

acid,

a

in

common

obtained

and

salt

point.

This

which has the advantage of being

salt,

non-deliquescent as

the

to

chloride

auric

of

solution

is

i.e.

HAuCl 4

,

may

the sodium salt of

compound which

the

acid

hydrochloric

be regarded

solution

of auric

and which can be obtained therefrom by

evaporation in yellow crystals having the formula

HAuCl 4 ,4H 2 O. With process,

respect to the

appears to

it

unnecessary into

chemistry of the toning

me

mystification

some of the

treatises

that a great

been

has

deal of

introduced

on photography.

Con-

A

sider the state of affairs in the toning-bath. film,

composed of the reduction products of

chloride

unaltered

immersed essential

and

silver

silver in

for

albumenate

chloride

and

together

results

that

should be neutral, and the gold

with

albumenate,

a solution of auric chloride.

good

silver

It

is

is

the toning-bath

salt is

accordingly

A A

354

THE CHEMISTRY OF PHOTOGRAPHY.

mixed

with

sodium is

it

such

carbonate,

well

known

substances

as

borax,

chalk,

Now

phosphate, or acetate.

that gold salts are most readily

almost any reducing agent, organic or

reducible

inorganic, will precipitate gold from a solution of

the chloride.

Thus, the action of sulphurous acid

or ferrous, sulphate

may

be taken as typical

[J]

:

2AuCl 3 + 3H 2 SO 8 +-3H 2 O = Au 2 + 3H 2 SO 4 + 6HC1

2AuCl 3

6FeSO 4 = Au + Fe

4-

2

The toning

solution

may

2

Cl 6 4

2Fe,(SO 4 ) 3

therefore

.

be looked

upon as containing a potential deposit of metallic gold ready to be precipitated on any reducing surface

that

same way

may

be bathed by

it

just

in

that the acid developing solution

the

may

be regarded as containing a potential deposit of metallic

silver.

silver chloride

image

The

reduction

products of the

and albumenate forming the

act as reducing agents, 1

positive

and thus become

1

If a reducing agent is put into the toning-bath, it reacts directly with the auric chloride^ and prevents the precipitation of gold on the image. This is why it is so constantly urged in all practical

manuals that no sodium thiosulphate should be allowed the toning-bath.

to get into

THEORY OF TONING PROCESS.

355

covered with the deposit of gold which imparts the

tone

the

to

The unchanged

print.

silver

and albumenate cause no reduction, and

chloride

therefore receive no deposit.

Much

stress

laid

is

upon the necessity

by

keeping the toning-bath

for

and the reason

neutral,

practical photographers

for

this

appears to be

that the presence of free acid checks the depo-

Free hydrochloric acid would

sition of gold. fact

play the part of a restrainer, and since this

must necessarily

acid

of the auric chloride,

some the

in

salt

acid

which

at

and

it

result

from the reduction have present

requisite to

is

once enters into reaction with

forms

a

soluble

chloride

v/ith

the liberation of an equivalent quantity of

some

milder acid, such as acetic, carbonic, &c. the or

of

function

the sodium

other salt which

solution.

doubt altered

in

is

The sodium turn

its

silver

acetate,

This

carbonate,

mixed with the chloride thus

helps to

albumenate

toning

formed no

decompose the unor

free

any

nitrate present in the film, converting these

pounds into

silver

is

chloride which

is

A A

silver

com-

dissolved 2

THE CHEMISTRY OF PHOTOGRAPHY.

356

out in

the

subsequent operation of fixing by

thiosulphate.

The

colour of finely-divided

by reduction from a

gold precipitated

solution of one of

its

salts

depends very much upon the state of concentration of the solution and the rate at which

This

a very old

is

it is

deposited.

observation due to Faraday.

If the deposition takes place very slowly

a red form of gold,

if it

we

get

takes place more rapidly

the gold particles appear blue, these differences

being doubtless due to different states of molecular

Now

aggregation.

albumenate

is

the reduction product of silver

and

naturally red,

the blue form of gold which

is

it

therefore

required in order

to give the print the desired tone.

reason that the toning-bath

is

It is for this

kept neutral

is

;

the

deposition of gold must take place as rapidly as possible consistently with uniformity in order to insure

the

form.

The accumulation

precipitation

of

the

of

a

blue-particled restrainer

hydrochloric acid in the toning-bath to

be

addition

avoided of

;

the

this salts

is

is

accomplished

referred

to,

like

therefore

by

the

and by the

FIXING OF PRINTS.

357

occasional neutralization of the bath solution with chalk.

