Full text of "The silver bromide grain of photographic emulsions"

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Monographs on the Theory of Photography from the
Research Laboratory of the Eastman Kodak Co.

No. 1

The Silver Bromide Grain

of Photographic

Emulsions

A. P. H.Trivelli and

S. E. Sheppard

ILLUSTRATED

D. VAN NOSTRAND COMPANY
NEW YORK

EASTMAN KODAK COMPANY

ROCHESTER, N. Y.
1921

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

Edited by
C. E. KENNETH MEES

and
MILDRED SPARGO SCHRAMM

Monographs on the Theory
of Photography

THE SILVER BROMIDE GRAIN or PHOTOGRAPHIC EMULSIONS.
By A. P. H. Trivelli and S. E. Sheppard.
Other volumes soon to appear.

GELATINE. By S. E. Sheppard, D. Sc. 2 volumes.
THE THEORY or DEVELOPMENT. By A. H. Nietz.

465033

Preface to the Series

The Research Laboratory of the Eastman
Kodak Company was founded in 1913 .to carry
out research on photography and on the pro-
cesses of photographic manufacture.

The scientific results obtained in the Labora-
tory are published in various scientific and tech-
nical journals, but the work on the theory of
photography is of so general a nature and occu-
pies so large a part of the field that it has been
thought wise to prepare a series of monographs,
of which this volume is the first. In the course
of the series it is hoped to cover the entire field
of scientific photography, and thus to make
available to the general public material which
at the present time is distributed throughout a
wide range of journals. Each monograph is in-
tended to be complete in itself and to cover, not
only the work done in the Laboratory, but also
that available in the literature of the subject.
A very large portion of the material in these
monographs, however, will naturally be original
work which has not previously been published.
The monographs are written by those specialists
in the Laboratory who are best qualified for the
task, each monograph being edited by the Direc-
tor of the Laboratory and by Mrs. Schramm,
who is the active editor of the series.

Rochester, New York
April, 1921

Preface

The fundamental units of the sensitive materials used in
photography are the small grains of silver halide which,
imbedded in gelatine, form the emulsion.

Since these grains are of very small size, and are, further-
more, precipitated in a colloid medium, they have usually
been treated simply as colloid aggregates.

As a result of a complete crystallographic study involving
photomicrographic work of a high order, it has been possible
not only to confirm the fact that the grains of high-speed
emulsions are definitely crystalline, but to identify their
crystalline form and to show that all the grains, though having
several distinct shapes, belong to the same crystalline class.
The grains being thus established as micro-crystalline, their
formation in the emulsion can be studied by the aid of the
recent physico-chemical theories of precipitation and especially
of the dispersion theory of Von Weimarn, according to which
the dispersity of the initial precipitate will be determined by
the concentration of the solutions and other physical condi-
tions. The changes in the dispersity of the original precipitate
during ripening, which will follow from the laws of surface
energy, is now found to be related to changes in the content of
adsorbed impurities, and in connection with this the effect
of ammonia upon the grains has been studied.

The catalysis of crystallization by nuclei is suggested as
an explanation of some of the effects produced by the
admixture of silver iodide with silver bromide in an emul-
sion, and the fact that traces of colloidal silver make the
grains color-sensitive is believed to be related to this.

A study of the relations existing between the sizes of the
grains and their photographic properties is reserved for a
future monograph.

Rochester, New York
April, 1921

The Silver Bromide Grain of
Photographic Emulsions

CONTENTS

PREFACE
CHAPTER

CHAPTER II

CHAPTER III.

CHAPTER
CHAPTER
CHAPTER

IV.

VI.

CHAPTER VII.
CHAPTER VIII.

CHAPTER IX.
CHAPTER X.

Page

The influence of ammonia on photo-
graphic emulsions and a theory of
ripening 11

Von Weimarn's theory and the deter-
mination of the dispersity of silver bro-
mide precipitates 27

Accessory factors influencing the disper-
sity of silver bromide emulsions . . 35

Crystallization catalysis 52

Capillarity and crystalline growth . . 57

Experimental study of the crystalli-
zation of silver bromide 75

The classification of silver halide crystals 95

The silver bromide crystals of photo-
graphic emulsions 99

The directions of most rapid growth in
silver bromide crystals, and the occur-
rence of anomalous forms . . . .107

The behavior of silver bromide and sil-
ver iodo-bromide crystals in polarized
light 121

Summary of crystallographic study of silver halide grains . 129

Alphabetical list of serial publications referred to, with the
abbreviations adopted in citations 132

Bibliography 133

Index of Authors 137

Index of Subjects 139

The Silver Bromide Grain of
Photographic Emulsions

CHAPTER I

The Influence of Ammonia on Photographic
Emulsions and a Theory of Ripening

The use of aqueous ammonia in the ripening of photographic
silver halide emulsions was introduced by Johnston 1 and is
well known to photographic technologists, particularly through
the later work of J. M. Eder. 2

Eder states that exposing a dry gelatino-bromide plate
for a few minutes to the vapor from strong ammonia imme-
diately before using in the camera results in a marked increase
in sensitiveness. On the other hand, Gaedicke 3 concluded
that fuming prior to exposure diminished the sensitiveness,
but that, subsequent to exposure and prior to development,
it increased the developability of the latent image, resulting
in an effective sensitizing. This action he considered to be
one on the latent image, not an acceleration of the action of
the developer. Sheppard and Mees 4 found that certain
plates gave a higher inertia, or lower speed, with ferrous oxalate
developer than with organic developers, while a larger group
gave practically the same speed with both developers. For
the latter, however, a slight fuming with ammonia increased
the inertia, i. e., decreased the speed, when ferrous oxalate
was used as developer. In addition to these relatively invisible
effects, the accounts of ' which exhibit rather contradictory
conclusions, it was observed by Eder that, if moist silver
bromide plates were exposed under a bell-jar to ammonia
vapor for a considerable time, they became more sensitive to
light and coarser-grained, ultimately forming a network of
coarse-grained silver bromide with relatively empty inter-
spaces resembling frost figures.

1 Johnston, J., Gelatino-bromide of silver emulsions treated with ammonia. Brit. J.
Phot. Almanac 1877: 95. 1877.

2 Eder, J. M., Beitrage zur Photochemie des Bromsilbers. Sitzungsber. Akad. Wiss.
Wien. 81: 679. 1880.

3 Gaedicke, J., Ammoniakraucherung bei Trockenplatten. Jahrb. Phot. 27: 62. 1913.

4 Sheppard, S. E., and Mees, C. E. K., Investigations on the theory of the photo-
graphic process.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

Englisch 1 found that a partial development of the latent
image was possible, for by treating an exposed plate with
strong aqueous ammonia the unexposed parts were apparently
more rapidly dissolved away than the exposed parts. He
attributed this to a lesser solubility of the exposed halide in
ammonia. Luppo-Cramer, 2 repeating the experiments, came
to a different conclusion. Under suitable experimental con-
ditions he found that the exposed portions at first apparently
dissolved out more rapidly than the unexposed, but that this
relation was reversed on continuance of the ammonia "devel-
opment." Luppo-Cramer modified Englisch's experimental
conditions by using ammonia vapor instead of aqueous
solutions. This has advantages in that the action is slower
and therefore can be better observed, and that there can be no
actual removal of dissolved silver bromide from the plate.
Proceeding in this way, and with the help of the microscope,
Luppo-Cramer concluded that ammonia development is
really a reaggregation or "ripening" process which proceeds at
different rates, according to the exposure to light of a given
part of the plate.

Luppo-Cramer considers that this supports the theory that
light brings about a certain disintegration of the silver halide.
He ascribed the "developability" with ammonia to the in-
creased "inner dispersity" of the silver bromide grains. He
finds that at first the exposed parts show a coarsened grain,
and concludes that, in consequence of disintegration, the
exposed silver bromide particles have on the whole a greater
solubility in ammonia, whereby at first an immediate Ostwald
ripening occurs. This, however, is reversed on further
treatment, the unexposed parts becoming later relatively
coarser-grained than the exposed parts. This he attributes
to the "disintegration by light" affording a greater number
of crystallization nuclei, whence, with greater number of
crystals formed, the ultimate size will be smaller, since the
mass of material per unit area is the same. Liippo-Cramer
later supported this view with experiments in which the
"chemical latent image" was completely (?) destroyed, but
could still be developed with ammonia.

It does not appear that this reasoning is either necessary
or sufficient. To begin with, if the first effect in the more
exposed parts is essentially an increased solubility and solution
of the disintegrated particles of the original silver halide
grains, whence come the subsequently invoked greater number

1 Englisch, E., Zeits. wiss. Phot. 2: 416. 1905.

2 Luppo-Cramer, Photographische Probleme, p. 83.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

of crystallization nuclei? And also, where, at the same time,
are the relatively larger . crystals which, according to the
Ostwald ripening theory, must be present to increase at the
expense of the smaller crystals? There appear to be mutually
incompatible requirements here, since the increased crystal
fragments must disappear by solution to give the postu-
lated initial Ostwald ripening in the more exposed parts, and
yet, to explain the apparent reversal effect later, must also
have been there all the time.

The experiments now to be described show, we believe,
that the facts are capable of a less involved explanation.
They show:

(a) That there is no ammonia development of the latent
image, properly so called, but only an ammonia development
of the visible image, no effect being obtainable with exposures
to light much lower than those which give the least visible
photochemical effect ;

(b) That the actual development can be more simply
explained by a simple recrystallization effect, not involving
directly, but only (if at all) as a very subsidiary factor, any
Ostwald ripening;

(c) That the development or ripening nuclei are due not
to disintegration, but to the photochemical decomposition
products of the silver halide probably colloid silver adsorbed
to silver halide and to similar decomposition products from
the reducing action of the ammoniacal gelatine on the silver
halide.

In addition to correcting what appears to us the incorrect
and unnecessary conclusions drawn by Luppo-Cramer in his
otherwise valuable and interesting papers, the experiments
are noteworthy because this ripening with ammonia affords
a cross-section of the ripening process in general, particularly
as convection currents within the emulsion are eliminated.
At the same time, it is believed that they indicate the causes
for some of the natural limitations and peculiarities in the
ripening process.

FUMING OF UNEXPOSED LAYERS

The experimental method followed was in the main similar
to that of Eder and Luppo-Cramer, namely, fuming with
ammonia vapor evolved from strongest aqueous ammonia.
Some side experiments with ammonia solutions applied direct
showed that far less control was obtainable in this way. The
aqueous ammonia S. G. 0.90-0.92 was contained in deep
crystallization dishes, the plates to be fumed being laid film

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

down over them, so that an unfumed surround was obtained
in each case. Care was taken that the distance between
the ammonia solution and the plate was kept constant, unless
purposely varied. For convenience the fuming was conducted
under a hood in a dark room. Illumination and inspection
during the experiments were facilitated by placing the care-
fully leveled dish over a dark-room lamp laid on its back, so
that red light was thrown upward through the dish and plate.
In all, some hundred experiments were performed, using Seed
23 and Seed Process Plates, Cine Positive film, and Lantern
Plates.

In repeating some of the experiments previously described,
an initial phase of the action of ammonia on the silver halide
emulsion was noted, which appears to have attracted little,
if any, attention. On fuming. Seed Process plates, unexposed
to light and unmoistened, in the manner described above, it
was observed that the first visible differentiation of the fumed
from the unfumed area is a uniformly diminished opacity;
this was such that in one hour the actual time varies both
with the kind of plate and with its relative moisture content
the film had become almost transparent by transmitted light,
but showed a light bluish gray turbidity by reflected light.

FIG. 1 FIG. 2

Print through fumed plate on Print on fumed plate through

unfumed plate. Crescent shows unfumed plate. Crescent shows

untreated portion. untreated portion.

The extent of this induced transparency is shown by the
photograph in Fig. 1, which gives the result of printing a
negative through a plate so treated on to another one unfumed.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

The part made transparent becomes, photographically, 1
greatly reduced in speed and density-giving power, as is shown
by Fig. 2, which gives a direct print from the negative shown
in Fig. 1 on the partially fumed plate. The unfumed sur-
round is overexposed long before the fumed part gives a devel-
opable impression. On development the image frequently
shows considerable irregularly distributed surface fog of a
dichroic nature. Microscopic investigation of the fumed
transparent area showed that in this state the emulsion has a
fine and very uniform grain, apparently considerably finer
than the original. The reduction of opacity, however, is

IbgJfedfe mm-

^ 0P .. *B^

FIG. 3
Ammonia fuming, early stage

due not solely to this diminution in grain size, but largely to
an approximation of the refractive index of the grains to that
of the gelatine.

The photographic speed- and density-giving power of an emulsion on exposure and
development must not be identified with the photochemical sensitiveness giving rise to a
visible image.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

It is very probable that at this stage a double compound
of silver halide and ammonia is formed. (See p. 25.) The
change of grain size is, however, not very pronounced as
compared with later stages. This is evident in that at this
stage the change is largely reversible. On being removed
from the ammonia atmosphere the emulsion regains opacity
to near its former value, this being accelerated by an air
current. The next phase (on continued fuming), is an irre-
versible ripening, in the sense of increase in size of grain, \m
which large crystal aggregates are formed. They commence
at isolated points (see Fig. 3), and radiate from these until the
respective recrystallization circles or domains meet, when

FIG. 4
Ammonia fuming, middle stage

boundaries which tend to be straight lines are formed, so that
the original recrystallization areas become polyhedral, as
illustrated in Figs. 4 and 5.

It will be seen that the final stage is a complete filling up of
the area fumed with a number of polyhedral cells enclosing a

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

sort of efflorescence of trichites and crystal aggregates. The
figures so far given are from natural size contact prints from
contact negatives made direct from the original preparations;
hence the appearance of the original preparations is reproduced
as accurately as possible. Photomicrographs dealing with cer-
tain aspects of this recrystallization process will be given later
in connection with the discussion of the theory. 1 At this
point it is necessary only to note that the beginning of nuclea-

FIG. 5
Ammonia fuming, final stage

tion, under the conditions given, is to a certain extent acci-
dental. In any case, it commences at the boundary where
the film is in contact with the ammonia container, but in the
fumed area dust particles or other casual nuclei seem to serve.
As scratching the sides of the container starts crystallization
from solutions, so a stress mark made with a glass rod on the
emulsion induces ammonia development along the trace. 2

1 Examples of this have been given by Eder (I.e.) and Luppo-Cramer, Kolloidchemie
und Photographic. XII. Koll.-Zeits. 9: 240. 1911.

2 Cf. Luppo-Cramer, Kolloidchemie und Photographic, I.e. This was confirmed in
the present investigation.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY
FUMING EXPOSED PLATES

The plates were exposed behind a scale negative a sensi-
tometer strip to a 100-watt lamp for definite times and at
definite distances. From density measurements of the scale
negative the relative exposures could be calculated, but it is
unnecessary to give these within this scale, because it was
found that precision in the matter of gradation within the
scale was neither important nor practicable to determine.
As has been pointed out under conditions of fuming, there
is no actual increase or decrease of material within a given
area, exposed or unexposed, but only changes of aggregation
or dispersity and of refractive index, which produce an appar-
ent change of density or opacity. Plates were exposed both
moist and dry. In preparing the moist plates the plate was
soaked for a brief period in water, and superfluous water
blotted off. The plates were weighed dry and wet to deter-
mine the amount of water absorbed.

The general effect of moisture was greatly to accelerate
the action of ammonia vapor. Although a definite propor-
tionality of effect could not be ascertained, it was evident that
excessive swelling in water produced more irregular effects.
The most marked difference between dry and moist plates, in
line with the acceleration, was the much coarser grain produced
in the wet or moistened plates, as will be evident from figures
to be given later.

EXPOSURE NECESSARY FOR AMMONIA DEVELOPMENT

A result of importance, in view of earlier pronouncements
on the ' 'development of the latent image by ammonia," was
that the exposures to light necessary to obtain a developed
image were of an entirely different order, being very many
times greater than those required to obtain a developable
image by ordinary development.

Thus with Seed Process plates the exposure necessary to
obtain ammonia development of an image of the scale was
some 150 times that necessary to give an image developable
with pyro-soda. With Cine Positive film, the corresponding
figure was about 250 times as long an exposure; and similar
results were obtained with other emulsions. (With lantern
plate, 190 times.) Under these conditions, which imply
exposures well toward the ordinary solarization limit, it
appears incorrect to speak of a development of the latent
image by ammonia. And, in fact, close inspection showed
that ammonia development of an image is possible only from
an exposure which is either the same or but little below the

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

threshold value of exposure to give a visible image. Of
course this threshold varies very considerably with the visi-
bility conditions, and it is generally possible to detect with
the microscope definite evidence of photochemical changes
well below the visible threshold of image formation.

Fig. 6 shows the slight indication of image formation
after ammonia-fuming a Seed Process plate, dry, for some
seventeen hours, the plate having received an exposure 130
times that necessary to give a full scale with pyro-soda devel-
opment. The ammonia development here has proceeded to

FIG. 6
Exposed plate, fumed dry; exposures in candle-meter-seconds

the stage of reversal already referred to, but has brought out
nothing further on the scale. An unfumed control plate as
well as the plate used showed a threshold visible image within
ajfield or two of the lowest developed by ammonia.

EFFECT OF MOISTURE

If the plate is moistened by swelling in water, fuming with
ammonia produces an effect earlier, but the developable

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

threshold exposure is much higher. Thus the result shown in
Fig. 7 was obtained after soaking a plate for one minute in
water and fuming fifteen minutes; but the ill-defined differenti-
ation or development covers only part of the scale covered in
Fig. 6 and implies about nine times as great an exposure, the
normal image being visible over a greater range. Further
development with ammonia in this case only filled the plate
with crystal aggregates and obliterated the primary differen-
tiation between exposed and unexposed portions. The faint
indication of an image obtained in this way is shown in Fig. 7.

FIG. 7

Exposed plate, fumed moist; exposures in
candle-meter-seconds

REVERSAL

The appearance of reversal observed by Luppo-Cramer is
clearly indicated in Fig. 8. It is to be noticed that more than
one type of reversal, as regards relative optical density, is
apparent in the process. The exposed portions, as compared
with the unexposed, appear at first more transparent, and

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

later, less transparent, than contiguous unexposed portions.
This is due, first, to the new silver bromide-ammonia complex
having a lower refractive index than silver bromide; second,
to the varying stages of dispersity of the new and old phases.
Reversal with increased exposure to light for the same time
of development (fuming) indicates that the optical opacity
at first increases with the number of nuclei, reaches a maxi-
mum, and then diminishes. (See Fig. 8.) Reversal with
increase of time of fuming is more apparent than real, being

FIG. 8
Exposed plate, showing appearance of reversal

dependent upon the relations between the stages of recrystal-
lization in two contiguous fields. Finally, this is affected by a
third factor partial or complete reconversion of the silver
bromide-ammonia complex into silver bromide, leaving
pseudomorphs of silver bromide by evaporation of ammonia.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

EFFECT OF AMMONIA-FUMING ON DIFFERENT EMULSIONS

As already noted, the rate and sensibility of ammonia-
fuming is very dependent in one and the same emulsion on
the actual state of the plate in respect of moisture content.
It is very difficult to bring different emulsions to the same
state in this respect, hence reliable comparisons between
different emulsions are anything but easy to obtain. It was
hoped at one time that ammonia-fuming might be used as a
method of investigation and control of the "grain" of an
emulsion when coated, somewhat in the manner of the etching
reactions in metallurgy; but it is evident that, even if it should
be possible, much more work on the control of conditions will
be necessary. Taken by and large, however, the results
showed that the finer-grained emulsions react, or rather
reaggregate, more rapidly on fuming with ammonia than the
coarser-grained ones. Their sensibility in the matter of the
development of an image by ammonia after exposure to light
appears to be entirely a matter of their photochemical sensi-
tiveness. The rate at which ammonia-ripening takes place
is a function of the size of the grain, the character of the
emulsion, and the moisture content.

THEORY OF AMMONIA DEVELOPMENT

Reference has already been made and certain objections
raised to Luppo-Cramer's theory that ammonia development
is due to a disintegration of the silver halide grains by light.
The fact that in the absence of light action the reaggregation
by fuming starts at the point of contact of the vessel used
with the emulsion layer, or within this at casual dust particles
or other nuclei, suggests that it is unnecessary to postulate
either disintegration of silver halide grains by light, or Ostwald
ripening. The simplest explanation is that reaggregation
and recrystallization are initiated by nuclei furnished by
light. Since the development practically only commences
with exposures giving the threshold of a visible image, it is
evidently unnecessary to look for these nuclei further than the
photochemical decomposition product, most probably colloid
silver adsorbed to residual silver halide, forming photo-halide.
Accepting this, and in view of the absence of ammonia devel-
opment for exposures much, if at all, below the visibility
threshold of light action, it appears that the nuclei in the
range of the so-called latent image are either not large enough
or possibly still too "protected" by residual silver halide to
function in ammoniacal recrystallization. This, however,
is only in line with the fact that the threshold values of

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

exposures above which it is possible to develop physically by
an acid silver developer is very much higher (particularly
in coarser-grained plates), than that for ordinary chemical
development.