After the print has been toned

is

it

washed and

by immersion in a solution of sodium thioThe chemistry of this operation has sulphate. fixed

already been sufficiently explained

;

but

is

it

im-

portant to remember that in the silver print at this

stage

reduction

we have an image composed of of

products

chloride coated with

silver

the

and

albumenate

a deposit of gold, the

re-

maining portions of the surface consisting chiefly of silver chloride formed by the action soluble

chlorides

in

the

toning-bath

of the

on

unaltered silver albumenate and the free nitrate film.

which

The

may have been

thiosulphate of

all this silver

chloride, but

retained

the

silver

the

by

course dissolves out

it is

without action on

the reduction product of the albumenate, so that the image

is

not affected by the fixing solution

so far as concerns this organic silver constituent.

But the reduction product of

known

to be

silver

chloride

is

decomposed by sodium thiosulphate,

THE CHEMISTRY OF PHOTOGRAPHY.

358

leaving a residue of metallic silver

inorganic silver constituent (photochloride)

image must therefore be decomposed bath, and the finished print

The

(p. 45).

of the

in the fixing

must be looked upon

as consisting of an organic silver

with

silver

compound coated The importance of thoroughly

1

and

gold.

washing out every trace of thiosulphate in order to

well

is

insure

known

to

after fixing

the permanency of the print

all

One

photographers.

of the

instability of silver prints

chief causes of the

is

the imperfect removal of the soluble thiosulphate or

the

formation of

OV Ag|Na.(S 2 3

owing

thiosulphate or to In

fixing-bath.

destruction therefore

is

of

the

irregularity of

order to

the

to

thiosulphate

to an insufficiency of

last

insure

trace

recommended

of the print

insoluble

action

the

in

latter

in

the

complete

of thiosulphate

after the final

immerse the

sodium

it

washing a

weak

solution of iodine in potassium iodide, the iodine

1

Much

of the i-ediiction product of the silver chloride

is

no doubt

re-chlorinated by auric chloride during the toning process, so that the proportion of metallic silver

extremely small.

left

in the finished print

must be

PLATINOTYPE PROCESS. the

converting

according to the well

2Na 2 S 2 O 3 The

+

I2

substitution of

for the

albumenized

been solved purpose

known

principles

which have been

former lectures.

We may use

salts,

means

in

^Nal.

accordance with the chemical

One

finely-divided platinum,

sufficiently

composed of

and the chemistry of

made

aid of previous

fact that the salts of

discussed

of the most promising of

platinotype process can be

by the

for this

and develop by ap-

these recent methods gives an image

ble

surface

a problem which has

ways.

uranic

ferric or

:

some other printing

silver is

in several

tetrathionate

reaction

= Na5 S 4 O 6 +

propriate

in

into

thiosulphate

359

this

readily intelligi-

considerations.

The

platinum are susceptible of

photo-decomposition has already been alluded to (p. 20).

This property was

by Herschel, and

the

made known

in

1832

photo-chemistry of the

platinum compounds has been since investigated

by Hunt, Gehlen, and many not permit

me

others.

Time

will

to relate the various steps in the

history of the process,

and

I

need only add that

it

360

THE CHEMISTRY OF PHOTOGRAPHY.

has been brought to

by Mr.

tion chiefly

present state of perfec-

its

Willis,

and that

it

has been

'thoroughly investigated by Pizzighelli and Hiibl.

The

first

directed

salts,

that the platinotype process

is

method of

which attention must be

point to

when

not a

Although the platinum

direct printing.

especially

is

the platinous condition,

in

are capable of being reduced to the metallic state

by the

action of light in the presence of organic

reducing agents, this property has not yet been

made

available

platinum

for

the

This

print.

direct

of

a

one of the problems

is

awaiting future solution.

production

The modern

platinotype

process depends upon the indirect reduction of the

by some

compound which first undergoes photo-reduction, and is then made to react with the platinum salt. The principle consalt

platinum

cerned use of

is

somewhat

similar to that

when a negative

chloride

(p. 338).

the image

is

is

which

intensified

by

is

made

platinic

In this case the metallic silver of

the reducing agent, and

if

we suppose

the platinum solution to act for a sufficient time,

the whole of the silver would be converted into

PLA7INOTYPE PROCESS. and the image replaced by

chloride,

in metallic

On

platinum.

its

361

equivalent

fixing with thiosulphate

the silver chloride would be dissolved away, and the

This

left.

image

platinum

the

illustrates

indirect production of a platinotype print [K],

modern process the photo-reducible com-

In the

pound

a ferric

is

of decomposition of which

The platinum

is

is

concerning which

it

familiar to

by

supplied

compound,

platinous

oxalate, the

salt, viz., ferric

desirable to give a

can by cautious heating to 300

two atoms PtCl.2

chloride,

water, but

Just in the

is

and

platinite,

salt

is

insoluble

acid,

H.,PtCl 6

crystallizing,

K.2 PtCl 4

.

in

and the H.,PtCl 4

.

solu-

regarded as containing

of chloroplatinous

solution

,

be made to

same way the hydrochloric acid

chloroplatinic acid,

little

leaving platinous

chloroplatinous

tion of platinic chloride

chloride

last

C.

soluble in hydrochloric acid,

is

contains

solution

of. chlorine,

This

.

chloroplatinite,

Platinum tetrachloride, PtCl 4

further information.

give off

(p. 16).

the reduction of a

potassium is

you

mode

we

.