The evident reversal on prolonging development is simply
a consequence of the variation of light transparency with the
phase of the recrystallization process and the dispersity of the
reaggregated silver halide. In the first noticed phase of
apparent homogeneous peptization and increased transpar-
ency, new crystal nuclei of an ammonia-silver halide complex
start to form about nuclei furnished by light. Then the size
of these nuclei increases, and at first therewith the opacity.
But in less exposed regions, and a fortiori in non-exposed ones,
the number of initial foreign nuclei is, at the start, propor-
tionately less; hence the grain size on reaggregation can
overtake that in the exposed regions where there are a greater
number of nuclei. 1 But a limit is set to this and a tendency
to neutralize the initial differentiation formed by the fact
that prolonged action of ammonia on the gelatino-silver
halide results in a chemical reduction, thus furnishing colloid
silver nuclei which are sometimes evident as a silver stain
after fixing, but which are generally developable with a silver
intensifier after careful fixation and washing. Luppo-Cramer's
chief argument for the disintegration hypothesis is the possi-
bility of ammonia development after destruction of the image,
as evidenced by incapacity for development with free silver,
the destruction being brought about by bromine. In repeating
these experiments it was found, first, that the threshold
exposure which could be differentiated by ammonia was
much raised by this treatment, and, secondly (as stated by
Luppo-Cramer), that the differentiation, or development,
is very imperfect after this treatment. This result is in no
way a necessary consequence of the disintegration theory.
It is equally well and perhaps better accounted for on the
colloidal silver nuclei theory here proposed. While bromina-
tion tends to re-halogenize the photochemical decomposition
product, the silver halide thus formed is not physically homo-
geneous with the original silver halide grains, but, as altered
material, may itself furnish nuclei for the ammonia recrystal-
lization. Further, and perhaps more effective, is the local
reaction on the gelatine.

1 If the plate is fumed moist, then dried out again, the visual opacity of the exposed
regions is usually higher than that of the unexposed region, and increases to a limit with
exposure. If the plate is fumed dry, and further dried out after fuming, the opacity of the
lower exposures is usually less than in the unexposed region adjacent, reaches a minimum,
and then increases again, but usually does not reach that of the adjacent unexposed region.
The phenomena are further varied by the nature of the emulsion and the original size of the
emulsion grains.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

The conception that the new phase formed by photo-
chemical decomposition could furnish the nuclei for this
recrystallization was tested indirectly in two ways. First,
a plate was given an exposure just sufficient to form a latent
image, i. e., one developable with a chemical developer, but
not, as already stated, with ammonia. This image was just
developed, very faintly, but not fixed. After washing and
drying, the plate was fumed with ammonia, whereupon a
well-defined image was developed up, showing that the silver
nuclei furnished by development could function as nuclei for
ammoniacal recrystallization. See Fig. 9.

FIG. 9

Normally exposed plate, developed to appearance of image in
diluted developer, then fumed with ammonia

As a second indirect support of the theory advanced, the
development of colloidal gold nuclei by ammonia-fuming can
be brought forward. By marking a plate with a glass rod
dipped in gold chloride solution and drying down, then washing
well and drying again, a very faint deposit of colloidal gold is

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

left. On fuming with ammonia, this is developed by recrystal-
lization on the traces of gold.

The minutiae of the recrystallization and reaggregation
process induced by ammonia-fuming will be discussed later
in connection with the general theory of ripening. Apart
from the general interest of the phenomena in question, the
phase of increased transparency first noted may be worth
investigating sensitometrically. If a homogeneous pepti-
zation is effected, the resolving power and solarization limit
should be markedly altered. Another point of interest is
the relation of this ammonia recrystallization to the ripening
of emulsions. The reaggregation or alteration of dispersity
will be discussed later, but it should be mentioned here that
the existence of direct chemical reduction of the silver halide
by ammonia and gelatine combined was found. It is probable
that this plays a determining role in the occurrence of both
ripening fog and aging fog in gelatino-bromide emulsions.

The chemical composition and constitution of the silver
halide-ammonia compounds is quite fully discussed by
Ephraim 1 in his studies on auxiliary valences. He concludes
that a maximum of three ammonia molecules can become
attached to the silver atom, so that for salts saturated with
ammonia the composition will be AgHal: 3NH3, while the
constitution may be any one of the following:

I. [Ag(NH,),X]; II. [Ag(NH 3 ) 6 AgX 2 ]; or III. [AgC^].

At ordinary temperatures the tri-amine is not stable,
passing over to the di- and mono-amines as the temperature
is increased or as the pressure of ammonia is diminished.

SUMMARY

1. The general course of development of silver halide
emulsions by ammonia was found to be similar to that de-
scribed by Eder and Luppo-Cramer.

2. It appears to be incorrect to speak of "development of
the latent image" in this connection, as the ammonia devel-
opment does not begin much, if at all, below the threshold of
the visible (print-out) image.

3. It is concluded that the process consists primarily in
recrystallization of the silver halide as a silver halide-ammonia
complex on nuclei furnished by the visible photochemical
image; it is therefore unnecessary to assume either a mechan-

1 Ephraim, F., Ueber die Natur der Nebenvalenzen. XIX. Ammoniakate des Silbers.
Ber. chem. Gesell. 51: 706. 1918.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

ical disintegration of the grains by light or Ostwald ripening
as factors in the effect.

4. The following results appear of importance for the
general theory of emulsions : The opacity to light of a mass
of silver halide increases at first on recrystallization with the
number of independent nuclei. Independent nuclei can be
furnished by foreign substances, such as colloidal silver or
gold, or probably even altered gelatine. Under the combined
action of ammonia and gelatine silver bromide is reduced
with production of colloidal silver.

CHAPTER II

Von Weimarn's Theory and the Determination of
the Dispersity of Silver Bromide Precipitates

The sensitive silver halide preparations used in photo-
graphy may be divided into two main classes:

A. Silver halide formed in the presence of excess silver salt.
This includes wet collodion, collodion emulsion, and most
printing-out emulsions. The function of excess silver salt
here is probably chiefly that of a chemical sensitizer, i. e., as a
halogen absorbent;

B. Silver halide formed in the presence of excess soluble
halide. This includes both positive and negative gelatine
emulsions for chemical development. While development
emulsions for printing (developing-out papers) and positives
depend chiefly upon silver chloride and combinations of
silver chloride and silver bromide in which the after-process
of ripening plays a relatively small part, the fundamentally
important negative emulsions are composed of silver bromide
and silver iodide, the silver bromide in considerable excess and
seldom used alone. By ripening is understood the increase
in speed and change in other sensitometric properties induced
by certain digestion processes, either by heat, with excess of
soluble bromide present (boiling process), or at lower tempera-
tures by ammonia. This treatment generally involves an
increase in the average size of the grains, or, in the terms of
colloid chemistry, a decrease in the dispersity. It was at one
time associated with the flocculation of colloid particles, 1 but
later has been more generally regarded as a case of Ostwald
ripening. By this is meant the growth of larger crystalline
particles at the expense of smaller ones, on the presumption
that the latter have a greater solubility. Before considering
either the general grounds for this thesis or its specific applica-
bility to photographic emulsions, it should be pointed out
that modern high-speed emulsions, relatively coarse-grained,
are not produced by the ripening of emulsions which would
otherwise be slow and fine-grained. The two types are pro-
duced under relatively different initial conditions, and, as
pointed out by Luppo-Cramer 2 and Mees, 3 are practically
discontinuous.

1 Cf. Quincke, G., Die Bedeutung der Oberflachenspannung fur die Photographic mit
Bromsilbergelatine und eine Theorie des Reifungsprozesses der Bromsilbergelatine. Jahrb.
Phot. 19: 3. 1905.

2 Luppo-Cramer, Photographische Probleme, I.e.

3 Mees C. E. K., The physics of the photographic process. J. Frankl. Inst. 179: 141.
1915.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

It may be said that fundamentally the production of such
different emulsions depends upon certain general principles
for regulating the dispersity, or average grain size, of a solid
precipitate. In a certain measure specific applications of
these principles have been familiar for a long time to both
analytical and industrial chemists. But it is only in recent
years, with the development of colloid chemistry, that they
have been reduced to definite and general laws, capable to a
certain extent of mathematical expression, and equally con-
cerned with the genesis of colloids and of crystals. While
the conditions for crystallization from liquid melts by cooling
have become known largely through the researches of
Tammann, 1 the determination of similar principles governing
the dispersity or internal subdivision of a new phase separating
from supersaturated solutions, particularly where the new
phase is solid, is due to the Russian investigator, von Weimarn/ 2
His contentions as to the "vectorial" character of phases
of matter usually termed amorphous will be noted later.
Meanwhile the kernel of his work will be briefly reviewed, as
it is of considerable experimental and practical importance.

It is a fact well known to chemists that not only does the
form and subdivision of a precipitate vary in different sub-
stances, but that variations in the conditions of precipitation
will alter the character of the precipitate for one and the same
substance. Thus, Stas 3 distinguishes certain "modifications"
of the silver halides, reference to which is made in many
text-books of photography. 4

Von Weimarn's first postulate is that the actual form and
internal subdivision of a new solid phase are determined by
two sets of factors :

1. Unilateral influence of the vectorial molecular forces
on the molecules forming the free (crystal) surface. By this
is meant a directing or orienting force of the molecules separ-
ating as a new phase on those forming the free surface of the
crystalline individuals of this phase. It is considered that
the molecules in the free surface of a crystal are imperfectly
oriented, or imperfectly ordered in respect of the space lattice
determining the crystal system and form. Since the smaller
the crystal, the greater its surface as compared with its volume,
the crystalline ordering would tend to be overwhelmingly
deviated from if it were not for this factor. It is considered

1 Tammann, G., Krystallisieren und Schmelzen.

2 Weimarn, P. von, Zur Lehre von den Zustanden der Materie.

3 Stas, J. S., Recherches de statique chimique au sujet du chlorure et du bromure
d'argent. II. Ann. chim. phys. V. 3: 145. 1874.

4 Eder, J. M., Ausfuhrliches Handbuch der Photographic, Vol. Ill, p. 13.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

as giving rise to a "capillary pressure" additive to the general
hydrostatic pressure on the surface, and increasing with
increasing dispersity. Hence, in general, it increases the
crystalline solid character, raising the melting point as the
size of the grain diminishes for substances for which pressure
does this, while for the relatively few substances, like water : ice,
the opposite obtains ;

2. The second factor or group of factors is considered to
be always tending to bring the substance to the fluid state
i. e., one of relatively unordered molecular movement and
is sometimes identified with ordinary dynamic surface tension.

The conception that the form and size of a crystal depend
upon a balance of internal and external forces is itself logical,
and will be considered from a slightly different angle later.
It should be noted at this point, however, that von Weimarn
contends that all so-called amorphous solid precipitates are
essentially crystalline in the character of their unit particles,
cellular and flocculent textures being due to secondary causes.
The crystallinity of the particles may be ultra-microscopic
(krypto-crystalline) , but it exists as a reality determining
the trend of their changes. 1 Since a crystal is regarded
essentially as a phase of definite composition, this standpoint
is in apparent contradiction with the general view of such
precipitates as "absorption-compounds," i. e., as phases of
variable composition. 2 The explanation is that the purity or
composition of a crystal is largely a function of its size. The
smaller the crystal, the larger its relative surface, and the
more it is liable to contamination with dissolved and adsorbed
foreign molecules. Hence, the composition of a solid crystal-
line dispersed phase is a function of the dispersity.

The composition of the surface layer may be expressed as
X n Y m Z p , i. e., X n molecules of the dispersed substance, Y m
molecules of the solvent (or dispersing medium), Z p molecules
of co-existing solutes. TV, m and p need stand in no rationally
fixed ratio; thus Y m may be nearly eliminated by drying,
while Z p will increase with the dispersity of X n proper. These
combinations form the class of adsorption compounds or
"capillary combinations."

It is desirable to add to this exposition of von Weimarn's
theory that, as a result of other work, we regard the
"attachment" of the components Z p and Y m as varying from
a state of true solution in the crystal to one of entirely super-

1 Cf. von Weimarn, 1. c., vol. I, p. 13.

2 Bemmelen, J. M. van, Die Absorption.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

ficial combination by chemical (residual) affinity. 1 In general,
however, the contamination will be ruled by the following
characteristics :

a. Equilibrium is reached very rapidly;

b. The reverse separation by washing with pure solvent proceeds
very slowly, adsorption being in most cases practically irreversible;

c. For low concentrations of the adsorbed substance (in the solution)
relatively greater amounts are adsorbed than at high concentrations.

Von Weimarn considers that these characteristics are
largely explainable by the fact that the surface layer of a
crystal behaves in a measure like a strongly compressed
viscous liquid.

Coming now to a more specific consideration of precipi-
tation, he points out that the actual aggregation of the mole-
cules of a new phase depends upon a considerable number of
proximate factors, e. g., its solubility, its latent heat of conden-
sation, the pressure on the medium, viscosity, and the
concentration of reactants. Of these, solubility and concen-
tration demand first attention, and it is assumed concerning
them that aggregation of a new phase may be divided into
two stages, the first (a) consisting in the formation of amicro-
scopic "germs" or nuclei, the second (b) in the growth of
these particles, chiefly by diffusion of dissolved molecules
into the sphere of their attraction. This division is common
to Tammann's theory of crystallization from super-cooled
melts and von Weimarn's theory of crystallization from
supersaturated solutions. The latter proposes the parallel
forms :

SUPERSATURATED SOLUTIONS SUPER-COOLED LIQUIDS

a. (1st stage)

Condensation pressure _ Super-cooling

Condensation resistance Latent Heat

~ T ~ T

= R ~ = P/S = E

O Li

Where W velocity of condensation

Q = total available molecules in solution

S = normal solubility of coarse-grained phase

Hence Q S = actual supersaturation

P/S specific supersaturation at initial condensation

b. (2nd stage)

V = - . Z(C-c) v = ? . Z(t-T), where

a a

Where V = velocity of crystallization v = velocity of crystal-

lization

1 Cf. Langmuir, I., The constitution and fundamental properties of solids and liquids.
I. Solids. J. Amer. Chem. Soc. 38: 2221. 1916; II. Liquids, ibid. 39: 1848. 1917.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

D = diffusion coefficient H = heat conductivity

d = diffusion path length d = diffusion path length

c = saturation concentration t = melting temperature
(solubility) for a given
dispersity
Z = total surface

The expression for the second stage is essentially identical
with that for the so-called Nernst theory of heterogeneous
reactions, 1 which may be considered an adaptation of Wilder-
man's formula. 2

Nernst's modification consisted in supposing that, in
heterogeneous chemical reactions, the reaction itself was
accomplished with practically infinite velocity and adjacent
to the boundary surface between phases, and that the velocity
measured was that of diffusion across a layer d having a
concentration gradient varying from saturation to that general
in the solution. 3 Hence, for the velocity constant k he sub-
stituted D/d, D being the diffusion coefficient.

This conception has been keenly criticized, 4 and in fact
is only partially adequate for a limited number of cases. Its
insufficiency will be specifically noted later. As regards the
second stage of crystallization, Wilderman's generalized
expression,

V = k Z (C-c) + K,

(where C-c expresses the concentration difference, or distance
from equilibrium, and K is a characteristic "instability con-
stant"), may be substituted without affecting von Weimarn's

thesis.

The following important deductions are made:
I. Provided that the product (volume x concentration) be kept constant
and sufficient time allowed, individual crystal magnitudes are inversely
proportional to W, the initial condensation velocity;

II. With increasing W the number of nuclei increases, but for very high
W adhesion of these occurs i. e., groups or clumps of crystal nuclei
cohere, forming a single crystalline aggregate termed by von Weimarn
aggregation crystallization.

In any case, the initial stage of separation of a new phase
is the formation of a colloid solution (suspensoid or emulsoid).
This, however, may be so transient as to escape notice, depend-
ing upon the relation between the velocity of initial conden-
sation and that of crystalline growth.

1 Nernst, W., Theoretische Chemie.

2 Wilderman, M., On the velocity of reaction before complete equilibrium and before
the point of transmission, etc. Phil. Mag. VI. 2*: 50. 1901.

3 Cf. W. Nernst, 1. c.

4 Cf. M. Wilderman, 1. c.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

The important factor here is not the absolute, but the
specific supersaturation P/S. Thus, with a given value of P
(say a few grains per 100 cc.) a very soluble substance (e. g.,
sodium chloride in water), will deposit nothing at first, even
for considerable supersaturation, since not only is the solu-
bility of coarse-grained crystalline sodium chloride very
considerable, but that of the amicrons is even greater. 1 Hence
the initial velocity of condensation is small compared with,
for instance, silver chloride.

However, the value of P, the absolute supersaturation,
is still of considerable importance. The resulting precipitate
will be very different, according as a given value of P/S (= V)
is due to large or small S. In the one case, a large amount of
the precipitate is formed, in the other, little. If V be large,
the former case gives a gelatinous precipitate, or gel; if V be
small, a large number of dispersed particles, therefore a
solution. Thus by suitable alteration of P or S, or both, we
can ensure the initial separation of the dispersed phase in any
desired form.

The stability of the new phase in the initial condition is
dependent, to a large extent, upon conditions expressed in
the formula for the second stage. 2 The smaller the existent
supersaturation Cc, and the smaller the value of V, the
greater the stability. Decrease of Z>, the diffusivity, helps
this. Hence, for stable suspensoid hydrosols there are
required :

Large values of P=QS/S and
Small values of 5

so that V may be large, giving many nuclei.

An example along these lines, worked out in detail by
von Weimarn, is barium sulphate. The solubility in water
at 18 C. is .00024 gms. per 100 cc. This is large enough so
that, with ordinary solutions of barium salts with sulphates,
the values of P do not give large values of V and hence barium
sulphate is obtained in an immediate microcrystalline form.
But from more concentrated solutions of more soluble barium
salts, e. g., barium sulphocyanide and manganese sulphate,
the barium sulphate may be obtained either as a cellular gel
or a translucent hydrosol.

Summarizing von Weimarn's postulates at this stage:
1. With very soluble substances, suspensoids are obtained only for large

values of V, resulting in a gel. If V be small, the suspensoid is

transitory.

1 The presumption of a higher solubility of finer grained particles will be discussed later.

2 It must be understood that these stages are, as regards the general rate of change and
the total mass in course of change, continuous and, to some extent, simultaneous. The
differentiation is mainly important as affecting the character of the precipitate.

32'

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

2. With substances of small but measurable solubility (from 4 to 10
gms. in 100 cc.), the suspensoid stage is reached for both large and
small values of V, the former resulting in a gel, the latter in a dilute
hydrosol.

3. With substances practically insoluble the suspensoid stage is not
recognizable for small values of V. With large values of V, dilute
sols are obtained.

THE DETERMINATION OF THE INITIAL GRAIN SIZE
OF PRECIPITATES IN RELATION TO

VON WEIMARN'S THEORY

As a first application of von Weimarn's principles, the
precipitation of silver bromide at different initial concentra-
tions was studied with V x C kept nearly constant. Equiva-
lent amounts of silver nitrate and potassium bromide solutions
were mixed at a temperature of 20 C. A deviation was
made, simulating emulsion practice, in that steady mechanical
agitation was employed. The vessels were dimensionally
similar cylinders with proportionate Wulff (centrifugal)
agitators. Under these conditions it was not found that the
dispersity of the precipitate was markedly affected whether
the silver nitrate was added to the potassium bromide or
conversely. However, the photochemical sensitiveness (visible
darkening) was higher in the latter case. 1

The results obtained are shown in the following table:

EQUIVALENT
CONCENTRATION DISPERSITY REMARKS

(Normal)

.0002 Clear hydrosol

. 0004 Clear hydrosol

.0010 Clear hydrosol

.0025 Cloudy sol, settles somewhat Decreasing

.025 Turbid suspension, settling dispersity

. 25 Flocculent ->crumbly ppt.

.75 F,occu,ent

1.50 Flocculent ppt. ->silt

3.00 Curdy -> crumbly ppt. Decreasing

4.50 Curdy, quasi-cellular voluminous gel dispersity

See outside curves in Figs. 10, 11, 12.

These results are mainly qualitative and will be studied
quantitatively later. They show, however, that as the initial
concentration of reactants is increased, the dispersity passes
through a minimum, the precipitate here obviously approach-
ing the crystalline condition. And for lower and higher
concentrations it tends to give colloid sols and gels respectively.

1 As has been indicated by Mees (The Physics of the Photographic Process, 1. c.),
many properties of the emulsion probably depend not simply on the dispersity (grain-size),
but on the distribution of sizes, or "dispersity gradient."

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

Now the extent to which the variation of dispersity thus
offered by change of initial concentration is photographically
useful is limited. For one thing, it is not practicable to keep
the volume concentration constant; for another, the practic-
able range of amount of precipitate per unit volume of
emulsion is limited. Hence, in any case, other factors must
be introduced; and these are superposed upon variation of
initial concentration within a certain limited practicable
range.

CHAPTER III

Accessory Factors Influencing the Dispersity
of Silver Bromide Emulsions

In von Weimarn's theory, the grain size of precipitates is
considered as being regulated essentially by the initial concen-
tration of the reactants. In point of fact, there are a number
of modifying factors, frequently as important as concentration,
which may, for convenience, be termed accessory dispersal
factors. These factors, in so far as silver bromide emulsions
are concerned, are:

1. The colloid emulsifying medium, and variation of its concentration

and condition;
2.- Effect of mixture of silver halides;

3. Addition of solubilizing reagents, in particular excess of bromide or
ammonia;

4. Addition of other soluble ingredients acting either on the silver halide
or on the gelatine or on both, and, modifying all these, temperature
and agitation.

Taking these up in detail:

1. EFFECT OF COLLOID MEDIUM UPON DISPERSITY

The influence of colloid media such as gelatine upon the
silver halide precipitate is far-reaching. To begin with, silver
halide precipitated in the absence of such a medium is, except
where certain special precautions are taken, practically imme-
diately reducible by developers. That is, it is not only
mechanically, but also chemically, unsuitable for photo-
graphic purposes. It was suggested by Sheppard and Mees 1
that the most probable explanation of this form of the pro-
tective function of gelatine is that it acts as a filter against
nuclei (development germs), and this view is strongly sup-
ported by Luppo-Cramer. 2 Apart from this, however, it
affects the dispersity (or size of grain) and the form and
composition of the individual grain.

Considering dispersity first, we have at present only
qualitative indications. It is well known that, in the presence
of gelatine, silver bromide is still obtained as a colloid hydrosol
at concentrations of the reagents which would otherwise give
a coarse-grained precipitate. Generally, it appears that if
the relation between concentration of precipitants and dis-

1 Sheppard, S. E., and Mees, C. E. K., I.e., p. 206.

2 Luppo-Cramer, Kolloidchemie und Photographic. XIII. Koll.-Zeits. 10: 182.
1912. Cf. Reinders, W., and Nieuwenburg, J. van, Gelatine und andere Kolloide alsVerzog-
erer bei der Reduktion von Chlorsilber. Koll.-Zeits. 10:36. 1912.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

persity of new phase is represented by a curve of the type
shown in Figs. 10, 11 and 12, i. e., passing through a minimum
dispersity, then the effect of a protective colloid is to flatten
out the curve and shift the minimum more or less considerably
to regions of higher concentration. This effect will in general
be the more pronounced the greater the concentration of the

Concentrations in Normalities

FIG. 10

gelatine, so the emulsion-maker can control the dispersity to
a considerable extent by varying the concentration of gelatine
present at mixing, supplying the rest as required.

It is probable that part at least of this effect is due to the
increase of viscosity, or inner friction, of the medium. Revert-
ing to von Weimarn's theories, it will be seen that the effect
might be attributed chiefly to this factor, which would have
the result of increasing the number of nuclei. It can be
shown, however, that while viscosity counts for much, the
protective action of the colloid is not due to this alone, since
solutions of different bodies of equal viscosity give very dif-
ferent results. There is here a definitely selective action,
dependent upon the colloid chemical character of the medium,
and largely specific in respect of the substance precipitated
or dispersed. Provisionally, we shall regard this as a capillary
chemical or adsorption effect, and discuss its nature more
fully both experimentally and theoretically.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

Technically, it is well known that different kinds of gelatine
are by no means equivalent for preparing silver halide emul-
sions. Without trenching on the colloid chemistry of gelatine,
or on the nature of the colloid condition of its sols and gels,
it is to be noted that the condition of a given gelatine will
depend to a considerable extent upon its thermal history, and
also upon its content of electrolytes. The affinity of gelatine

Silver Halide Emulsion
With Increasing Gelatine

1*34
Concentrations in Normalities

FIG. 11

for water as shown particularly by its absorption as gel, and
also by its behavior as sol is increased greatly by small
amounts of acid and alkali. Again, it is affected by salts,
some of which increase, others decrease, its affinity for water.
In considering the effect of additions upon a gelatino-halide
emulsion, not only the direct effect on the silver halides must
be considered, but also the indirect effect, by way of their
action on the gelatine.

We may at this point anticipate the results given in a
later chapter by stating that while it is very possible, even
probable, that in the preparation of emulsions, particularly
in the ripening process, a combination of some kind between
the gelatine and the silver halide occurs, we know little or
nothing as to its character or extent. We have found, follow-
ing Eder, 1 that silver bromide precipitated in gelatine and

1 Eder, J. M.. 1. c.. Vol. Ill, p. 11.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

centrifuged out at 1,000 to 2,000 revolutions per minute
carries with it, after rapid washing with warm water, about
tw T o per cent of gelatine. Similar figures were obtained with
silver halide emulsions. However, the greater part of this is
probably mechanically retained and it is most probable that

Silver Halide Emulsion
With Increasing Gelatine

1 Z 3 4

Concentrations in Normalities

FIG. 12

the amount of "combined" gelatine in the ripened high speed
emulsions is of the order of the dyes retained in sensitizing.

For low-speed and positive emulsions the nature of the
combination between the gelatine and the silver halide is
probably even less definable. Nor do we know whether or
not there is a solution or adsorption of the gelatine as a whole,
or whether there is a selective (preferential) solution, sorption
or combination of amino-acid anhydrides which may be
considered as the potential structure-units of the gelatine
solution aggregate, or of protein derivatives.

Influence of Temperature. The general conclusions outlined
here the shape of the dispersity-concentration curve, etc.,
are much modified by change of temperature. By reducing
the amount of gelatine and raising the temperature to near the
boiling point, the zone of minimum dispersity is markedly
enlarged, so that by adding the first part of the precipitating

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

silver solution drop by drop, thus forming (according to the
principles discussed), a nucleus solution, then adding the
remainder relatively rapidly and stirring well, a well denned
crystalline precipitate is obtained. The principle is, of
course, familiar in analytical chemistry in the control of
precipitations for gravimetric analysis. 1

Colloid Medium and Crystal Form. We must, however,
notice that the effect of a colloid medium upon the dispersity
of a precipitate is closely connected with its effect upon the
form of a crystal. This will be readily understood in view of
the fact that it affects not only the condensation but the rate
of crystal growth.

This may be studied in its most pronounced form when
crystallization occurs at rest. Generally, of course, agitation
tends to diminution of the size of crystals, quiescence to
increase. The object being to obtain a uniform, relatively
fine-grained material, silver halide emulsions are continu-
ously and thoroughly stirred.

Studying, for maximum contrast, the conditions of crystal-
lization at rest with a colloid present affecting crystal growth,
there are, from one point of view, the following possibilities: 2

a. Total inhibition of crystallization;

b. Suppression of some of the lines of growth;

c. Extension of the crystal to abnormal proportions, forming a compound
crystal ;

d. Gyrating and curving direction of growth.

Of these, the first need not be considered here. The others
will be discussed in order.

(b) Suppression or repression. It is supposed that currents
are set up to and from growing crystals i. e., micro-convec-
tion currents due to gravity changes, as well, probably, as
convergence of diffusion lines, which are again affected by
the rate of crystallization and viscosity. These currents are
likely to become more accentuated and well defined for a
medium at rest, and one of high viscosity. 3 Hence, any
tendency to irregularity of growth will be facilitated in so far
as these currents are favored. This important question will
be taken up later. 4

1 Cf. Brother, G. H., Suggestions on some common precipitates. J. Amer. Leather
Chem. Assoc. 13: 159. 1918.

2 Bowman, J. H., A study in crystallization. J. Soc. Chem. Ind. 25: 143. 1906.

3 Viscosity must have a limiting or maximum influence here, for it tends to decrease
gravitational convection currents.

Cf. Chapter IX.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

crographs (Figs. 13-16) of silver halide ripened in situ by
fuming gelatino-bromide plates with ammonia illustrate both
(b) and (c). In some cases there are filiform and dendritic
structures, due largely to factors of the type (b) i. e., greater
rapidity of condensation with very imperfect orientation. In
others, there is a definite tendency to form an imperfect

FIG. 13
Crystal aggregate, magnified 40 diameters

skeletal example of a much larger compound crystal, the
constituent crystallites being oriented in planes at definite
angles to each other.

In some cases, within the same diffusion sphere (or con-
densation-halo) it will be seen that on one side the condensation
tendency has prevailed, giving a dendrite, and on the other,
the orientation tendency, giving an aggregate-skeleton, or
compound crystallo-crystalline aggregate.

These types of growth (b) and (c) at rest are both compat-
ible with what Bowerman terms the relay principle, i. e., a
growing point is pushed forward into the supersaturated
mass and becomes a new focus or nucleus.

(d) Finally, curving or gyrating occurs where the crystal-
lizing force is so nearly balanced by the resistance to growth

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

that this takes place along the lines of least resistance. There
are reasons for stating that in a system at rest this will generally
approach a logarithmic spiral.

Taken altogether, the phenomena in a colloid gel (at rest)
indicate that the supply, as fixed by the diffusion potentials
of the supersaturated field, is more or less divided in distribu-
tion between local condensation and uniform orientation of
the crystallizing molecules. There results, then, a particular

FIG. 14
Crystal aggregate, magnified 40 diameters

equilibrium between the former tendency to increased density
in phase (condensation) and the latter tendency to oriented
or ordered distribution in phase (crystallization per se), which
is possible only in systems at rest, where the diffusion sphere
may be large 1 (cf. Fig. 5). Agitation destroys this condition,

1 The principle here outlined is an application of Gibbs' theorem on the homology of
macro-canonical and micro-canonical ensembles, as given in Elementary Principles of
Statistical Mechanics. On formation of skeletons and crystal growths in general, see R.
Brauns, Chemische Mineralogie, p. 130, and O. Lehmann, Molekular Physik, Vol. I, p. 337.
Lehmann's explanation of the local concentration gradients along the lines of auto-intensi-
fication applies to the growth forms (dendrites), but our contention is that these are char-
acteristically antithetic to the skeletal forms, and that this difference is explicable on the
differentiation pointed out by E. Riecke (Ueber Wechselwirkung und Gleichgewicht
trigonaler Polysysteme, Ann. Physik. IV. 3: 543. 1900). Riecke considers that the
growth of a crystal depends, inherently, on the exertion of both attractive (condensing)
and orienting (ordering) forces by the original nucleus on the crystallizing substance.
This subject will be discussed more fully in a later chapter.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

i. e., reduces aggregation-crystallization to a relative mini-
mum depending upon chance of collision of crystal nuclei,
which increases with total concentration of reactants, and
decreases with the viscosity of any colloid present.

Actually, then, agitation under certain conditions may
favor crystallization, for by eliminating certain types of
aggregation-crystallization it increases the chances of devel-
opment of individual crystals. The system to which the
crystal belongs can be supposed to be determined inherently
by its chemical constitution, or, if polymorphic, by conditions
of pressure and temperature.

FIG. 15

Ammonia recrystallization of silver bromide, exfoliatory
aggregation, magnified 30 times

The facts just described may be partially accounted for
by certain considerations of the influence of capillarity upon
crystal form, a topic which will be discussed in a subsequent
chapter. (See pp. 57 et seq.)

2. EFFECT OF MIXTURE OF SILVER HALIDES

The Condition of Co- precipitated Silver Halides. When
two relatively insoluble compounds with a common ion, such
as silver chloride and silver bromide or silver bromide and
silver iodide, are precipitated together, the proportions in
which they are formed in the precipitate depend upon :

1. The solubilities of the compounds in water;

2. The solubilities of the compounds in excess of the precipitants;

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

3. The relative proportions and absolute excesses of the precipitants;

4. The possible formation of definite compounds in between the precip-
itated substance.

These conditions, expressed in terms of the mass law,
determine true equilibrium for a given temperature. 1 We
must, however, bear in mind the possibility of the end-state

FIG. 16

Silver bromide aggregate, recrystallized by ammonia fuming.
Compound dendritic and cubic aggregate structure

in a given case being a "false equilibrium," owing to quasi-
mechanical adsorption factors, etc., so increasing the inner
friction that true equilibrium is not reached. 2 There are

1 See F. W. Kuster, Ueber Gleichgewichtserscheinungen bei Fallungsreaktionen. Zeits
anorg. Chem. 19: 81. 1898.

2 Duhem, P., Traite elementaire de mecanique chimique, fondle sur la thermodyna
mique. Vol. I, p. 4.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

several phenomena in the formation and ripening of silver
halides which point to this as a probability. The experi-
mental test of false equilibrium is that different end-states
are reached on proceeding from opposite directions. It is
possible that the discontinuity between low-speed and high-
speed emulsions depends to some extent upon this condition.

An important investigation in which the precipitation
relations of the silver halide pairs, silver chloride-silver bromide
and silver bromide-silver iodide, were examined is due to
Thiel. 1 The object of the research was the investigation
of reversible electrodes of the second kind with mixed depolar-
izers. The so-called second kind of galvanic elements are
concentration cells in which the electrodes are of the same
element, having the same solution tension, but bathed in
solutions of a slightly soluble salt of the electrode initial in
equilibrium with a soluble salt of another metal with the same
anion. These combinations are then reversible with respect
to the anion, and the polarization-preventing salt of the elec-
trode metal is termed a depolarizer.

Thiel pointed out that elements of the second type might
be formed in which the metal is surrounded by a depolarizer
which is not a single solid body of constant valency, but a
homogeneous mixture of the body giving the anion with
another, thus being analogous to an amalgam. If silver
bromide-silver chloride, silver bromide-silver iodide, silver
chloride-silver iodide formed homogeneous mixtures, this
objective could be realized with silver. Conversely, potential
measurements with such combinations give information as
to the homogeneity of the mixtures of the co-precipitated
silver halides under varying precipitation conditions; and
this is naturally the consequence of present interest. Thiel's
observations and results of most importance in this connection
were as follows:

1. When mixtures of silver bromide and silver iodide were
precipitated together, noteworthy peculiarities in the color
were observed. While the precipitated silver bromide was
pale yellow, and the silver iodide only a slightly deeper yellow,
mixtures showed a much deeper color, varying from lemon
yellow to that of egg yolk. No direct connection between
the color and composition could be observed, as the color
obtained in mixtures of different proportions would often be
the same, while it might vary in equivalent mixtures;

2. In the precipitation of pure silver iodide the presence
of a trace of free iodine was evident. To keep this down as

1 Thiel, A., Umkehrbare Elektroden zweiter Art mit gemischten Depolarisatoren.
Zeits. anorg. Chem. 24: 1. 1900.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

much as possible, the acidity (from sulphuric acid) was kept
as low as was compatible with obtaining pure precipitates. 1
The color in the solution was discharged by a few drops of
sodium thiosulphate.

From this Thiel at first concluded that the color of the
mixtures might be due to a trace of free iodine in the precipi-
tate. However, treatment with thiosulphate failed to remove
it, so it is probable that it depends in some way upon the
condition of the precipitate presumably the dispersity;

3. Investigation of the pair silver chloride-silver bromide
showed that they formed homogeneous mixtures in all propor-
tions that is, have unlimited miscibility;

4. On the other hand, silver bromide-silver iodide have'
only limited miscibility. In this case, silver bromide in
excess is able to dissolve silver iodide up to thirty per cent,
whereas silver iodide is able to dissolve silver bromide only
up to five per cent. Thus, if we have altogether ten millimols
of silver halides precipitated and saturation for silver iodide
just reached, the precipitate would contain three parts silver
iodide to seven parts silver bromide. If now sufficient potas-
sium iodide were added to the solution to form four parts
silver iodide at equilibrium in the precipitate, the solid phase
would consist of a saturated solution of silver iodide in silver
bromide (2.5 : 5.9) and a saturated solution of silver bromide
in silver iodide (0.1 : 1.5);

5. On comparing the amounts of silver bromide-silver
iodide in mixed precipitates with the theoretical quantities
given by the solubility relations, 2 it was found that more of
the silver than calculated was always present. There is a
tendency to preferential precipitation of the less soluble
component.

In so far as these results bear on photographic emulsions
it may be noticed that the presumption formerly was that
either specific addition compounds of the silver bromide and
silver iodide were formed (Eder) or that silver bromide and
silver iodide were miscible to an unlimited extent. 3 Supposing
the precipitates crystalline, Thiel remarks that the behavior
of silver chloride-silver bromide mixtures indicates a high
but not perfect degree of isomorphism, while with silver

1 In precipitating silver halide the solution must be acid rather than neutral or alkaline,
as otherwise contamination with silver oxide may ensue. In any case, the only permissible
alkali generally would be ammonium hydroxide, which redissolves silver oxide. (Compare
the two methods for making emulsions, on p. 27.) On the other hand, where iodides are
used it is evident that if free, strong acid (high hydrogen-ion dissociation) is present, free
iodine, which is strongly adsorbed by silver iodide, will be formed.

2 Cf. Thiel, 1. c., p. 60-63.

3 Bancroft, W. D., 1. c., p. 650.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

bromide-silver iodide mixtures there is either only a low grade
of isomorphism, or isodimorphism.

Although the proportion of silver iodide in silver halide
emulsions is well within the range of homogeneous mixture
(or solid solution), it is probably sufficient to markedly affect
the crystalline habit and growth in view of the generally
anomalous behavior of the silver bromide-silver iodide mix-
tures of the silver bromide. 1

While it is not entirely permissible to compare separations
from liquid melts with precipitations from supersaturated
solutions, yet a contingency for the same substances is obvious.
Thus it is of interest to note that Stoltzenberg and Huth 2
concluded from thermal analysis of fused silver halides that
all three are capable of forming a liquid crystal phase, the
transition temperature of regular silver bromide to liquid
crystal being 259 C. Below the transition points they show
considerable plasticity. As against this, Tubandt and Lorenz 3
conclude, from their studies of the application of conductivity
determinations to polymorphy of single compounds and to
the state of binary salt mixtures, that there is no definite
evidence for a liquid crystal phase with the silver halides.
Further, as regards mixtures of the halides, they find that:

1. Monkemeyer's 4 conclusion of unlimited miscibility
between silver bromide and silver iodide is incorrect. There
is only a restricted miscibility from both sides, with series of
regular and hexagonal mixed crystals;

2. The saturation line of regular mixed crystals cuts the
crystallization curve as the proportion 80% silver bromide-
20% silver iodide. It is probable that here the combination
4AgBr : Agl separates. The region between 20 and 100%
silver bromide probably consists of homogeneous mixtures
of this compound (4AgBr : Agl) with silver bromide. Thus,
in this region, no transition phenomena are observed, which
indicates stability of form;

3. Silver iodide-silver chloride gives complete mixed
crystals up to 90% silver chloride, with marked transition
phenomena;

4. The crystals from melts containing 80% and more
silver bromide are strongly birefringent;

1 Cf. Chapter IX.

2 Stoltzenberg, H., and Huth, M. E., Ueber kristallinisch-flussige Phasen bei den
Monohalogeniden des Thalliums und Silbers. Zeits. physik. Chem. 71: 641. 1910.

3 Tubandt, C., and Lorenz, F., Das elektrische Leitvermogen als Methode zur Bestim-
mung des Zustandsdiagramms binarer Salzgemische. Zeits. physik. Chem. 87: 543. 1914.

4 Monkemeyer, K., Ueber die Bildung von Mischkrystallen der Blei-, Silber-, Thallo-
und Cupro-halogene aus Schmelzfluss. Neues Jahrb. Mineral. Geol. 22: 1. 1906.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

5. It may be concluded that the only alternative to the
presence of the combination 4AgBr : Agl is that a third stable
modification of silver iodide is present in the region 80% - 100%
silver iodide, such as that indicated by Tammann 1 as stable at
very high pressures. From the conductivity phenomena
this is not probable.