Now acid

on mixing a

with potassium

get potassium chloro-

In practice this salt

is

con-

362

THE CHEMISTRY OF PHOTOGRAPHY.

veniently prepared by reducing the chloroplatinate

with moist, freshly-prepared cuprous chloride

K On

2

PtCl 6

K

Cu 2 Cl 2 =

+

2

+

PtCl 4

:

2CuCl 2

.

and evaporating the solution the

filtering

crystals of the chloroplatinite separate out.

order

In

printing,

it

to

sized

first

is

paper

prepare

so

for

as

to

platinotype

sensitizing solutions from being absorbed,

then coated with a solution containing late

and potassium

acid with

oxalic

chloroplatinite,

thoroughly dried, and tive in the usual is

to produce an

image

and

ferric

made

sensitized

is

oxa-

slightly

paper

is

then exposed under a nega-

is

The

way.

chemical reduction, a film

The

acid.

the

prevent

effect of this

in ferrous

this

exposure

oxalate by photo-

image being imbedded

in

of potassium chloroplatinite, because the

latter salt

does not undergo any appreciable amount

of decomposition during the short exposure that is

necessary to decompose the

this point the

the process.

water but

is

platinum

Now

salt

ferric salt.

Up

to

has taken no part in

ferrous oxalate

is

insoluble in

soluble in potassium oxalate, so that

DEVELOPMENT OF PLATINOTYPE. on immersing the print (generally used

solved

out,

moment

the

at

a solution of this salt

the ferrous oxalate

warm)

and

in

363

of

its

is

dis-

solution

reacts with the chloroplatinite, reducing the latter

to the metallic state

3K

2

PtCl 4

:

= 3 Pt + + CFeC + Fe Cl 6 + 6KC1. 4

2

2Fe 2 (C A) 3

2

The amount

of ferrous oxalate produced on any

part of the ferric-coated surface ing,

proportional

is,

broadly speak-

amount of

to the

light

The high

reaches that part of the surface.

which lights

of the object, corresponding to the densest parts of the negative, thus correspond with the smallest

amount of

ferrous oxalate,

and the deepest shadows

of the object similarly give rise to the formation of the largest

amount of

ferrous salt, while the half-

tones produce an intermediate quantity of ferrous oxalate. in

ferrous salt,

immersed the

In this

in

manner the image

and the

latter,

is

impressed

when the

print

is

the potassium oxalate bath, reduces

chloroplatinite

to

that the ferrous image

a proportional extent, so is

replaced

by

its

platinum

THE CHEMISTRY OF PHOTOGRAPHY.

364

The potassium

equivalent.

oxalate solution thus

causes the image to be developed, and at the same

time partially fixes unaltered ferric solved out

by removing most of the

it

The

salts.

print with

by repeatedly washing the

dilute hydrochloric acid.

add that an image platinum

latter are finally dis-

is

It is

hardly necessary to

composed of

finely- divided

practically indestructible

by

all

those

agents which are capable of affecting a silver print.

A

number of

printing processes, and especially

the photo-mechanical methods, are based on the action of light (p.

23).

upon a

The change

film of chromatized gelatine

of colour undergone

solution of potassium or

when spread over a to light

ammonium

devised by

Mongo Ponton

in

dichromate

and exposed

surface of paper

was made use of

by a

a printing process

The property

in 1839.

possessed by a mixture of potassium dichromate

and gelatine of becoming insoluble the influence of light was

in

water under

first utilized in

process described in 1852 by

Fox

a printing

Talbot, and the

conversion of such a film into a coloured print by

AUTOTYPE PROCESS.

365

mixing the gelatine with a pigment was l

The

&55 by Poitevin.

effected in

nature of the chemical

change undergone by the chromatized gelatine was first

suggested by

has since been

Swan

in 1870,

and nearly all that

made known concerning

the photo-

chemical action of chromium compounds upon organic colloidal substances

is

due to the

1 gations of Eder published in iS/S.

investi-

A discussion

of the various processes depending upon the use of

chromatized gelatine would take us too

far

from

the domain of chemistry on the present occasion.

One

of the best

known

is

that

made

use of by the

Autotype Company, specimens of whose work have been forwarded for your inspection. as at present worked,

is

This process,

the outcome of a

method

of printing with pigmented gelatine which was successfully applied

by Swan

description will enable

in

1864,

first

and a brief

you to see how the action

of light upon chromatized gelatine

is

practically

utilized for the production of the coloured

prints

before you. 1

Ucber die Reactionen der Chromsaure itnd der Chromate auf

Gelatin, Gtimnri, Zncker, &>c.