Now, although we can not infer directly that the molecular
state of a solid phase from a homogeneous liquid melt will be
identical with a solid phase of the same composition obtained
by precipitation from supersaturated solution, yet it is very
probable that the state of the solid phase from the liquid
melt represents the equilibrium condition to which that from
the supersaturated solution tends to approach. Hence it is
possible that some of the color anomalies of silver bromide-
silver iodide mixtures observed by Thiel may depend upon
the completeness of formation of the compound 4AgBr : Agl ,
and that this again may be concerned in ripening phenomena.
It is particularly interesting to note that birefringence in the
crystals of silver halide emulsions is now well established. 2
In regard to the general influence of a co-precipitate like silver
iodide upon silver bromide or silver chloride, Tubandt and
Lorenz remark that the ' 'simple silver iodide molecules may
transfer their oscillation condition to simple silver bromide or
silver chloride molecules executing other but similar oscilla-
tions, and thus help order them in the same space lattice."

3. ADDITION OF SOLUBILIZING AGENTS

The influence of solubilizing agents on the dispersity of
silver bromide in emulsions need not be discussed here, as it
consists in intensifying or otherwise modifying the saturation
factor of von Weimarn's theory; and also because it will be
considered in more detail from the point of view 1) of ther-
modynamic theory in the discussion of capillarity and crystal
growth (Chapter V) , and 2) of molecular theory in the discussion
of crystallization catalysis (Chapter IV).

4. ADDITION OF SOLUBLE INGREDIENTS
OTHER THAN SILVER HALIDES

The concluding statement of section 2, with regard to
mixed silver halides, indicating what may be termed a mutual
induction effect in crystallization, brings us to the heart of
the problem of photochemical sensitizing of the silver halides,
including both ripening effects and the so-called optical sensi-
tizing by the use of certain dyes.

1 Tammann, G., Das Zustandsdiagramm des Jodsilbers. Zeits. physik. Chem. 75.
733. 1911.

2 Cf. Chapter X.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY
CHEMICAL, OPTICAL AND PHASE SENSITIZERS

The original photochemical conception of sensitizers, due
to Vogel, 1 was that they acted as absorbents of decomposition
products, e. g., silver nitrate as absorbent of bromine from
silver bromide gives silver -+- bromine, a function, as pointed
out by Bancroft, 2 similar to that of depolarizers electrochem-
ically, and as such indicated by Grotthus. 3

At first gelatine was supposed to be superior to collodion
as a medium because of higher halogen absorption power.
But it was pointed out by Luppo-Cramer 4 that the superiority
could not rest on this property alone, and that ordinarily
chemical sensitizing of this form probably plays only a small
part in the field of gelatino-halide emulsions. The discovery
of optical sensitizers by Vogel brought forward a new photo-
chemical problem. Why should certain quite a limited
number dyes make silver halides sensitive to their own
absorption region? There have been two explanations for
this. One is that a chemical decomposition of the dye is
effected, which is either extended to the silver bromide before
development, or provides a nucleus for development. The
other supposes that the internal vibrations of the dye molecules
in absorption of light affect the silver halide in the same way
as its own direct absorption of light.

It is evident that the crux here is very similar to that for
the developability (latent image issue) of the silver halides
per se. This is a region where chemical and physical change
overlap. The theory of radiation-transformation, or radiation
catalysis, has received powerful support from the work of
Chapman and his collaborators 5 on the photochemical induc-
tion of chlorine, 6 and has been extended to chemical catalysis
in general by Lewis. 7 In the case of the silver halides and
dyestuffs it has been made more precisely applicable by the
work of Stark and his collaborators 8 on "latent fluorescence"
and "ultra-violet fluorescence."

1 Vogel, H. W., Handbuch der Photographic. 4th edition, Vol. I., pp. 193-195.

2 Bancroft, W. D., The electrochemistry of light. J. Phys. Chem. 12: 209, 318, 417.
1908; and 13: 1, 181, 449, 538. 1909.

3 Grotthus, F. von, Physisch-chemische Forschungen.

4 Luppo-Cramer, Photographische Probleme, 1. c., p. 33.

5 Burgess, C. H., and Chapman, D. L., The interaction of chlorine and hydrogen. J.
Chem. Soc. (Trans.) 89 2 : 1399. 1906. Chapman, D. L., Chadwick, S., and Ramsbottom,
J.E., The chemical changes induced in gases submitted to the action of ultra-violet light.
J. Chem. Soc. (Trans.) 91 1 : 942. 1907. Chapman, D. L., and MacMahon, P. S., The
interaction of hydrogen and chlorine. J. Chem. Soc. (Trans.) 95 1 : 135. 1909. Chapman,
D. L., and MacMahon, P. S., The retarding effect of oxygen on the rate of interaction of
chlorine and hydrogen. J. Chem. Soc. Trans.) 95 1 : 959. 1909.

6 See also Sheppard, S. E., Photochemistry.

7 Lewis, W. C. M., Studies in Catalysis. V. J. Chem. Soc. (Trans.) 109: 796. 1916.

8 Stark, J., Zur Energetik und Chemie der Bandenspektra. Physik. Zeits. 9: 85. 1908;
Steubing, W., Fluoreszenze und lichtelektrische Empfindlichkeit organischer Substanzen
Physik. Zeits. 9: 493. 1908.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

Recent research on selective absorption and fluorescence
has shown that the absorption spectrum and the fluorescence
spectrum of a substance are potentially equivalent, but that
much of the fluorescent spectrum is easily rendered "latent"
owing to re-absorption and degradation by adjacent layers of
the same molecules. The fluorescence spectrum is excited
in its maximum extension and intensity for layers practically
one molecule deep. This is strikingly borne out by R. W.
Wood's work 1 on the resonance spectra of sodium, potassium,
etc.

Again, fluorescence is not limited to the visible region, but
may exist in both the ultra-violet and the infra-red. Stark's
theory is substantially that optical sensitizing is due to ultra-
violet fluorescence from a layer of dye one molecule thick, and
that excess of dye interferes by excessive absorption.

However, there is one difficulty in the way of this hypoth-
esis. The dyes which sensitize are quite limited. Yet prac-
tically all dyes containing a benzene ring with unstable
auxo-chromes or auxo-fluors should be capable of ultra-violet
fluorescence. It appears probable that something more is
necessary namely, a marked and selective capacity for
sorption and solid solution by the silver halide. The import-
ance of this will be more fully evident on considering, on the
one hand, the theory of the nature and genesis of the crystalline
condition; and on the other, the experimental researches
(particularly those of Retgers, Reinders and Marc), on the
influence of additions on crystallization.

It is agreed that a crystal is a homogeneous assemblage of
ultimate particles of a substance, such that each particle is
similarly situated and similarly environed by identical par-
ticles, and that the typical distribution may be referred to a
regular group of points or space lattice.

It may be pointed out at this stage that one reason for the
importance of crystallization in the formation of sensitive
halide emulsions is the increased probability of the extension
by resonance of the perturbation (from light) of a single
molecular layer throughout the mass of a uniform crystal.

Further, however, it is not agreed as to whether the
"ultimate crystalline particles" are molecules or associated
aggregates of molecules, or, as indicated by X-ray crystal
analysis, whether crystallization involves loss of molecular
individuality. 2 In any case, it appears to be a manifestation

1 Wood, R. W., Researches in physical optics.

2 Cf. I. Langmuir, 1. c.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

of the same intra-molecular force which we term "chemical
affinity" and which is now identified with the radiation field
proper to atom or molecule. 1 A consistent theory of the gene-
sis of crystallization based on this view has been suggested by
Beckenkamp. 2 He considers that crystallization is regulated
by approach to equilibrium of the inner radiation fields of
atoms or molecules, in particular by very short wave-length
ether vibrations (wave-length fractions of molecular diameter) .
Interference between these forms a system of stationary waves,
(the permanence of which would obviously depend upon
reduction of thermal energy, e. g., molecular kinetic energy),
the nodal points of which by mutual attraction (resonance)
determine the crystallization of simple forms, twinning, etc.
An analogy will make this clearer. It is supposed that the
atoms (or molecules) are marshalled in symmetrical space
lattices by stationary ether wave systems, just as the particles
of talc or lycopodium are arranged in symmetrical patterns
by stationary sound wave systems in Lissajou's experiments.
As differing from the analogy, however, the determining
stationary wave system is not external, but internal and
inherent, emanating from the atoms (or molecules) themselves.
In other words, a crystal is a concrete manifestation of an
actino-chemical equilibrium. It becomes conceivable, then,
that if a substance is to act effectively as an optical sensitizer,
in that its specific absorption is to disturb the actino-chemical
equilibrium, it must also be able to accommodate itself to the
space lattice of the material sensitized. Now it will be seen
later that, to effect this, substances must either be so consti-
tutionally similar as to be more or less isomorphous, or be
capable of the colloid condition, wherein the tendency to
pronounced crystalline form is a minimum, that to molecular
net-works (gels) a maximum.

The operation of a sensitizer may be regarded in respect
of either wave-length or phase. It would be very possible for
a crystalline substance to absorb only a limited amount of
light of suitable wave-lengths, but of such irregular phase
ordering that the equilibrium radiation field of the crystal
would get only partially in resonance. We can conceive then
that a non-crystalline body in solid solution or absorbed would
increase the photochemical sensitiveness, acting as a resonance
complement. Thus colloid silver acts as a panchromatic

1 See Wyckoff, R. W. G., The nature of the forces between atoms and solids (J. Wash.
Acad. Sci. 9: 564. 1919.) particularly Classification of Crystalline Solids. "The crystalline
state furnishes the greatest condensation of the fields about the individual particles (atoms
or molecules, depending upon the type of solid)."

2 Beckenkamp, J., Der tetrakishexagonale oder oktaedrische Typus der Kristalle.
Ann. Physik. IV. 39: 346. 1912.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

sensitizer for silver halides. The behavior of traces of calcium,
bismuth, etc., in developing phosphorescence in the alkaline
earth sulphides is a similar case. 1 If the above conception
is true it is possible that silver iodide acts both as a wave-
length sensitizer and as a phase-sensitizer for silver bromide,
as well as acting as an independent sourc^ of silver nuclei by
its direct photolysis. However, these contributions are likely
to be of less total importance than its stabilizing influence as a
crystallization buffer substance on the crystallization of co-
precipitated silver bromide.

It has been shown that a fundamental relation exists
between photochemical catalysts, positive and negative, and
crystallization catalysts. This relation may be summed up
as follows: Crystallization is a process of approach to a
complete (static) equilibrium of the radiation fields (chemical
affinities) of the component atoms and constituent molecules.
The attainment of static equilibrium may be accelerated or
retarded by alien substances, or crystallization catalysts,
which, from the actino-chemical nature of crystallization,
are consequently likely to be also photochemical catalysts,
affecting the transformation and redistribution of incident
light energy. That is, they affect the way in which light
energy is redistributed by a molecule or aggregate M. This
may be represented thus:

Fluorescence } Distribution

Phosphorescence I of energy

Light energy M

Photochemical change > largely
Photo-electric effect | determined
Heat J by catalysts

Cf. S. E. Sheppard, 1. c., p. 400.

CHAPTER IV

Crystallization Catalysis

We can conveniently include all the effects of additive
substances upon the crystallization of a new phase under the
term crystallization catalysis. By this we shall understand
both the positive actions leading to fully developed crystals
and the negative ones retarding crystallization and leading
to well developed colloids. It follows from what has been said
that the seat of crystallization catalysis is primarily the
interface between growing crystal and mother liquor.

In the simplest case, positive catalysis of crystallization is
effected by substances which form more soluble, but readily
dissociable compounds with the crystallizing substance.
Thus the soluble bromides and ammonia act in this way to
silver bromide. Substances forming very stable soluble
complexes, such as thiosulphates and cyanides, do not have
this effect. On the contrary, especially if they form less
soluble stable complexes, such bodies are strong negative
catalysts, such as, with silver bromide, bodies like mercuric
bromide or lead bromide (or other mercuric and lead salts).
These substances, it should be noted, produce strong congela-
tion of silver halide hydrosols. A more complex case of
crystallization catalysis exists where a consolute substance
affects the habit of the growing crystal. A striking example
is the effect of urea upon the crystallization of sodium chloride.
In water alone sodium chloride crystallizes in cubes, but if
urea be added, octahedra are formed. 1 An important and
interesting series of investigations on crystallization from
aqueous solution have been made by Marc and collaborators. 2
Marc found that, while with some substances the rate of
solution is equal to the rate of crystallization up to the greatest
velocities of stirring, there are others which show a great
difference; and with these the rate of crystallization may be
only one-sixteenth that of solution, indicating a slow change
practically independent of diffusion as to speed. In any case,
however, crystallization may be brought to a standstill prema-
turely by the presence of dyes, while the rate of solution of
the crystals is either not affected or only somewhat retarded.
The taking up of dye by the crystals in general follows the

1 Cf. Retgers, J. W., Beitrage zur Kenntnis des Isomorphismus. V. Zeits. physik.
Chem. 9: 257. 1892.

2 Marc, R., Ueber die Krystallization aus wasserigen Losungen. Zeits. physik. Chem.
79: 71. 1912.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

adsorption formula, but shows the phenomenon of ''saturation"
i. e., with concentrations above a certain value there is no
increment of adsorption. Marc considers that the surfaces
of the crystal become saturated, and attributes the checking
of crystal growth to the slowness of diffusion of the dye in the
solid crystal, with which it tends to form a solid solution.

Generally, colloids are readily adsorbed, 1 but not so
crystalloids unless either isomorphous or chemically com-
bining. Between the amounts of different substances which
can saturate a given crystal surface, 2 quantitative relations
exist which are conserved for other surfaces. The saturation
indicates that an absolute minimum of surface energy is
reached. Such saturated surfaces have lost all "free" surface
energy, and therewith the capacity to act as germ or catalyst.

This last corollary perhaps indicates a relation to photo-
graphic solarization, where the halide grains progressively
lose capacity to function as a "germ" for chemical development.

That colloidal silver can be taken up by crystallizing silver
halides was shown by Reinders. 3 He concludes that colloidal
silver forms solid solutions with the silver halide, not simply a
surface adsorption layer. The photo-halides are normal
salts of silver colored by small amounts of colloidal silver, the
color depending upon the dispersity of the latter. Certain
dyes, and albumenoids such as gelatine, are also absorbed
by the crystallizing silver halides, and it is noteworthy that
gelatine and similar colloids check or completely prevent
the taking up of colloidal silver. This is a confirmation of
Sheppard and Mees' filter theory of the value of gelatine as
an emulsifying medium. Colloidal gold behaves similarly
to colloidal silver.

The panchromatizing effect of colloidal silver is very
probably responsible for a remarkable panchromatizing
effect discovered by J. G. Capstaff of the Eastman Re-
search Laboratory. Mr. Capstaff found that, if an ordinary
dry plate or film 4 be bathed a short time in a two per cent
sodium bisulphite solution (NaHSO 3 ), then subjected to
prolonged washing in faintly alkaline water and allowed to
dry spontaneously, it becomes more or less panchromatically
sensitized. Although the result is not yet capable of precise
control, it has been found that the extension of spectral

1 Marc, R., Ueber Absorption und gesattigte Oberflachen. Zeits. physik. Chem. 81: 641.
1912.

2 Marc worked with micro-crystals, so this does not specify a habit surface.

3 Reinders, W., Studien iiber die Photohaloide. Zeits. physik. Chem. 77: 213 and 357.
1911.

4 Most of the experiments were made with Eastman Portrait Film.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

sensibility runs parallel with duration of washing with ordin-
ary (hard) tap water. Thus, only a slight extension of sensi-
bility was found after five to fifteen minutes' washing; very
considerable after one to five hours' washing; and after
twenty-four to thirty hours' washing, sensibility was extended
to nearly SOO/nfi. (See Fig. 17.) The time of washing may

FIG. 17

Capstaff panchromatizing effect, showing stages in the development
of color sensitizing with time of washing in hard water

be greatly shortened, five to ten minutes being sufficient to de-
velop strong sensitizing action, if the wash water is made faintly
alkaline with sodium carbonate (Na 2 CO 3 ). (See Fig. 17, 4.)

The nature of the sensitizing action will be evident from
Fig. 17 1 (1, 2 and 3), which shows phases of its progression to

Taken with a wedge spectrograph.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

full panchromatic sensibility. It is evident that the sensitiz-
ing action differs markedly from that conferred by sensitizing
dyes, since the action commences not with a new band, but by
lateral extension toward the red end of the usual sensitivity
curve. There is at the same time a small decrease in the blue
sensitiveness, but this is more than compensated for by the
increase in general sensitiveness; hence the speed to white
light is greatly increased.

As regards chemical conditions, those at present evident
are as follows:

a. The effect can be induced by sulphurous acid, as well as by acid bisul-
phites; it is then due immediately to sulphurous acid (H 2 SO 3 );

b. If this is washed out with distilled water, little or no sensitizing action
is observed;

c. An alkaline after-bath or wash is necessary to develop the induced or
presensitizing effect;

d. A very small amount of soluble bromide e. g., potassium bromide
(KBr) at a concentration of .004 per cent in the sensitizing bath,
is able to kill the effect;

e. "Chemical fog" increases progressively with the sensitizing effect
although not reaching very high values e. g., D = 0.6 at the limit.

Provisionally, the existing facts appear to be compatible
with the view that a small amount of reduction of ionic to
metallic silver is effected by or in the presence of sulphurous
acid, as a presensitizing effect; that this is inhibited by soluble
bromide; and that the alkaline after-bath is necessary to
peptize this silver to higher dispersity, by which the pan-
chromatizing effect is fully developed. 1

In view of the similarity of the adsorption phenomena of
the colloidal metals and dyes, particularly the fact that silver
halides can be panchromatically sensitized with colloidal
silver, there seems a measure of probability in the view that
exposure to light produces a substance which is itself capable
of accelerating the reaction to light of the wave-length in
question. Such a presensitizing effect would be an example
of a specific auto-catalysis. However, we are straying some-
what from the main theme. It is sufficient to point out
that there is some probability that in ripening a certain amount
of adsorption of gelatine, or of hydrolytic derivative of gelatine,
occurs. Also, there is a possibility that a very slight initiation
of reduction occurs, which, however, is rapidly succeeded by
spontaneous fogging. Again, it is equally possible that the
most important fact in ripening is a recrystallization, involving

1 It is interesting to note that a visible yellowish-orange discoloration increases pro-
gressively with the washing treatment; this runs approximately parallel with the sensi-
tizing action.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

a purification of the silver halide. Bancroft's theory 1 that
ripening involves progress of the gelatino-silver halide grain
to a certain optimum composition of silver halide-gelatine-
water, but that therefore size of grain is entirely unimportant,
would, according to this view, be only partially correct. For
first, only by progress from the suspensoid colloid to a micro-
crystalline suspension of lower dispersity could adsorbed
free bromide be completely washed out. It is a mistake to
suppose that any amount of washing will completely remove
every trace of a stabilizing electrolyte from a colloid precip-
itate. Several peculiarities of emulsions are no doubt connected
with this.

Secondly, any further phenomena of purification e. g.,
degelatinization can not be independent of the size of grain
in more advanced recrystallization as in negative emulsions,
particularly high-speed emulsions, because purification and
increase of size go largely together, as is brought out by
Marc's investigations. What seems most probable for the
critical stage, when an emulsion is nearing the point of "going
over" i. e., becoming liable to spontaneous fog is that a
process of degelatinization of the silver halide crystal is going
on pari passu with a taking up of colloidal silver formed by
interaction of silver halide with decomposition products of
the gelatine; for, as shown by Reinders, taking up gelatine
excludes the taking up of colloidal silver. The point at which
the protective effect of the gelatine is passed (due to weakening
by hydrolysis) is the point of going over.

This view would seem in some degree incompatible with
Reinders' observation that silver halides crystallizing from
gelatine solutions are much more sensitive i. e., that taking
up gelatine increases sensitivity. But it must be reiterated
that this refers to photochemical sensitiveness i. e., photo-
lytic production of visible darkening or coloration. Photo-
graphic developability is a phenomenon of another order, in
which excess of gelatine in the grain will impede the chemical
reduction of the grain by the developer by lessening the contact
action of the nuclei, while presence of a minimum trace of
colloidal silver may lower the quota of developability per
grain to be added by exposure to light.