Wien, 1878.

366

THE CHEMISTRY OF PHOTOGRAPHY.

In the autotype process the

and gelatine are mixture, and the

first

made

is

spread as a uniform film

over the surface of paper.

dry

is

The

placed under the negative which

The image

at this stage

when

sensitive film

printed from and exposed to

way.

homogeneous

being sensitized with

latter, after

potassium dichromate,

matter

colouring

into a

light in is

is

to be

the usual

invisible because

the insoluble portions of the film do not differ in

appearance from those portions which are unacted

upon

by

chromatized

dependent

Now

light.

gelatine

upon

the

the is

extent to which the

rendered

amount

of

insoluble light

is

which

reaches any part of the sensitive surface.

The

high lights of the object, corresponding to the darkest parts of the negative, therefore give the smallest quantity of insoluble gelatine, the shadows

of the object give the largest quantity, and the halftones an intermediate quantity of insoluble gelatine.

But the whole surface of the the negative

is

obviously rendered

insoluble, so that development

the back

film in contact with

of the film.

When

more or

less

must take place from

the printing operation

A UTO TYPE PROCESS. is

over,

paper

the

latter

is

therefore

support which

holds

367

transferred

it

during

to

a

develop-

ment, the surface which was in contact with the negative being held by the support, and the film

being thus stripped off the paper to which originally attached.

the film

is

now

On

treatment with hot water

that

is

to say the various por-

tions of the gelatine are dissolved

Where

to their solubility.

away according

the light has penetrated

most, there most of the gelatine will be the least light has been transmitted

left

by the

the smallest quantity of gelatine will be forth.

At

was

attacked from behind and develop-

ment takes place

so

it

this

stage the developed

consequently an image in a gelatine film.

relief

:

where

negative, left,

and

print

is

on the surface of

The developed

print

on being

again transferred to a white surface thus gives a coloured picture in which the depths of colour are proportional to the depth of pigmented gelatine

through which the white light has to pass

The high the

lights

thinnest

layers,

and

of

the

layers, the

object

correspond

[L].

to

shadows to the thickest

the half-tones to layers

of coloured

THE CHEMISTRY OF PHOTOGRAPHY.

3 68

gelatine of intermediate

thickness.

the print

colour being of course

is

determined gelatine.

formed,

its

the

by

This

pigment

is

mixed

mode

way

with

explanation will suffice to

intelligible the principle of the It

In this

the

make

of working.

perhaps necessary to add that the image

laterally reversed

by the processes of

is

transference,

but this can be obviated by the use of an inter-

mediate temporary support. this

modification

is

due

to

through whose kindness you tunity of seeing

how

At

Mr. will

introduction of J.

R.

Sawyer,

have an oppor-

the whole autotype process

practically carried out. 1

The

is

1

the conclusion of the lecture a demonstration of the process

was given by a representative of the Autotype Company.

APPENDIX TO LECTURE

A (p.

A glass plate

332).

is

IX.

coated with a film

of collodion salted with strontium chloride, and

bath.

One

is

red light) in a silver nitrate half of the plate (wet with the bath

then sensitized

(in

then immersed in a strong solution of sodium thiosulphate, and the other half in a very solution)

weak

is

solution of the

same

salt.

The

first

half of

the plate becomes quite clear, and the second half receives a coating of the dark insoluble double salt.

B

(p. 334).

A

design in metallic silver

duced, as in

p. 180.

is

pro-

The paper

is Experiment H, washed, immersed in a sodium thiosulphate bath, and then again washed with water. On brushing

over the design with a solution of mercuric chloride gradually disappears. The paper is then washed

it

in water

and torn into

five or six strips for sub-

sequent experiments.

C last

(p. 334).

A portion

experiment

is

of the paper used in the brushed over with dilute am-

B B

THE CHEMISTRY OF PHOTOGRAPHY.

370

monia, when the design again comes out dark on a white ground.

D

Another portion of the paper from

335).

(p.

Experiment B

ammonium

is

brushed over with a solution of

sulphide,

and another

strip

with sodium

sulphite.

E (p.

A solution of potassio-ferrous oxalate

335).

prepared by adding a cold saturated solution of ferrous sulphate to about 3 4 times its volume is

of a cold saturated solution of potassium oxalate. Another strip of the paper from Experiment B is

then brushed

over with this solution, when, as

before, the design comes out dark.

A

F (p. 336). solution of potassium cyanide is run into a solution of silver nitrate until the precipitate strip

from Experiment

this solution

G

formed

first

;

B

just is

dissolved.