1 Bancroft, W. D., The photographic plate, 1. c., p. 650.

CHAPTER V
Capillarity and Crystal Growth

The original work in this direction is due to Gibbs. 1 From
his investigation of the equilibrium of heterogeneous sub-
stances he deduced that the forms which are in equilibrium
for crystals under the influence of capillary forces are those
in which the surface energy is at a maximum or a minimum.
Assuming that each crystal face has its specific capillarity
constant, measured by the work of increasing the face by the
unit of area, this deduction may be expressed by assuming
that

A^H- A 2 S 2 + A 3 S 3 + --..A n S n

is a maximum or minimum, the areas of different faces being
denoted by S lt S,, etc., the capillarity constants by AI, A 2 ,
etc. Gibbs, however, qualified this theorem by the statement
that the tendency of a crystal to take up the form set by this
capillarity equilibrium is inversely proportional to its linear
dimensions. He states that "On the whole, it seems not
improbable that the form of very minute crystals in equilibrium
with solvents is principally determined ... by the
condition that

A^.4- A 2 S 2 + ....A n S n

shall be a minimum for the volume of the crystal,
but, as they (the minute crystals) grow (in a solvent no more
supersaturated than is necessary to make them grow at all),
the deposition of new matter on the different surfaces will be
determined more by the nature (orientation) of the surfaces
and less by their size and relations to the surrounding
surfaces." 2

It is in fact probable that the surface energy principle
ceases to be regulative for crystals of a given substance above
a certain size i. e., beyond a certain dispersity though
we do not know whether this is absolute or is relative to the
nature of the substance. Although Gibbs was the originator
of this principle, it is better known from the work of Curie 3
and Wulff. 4 Since it is closely interwoven with all questions
of crystal size and growth, and with the so-called "Ostwald

1 Gibbs, J. W., Scientific papers, Vol. I., pp. 320-326.

2 Gibbs, J. W., 1. c., p. 325, footnote.

3 Curie, P., Sur la formation des cristaux et sur les constantes capillaires de leur dif-
ferentes faces. Bull, frang. mineral. 8: 145. 1885.

4 Wulff, G., Zur Frage der Geschwindigkeit des Wachstums und der Auflosung der
Krystallflachen. Zeits. Kryst. u. Mineral. 34: 449. 1901.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

ripening," it is desirable to consider more closely its conse-
quences and the experimental results. Considering a crystal
in its mother liquor, only the surface energy is variable, and
its growth will be in that form for which the total surface
energy is a minimum. Furthermore, each surface must have
its own specific capillarity, otherwise a sphere would be formed.

For substances like silver bromide and silver chloride
crystallizing in the regular system, the condition for a right
quadratic prism is as follows: Let x be the side of the base,
y the height of the prism, A the capillarity of the (equivalent)
side faces, and B of the base, then the surface energy E =
4xyA = 2x*B.

Since the volume of the prism is V = x 2 y = constant, we
have $>(x 2 y) = o, and equilibrium will occur when is a
minimum. The necessary condition for this is A (xby -\- y&x) -\-
Bxkx = o for any variations satisfying V = constant, i. e.,
xby + 2ybx= o. This gives at once Ay= Bx; or x/y = A/B,
which means that the capillary constants of the prism sur-
faces and bases are inversely proportional to the lengths of
the sides. Similar calculations can be made for a cube or an
octahedron. A regular octahedron can occur only if AIOO:
Am > V~J; a cube if AI OO : A m < I/ Vs.

Wulff and Hilton 1 have reduced the principle of minimum
surface energy to another form, formulating a generalized
connection between the capillarities of the various faces and
their distances from the center of the crystal for undisturbed
growth. This is expressed in the following theorem:

The perpendiculars on the faces of a crystal from a certain
point within it are proportional to the capillarities of the
faces, thus:

H! : h 2 : h 3 . . . .h n = k l : k 2 : k 3 : . . . .k n , where h = the
perpendiculars, k = the capillarities, and n the total number
of faces.

This holds for 22 of the 32 crystal classes. But for the
other ten there are an indefinite number of points equidistant
from all faces of the same form. Wulff 2 considers that his
work on the rates of growth of the faces of Mohr's salt,
(NH 4 ) 2 Fe(SO 4 ) 2 .6H 2 O, established the theorem, but his
proof has been called into question by Hilton; 3 and it has
been pointed out by Friedel 4 that Wulff obtained the same
results with crystals of different habits, thus showing that a
distorted crystal does not tend to approach an ideal form.

1 Hilton, H., Mathematical Crystallography.

2 Wulff, G., 1. c.

3 Hilton, H., 1. c., p. 105.

4 Friedel, G., Examen critique de la thSorie de Curie-Wulff sur les formes crystallines.
J. chim. phys. 11:478. 1913.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

Marc and Ritzel 1 reached the same conclusion from their
work, which showed that there are different velocities of
solubility on the octahedral and the cubic surfaces of these
crystals. Since in the case of silver bromide crystals in
photographic emulsions one is dealing only with octahedral
faces, the Gibbs-Curie-Wulff law may be applied without
limitation, providing that no other crystallization conditions
arise, and that the crystals are obtained from very slightly
supersaturated solutions. Ostwald considered that his experi-
ments on red and yellow mercuric oxide 2 gave a quantitative
proof that the solubility (equilibrium condition) is a function
of the size of grain, and this was supported by Hulett's experi-
ments 3 with gypsum (CaSO 4 ).

Hulett placed aqueous solutions of gypsum in contact
with large gypsum plates and found that equilibrium occurred
when the concentration reached 15.33 millimols per litre.
Then, if very fine gypsum powder was added to the saturated
solution, the concentration increased, in one case reaching
18.2 millimols per litre. This high solubility decreased very
rapidly at first, then more slowly, until after 168 hours the
concentration again became 15.33 millimols.

A similar experiment with very finely powdered baryta
showed a sudden increase of about 80 per cent in the saturation
concentration, which, as with gypsum, decreased to the normal
amount after long standing.

The size of the gypsum grains varied from 0.2 to OA/JL.
The baryta grains averaged about 0.1 AI. These dimensions
are at the limit of microscopic resolving power, and should
therefore be accepted with caution.

The experiment with gypsum was further complicated by
the presence of a monoclinic <-dihydrate and a rhombic
/3-dihydrate, into which the o<r-dihydrate passes over when
left for a considerable time in the concentrated solution.
The solubility of the /3-dihydrate is approximately 30 per cent
less than that of the ^-dihydrate.

The results of Hulett's experiments have been mathe-
matically worked out by Valeton, 4 as follows:

If r is the length of a certain crystal edge, the volume of a
crystal grain of the form under consideration ur\ the surface

1 Marc, R., and Ritzel, A., Ueber die Faktoren, die den Kristallhabitus bedingen.
Zeits. physik. Chem. 76: 584. 1911. Cf. critique by Kuessner, H., ibid. 84: 313. 1913;
and Ritzel's reply, ibid. 86: 106. 1913.

2 Ostwald, W., Ueber die vermeintliche Isomerie des roten und gelben Quecksilberoxyds
und die Oberflachenspannung fester Korper. Zeits. physik. Chem. 34: 495. 1900.

3 Hulett, G. A., Beziehungen zwischen Oberflachenspannung und Loslichkeit. Zeits.
physik. Chem. 37: 385. 1901.

4 Valeton, J. J., referred to by Gross, R., Sammelkristallization in Beziehung zum
Atomfeld der Kristalle. Jahrb. Rad. u. Elekt. 15: 270. 1918.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

ur 2 , the energy per cu. cm. i and the surface tension per sq.
cm. 7, then the total energy contained in a grain having length
of side r is

E r = iur s -f- 7 ur 2 .

Further, if g is the specific gravity of the crystal and M

M
the gram-molecular concentration of the solution, then --is

~l\ /f

the molar volume and r the number of crystal grains

gur 3

contained in one mol. The total energy in one mol is then

. M Mi

u r = i h 7 - 3 wr 2 .

g g vr *

If one takes = K,

. M , M

then u r = i h y K - - .

g gr

For grains having the edge-length r = ^ ,

. M
- * T .

The difference u, u^ is the work necessary to powder
one mol of infinite size to grains of a size corresponding to
the edge-length r.

Now, if the very large grains are in equilibrium with
concentration c^ , then there is, according to thermodynamic
conditions for equilibrium, a concentration c for saturated
solutions in contact with the grain size r, of which the osmotic
work in one mol is increased by an amount corresponding
to the difference between c and c^. This is developed as
follows:

In the case of dilute solutions the osmotic work may be
represented approximately by the equation

RT = , where

p = osmotic pressure, T = absolute temperature,
R = gas constant.

Therefore,

RTlnci = RTlnc^ + jK

7K M

or Cl = c^ . e RT ' " .

On the basis of these relations Vale ton has worked out a
diagram (Fig. 18) by substituting the pairs of values found by

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

Hulett for grain sizes 2 and 0.2 and the corresponding concen-
trations of 15.33 and 18.2 millimols per litre. 1

It will be remembered that the conclusion that
20 1 solubility is a function of the size of grain is incor-

porated in von Weimarn's theory of precipitation.
It has also been very generally accepted as apply-
ing to the ripening of photographic silver halide
g \ emulsions. As already stated, procedure here is
' broadly divided into (a) the boiling process, using
excess soluble bromide in slightly acid solution, and
(b) the ammonia process, carried out at lower tem-
peratures. In either case, a particular type of sol-

123456789 10

RADIUS OF PARTICLES IN /.

Fig 18

vent for silver bromide is present, the action of which will be
considered specifically. The presence of solvents tending to
form complexes does not necessarily affect the argument as
to Ostwald ripening.

The first to attempt a microscopic and semi-quantitative
survey of the photographic ripening process were Bellach and
Schaum. 2 As a first result Bellach observed that in certain
stages of ripening, beside relatively shapeless to spherical
grains, definite crystalline polyhedra, apparently tetragonal,
were present. As pointed out by Bancroft, 3 this had been
previously observed by Banks 4 and has been fully confirmed
by other observers. 5 Bellach at first assumed that this
occurred only in mixed emulsions, but later found it in pure
silver bromide emulsions prepared by himself. Crystallization
was observed after a certain time both when the boiling process
was employed, and with ammonia ripening.

At the same time, the average size of grain increased, the
photomicrographs showing this to concur with the disap-

1 For a critique of this work, see Gross, R., 1. c.

2 Bellach, V., Die Struktur der photographischen Negative, 1. c.

3 Bancroft, W. D., The photographic plate, 1. c.

4 Banks, Remarks in discussion of paper by Hurter and Driffield on the latent image.
Phot. J. 22: 159. 1898.

5 Dyer, Note under Emulsionsbereitung. Jahrb. Phot. 18: 437. 1904. Sheppard, S. E.,
and Mees, C. E. K., 1. c., p. 51. Liippo-Cramer, Phot. Prob., 1. c.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

pearance of small grains. There appears to be evidence that
for certain straight silver bromide emulsions a definite Ostwald
ripening occurs. It must be noticed, however, that in practice
variable amounts of silver iodide are also present, and the
influence of this will have to be considered separately. We
may note that per se it appears to show little or no ripening
in gelatine. 1

EXAMPLE OF OSTWALD RIPENING WITH MERCURIC IODIDE

An apparently well developed example of Ostwald ripening
was observed by us in an experimental study of the photo-
chemistry of mercuric iodide. Previous investigators (notably
Luppo-Cramer), working with this compound observed that,
when precipitated in gelatine, it appears first as the yellow,
unstable modification, which crystallizes in the rhombic
system and which passes over to the stable red iodide, crystal-
lizing in the tetragonal system, normally below 127 C., 2
the transition temperature. Luppo-Cramer tried various
colloid media, finding that with gum arabic the red stable
form is immediately produced which indicates a lower
protective effect. When the iodide is precipitated in gelatine
in presence of excess of potassium iodide, it appears first as a
yellow, very finely divided colloidal suspension, which on
being digested at 70-90 C. in presence of excess of potassium
iodide passes over to the regular form.

The emulsion used in our experiments was in general
prepared as follows:

a. 10 gms. soft gelatine in 400 cc. H 2 O

b. 10 gms. HgCl 2 " 100 cc. H 2 O (hot)

c. 10 gms. KI " 50 cc. H 2 O

d. 20 gms. hard gelatine " 60 cc. H 2 O

Emulsion H - 24

(a) and (b) are mixed together at 60-70 C., then (c) is
added with careful stirring. The emulsion was ripened by
heating at 70-90 C. for 60 to 190 minutes. Samples were
removed at definite intervals for centrifugal analysis. The
setting gelatine (d) was added before washing and coating.
The progressive change or ripening effected could be followed
visually by the color change from yellow through salmon pink
to deep red. At the same time the increase in size of grain

1 Luppo-Cramer, Phot. Prpb., 1. c., but this depends on presence of excess of soluble
iodide. From this crystallization occurs readily.

2 Reinders, W., Ueber die Bildung und Umwandlung der mischkrystalle von Queck-
silberbromid und Quecksilberjodid. Zeits. physik. Chem. 32: 494. 1900.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

and probably also in density was evident on centrifuging
samples of the emulsion at different stages. The following
table illustrates this (see Fig. 19) :

CENTRIFUGE
NO. TEMPERATURE TIME OF COOK TIME R. P. M.

1 43 C. 5 mins. 1000

2 78 C. 5 mins. 5 mins. 1000

3 75 C. 15 mins. 5 mins. 1000

4 85 C. 40 mins. 5 mins. 1000

5 88 C. 60 mins. 5 mins. 1000

6 90 C. 80 mins. 10 mins. 1000

7 90 C. 100 mins. 10 mins. 1000

8 90 C. 120 mins. 10 mins. 1000

9 90 C. 140 mins. 10 mins. 1000
10 90 C. 140 mins. 10 mins. 1000

The progress of ripening is also shown microscopically in
an increase in size of grain and concurrent disappearance of
smaller grains. This is shown by the accompanying photo-
micrographs. (Fig. 20.) The changes are also shown by
the centrifugal separations.

Here, then, there appears a definite example of Ostwald
ripening, in the sense of the "eating up" of smaller grains by
larger ones, fairly well formed tetragonal octahedra of mercuric
iodide being formed. At the same time the emulsion acquired
higher sensitiveness and density-giving power, although, of
course, still much inferior to silver bromide. It must be
considered, however, before regarding this as establishing a
clear case of Ostwald ripening, that another factor is present.
This is the initial appearance of mercuric iodide in the yellow
form, belonging to the rhombic system, and stable only above
129.5 C. The colloid gelatine has acted here as agent of
retarded transformation. In this connection it is important
to note that silver iodide also is polymorphic, crystallizing
in the hexagonal form below 145 C., and in the regular above
this temperature. There is, therefore, an obvious possibility
that silver iodide precipitated in gelatine or, generally speak-
ing, under conditions favoring retarded transformation, may
occur initially in the unstable form, passing over, similarly to
mercilric iodide, to the stable form on digestion. There are
certain phenomena in straight silver iodide emulsions which
point to this. Reciprocally, if precipitated with silver bromide,
this potentiality may be of importance.

The condition or mode of combination of silver iodide with
silver bromide, important per se, may, however, be only
accessory to two other important roles of this substance in
emulsions. First, as relatively less soluble in either excess
potassium bromide or in ammonia, it will function in recrystal-

Fig. 19

Ripening of mercuric iodide emulsion. Progressive accumulation of the
red stable form shown by centrifuging

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

X 1000. 0-5 minutes.

.
V * *

* *

X 1000. 160 minutes.

X 1000. 5-15 minutes.

X 1000. 120 minutes.

X 1000. 40 minutes. X 1000. 60-80 minutes.

Fig. 20

Progress of ripening, showing increase in size of grain

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

lization as a buffer substance, tending to conserve and regulate
the number of nuclei. It has been shown in Chapter I that
relatively foreign substances can act as nuclei for recrystallizing
silver halides, and one function of silver iodide is very probably
of this character. Another may well consist in the greater
adsorptive power of silver iodide, which can be operative both
in emulsion preparation in the matter of taking up other
sensitizers, and in development after exposure in affecting
the adsorption of the developer. 1

Before concluding this discussion, it is desirable to point
out that the principle of minimum surface energy can lead
to other phenomena than Ostwald ripening in recrystallization ;
that in fact this is by no means the necessary consequence.
Since this is of considerable importance in regard to the
possibility of making fairly fine and uniformly grained emul-
sions of high speed we shall include a brief account of the
effect of the principle of minimum surface energy upon the
variability of habit, or of the preferential growth of certain
faces of the crystal.

It has been shown that this principle necessarily implies
different capillarity constants for different faces. This,
however, involves different solubilities for different faces, for
otherwise it appears impossible to conceive how a distorted
crystal can assume the equilibrium form (with minimum
surface energy) unless certain faces dissolve while others
grow. The most complete discussion of this experimentally
still unsettled point is due to Ritzel 2 and Kuessner. 3 Ritzel
applied Freundlich's corrected form of Ostwald's formula for
the solubility of small particles as related to surface tension. 4
For a substance crystallizing in cubes the formula gives the
solubility of a cube (C w ) of length of side A in relation to that
of an infinitely extended cube as (reference being to perfectly
developed forms) :

M 4$w
r _ r P RT ' Pa

^W - V ^W' cr , *

where M = Molecular weight,

R = Gas constant (.8316 x 10' 8 ),

T = Absolute temperature,

S w = Surface tension on cube face,

P = Density.

1 See Sheppard, S. E., and Meyer, G., Chemical induction and photographic develop-
ment. J. Amer. Chem. Soc. 42: 689. 1920; Phot. J. 69: 12. 1920.

2 Ritzel, A., Die Kristalltracht des Chlornatriums in ihrer Abhangigkeit vom Losungs-
mittel. Zeits. Kryst. u. Mineral. 49: 152. 1911.

3 Kuessner, H., 1. c.

4 Freudlich, H., Kapillarchemie, p. 47.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

Following Wulff and Hilton, it is more convenient to take as
characteristic parameter of the crystal not a but the distance
of a surface from a middle point, A = a 1 2, when the formula
becomes

M 2 S W

r _ r e RT PA

\-/ ^tf "" " ^- X W ^ *

While for octahedra, taking the distance B from a middle
point, since total surface = 12 V 3 . B 2 , mass m = P. 4 V 3 . B 3 ,

M_ 2S<,

,> RT ' PB

where S G = surface tension of octahedral surface. Kuessner
finds that, for equilibrium, the parameters A and B must be
proportional to the respective capillarity constants. The
minimum of surface energy then entails, however, an equality
of solubilities of all faces, otherwise the combination could
not be in equilibrium. It is found, by executing a cyclic
process of transference and equating work terms, that the
same solubilities must be possessed by
1 . A cube of parameter A ;

2. The cubo-octahedron, with parameter A for the cube
face, and A.S /S W for the octahedral face; and

3. The octahedron with parameter B = A.S / S w .

But this leads to a contradiction. It is shown that under
given conditions only one form can be stable, since the mini-
mum of surface energy can exist for only one configuration.

The single stable form is a cubo-octahedron if - is between

v"s and I/ V~3; a simple hexahedron if S >> V~3 S w ; a simple
octahedron if S < S w I V^ ; and only then is thermodynamic
stability ensured.

Finally, it is pointed out that a paradox results in that the
stable form is that with the greater solubility; for only then
can the other be transformed into it by way of the cubo-
octahedron. There is, however, no real contradiction here,
in that the stable, more soluble form may dissolve in a com-
mon solution and the other less stable form will grow, for this
must by growth transform into the stable form, so that the
total result would be a single crystal of the stable form. Hence
it follows that the principle of Ostwald ripening, by solubility
decrease with size of crystal, must be applied with caution.
It will be seen that recrystallization may well occur between
cubical and octahedral forms of a regular system without
reference to growth of large crystals at expense of small. If,

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

as suggested in this paper, the importance of recrystallization
in ripening is less concerned with increase of size (size being
principally determined by initial condensation conditions)
and more with the inner 'adjustment of composition, the
possibilities indicated here are significant. 1

Equally relevant to the problem of the scope and function
of crystallization in silver halide emulsions is the relation of
dispersity to the twinning of crystals. It has been shown
experimentally by Johnsen 2 that, compared with simple
forms, twin crystals are a labile phase. Pawlow 3 has tried to
show that the free energy of twins is greater than that of
single crystals of equal mass, but smaller than that of two
simple crystals which, combined, would be of equal mass.
It will be seen that twin forms might occur as an intermediate
stage between ultra-microscopic crystals and larger micro-
crystals, or more generally, as pointed out by Niggli, 4 we
can make the following statement: In a system of definite
dispersity (surf ace/ volume), twins represent a labile phase
relative to single crystals; or, a fine crystalline precipitate
consisting entirely of twin forms will be of higher dispersity
than a precipitate of single crystals of the same individual
mass.

Reverting to von Weimarn's analysis of the crystallization
process, it appears probable that the genesis of twin forms
may be predicated as entirely determined in the amicroscopic
stage of the dispersed phase. This conception is entirely in
harmony with the influence of solution factors on twinning,
regarded as operative by way of surface forces. The influence
of these solution factors is necessarily greater, the higher the
dispersity.

We have seen that the initial formation of the labile
(yellow) rhombic form of mercuric iodide in gelatine, which
is stable only above 129.5 C., and which is converted into
the stable (red) form on keeping the emulsion melted, is in
line with Ostwald's law of stages. According to this a new
phase appears first in the form involving the least loss of free
energy. Now since the regular form of silver iodide is stable

1 The results of the present investigation (Chapter VIII), of the crystal forms of silver
bromide in normal gelatine-bromide emulsions show only octahedral forms. Hence, either
the above-noted factor would be imperative for silver bromide emulsions, or limited to the
submicroscopic stage. The reported preparation of fine-grained highly ripened silver
bromide emulsions in albumen is interesting in this connection. See Lehmann, E., and
Knoche, P., Plate-grain and albumen emulsions. Brit. J. Phot. 61: 759. 1914.

2 Johnsen, A., Untersuchungen iiber Kristallzwillinge und deren Zusammenhang mit
anderen Erscheinungen. Neues Jahrb. Mineral. Geol. 23: 237. 1907.

3 Pawlow, P., Ueber die Bildung, das Gleichgewicht, und die Veranderungen des Kris-
talles im isothermen Medium. Zeits. physik. Chem. 72: 385. 1910.

4 Niggli, P., Kolloidchemie und Zwillingskristalle. Koll. Zeits. 10: 268. 1912.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

only above 145 C., an initial appearance of silver iodide in
this form would mean a greater relative retention of free
energy than in the case of mercuric iodide, but might tend to
happen more readily in the presence of greater protection
afforded by co-precipitation with excess silver bromide.
Silver bromide alone has not been obtained in any other than
the regular system. But the tendency to assume a form
proper to a relatively labile system might limit the effect to
twinning as the immediate sequence of the suspensoid stage.
Indeed, Mugge 1 has suggested that twinning in any case
indicates an accommodation of the crystal to a different space
grating, corresponding to an earlier or later energy condition.
Since this would involve a condition of internal strain, it should
result in an optical anomaly, such as birefringence in uniaxial
crystals, a phenomenon which has been shown to occur in
silver bromide emulsion crystals. 2

It is possible (though this awaits determination), that this
occurrence of optical anomaly, or, better, of anomalous optical
activity, is the very focus of ripening in relation to speed, etc.
If so, and if it should be found to be connected with twinning,
the conditions determining the occurrence and governing
the relatively temporary fixation of this labile stage merit
earnest consideration. 3

Going back to the broadest generalization governing the
morphogenesis of a new phase, we have Ostwald's law of
stages. Now there are two ways in which the law could
operate in crystallogenesis. One form may be stated thus.
Every given crystal individual in its growth (evolution to
most stable form) tends to pass through every stage of its
possible range of forms; each transition reduces its free
energy, but each transition is such as to make the necessary
reduction of free energy a minimum at each step. For example,
a substance which normally crystallizes in the regular system
as rigid crystals, but at higher temperatures and normal
pressures in the rhombic, might also be capable of plastic
and liquid crystalline forms at both higher temperatures and
pressures, as asserted of silver iodide. In forming a new
phase, especially of a new component, the individuals would
then pass through the sequence;

Liquid 7* liquid - plastic -> rhombic -* tetragonal
droplet crystal crystal crystal crystal,

1 Mugge, O.,Ueber die Zwillingsbildung der Kristalle. Fortschr. Mineral. 1:38. 1911.

2 The interpenetration of gelatine and the co-crystallization of silver iodide have also
to be considered in this connection.

3 Because indicating that the intensified development and stabilization of this stage is
the direction in which high speed fine-grained emulsions must be sought. In existing
emulsions, examined microscopically, but few instances of twinning were observed. But
investigation of the sub-microscopic stage is lacking.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

the latter stable stages being chronologically later. This
represents a linear evolution.

On the other hand, there is an alternative possibility.
At the initiation of the new independent component, nuclei
of every possible phase of the new component are formed
simultaneously, the transitions observed representing the
relative dominance of each there present with resorption of
the regressive stages, and the progression of form being deter-
mined chiefly by factors influencing dispersity in the amicro-
scopic state. At any relatively permanent stage, all possible
forms are present, but in quantities determined by conditions
regulating dispersity-equilibrium.

This view indicates that we should generally observe and
study not the apparent linear evolution in time of pseudo-
individual crystals, but the mutation in space of the collective
mass. Where we expect to see the chronologically sequent
steps of a linear evolution we really section off displacements
of the mobile equilibrium between simultaneous forms, the
equilibrium among which is above all determined by the
tendency to forms of minimum free surface energy. 1 It will
be seen, as an important consequence, that according to this
view the new component and new phase, at their very incep-
tion (in the amicroscopic suspensoid condition), must be
regarded as heterogeneous.

For an average uniform degree of dispersity (isopsegmaty) ,
the individual particles will consist of arbitrary crystalline
aggregates, polysynthetic twins, twins, and single crystals. 2
From this start progress to equilibrium by reduction of free
energy will lie in displacement in favor of single crystals,
aggregates and twins reducing to these as shown by Johnsen's
experiments.

CONDITIONS FAVORING TWINNING
AND OTHER MULTIPLE FORMS

Since twins are not a stable form, it is important to consider
the conditions favoring their occurrence. Following von
Weimarn's analysis and Johnsen's specific experiments, it is
evident that their occurrence is primarily determined in the
amicroscopic stage, partly by collision of ultra-microscopic
crystals. Barmhauer 3 has pointed out that in the evaporation

1 With the progress of dispersimetry, this mutation theory will find a large field in
metallurgy. The isolation of single quasi-individual crystals, instructive as it may be,
indicates the resultant arrest of one line of mutation. We require also the mass resultant.

2 Together, of course, with "dissolved" (molecularly dispersed) molecules (or the
mother phase , liquid crystallites and droplets, and, if another crystalline system is poss-
ible, a duplicate set of aggregates, etc.

3 Barmhauer, H., Die neuere Entwicklung der Krystallographie, p. 121.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

of unsaturated solutions of potassium sulphate (which allows
slow crystallization), only simple crystals are formed, whereas
by rapid cooling of a hot concentrated solution, giving high
supersaturation and rapid crystallization, there are formed a
great number of twins. Further, infiltration of alcohol into a
cold concentrated solution gives rise to great numbers of the
most multifariously shaped twins, and if a cold concentrated
solution thickened with gelatine is evaporated, there is again a
large production of variously shaped twins and triplets.

Production of twins, etc., is less easy with less soluble
substances, which of course agrees with the influence of a
higher degree of supersaturation, and it is obviously possible
that this would be affected by the solubilizing factors in
emulsion-ripening. A supersaturated solution probably
already contains the whole permutation of crystal germs
in equilibrium with molecularly dispersed substance (dissolved
molecules), and hence not growing, but contributing to the
"colloidal" properties of such solutions. On rapid cooling
or increased supersaturation, the stages are fixed under the
dominance of primary inoculation. And here the nature of
the medium and the initial concentration play the chief role.
Under the foregoing conditions, a special type of twinning
may predominate, corresponding to specific alteration in
solubility of single faces, as already noticed. Preferential
twinning on specific faces has been frequently observed by
mineralogists.

The effect of inoculation was studied by Johnsen in relation
to degree of supersaturation with enantiomorphous crystals
of sodium uranyl acetate, with the following results:

SUPERSATURATION NO. OF INOCULATION EFFECT

(1 = SATURATED) CRYSTALS D/L

L. D.

.00 13 29 2.23,

.03 10 23 2.30

.09 10 25 2.50

.14 7 22 3.14

.20 1 40 40.00

.26 1 45 45.00

1.31 1 45 45.00

1.34 labile 14 11 .79

1.34 labile 5 20 4.00

It will be seen that the effect increases at first with the
degree of supersaturation, and decreases immediately when
the solution is labile. This comprises the view of the prede-
termination of forms in the highly dispersed stage.

In general, then, the twinning conditions are closely
related to those conditions of separation of a new phase

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

which lead to colloids, and the control of twinned forms is
essentially a problem of dispersoid chemistry, depending
upon the methods used to produce systems of great dispersity.

Becke 1 has pointed out that in general twinned crystals
grow more rapidly than simple ones. This may be attributed
to the multiplication of the force of crystallization, or rate of
growth, at the boundary where common directions of growth
radiate.

These conditions have also a bearing on the formation of
pseudomorphs and mimetic twinning. Increased super-
saturation will usually involve nearer approach to a transition
point, and consequently, as suggested by Mugge, increased
tendency to orientation in an altered space lattice.

The conception that, just before crystallization, solutions
consist often not only of different particles of one modification
but of polymorphic particles of different modifications is in
agreement with Smits' theory of allotropy. 2 He has shown
that, where two modifications of a substance (e. g., mercuric
iodide, silver iodide) may exist, under certain conditions both
modifications are present in a definite equilibrium over a wide
temperature interval about the transition point (compare p. 62).
The transition point is thus a point of separation of a mixture,
analogous to the ' 'cracking" temperature of an oil-water
emulsion.

SUMMARY

Our review of the factors in the preparation and ripening
of silver halide emulsions thus returns to the point of departure.
Beginning with the dispersion theory of von Weimarn, it
connects up the peculiarities of "slow" and "rapid" emulsions
with this analysis. But while the slower positive emulsions
remain short-circuited in the region of the purely colloidal
phenomena of peptization and pectization, high-speed emul-
sions require a traverse of very definite crystallization phe-
nomena. In this connection it is suggested that decreased
dispersity and increased grain size is determined chiefly by
initial precipitation conditions, and that ripening by way of
recrystallization depends mainly on elimination of adsorbed
impurities. It is pointed out that degelatinization favors
adsorption of colloidal silver, setting a limit to ripening.
The hypothesis of Ostwald ripening is discussed in connection
with the law of minimum surface energy, and it is shown that

1 Becke, F., Ueber die Ausbildung der Zwillingskristalle. Fortschr. Mineral. 1: 68.
1911.

2 Smits, A., Eine neue Theorie der Erscheinung Allotropie. Zeits. physik. Chem.
76: 421. 1911.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

mere increase in size is not the sole outcome of this, but that
equilibrium relations between different habits are involved.

At this point, leaving the field of colloid chemistry for
that of crystallography, the relation of twinning and habit
variation to the initial conditions of dispersity is discussed,
and it is shown that conditions similar to those regulating
the colloid state determine the formation of twinned crystals
and anomalies of crystallization. The relation of twinning
to intra-crystalline strain and anomalous optical activity is
pointed out, and the suggestion made that this relation may
be of great importance in the theory of emulsions and their
preparation. Thereby the problem is brought back into the
ambit of the analysis of initial precipitation and colloid chem-
ical regulation of dispersity. The gamut of silver halide
emulsions from the slowest gas-light to ultra-rapid may be
conceived as disposed on a helix, the axis of which is this
colloid chemical regulation of dispersity that is, ratio of
surface to volume since all the auxiliary factors of sensitizers
and desensitizers, of size of grain, of individual crystal habit
and eventual twinning, of optical anomaly and strained space-
lattice, are dependent thereon.

None the less, although this dispersoid theory envisages
and embraces emulsion phenomena in their entirety, collect-
ively and distributively, it will be obvious from the foregoing
that its function is limited to that of a regulative principle,
operating statistically through the principle of minimum
surface energy. The intimate relation between grain structure
and photographic properties is, however, fundamentally a
matter of crystallographic investigation, and as such is dealt
with in the following chapters.

Considering that initial precipitation conditions determine
very much the type of emulsion in the case of development
emulsions precipitated with excess halide, and considering
also the ripening or after treatment, the following factors are
involved :

(a) The precipitate is [(AgBr) x : (Agl) y ] m (KBr) n (Gel 3 : H 2 O) ;

(b) Silver bromide-silver chloride readily form continuous solid solutions
in all proportions;

(c) Silver bromide-silver iodide form only restricted solid solutions; it is
probable that mixtures containing up to 20% silver iodide contain the
compound four parts silver bromide-one part silver iodide, or tend to
form this compound;

(d) Silver iodide acts as a crystallization buffer-substance, a brake on
speed of recrystallization. In particular, remaining undissolved by
the ripening agent, it tends to conserve the original number of nuclei,
and hence restrict increase in size of grains. It can also affect adsorp-
tion;

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

(e) Sorption of the soluble additions is higher, the higher the dispersity;

(f) This sorption, lowering reactivity in development, is not entirely
removable by washing;

(g) It is reduced by recrystallization in ripening;

(h) Ripening in colloid silver halide emulsions is mainly a flocculation
phenomenon;

(i) Ripening in suspensions proper is mainly a recrystallization process;

(j) This recrystallization process increases homogeneity of the silver
halide in the grain, reduces absorbed bromide, and probably gelatine;

(k) Recrystallization, in so far as it affects ripening, is limited by formation
of saturated surfaces, and very probably by colloid silver formation
(incipient reduction). It is not known at what stage colloid silver is
formed, but it may occur early, and thus afford nuclei for recrystalli-
zation, as shown in ammonia development (Chapter I) in the case of
ammonia-ripened emulsions;

(1) When the experimental conditions regulating the three primary
factors (1) dispersity-distribution, (2) recrystallization, and (3)
sorption, (both adsorption and desorption), are completely known,
scientific control of the characteristic curve i. e., of speed, latitude,
and density will be possible.

These results show that a consideration of the dispersity
and distribution of the silver halide precipitate is insufficient
to account completely for all the facts. It therefore becomes
necessary to study intensively the crystalline form and habit
of the individual silver halide grains, and thus endeavor to
determine in what way this factor of fundamental structure
is related to the facts reviewed in the preceding pages.

CHAPTER VI

Experimental Study of the Crystallization
of Silver Bromide

Microscopic examination of emulsions used for sensitive
photographic plates reveals a definite crystalline structure of
at least a large number of the silver bromide grains. F. W. T.
Krohn, 1 who was apparently the first to recognize this struc-
ture, made his observations between 1892 and 1901, though
his conclusions were not published until 1918. Banks' is the
first recorded observation. 2

FIG. 21

Special silver iodo-bromide emulsion, magnified 1350
diameters

1 Krohn, F. W. T., The mechanism of development of the image in a dry-plate negative.
Phot. J. 58: 193. 1918.

2 Banks, E., Remarks in discussion of Hurter and Driffield's paper on the latent
image. Phot. J. 22: 159. 1898.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

Bellach 1 and Luppo-Cramer 2 observed this structure
somewhat later, though their published accounts, which
appeared in 1903 and 1907 respectively, preceded Krohn's
description.

In Figs. 21 and 22 are shown photomicrographs of an
emulsion (magnified 1350 diameters 3 ), which was prepared

FIG. 22

Special silver iodo-bromide emulsion, between crossed nicols.
Magnification, 1350 diameters

for this special purpose, and which is distinguished by a
number of relatively very large grains, the largest having a
diameter of 6 - 8^. Fig. 23 is a photomicrograph of a
"Radio-bromide" emulsion (magnified 2,500 times), made by
Guilleminot of Paris, and Fig. 24 shows a similar magnifica-

1 Bellach, V., 1. c.

2 Luppo-Cramer, Photographische Probleme, p. 51.

3 For description of the apparatus used see pp. 82-83.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

tion of Seed 30 emulsion prepared by the Eastman Kodak
Company.

It will be noticed in all these figures that the largest grains
are polygons with angles of 60 and 120. There is an obvious
tendency to round off the edges and corners in the small
grains a phenomenon which is repeatedly observed in the

FIG. 23

Guilleminot's Radio-bromide emulsion,
magnified 2500 diameters

formation of crystals in a colloid matrix so that the smallest
grains generally appear more or less spherical.

It will also be noted that there are apparently two kinds
of grains, those which are clear and therefore absorb little
transmitted light, and those which appear nearly black and
therefore absorb considerable transmitted light. In the
original negative, however, these "dark" grains do not show

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

uniform light absorption. Instead, there seems to be a net-
work of more or less dark portions, the details of which are
not shown in the reproduction. As may be clearly seen in
Fig. 21, these dark bodies exhibit the same crystallographic
habit as the transparent grains, so that, for the present at

FIG. 24
Seed 30 emulsion, magnified 2500 diameters

least, there is no justification for the assumption that these
represent two different substances.

The ratio between the number and the size of the round
and the polygonal grains and between the clear and the opaque
grains is not constant, but may vary considerably in the
different emulsions. Thus, for instance, Luppo-Cramer 1
prepared a photomicrograph of an emulsion in which there

1 Luppo-Cramer, Photographische Probleme, p. 54.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

were no well-developed crystal forms, all the grains being
spherical. This must not be interpreted as meaning that
these spherical bodies are not crystals, however, for that which
determines whether or not a given body is crystalline is
structure, not habit. The typical polygonal form of a well
developed crystal is merely one manifestation of its structure.

The above-mentioned emulsions (and this may be said of
practically all highly sensitive emulsions), contain in addition
to the silver bromide a certain quantity of silver iodide, which
varies in different emulsions. But there is never any indica-
tion that either the bromide or the iodide crystals are precip-
itated alone i. e., without an admixture of the other.

In investigating this question, 122 photomicrographs of
as many emulsions, prepared in various ways and magnified
2,500 times, were examined without finding one instance of
separate precipitations of the iodide and of the bromide,
i. e., of hexagonal silver iodide and regular silver bromide.
For silver iodide is polymorphous and, as the p.-t. diagram
of Bakhuis-Roozeboom 1 shows, the stable phase at normal
temperatures is hexagonal, the transition point into regular
silver iodide being about 145 C. And it is very improbable
that silver iodide for emulsion purposes is precipitated at
145 C. or higher. 2 Renwick 3 is of the opinion that the
hexagonal silver iodide determines for the most part the
crystalline form of the silver iodo-bromide grains, and thus
forces the silver bromide to crystallize according to the hexa-
gonal system. But this is not in agreement with the facts,
as will be shown in later paragraphs.

Thiel 4 has determined by electrical measurements that,
at 25 C., silver iodide can form solid solutions with silver
bromide up to 30 molar per cent. This opens the question
as to whether all the indications of isomorphism mentioned
by Mitscherlich actually occur below these limits. As a
means of investigating this, very carefully purified silver
iodide was dissolved in ammonia (D. = 0.897) and the
solution allowed to evaporate at room temperature in an open
vessel. The crystalline precipitate obtained showed micro-

1 Bakhuis-Roozeboom, Heterogene Gleichgewichte, Vol. I, p. 128. See also Gmelin-
Kraut, Handbuch der anorganischen Chemie, Vol. II, p. 115.

2 It may, however, tend to occur initially, at high dispersity, in the unstable regular
form, as is the case with mercuric iodide precipitated in gelatine. (See Chapter V, p. 62.)
Being, in this form, isomorphous with silver bromide, co-precipitation with the bromide
would favor this condition. However, at ordinary temperatures this form would be ther-
modynamically unstable, and, on any application of heat, would tend to pass to the stable
form, thus setting up strain in the associated silver bromide crystals.

3 Renwick, F. F., (Discussion of Krohn's paper). Phot. J. 58: 195. 1918.

4 Thiel, A., 1. c.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

scopic tabular crystals of the regular system of the form {100}
and the combination (ill) with {100} , which became faintly
brownish in the light. Thus at room temperature silver
iodide separates from ammoniacal solution in a metastable
phase.

Mixtures of silver iodide and silver bromide which were
dissolved in concentrated ammonia crystallized in octahedra
of the regular system. For silver bromide alone the regular
forms {ill} and {100} and combinations are known. Hence
it may be said that regular metastable silver iodide can,
within certain limits, produce isomorphous mixtures with
regular stable silver bromide.

Since the quantity of silver iodide in emulsions falls below
these limits, silver iodide crystals of photographic emulsions
may very well belong not only to the same crystal system, but
also to the same crystal class as silver bromide. The classi-
fication of silver bromide would then be determinative for the
silver iodo-bromide crystals of emulsions.