Another

then brushed over with

the design comes out dark.

A

(p. 340).

is

silver

design

is

produced- as

in

Experiment B, and the paper is then torn into two strips, one of which is floated on a solution of cupric chloride and the other on a solution of ferric chloride.

H

In both cases the design gradually bleaches. solution of silver nitrate is added (p. 343).

A

to a solution of

albumen

in

water

in

order to show

the formation of a curdy precipitate. I (p.

by

351).

floating

it

Paper is coated with silver tartrate on solutions of potassium tartrate

APPENDIX TO LECTURE and

IX.

371

The experiment

silver nitrate alternately.

is

performed as described in the text.

A

painted on a sheet of paper with a strong solution of ammonio-ferrous sulphate thickened with starch paste. When the design J (p. 354).

is

design

nearly dry the paper

floated, face

downwards, on an ordinary gold toning-bath solution till the marking comes out of a brownish colour. The is

is

experiment succeeds better is

warmed

K

if

the gold

solution

before using.

(p. 361).

A

in

design

paper, as in

silver

is

produced on

The paper

is torn in Experiment one face and half, downwards, for portion floated, some minutes on a solution of platinic chloride.

B.

Both halves are then passed through a

dilute 'nitric

when the

unplatinized portion of the design is completely dissolved away, while the platinized portion is not attacked, showing that the silver of the design has, in the latter case, been acid bath,

replaced

L by

by platinum. The increase

(p. 367).

increasing the thickness

in the

depth of shade

of the coloured film

can be easily shown by placing a thin sheet of coloured gelatine in the path of the electric beam,

and then folding quadruple

it

so

as

to

get

double

thicknesses.

B B 2

and

INDEX.

INDEX. AUTHORS QUOTED. action of dyes, 306 alkaline development, 175

ABNEY,

gelatine-chloride emulsion,

345

UAGUERRE,

early experiments, *5 Draper, J. W., absorption and decomposition, 290

red-sensitive silver bromide,

294

spectrum photography, 253 spectrum on mixed haloids, 259 theory of orthochromatic action, 315

thiosulphate in developer, 1

action of dyes, 306 alkaline development, 176 action of chromates on gela-

EDER,

reversals in spectrum, 272 solarisation, 213, 216, 224

88

tine, 365 development of pressure marks, 191 different forms of silver bromide, 292 size of silver bromide particles

BAILEY and Fowler,

silver sub-

oxide, 42 Beccarius, darkening of chloride, 37 colours on Becquerel,

solarisation, silver

emulsion,

120,

theory

of

216 orthochromatic

action, 315 silver

chloride, 322

Bennett, ripening of emulsion, 121

214

solarisation,

in

124

weakening of image, 341 Eder and Pizzighelli, ammonia process of ripening, 129

image transference, 349 and Toth, intensifier, 337

Blanquart-Evrard, albumen dry plates,

92

calotype, 89,

FARADAY, forms of 90

and Sayce, collodion emulsion, 105 Bothamley, orthochromatic pho-

Bolton

tography, 309

CAREY LEA, development by mercury salts, 165 development of pressure marks, 191 ferrous

oxalate

tine, 364 improved process, 89 prin's on silver chloride, 83 Frankland and Lockyer, pressure and gaseous spectra, 301 Friedheim, silver suboxide, 42

GAUDIK, emulsion proposed

image transference, 349 photochromy, 322, 325 photosalts of silver, 59 i eduction of silver chloride light,

Fischer, silver subchloride, 40 Fox Talbot, chromaiized gela-

developer,

186

by

gold, 356 Farmer, weakening mixture, 34!

43

by, 105 Gehlen, photo- decomponition of

platinum salts, 359 Goddard, improvement of Daguerreotype, 88

INDEX. HERSCHEL, bleaching by

potas-

sium iodide, 209 colours on silver chloride,

Ponton, printing with chromates, 364

322 iron salts, 13 -

photo decomposition platinum salts, 359 sodium thiosulphate for ing, 84,

Hodgkinson,

33

W.

Poitevin, pigment printing, 365 positives on glass, 217 silver as a sensitizer, 150

of

READE, fix-

1

R., silver oxy-

chlcride, 56

Hunt, absorption of oxyren by darkening silver chloride,

J. B., gallic acid developer, 89 Riche, reduction of silver chloride by light, 44

Russell, Colonel, acid

development, 173 alkaline developer, 104, 106 collodion dry plates, 96 tannin process, 98

photo-sensitiveness of plati-

num JANSSEN,

salts,

359

solarisation,

Le Gray, collodion process, 92 waxed paper process, 90 Lermontoff, micro-galvanic deposition of silver, 163

Liebig and Regnault, pyrogallol developer, 1 66 R. L., gelatine emul-

sion, 115

Mansfield, ripening of emulsion, 121

Monckhoven,

Van.,

ammonia

process of ripening, 129

image on 349

silver

of light on silver chloride, 38 photography in spectrum,

SCHEELE, action

214

LASSAIGNE, bleaching by potassium iodide, 209

MADDOX,

and alka-

line

albumenate,

243 Schramrn, action of light on halogens and hydrocarbons, 29 Schulze, J. H., action of light on silver salts, 36 Scott Archer, collodion process, 92 mercuric chloride intensification,

334

Seebeck, colours on silver chloride,

322

Selle, intensifier,

337

Wharton, collodiochloride emulsion, 105 Spring, chemical change by presSimpson,

sure,

192

.