EARLY CRYSTALLOGRAPHIC INVESTIGATIONS
OF SILVER BROMIDE

All crystallographic investigations of silver bromide
confirm the existence of regular silver bromide. The crystals
of natural silver bromide (bromyrite) show forms {ill} and

{100.}

Investigators have interpreted their observations differ-
ently, as shown in the following:

Roscoe and Schorlemmer 1 reported that silver bromide
crystallizes from aqueous solutions of hydrobromic acid and
mercuric nitrate in octahedra; Bellach 2 described "tetrahedral
forms" of silver bromide; and mention is made of hexagonal
forms of silver bromide, as follows :

Elsden 3 says: "The crystals have no influence upon polar-
ized light when lying flat, but they appear to be doubly
refractive when the rays pass obliquely through them, as
if they belong to the hexagonal system"; Baur 4 records that
amorphous silver bromide, dissolved in concentrated ammonia,
is precipitated in hexagonal tablets upon diluting the solution

1 Roscoe, H. E., and Schorlemmer, C., Treatise on chemistry, Vol. II., p. 472.

2 Bellach, V., 1. c.

3 Elsden, J. V., On the formation of a chemical compound of ammonia with silver
bromide. Phot. News 25: 174. 1881.

4 Baur, E., Silber in Abegg's Handbuch der anorganischen Chemie, Vol. II, Partll,
p. 684.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

with five parts of water; and Renwick 1 writes: "Silver bro-
mide could occur both in the cubic and in the hexagonal
crystalline systems." Krohn 2 also mentions hexagonal silver
bromide, but he probably means by that octahedral silver
bromide of the regular system which crystallizes in hexagons.
The only classification of silver bromide crystals is by
Groth, 3 who assigns silver bromide crystals to the hexakis-
octahedral class of the regular system. (See the concordance
of symmetry classes, p. 131.)

THE PREPARATION AND EXAMINATION OF THE
MICROSCOPE MOUNTS 4

The Materials Used. The following chemicals were used
in the precipitations:

Silver nitrate made by the Eastman Kodak Company;

Potassium bromide 'Analyzed,' from Kahlbaum;

Potassium bromide U. S. P. IX from Merck;

Ammonia (D = 0.897) purified double-distilled, from Powers- Weight-

man-Rosengarten Co., Philadelphia;

Distilled water from the laboratory.

These substances are recognized as the purest utilized for
practical purposes. A special chemical analysis was not
undertaken, because it was believed that possible impurities
have no effect on the classification of crystals. Indeed,
impurities are sometimes desirable, since their presence may
alter the free energy between the crystal surfaces and the
mother liquor, and new forms thus appear. Since the class
to which a crystal belongs is determined by the highest sym-
metry common to the different crystals, it is desirable to have
quite a large quantity of crystals of various forms. For this
reason a large number of crystals were precipitated in the
presence of various supplementary agents, which, however,
led to no different result than that already obtained by the
crystallization of the silver bromide from unadulterated
solutions.

Preparation of Crystals. A solution of potassium bromide
was added to a silver nitrate solution, the precipitated amor-
phous silver bromide was washed several times in a beaker
with boiling distilled water, and the surface liquor removed
by decantation. Finally the precipitate was washed on filter

1 Renwick, F. F., 1. c.

2 Krohn, F. W. T., 1. c.

3 Groth, P., Chemische Krystallographie, Vol. II., p. 200.

4 Higson has recently published an article (Photomicrography in photographic re-
search, Phot. J. 69:140. 1920 , in which a similar method of preparing microscope mounts
is described.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

paper and dried. The silver bromide in slight excess was
put in a bottle with ammonia, and, to obtain equilibrium,
was allowed to stand for a week (being frequently shaken)
at a temperature of 20 C. This stock solution was then
used in the preparation of the silver bromide crystals.

After being washed the crystals were placed on a slide,
dried, covered with Canada balsam and a cover-glass, and
heated in a drying oven at 70 C. for at least twelve hours.
If microscopic examination showed that new forms had
appeared, the precipitation was repeated and the new prep-
aration heated, often as high as 90 C.

As a result of this treatment the crystals are so massed
together that it is possible to observe and photograph only a
few isolated crystals. Better results are obtained if, after
washing, the crystals are suspended in a three per cent water
solution of gelatine and then spread on the slide.

In all, 192 slides of silver bromide alone and 73 of silver
bromide crystals precipitated in the presence of various
foreign substances were prepared. Each preparation con-
tained on the average about 2,000 crystals, so that altogether

more than 500,000 crystals were
prepared.

The best and most interesting
preparations were selected and care-
fully examined under the micro-
scope. The slide under observation
was moved back and forth in such
a way as to give the effect of moving
the objective in the manner shown
in Fig. 25. The distance between
FlG - 25 the two successive back and forth

Method of examining the movements was thus in every case
S a ^sTa S ,s f u e r t; much smaller than the diameter of
microscope. the microscope field.

Apparatus used.

a. In the examination. The instrument used in the
investigation was a large model Zeiss microscope with an
illuminating attachment and a Bausch and Lomb camera
mounted on a large horizontal standard. An arc lamp of 12
amperes and 110 volts, with a rheostat and an Abbe aplanatic
condenser (N. A. = 1 .40) served as the illuminant.

For the 800 diameters magnification a Zeiss 2 mm. oil-
immersion apochromat (N. A. = 1.4) and Zeiss compensating

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

ocular No. 4 were used: for magnification of 1350 diameters,
a Bausch and Lomb 1.9 mm. oil-immersion objective
(N. A. = 1.3) with Zeiss compensating ocular No. 6; for
magnifying 2,500 times, a Bausch and Lomb 1.9 mm. oil-
immersion objective (N. A. = 1.3) and Zeiss compensating
ocular No. 12. In every case a Wratten G filter was used,
arid the degree of magnification was ascertained by means of
a Bausch and Lomb stage micrometer ruled to 10/* and lOOAt.
b. In preparing the photomicrograph. Since silver bromide
is yellow, and it is desirable to show as many details as possible
in the photomicrograph, a yellow filter (Wratten G) was

FIG. 26

Photochemical decomposition on the octahedral surfaces
of silver bromide crystals. Magnification, 2500 diameters

used. There is an additional advantage in this, as the silver
bromide decomposes very rapidly in the strong illumination
in the microscope, and shutting out the blue and violet rays
retards this decomposition, although it does not entirely
prevent it.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

Figs. 26 and 27, which are photomicrographs of two
different crystals taken after less than two minutes' exposure
to unfiltered light, show how rapidly decomposition proceeds
in unfiltered light. This photochemical decomposition does
not take place simultaneously over the entire surface of the
crystal, but begins in isolated points from which it spreads

FIG. 27

Photochemical decomposition on the octahedral surfaces
of silver bromide crystals. Magnification, 2500 diameters

over the whole crystal until the crystal disappears (as has
been observed by Lorenz 1 ). In this respect, therefore, the
direct photochemical decomposition of silver bromide crystals
seems to proceed in a manner comparable to the formation
of the developed image as demonstrated by Scheffer 2 and
Hodgson. 3

1 Lorenz, R., Kolloidchemie und Photographic. Koll. Zeits. 22: 103. 1918.

2 Scheffer, W., Microscopical researches on the size and distribution of the plate grains.
Brit. J. Phot. 54: 116. 1907.

3 Hodgson, M. B., The physical characteristics of the elementary grains of a photo-
graphic plate. J. Frankl. Inst. 184: 705. 1917.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

The presence or absence of colloids such as gelatine does
not affect the progress of this photochemical decomposition,
and it will be seen that most of the photomicrographs contain
dark spots caused by this decomposition.

The use of the Wratten G filter made it necessary to use
yellow-sensitive plates; and as it was desirable to obtain
negatives which would intensify delicate details, Wratten
panchromatic plates (M-plates), were used and developed
for 2^ minutes at a temperature of 18-20 C., in the following
developer :

Metol 2.2 gr.

Hydroquinone 8.8 gr.

Sodium sulphite 4.8 gr.

Sodium carbonate 4.8 gr.

Potassium bromide 0.88 gr.

Water to lOOO.OOgr.

(It was demonstrated in the physical section of this labora-
tory that it is possible under these conditions to obtain a
degree of development where 7 = 2.4 without danger of
appreciable fog.)

SILVER BROMIDE CRYSTALS

Of the various known solvents of silver bromide, such as
hydrobromic acid, potassium bromide, mercuric nitrate,
ammonia, etc., ammonia has been found the most convenient.
Not only can the various crystal forms of silver bromide in
photographic emulsions be accurately identified when crystal-
lized from ammoniacal solutions, but also a large quantity of
other forms which are valuable for crystal determinations.

METHODS OF PREPARATION

There are three different methods for crystallizing silver
bromide from ammoniacal solution: (a) by diluting the
solution with water; (b) by evaporation of the ammonia;
(c) by cooling the solution.

The first method was used by Elsden in his above-men-
tioned work on silver bromide; the second was utilized by
Reinders 1 for obtaining crystallized photo-chloride; and the
third was undertaken at our suggestion by Mr. Schneider.

None of these methods gave uniform results, crystals of
various forms being always obtained. No rule could be
established for the appearance of any one crystal form, for
everywhere were all possible grades and transitions. This
was probably due to the impossibility of obtaining identical

1 Reinders, W., 1. c.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

conditions throughout the solution during the process of
crystallization. In the more concentrated solutions crystals
of silver bromide-ammonia compounds, which frequently
produced singularly beautiful pseudomorphs, also appeared.

The methods of preparation will be taken up in order, and
the silver bromide-ammonia pseudomorphs and means of
detecting them will be treated in a later section.

(a) Dilution of ammoniacal solution. The dilution was
made as rapidly as possible and at room temperature (about
20 C.). The crystals separated throughout the solution
and fell to the bottom. In reflected light they showed a
lively play of colors, due to the interference phenomena of
thin layers. (The majority of these crystals were laminate.)

For the various conditions under which the precipitation
was made and the results obtained, see the following table:

Original Solution

% NH 3

- .L/11UUOI1

%, AgBr with H 2 O

29.4

0.4 1:2

29.4

0.4

:2 1 A

29.4

0.4

29.4

0.4

29.4

0.4

:10

23.5

0.4

23.5

0.4

23.5

0.4

:10

20.5

0.34 1:5

20.5

0.34 1:10

17.6

0.26 1:10

14.7

0.25 1:10

Numbers of

Special Well-defined Preparations
P'orms Dimensions Kxamined

d, s, p, 1 1,2,3

d,p,

1, 2, 3

2 3

2,3

2, 3

s = skeleton

1 == needle

2 = plate

3 = ordinary

crystal

Crystal
Faces

O, C

o
o
o
o
o
o
o
o
o
o
o

= octahedron
C = cube

1 = lamelliform

p = pseudomorph

d = dendrite

As is evident from the above, octahedra predominate.
Only the first preparation contained a few cubes, and they
were very irregular. The first preparation also showed the
greatest variety of crystal forms, which is readily compre-
hensible, since it was impossible to obtain identical crystal-
lization conditions throughout the solution because of
concentration changes due to the evaporation of the ammonia.
In order to have the conditions as nearly uniform as possible,
however, the crystallization was carried out in closed vessels.

The dilutions indicated in the table are about the limits
within which one can obtain well-developed crystals.

(b) Evaporation of ammonia. Eleven different concentra-
tions of ammonia were prepared, each being diluted with ten

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

per cent more water than the preceding one. They were
treated with a slight excess of silver bromide and left in closed
vessels for a week. When equilibrium was established, several
drops of the solution were put on a slide, and the ammonia
allowed to evaporate. Then the crystals were heated. In
this way thirty-five slides were prepared. The highest con-
centrations gave pseudomorphs in addition to octahedra in
the form of the usual crystals and plates. Also, there were
the same dendritic and lamellate-formations as in the diluted
ammoniacal solution.

(c) Cooling the ammoniacal solution. A given quantity
of ammoniacal silver bromide solution was put in a bottle
and enough boiling distilled water added to fill the bottle.
It was then sealed so that no air would be in contact with the
liquid. The cooling was accomplished by means of a stream
of ice-cold water, and the rate of cooling was regulated by
using bottles of different volumes. Too rapid cooling produced
too small crystals, and too slow cooling gave only well-devel-
oped crystals. Very good results were obtained by using a
50 cc. bottle and a 10 cc. and a 14 cc. pipette. Crystals
obtained under these conditions showed the greatest variability
of forms, i. e., skeletons, lamellate-formations on the crystal
surfaces, etc.

The various <3j|nditions under which the crystallization
was carried out anwven in the table below :

Solution Degree of
% NH 3 % AgBr Cooling (C.)

Rapidity Crystal I
of Cooling Faces

Spt
Foi

95-

96-

18 0.01

95-

a, b, c r, c, o

76 0.02(-)

95-

a, b c, o

94-

35 0.02( + )

95-

a, b, c o, p

96-

94 . 03

95-

a, b, c o, p

95-

88 0.06

95 -

a, b, c o

a =

vessel of 50 cc.

octahedra

b =

vessel of 10 cc.

c =

cubes

c =

slow cooling at

r =

rhombic dodecahedra

room temperature

P =

pentagonal dodecahedra

in a 50 cc. bottl

skeletons

i =

lamellate-formations

Forms Dimensions

s, 1

Numbers
of Prepar-
ations

21
12

1, 2, 3
1, 2,3

1,2,3
2, 3

1 = needles

2 = plates

3 = ordinary crystals

SILVER BROMIDE-AMMONIA COMPLEXES

According to Bodlander and Fittig, 1 silver bromide in
ammoniacal solution is present as the complex compound
Ag(NH 3 ) 2 Br / , in which Ag(NH 3 ) 2 is the complex cation.

1 Bodlander, G., and Fittig, R., Das Verhalten von Molekularverbindungen bei der
Auflosung. Zeits. physik. Chem. 39: 597. 1902.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

These silver bromide-ammonia compounds separate from
solution in a solid phase, the following compounds being
produced:* Ag 2 (NH 3 ) 3 Br 2 ; AgNH 3 Br; Ag(NH 3 ) 3 Br. There-
fore, in the crystallization of silver bromide from ammoniacal
solutions, it is not impossible to obtain a complex compound
which may lead to entirely erroneous results. But Bodlander
has shown that these complex compounds are very unstable,
and in contact with air or water dissociate into their constit-
uents, thus forming pseudomorphic forms of silver bromide.
The following characteristics were tested as to their
reliability for distinguishing the silver bromide from the
ammonia compound :

AgBr Agn(NHz)mBm

1. Yellow. 1. Colorless.

2. Sensitive to light. 2. Insensitive to light.

3. Unchanged in ammonia-free 3. Becomes turbid in ammonia-free
water. water.

4. Microscopically unchanged after 4. Becomes opaque microscopically
being heated to 70 C. after being heated to 70 C.

5. Shows simple refraction between 5. Shows double refraction between
crossed nicols. crossed nicols.

1. The yellow color of silver bromide crystals is so intense
that it can be perceived even in crystals of +1^ in diameter,
and it is, therefore, a useful means of distinguishing silver
bromide from the colorless ammonia-compounds.

2. The complex silver bromide-ammonia compounds are
unchanged after being in the sunshine in the presence of
ammonia for a day. As soon as the ammonia-pressure becomes
too low, however, a photochemical decomposition of the
disengaged silver bromide sets in. This gives the impression
that the complex compound is sensitive to light, as the light-
decomposed ammonia compound is very similar to photo-
chemically decomposed silver bromide. Therefore, this test
is not recommended for practical purposes.

3 and 4. These tests, the effect of ammonia-free water
and the effect of heat, may be combined. First, the crystals
are well washed with distilled water; if they remain unchanged
during the w r ashing (the ammonia-compound becomes turbid),
they are heated to 70 C., or even higher, for at least twelve
hours. In this heating the pseudomorphic crystals become
opaque, due to loss of ammonia, and may therefore be readily
detected, as the silver bromide crystals are unaffected.

5. The double refraction of light by the various silver
bromide-ammonia compounds has not been definitely estab-

1 Ephraim, F., 1. c.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

lished. Elsden 1 examined only one of the complexes which
one is not known and found that it belonged to the tetragonal
system. Hence too little is known regarding this test to
consider it reliable.

SUMMARY OF OBSERVATIONS

Crystal forms observed. In the various preparations
the forms observed were as follows: {ill 1 /, {HO/, {100} ,
(210), of which {ill} and {lOOJ appeared as single crystals,
and of which the following combinations were found: /I 111 +
(100); {111} + {110}; {111} + {210}.

The rhombic dodecahedra, which were seldom seen,
occurred only as small strips on the edges of the octahedra.

Special forms observed. Among the forms observed,
needles, plates, "dark" grains and pentagonal dodecahedra
merit special mention.

Silver bromide needles. In a microscopic study there was
found a developed and fixed photographic plate which under
the microscope showed very remarkable developed grains in
the form of needles. Figs. 28 and 29 show two such grains,
magnified 2,500 times, which measured respectively 21/* and
12 /* in length.

FIG. 28

Needle-shaped grain occurring
in a silver iodo-bromide emul-
sion; enlarged 2500 diameters.

FIG. 29

Needle-shaped grain occurring
in a silver iodo-bromide emul-
sion; enlarged 2500 diameters.

The usual developed grains of photographic plates are
somewhat larger than the original silver iodo-bromide crystals,
and generally have a different form. (A photomicrograph,

1 Elsden, J. V., I.e.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

published by M. B. Hodgson, 1 shows developed grains which
retain the form of the original crystals only in those cases
where the crystals were very large. The change in form is so
slight in proportion to the size of the grains that the form of
the crystal is but little affected.)

When there is an accumulation of silver iodo-bromide
crystals in the emulsion, or when the distance between the
crystals is very small, the developed grains coalesce, which

FIG. 30

Crystalline needles in a silver iodo-bromide emulsion,
enlarged 2500 diameters

makes them appear very much larger than the original crystals.
The needles shown in Figs. 28 and 29 may be linear aggre-
gations of silver iodo-bromide crystals, or may be developed
from small needles. *

A microscopic examination of the original emulsion showed
various needle-shaped crystals, some of which were photo-
graphed. (Figs. 30 and 31.) However, needles were found

Hodgson, M. B., 1. c.

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

not only in this emulsion but also in a large number of com-
merciaf plates, though in the latter the needles were not of
such unusual size. 1 Needles were also found in emulsions
containing silver bromide alone. Between crossed nicols
these needles show exactly the same optical behavior as the
other grains. This observation is not new, as Luppo-Cramer 2
published a photomicrograph of an emulsion needle in 1907. 3

FIG. 31

Crystalline needles in a silver iodo-bromide emulsion,
enlarged 2500 diameters

To make a more accurate determination of the crystalline
form of these needles, various crystalline precipitates were
examined microscopically and needles were found which
showed not only octahedral faces (Fig. 32) but cubical faces
as well. 4 Where there were cubic faces, the combination

1 The lengths of the needles in the different emulsions varied from 3 /A to 25 U .

2 Luppo-Cramer, Photographische Probleme, p. 49.

3 Wallace also refers to the presence of "spicular crystals" in certain emulsions.

4 There was abundant needle-formation in a preparation which contained 0.05%
aluminium bromide, and good needle-formation in one to which 0.5% strontium bromide
had been added.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

111) + {100} frequently appeared (Fig. 33). Needles
were also obtained by subliming silver bromide, but their
faces could not be determined on account of the rounded
corners. Octahedral needles were also found in one of the
above-mentioned preparations of silver bromide from potas-
sium bromide solution. The needles are, therefore, not unique

FIGS. 32 and 33

An octahedral and a cubic needle of silver bromide, precipitated

from ammonical solution. Magnification of 2500 and

800 diameters, respectively.

modifications of silver bromide, but must be regarded as the
result of a special development of the silver bromide crystals.

Silver bromide plates. It was to be expected that, in addi-
tion to the ordinary silver bromide crystals, developed more
or less in accordance with the three co-ordinates, and the
needles, which show a marked growth in only one direction,
plates, or crystals which develop in two directions, would
also appear. This was true in most cases where the silver
bromide was crystallized from ammoniacal potassium bromide
solution. There were relatively few instances where the
crystals developed well in all three directions.

That most of the crystals are plates may be shown as
follows :

1. By focusing down with a 2 mm. Zeiss oil-immersion
apochromatic objective (N. A. = 1.4) and compensating
ocular No. 12 with the diaphragm of the condenser
(N. A. = 1.4) wide open. The small depth of focus of this
system made it possible to obtain a very sharp focus on the
thin surface of the object. The crystals well developed in
three directions were distinguished in that they could be
sharply seen in more than one focus. Plates, on the contrary,
appeared suddenly sharp and distinct, and vanished almost
immediately on changing the focus;

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

2. When the crystals are in Canada balsam, one may turn
them over and look down at right angles to the edges of the
crystals, most of which are thinner than one ^;

3. The lively play of colors in the silver bromide crystals
in reflected light and the pale colors in transmitted light, in
which gray of the first order in Newton's color series was often
identified, is explainable only by the interference phenomena
of thin layers.