Muthman,

NIEPCE de

silver suboxide, St. Victor,

42

albumen

on

glass process, 91 colours on silver chloride,

Niepce, Joseph Nicephore, bitumen process, 27, 85

Starts, photochromy, 323, 325 Stas, molecular aggregation of silver haloids, 127

Stokes, use of quartz, 250 Stuart Wortley, ripening emulsion, 121

TATJPENOT, collodion dry

PIZZIGHELLI and Hiibl, platinotype, 360 Pliicker and Hittorf, pressure and gaseous spectra, 300 Pditevin, colours on silver chloride,

322

of

Swan, chromatized gelatine, 365 plates,

95 action of dyes, 304 function of sensitizer, 149

VOGEL,

plate in sodium solution, 204

sulphite

INDEX. and Eder, displacement of absorption-band, 313

Yogel

377

Wetzlar, silver subchloridc, 40 Willis, platinotype,

360

\Vohler, silver suboxide, 42

WARNERKE,

development

of

pressure marks, 191

Wedgwood

and

Davy,

early

photographic experiments, 82

287 and reversal, 278 by halogens, 299 intramolecular, 302 specific,

of

silver

Chlorine, liberation from silver chloride, 38

decomposition

of, ii

haloid

agents, 206 as a. substratum, 91

Albumen

343

of,

TO.

Chlorine-water,

molecule, 297 Accretion, development by, 163 Acid development, 158, 172 Acids, minerals, as destructive

formula

Albumenized paper, 342 Alcohol and ether as sensitizers,

Chloro-auric acid, 353 Chloroplatinous acid, 361

Chromates, photochemical decomposition of, 21 Chromatized gelatine, use of, 365 Collodion, chemical nature of, 93 dry plates, 95 plate, reversal in, 220 sensitizing of,

image

introduction

of,

104

process of ripening,

129

Astronomy, applications photography to, 134 Autotype process, 365 development in, 366

of

of,

salts,

Curve of

26

Blue, aniline, light transmitted by, 248

46

visual, of solar spectrum, 25 1 Curves for mixed hdloids, 259 Cyanine, action of, 310, 318 intensity,

DAGUERREOTYPE,

CALOTYPE

process, 89 Cellulose nitrates, 93 Chemical action, reversed, 201 force,

supposed,

spectrum, 245

Chemical photometry,

16, 19

by

light, 27 Colours of natural objects, 247 Concurrent reversal, 264 salts, photochemical decomposition of, 1 7 Cuprous oxychloride, 57 sal's, analogy with silver

Bleaching action of potassium iodide, 209, 211

Chemical

203

in,

introduced, 92 Colour, nature of, 239 Colouring matters bleached

Cupper

BlTUMENOUS substances, photooxidation

94

process, fading of invisible

75

Alkaline development, 166, 170, 186

Ammonia

silver

chloride, 322

SUBJECTS REFERRED ADSORPTION and decomposition,

on

colours

ZENCKER,

in

visible

fading of in-

image process, 86, 88

in,

sensitizer in,

202

150 development of, 156 and Decomposition absorption, 288 plate,

INDEX.

37 8

of

Deposition

silver,

micro-

Developer,

a

test.

189

microchemical

contributes to sensitiveness,

185 function

in solarisation,

of,

Developers, nature

143, 146 acid and alkaline of,

Development, compared, 172

alkaline introduced, 104

by ammonium

pyrogallate,

spectra, 301 Gelatine, action of light on chromatized, 23 as a sensitizer, 131, 153 chromatized, use of, 364 nitrate,

emulsion

with

silver

compared

with

of,

117

collodion emulsion, 119 emulsions, 113 plates, cause of longevity,

206

66

by ferrous oxalate, 186 by ferrous sulphate, 158 by mercury vapour, 156 of pressure marks, 191 Dissociation, photochemical, 24 Draper's law, 290 Dry plates, collodion, 95 lading of invisible image

Dyes,

acid as a sensitizer, 152 of pressure on

effects

compound

227

1

GALLIC Gases,

galvanic, 163

in, 205 effect of,

on photographed

spectra, 306 sensitizing action of, 305

ELECTROLYTIC

decomposition of silver bromide, 174, 175 Emission spectra, analogy with photographed, 301 Emulsion, collodion, 105 collodio-chloride, 345 gelatine-chloride, 345 mass washed, 112 Emulsions for printing, 345 gelatine, 113

ripening

of,

washing

of,

in, 121, 122 108, 123

Eosin, action of, 317 Erythrosin, action of, 310

FERRIC

oxalate, photochemical decomposition of, 16 Ferrous oxalate as a developer, 1 86 Ferrous sulphate as a developer,

.'5*

Fixing agents, 331 Fixing, explanation of, 331, 332 Fluorescence, 242 Fraunhofer lines, 249

of, 114 Gelatino-bromide emulsion, applications of, 133 causes of sensitiveness, 131,

properties

185 , paper, 346 plate, reversal in,

222

ripening of emulsion, 122 Glass, absorption of light by, 250 as a substratum, 91 coloured, light transmitted by, 249 Gold, colour of precipitated, 356 salts,

photochemical decomposition of, 20

reduction

of,

354

toning with, 352 Graphical representation of intensity, 251 Green, iodine, light transmitted by, 248

HALOGENS,

liberation of mixture

263 specific absorption by, 299 Hydrijdic acid, photochemical decomposition of, 1 1 Hydrogen as a sensitizer, 75 of,

and chlorine, action of on mixture of, 9 Hydroxyl, reversal

IMAGE,

in,

light

277

intensification of, 333

invisible or latent, 146

on albumenized

silver,

reversal of, 208

transference, 349

344

INDEX:

379

Images, positive and negative, 145 Instantaneous photography, 133 Intensification of image, 333 Intensifier, ferricyanide, 337 mercuric chloride, 333 potassio-silver cyanide, 336 uranium, 337

Molecular aggregation of silver haloids, 127

representa-

Nitric acid, function of, in collodion emulsion, 120

Intensity, graphical tion of, 251

visual and photographic, 243 Invisible image, destruction of,

198 by mineral acids, 206 by ozone, 205 nature of, 146 relapse of, 197 Iodine, removal of thiosulphate, by, 359

Iron developer, 158

development of, 143 photochemical reduc-

prints, salts,

tion of, 12, 14, 15

LATENT

image, nature

of,

146

Lead compounds, photo-oxida-

NATURAL

of,

OPTICAL change during

ripen-

ing, 291 Orthochromatic

theory

of, 311. 315 Orthochromatic

object

of,

action,

photography,

304, 309

Oxidation promoted by light, 25 Oxidizing agents, function of, in causing reversal, 223 increase of reversal by, 277 retardation of photo-decomposition by, 39 retarded reversal by, 281

Oxygen, absorption of, during darkening of silver chloride, 53

ferricyanide intensifier, 337 Light, components of white, 236

Ozone

nature of, 238 Longevity of invisible image on gelatine plates, 206

colours

Niepceotype process, 91

26

tion of,

objects,

247 Nickel sulphate, crystalline modification of, 8

as

a destructive agent,

205 reversal by. 284

PAPER, albumenized, 342 as a sensitizer, 75

Materia photographica, 6

Maximum

of decomposition in spectrum, 256

Mercuric chloride intensifier, 333 Mercurous salts, analogy with silver salts,

Mercury

46 development by,

salts,

photo-decomposition 24 Mercury vapour, development by, 156 Mineral acid, reversal in, 275, 281 165,

of, 1 8,

Mixed

haloids, sensitiveness of,

261 Moisture, accelerates photo-decomposition of silver chloride,

52

gelatino-bromide, 346 waxed, process, 90 Phosphorus, photophysical modification of, 8 Photochemical action, 7 Photochloride, photochromatic property of, 322, 325 Photochvomatic impression, 323 Photochromy, 321 Photographed spectra, analogy with emission spectra, 301 Photography, astronomical, 134 beginning of, 82 instantaneous, 133 Orthochromatic, 304, 309 scientific advancement of, 6 spectrum, 253 Photometry, chemical, 16, 19

INDEX. Photo-oxidation in spectrum, 269 Photophysical action, 7 Photo-reduction, nature of, 24 Photosalts of silver, 59

Refraction spectrum, characters of, 241 Relapse of invisible image, 197 Restrainers, nature of, 160

Pigment printing, 365

physical and chemical, 161, 185 Reversal and absorption, 278 by ozone, 284

Platinotype, 359

development of, 363 Platinum salts, photo-decomposition of, 20 Polymerisation caused by light, 28 Positives, direct on glass, 217 prints on paper, 342 Potassio-silver

cyanide intensi-

336 Potassium bromide, excess

by potassium iodide, 217 Reversal in hydroxyl, 277 in mineral acid, 275, 281 in mixed silver haloids, 264 in potassium bromide, 275 in potassium dichromate,

277

fier,

re-

quired in emulsion, 128 restraining action of, 168

in potassium

in spectrum, in spectrum

reversal in, 275

in,

277

sensitizer for gelatine,

366 iodide as a bleaching agent, 209, 211 as a reversing agent,

217 photo-oxidation reversal

by,

in

of,

25

spec-

trum, 271 permanganate, reversal in, 277 Preservatives, 98,

267 in

solutions,

272

chloroplatinite, 361 cyanide for fixing, 331

dichromate, reversal

permanganate,

277

no

Pressure marks, development of, 191 Printing process, albumen, 342 gelatino-bromide, 346 pigment, 365 Pyrogallol as a developer, 166

of chemical action, 2OI of image, 208 region of strongest, 269 retarded by oxidizers, 280

Reversing agents, direct action of, 283 Ripening by ammonia, 129 of emulsions, in, 123 of silver bromide a physical process, 123, 125 of silver haloids, optical

change during, 291 action of light on, 9 Sensitiveness, partly due to developer, 185 Sensitiveness, relative of silver haloids, 192, 195 Sensitizer, action of, in solarisa-

SELENIUM,

tion,

219

Sensitizers,

action

in

different

processes, 150

REDUCIBILITY of silver haloids, 193 Reduction product, destruction by iodine, 266 formed by chemical

method, 190 image formed of, 154 products, definition of, 49 organic silver, 351

theory of, 74, 78 Silver bromide, cause of molecular growth, 127 cause of sensitiveness, 258, 293 electrolytic decomposition of, 174, 175

oplical

properties emulsion, 124

in

INDEX. Silver

bromide photo-chemical decomposition, 57 red-sensitive form, 294 size of particles in 1 20,

ulsion,

em-

124

bromo-iodide, 264 chloride, action of solvents on darkened, 45

and

iodide, crystalline

modifications

of,

8

emulsions, 345

photochemical decomposition of, 37, 50 photochromatic property of, 321 transformation

into

bromide, 349 with magof, nesia, 63

compound

growth of deposit, 161, 171, 174 haloid

molecule, specific absorption by, 297 Silver haloids, absorption intramolecular, 302 action of dyes on local sensitiveness of, 309 action of spectrum on

mixtures of, 258 comparative action of light on, 76 different states'of molecular

aggregation,

127 heats of formation

3Si

Silver iodide,

image on, 150 photochemical decomposition, 58 nitrate as a sensitizer, 152 compound with albumen, 343 compound of, with gelatine, 117 oxychloride, supposed, 56 photosalts of, 59 print, composition of ima?e, 357 conditions of permanency, 358 fixing of,

ful,

42

Sodium

chloride as a fixing fgent, 83 chloro-aurate, 353 sulphite, plate exposed in,

204 thiosulphate as a fixing agent, 84, 331 in developer, 188

from

removal 359

optical change during ripening, 291

prints,

Solar spectrum, curve of visual intensity in, 252 fixed lines in, 249

212

Solarisation, 208, of, 82

357

salts, actions of light on, 35 existence subchloride, doubtful, 40 suboxide, existence doubt-

favoured by preliminary exposure, 227 function of developer

in,

order of darkening by light, 76 order of reducibility,

function

]93 of order sensitiveness,

in collodion plate, 220 in gelatino-bromide plate,

79 photo

phases

227 of

sensitizer

in,

219

222 -

decomposition

of, accelerated, 71 relative sensitiveness

of, 192,

when

mixed, 261, 265 spectrum photographed on, 254

214, 220

by

oxidizing

agents, 223

retarded by reducing agents,

195

sensitiveness

of,

promoted

277 Spectra of gases, effects of pressure on, 301

photochromatic, 324

INDEX. action

Spectrum,

of

tures

of,

silver

on mixhaloids,

Toning-bath, chemistry 355

of,

353,

258 characters of, 240 colours of, 238 displacement of absorption-

band

in,

maximum in,

URANIUM

313

prints,

337 development of, 144

salts,

photo-decomposition

of decomposition

of,

inlensifier,

19

256

normal, 241

photographed on loids, 254 photography, 253

silver ha-

photo-oxidation .277

in,

reversal

iodide

by in,

sity,

potassium

271

243

WASHING for, 97,

Water,

of dry plates, reason 08, 122

1

decomposition

by

chlorine, II

Waxed

245 Sulphur, photophysical modification of, 8

Thallous

by, 156 Visual and photographic inten-

269,

reversals in, 267, 272 supposed chemical force in,

TANNIN

VAPOUR, development

as a preservative, salts,

silver salts,

46

analogy

paper process, 90 Weakening agents, 339, 341 White light, composition of, 236

98 with

ZINC

selenate, crystalline fication of, 8

THE END.

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