How thin these crystals may be is shown by the fact that,
according to the well-known formula,

"air _ XAgBr (where n = refractive index,
"AgBr Tair~ ^ = wave-length of light),

the wave-length in silver bromide is less than half as long as
the wave-length of the corresponding colors in the air. Now,
if it is remembered that air layers 0.3^ thick can produce
marked interference colors, it may be readily understood
that silver bromide plates 0.13^ thick can produce the same
effect. (These facts first directed our attention to plate-
formation in photographic emulsions.) Koch and du Prel 1
stated that whether the silver bromide crystals in photo-
graphic emulsions are plate-shaped or tetrahedral is yet to
be determined.

After this form had been verified, Krohn's article 2 was
published (in 1918). In this article Krohn stated that previous
to 1901 he had been able to observe emulsion grains from all
directions because of their Brownian movement, and had
come to the following conclusion : "The crystals are lammellar
and almost hexagonal and are probably imperfectly developed
octahedra such as one gets with chrome alum crystallized in
a shallow dish;"

4. Measuring the total volume of silver bromide in a
photographic emulsion, determining the number of grains in
one cc. of the emulsion, and calculating the mean diameter
of the grains gives the data for computing the average thick-
ness of the silver bromide crystals in the emulsion. To do
this, a given quantity of emulsion was spread on a definite
area of film. Pieces were then taken from five different
regions of the film and by means of a microtome three cross
sections of each of 5^ (dr 0.06) in thickness, were prepared
and used for the microscope preparations. This process was
repeated for five different emulsions, each containing a dif-

1 Koch, P. P., and du Prel, G., Ueber das Korn der photographischen Platte und eine
Methode zu einer Untersuchung. Physik. Zeits. 17: 536. 1916.

2 Krohn, F. W. T. t 1. c.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

ferent quantity of silver bromide, so that there were 75 prep-
arations in all. One hundred and fifty photomicrographs
(magnification 1176.4 diameters) were taken, using a para-
boloid condenser. The use of dark field illumination makes
it possible to define each grain sharply, so that the number
of grains in the emulsion can be counted. The number in
these emulsions varied from 16 to 43 x 10 10 per cc. One
emulsion contained 16 x 10 10 dr 0. 5 x 10 f> grains. The diameter
of the grains averages about 1 . 5/^. Assuming that all the
grains are truncated tetrahedra, the average thickness may
be reckoned as 1/14 of the diameter. In other words, most
of the grains in photographic emulsions are plates.

The dark grains. As shown in Fig. 22, these grains show
very intense light between crossed nicols. This may be due
either to a thicker layer of silver iodo-bromide or to a greater
reflection of light. Also, there is a layer-by-layer variation
in the definition of these dark grains as the focus of the micro-
scope is changed. All this indicates that these dark grains
are crystals more or less well developed in all three dimensions,
and that the effect of dark color is probably produced by the
strong light-reflections in the crystal, due to the unusually
high refractive index, as has been observed in crystals of
thallium salts and of gold-barium acetate.

The pentagonal dodecahedra will be discussed later. (See
page 95.)

FACES OBSERVED

The following faces were observed in the different crystal
forms :

Usual crystals \ 1 1 1 \

Combinations <{lllf> +

UH}- +

Plates \\\\\

Combinations "{Ill -(- \ 100

+ ^210
UOO

Needles ml

Combinations \\\\\ + j 100

Etching.

In order to study the etch-figures which appeared on the
larger octahedral faces after treatment with ammonia, the
crystalline precipitate obtained by cooling the ammoniacal
solution was kept in the closed bottle at room temperature
without removing the ammonia. The solubility increased
with the higher temperature and very beautiful etch-figures,
in the form of triangles with rounded corners, resulted. It
was thought that these figures would aid in determining
symmetry-ratios, but in this case they gave no criterion for
classification.

CHAPTER VII

THE CLASSIFICATION OF SILVER BROMIDE CRYSTALS

From the crystal forms observed, it would seem that the
symmetry-relations of silver bromide crystals are less than
is now assumed. An accurate method of investigating this
relation by the determination of different physical constants
in various directions in the silver bromide crystals, which
would be adapted to the extremely small dimensions of the
silver bromide crystals, is not yet perfected.

The pentagonal dodecahedra obtained by cooling the
ammoniacal solution are so important for the classification

of silver bromide that they merit
closer study. These forms oc-
curred only in combination with
the octahedra, as shown in Figs.
34 and 35. This combination is
well known in SnI 4 , FeS 2 , CoAsS,
Cs 2 Al 2 .(SO 4 ) 4 .24H 2 O and (NH-
(CH 3 ) 3 ) 2 .A1 2 (SO 4 ) 4 .24H 2 O. Un-
fortunately, the combinations of
pentagonal dodecahedra and oc-
tahedra are not very clear in the
accompanying reproductions, in
which the combinations resemble
quintettes. But a careful study
of the original preparations leaves
no doubt that these crystals,
especially those in Fig. 35 d and
f, are combinations of true pentagonal dodecahedra with
octahedra.

The pentagonal dodecahedra are distinguished from the
usual silver bromide crystals in that there is a strong tendency
to plate-formation, while the faces ABF, BGF, BCG, CHG,
CDH, DJH, DEJ, EKJ, EAK and AFK show a much greater
rapidity of growth in the direction of their normal than the
other faces. The resulting lack of development of these latter
faces produces a five-sided pyramid which in Fig. 35a is lying
with one of the triangular faces on the slide.

In Fig. 35a and b are pictured two combinations {ill} "i"
[210} in which the upper and lower pyramids are about the
same size. Since, as Fig. 34 shows, the corners of the two
pyramids do not coincide, but are about 36 apart and are

FIG. 34

Diagram showing fully de-
veloped combination of octa-
hedron and pentagonal dode-
cahedron-.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

FIG. 35

Combinations of octahedra with pentagonal dodecahedra of silver
bromide. Enlarged 800 diameters

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

flattened, the resulting truncation produces a decagon in
projection.

The occurrence of pentagonal dodecahedra shows that
silver bromide crystals belong only to the tetrahedral-
pentagonal-dodecahedral or the dyakisdodecahedral class. 1
However, there is no evidence of the hemihedrism of silver
bromide crystals. What have been described by many
authors as tetrahedra of silver bromide in photographic
emulsions are triangular octahedral plates (see below). Inas-
much, therefore, as this hemihedrism is not proven, we must
place silver bromide in the dyakisdodecahedral class.

THE CLASSIFICATION OF SILVER CHLORIDE
AND SILVER IODIDE

It has been mentioned that silver iodide in limited quanti-
ties can form homogeneous mixed crystals with silver bromide.
This indicates that the metastable silver iodide, crystallized
in regular form from solutions at ordinary temperatures, may
be placed in the same class as silver bromide.

Groth 2 placed silver chloride in the hexakisoctahedral
class, in accordance with his observations of the faces of
natural crystals. But these crystals may be as logically
placed in the dyakisdodecahedral class. Furthermore, Thiel 3
has demonstrated, by the continuous change of potential of
silver chloride-silver bromide mixtures, that silver chloride
forms homogeneous mixtures with silver bromide in all pro-
portions. So in all probability silver chloride belongs to the
same crystal class as silver bromide. 4

THE POSSIBILITY OF MODIFICATIONS OF SILVER BROMIDE

Even though the reasons for assuming the existence at
ordinary temperatures of a stable or metastable hexagonal
silver bromide in addition to the regular silver bromide are
insufficient, the fact that silver bromide crystallizes in the
dyakisdodecahedral class indicates that it is possible for it to
crystallize in right and left pentagonal dodecahedra, so that a
kind of enantiomorphism, as in quartz, sodium chlorate, etc.,
may exist. This is not to be interpreted in a purely geomet-
rical sense, i. e., that optical differences, such as the opposite

1 See concordance, p. 131.

2 Groth, P., 1. c., Vol. I., p. 200.

3 Thiel, A., 1. c.

4 Crystals were also identified from silver cyanide and silver sulphocyanide as octahedra
of the regular system. These investigations, however, have not been carried further.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

rotation of circularly polarized crystals, can occur here, but
rather, as Marbach 1 found in the case of pyrite and cobaltite,
that these crystals belong in the thermo-electric electromotive
series partly beyond positive antimony and partly beyond
negative bismuth: so that a pyrite crystal of the first kind
which is united with a similar crystal of the second kind
produces a stronger thermal current than antimony with
bismuth. In this sense it is possible for silver bromide to
form two modifications.

1 Marbach, cited by Groth in Physikalische Krystallographie, p. 193. (Third Edition.)

CHAPTER VIII

The Silver Bromide Crystals of
Photographic Emulsions

As is well known, in photographic emulsions the silver
bromide is precipitated in the presence of a protective colloid
(gelatine) and therefore one has to do with a case of colloidal
precipitation. Since there are distinct silver bromide crystals
in the melted emulsion and in plates, a transformation in the
sense of colloidal silver bromide to crystalline silver bromide
must have taken place.

This transformation is, however, simply a special case
of the thermodynamic principle according to which the system
tends to reduce its surface-energy, 1 and which (as has been
indicated in Chapter V), is the starting point of the Gibbs-
Curie-Wulff law; for, as stated, this conversion takes place
more quickly in the presence of a solvent of silver bromide,
such as potassium bromide, ammonium hydroxide, etc.
Further, the rapidity of the transformation increases with
rise in temperature. Whether the colloidal silver bromide
consists of an aggregation of unusually small crystals, or of
purely amorphous silver bromide i. e., in the molecular
state makes no difference, since the surface-energy of colloids
is very large in proportion to the surface-energy of the single
silver bromide crystals. It is probable therefore that the
velocity of solution of colloidal silver bromide is greater
that that of the crystalline silver bromide. Thus a trans-
formation occurs in the solvent by which the crystalline
silver bromide is formed while the colloidal form disappears.
This is essentially the same as the principle discussed above
to indicate the relation between the size of the crystal and
the decreased solubility, which in photographic literature is
known as "Ostwald Ripening." 2

Therefore in photographic emulsions one finds, certainly
for a number of the grains if not for all, the same conditions
as in ordinary processes of crystallization; and this does not
prevent our applying the Gibbs-Curie-Wulff law to at least
the larger silver bromide grains which have definite crystalline
structure.

1 In general, small crystals dissolve more quickly than larger ones because, in the
transformation of many smaller crystals into one larger, the surface energy is diminished.

2 Ostwald, W., 1. c.

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

The question of the influence of gelatine on crystallization
processes, already discussed in part, will be dealt with in
another relation in a later chapter.

FORMS OF SILVER BROMIDE CRYSTALS IN EMULSIONS

The forms of the silver bromide crystals in photographic
emulsions are very varied, though all belong to the same
crystallographic system and the same class. In 122 different
emulsions which were examined at a magnification of 2,500
diameters, only octahedra could be positively identified.
Higson (1. c.) says he found cubic crystals in silver bromide
emulsions. In the work described here this observation could
not be confirmed, and the presence of cubes and of combina-
tions of cubes with octahedra or other forms could never be
definitely determined. It is possible that Higson's cubic
crystals were obtained from precipitations from dilute am-
moniacal solutions.

The octahedra appear:

a. In crystals more or less well developed in three directions
dark grains;

b. In plates, which are most markedly developed in two
directions; and

c. In needles, which are developed principally in one
dimension.

(Those grain-aggregations and groups which come only
within the range of probability will be disregarded.)

Each of these forms exhibited variations, for which the
plates are especially noticeable, though the needles and
ordinary crystals showed similar differences to a less marked
degree. We will, therefore, limit our discussion here to the
plates, since they are in the majority in photographic
emulsions.

Plate-forms are so numerous that it is impossible to describe
them all. Here we shall discuss only definite types as observed
in the emulsions, remembering that the other forms represent
all possible transitions between the types described, and that
all are variations of one and essentially the same crystal form.

Fig. 36 represents an octahedron (ABCDEF) which is
lying with one face on the paper. In parallel projection a
hexagon is obtained. Now, if the capillary constant between
the mother liquor and the crystal is different for different
faces, 1 so that, for example, Kj of the face AED and K 2 of

1 Cf. Chapter V.

100

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS^

the face BCF are much smaller than K 3 of the face ACD,
K 4 of the face CDF, K 5 of the face DFE, K 6 of the face FEE,
K 7 of the face EBA, and K 8 of the face BAG, then the condi-
tions necessary for the formation of a plate or tablet are
established. 2

An equilateral regular hexagon is produced if

Ki = K 2 , K 3 - K 4 - K 5 = Ke = K 7 = K 8 ,

K, < K 8 .

These plates are thinner the greater the ratio K 3 /Ki. For
one emulsion the mean value of K 3 /Ki has been determined
as 14.

On the other hand, if

1C = K 2 , K 3 = K 5 - K 7 , K 4 - K 6 = K 8 , and

K! < K 3 < K 4 < 2K 3 ,

a scalene but otherwise regular hexagon will result, as shown
in Fig. 37. Here AED is the upper and BCF the lower
octahedral face of the tablet.

FIG. 36

Diagram showing the
formation of a tabloid
equilateral hexagon from
an octahedron.

FIG. 37

Diagram showing the
formation of a tabloid un-
equilateral hexagon from
an octahedron.

If the following conditions obtain during development,
K 4 ^ 2K 3 , a triangular plate will be formed, as shown
in Fig. 38. Since the growth of th.-ee of the side-faces is
two or more times as rapid as that of the other three, the
latter are suppressed.

Another scalene but otherwise regular hexagon which is
often to be seen in emulsions is formed when

K! - K 2 , K 3 = K 4 = K 6 = K 7> K 6 = K 8 , and
K! < K 3 < K 5 < 2K 3 .

2 |For the sake of simplicity, Ki is*assumed as equal to K2. Small differences between
Ci and K2 obviously do not affect this and the following results. It is also assumed that

Ki and K2 obviously

K does not change during growth.

101

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

Irregular forms appear when K 3 , K 4 , K 5 , K G , K 7 , and K 8
are more or less unequal.

One very remarkable form occurring in emulsions is the
trapezoidal lamina (Fig. 39) which appears in the most varied
modifications. The conditions for its formation are:
Kx = K 2 , K 3 - K 5 = K 7 =K 8 , K 4 - K 6 ,
K! < K 3 and K 4 ^ 2K 3 .

FIG. 38

Diagram showing the forma-
tion of a tabloid equilateral tri-
angle from an octahedron.

Diagram showing the forma-
tionof a tabloid trapezium
from an octahedron.

Modifications result when :

K 5 = K 8 , K 5 < K 4 ; K 8 < K 4 , K 3 = K 7 < K 4 , or if
under the same conditions:

K 3 - K 7 , K 3 < K 4 , K 7 < K 4 , K 5 - K 8 < K 4 , etc.

A pentagon (Fig. 40) with angles of 60 and 120 and
therefore of a form quite different from the faces of the pentag-
onal dodecahedron appears when:

IVl = X\-2 K-3 == K-4 == KG .K-- = K-8)

K: < K 3 and K 5 ^ 2K 3 .

Modifications are obtained when K 3 , K 4 , K 6 , K 7 , and K 8
are more or less unequal, but always remain smaller than
K 5 .

A rhombus (Fig. 41) with angles of 60 and 120 appears
when two parallel side faces are suppressed on account of too
great velocity of development, when therefore:

K! = K 2 , K 3 = K 4 - K 6 - K 7 , K 6 = K 8>

K, < K 3 and K 6 ^ 2K 3 .

102

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

Modifications may appear when K 3 , K 4 , K 6 , and K 7 are more
or less varied, but always remain smaller than K 5 .

FIG. 40

Diagram showing the forma-
tion of a tabloid pentagon from
an octahedron.

FIG. 41

Diagram showing the forma-
tion of a tabloid rhombus from
an octahedron.

A different development phenomenon is responsible for
the needle formations, which develop when the free energy
of the base is greater than that of the prism-faces, so that the
crystal grows most rapidly in the direction of the base. The
multiplicity of plate-forms is induced by the greater velocity

of growth of one side-face between
two other side-faces having an inferior
development. The formation of plate-
shaped crystals by convection-cur-
rents in the mother liquor is not pos-
sible in silver bromide emulsions, since
the grains are suspended and in slight
Brownian movement, which reduces
the convection-currents, on the one
hand, and these are further reduced
by the viscosity of the mother liquor
on the other. An octahedral needle
results when two contiguous side-faces
develop more rapidly than the other
side-faces. (Fig. 42.) However rapid
this growth may be, these latter faces
can never disappear. But if the capil-
larity constant of one of these rapidly
growing faces decreases during growth,
the development of the entire needle in one direction is
interfered with.

FIG. 42

Diagram showing the
formation of a needle from
an octahedron.

103

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

Among needles, one must differentiate between well-
developed needles of approximately equal thickness and
width, and plate-like needles. The first type appears as
octahedra when Ki = K 2 = K 5 = K 8 . The second appears
as octahedra when Ki < K 5 . These distinctions could not be
made in photographic emulsions, however, because of the
limits of microscopic resolving power.

The conditions for the formation of a needle are:
Id = K 2 , K 3 - K 4 - K 6 = K 7 , K 5 = K 8 , and K 3 < K 5 .

Now it is possible that needles do not develop with equal
rapidity in two opposite directions e. g., K 3 and K 4 may
be very much greater than KG and K 7 . But this does not
affect the results in the least.

Only slight indications of tabular octahedral twinning,
such as was frequently observed in ammoniacal silver bromide
crystal-formation (Fig. 43), were to be seen in completed
photographic emulsions. 1

THE RELATION BETWEEN THE LIGHT-SENSITIVITY AND THE

SURFACE-ENERGY OF SILVER BROMIDE CRYSTALS

IN EMULSIONS

Only very general statements can be made on this subject,
as very little is known about it. If a change in light-sensi-
tivity is produced by an increase or decrease of the surface-
energy of silver bromide emulsion crystals, then in general
it may be assumed that larger emulsion grains have a different
sensitivity from smaller. In practice, indeed, it often seems
that the coarse-grained emulsions are more sensitive than
the fine-grained. But that this is not always the case is
shown by the following:

An experimental emulsion was prepared, the grains of
which measured up to S/JL in diameter and which had an H.
and D. speed of only 38. In comparison with this emulsion
a "Royal Standard Lightning Plate" from Kodak Ltd. was
tested, the grains of which averaged up to 2.8^ in diameter,
and of which the H. and D. speed was 728. Thus it appears
that emulsions containing grains of approximately one-third
the linear dimensions are more than nineteen times as sensitive.
This is true also of individual grains in the same emulsion.
After a quantitative investigation of this question, Koch and
du Prel 2 concluded that it is not possible to formulate a definite
relation between grain-size and sensitivity with the information
at present available, but that it is certain that the largest
grains in an emulsion are by no means the most sensitive.

1 See Chapter V, p. 68.

2 Koch, P. P., and du Prel, G., 1. c.

104

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

FIG. 43

Twin forms on octahedral faces of silver bromide. A-E, magnified
800x; F, 2500x

105

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

Lehmann and Knoche 1 also demonstrated that ripening
increases the sensitivity of silver bromide-albumen emulsions
without noticeably altering the size of the grain.

Now, if one considers the ripening process as a purely
thermodynamic one, entirely independent of light-sensitivity, 2
(i. e., as a process in which the surface-energy of the grains
tends to reach a minimum), Liippo-Cramer's remarks 3 to
the effect that the sensitivity of colloid emulsions may be
increased to but a relatively slight degree, and that the initial
stage of emulsion-making (the conditions under which the
silver bromide is precipitated), is of much greater importance
than the later steps of the process, become more intelligible.

In other words, the increase in sensitivity is determined
not so much by the ripening process as by the conditions
under which the silver bromide precipitation was accomplished.
(See particularly Chapter II, p. 27.)

Luther's statement, made independently, that the laws of
thermodynamic equilibrium are not applicable to photo-
chemical equilibrium, and that therefore there is no definite
connection between thermodynamics and photochemistry, 4
is in entire agreement with this view.

Thus we come to the conclusion that the high sensitivity
of some photographic emulsions may be somewhat influenced
by variations in the surface-energy, but can certainly not be
entirely determined thereby. In agreement with Liippo-
Cramer's statement above, we must very probably seek these
conditions in the crystal structure of the silver bromide on
the surface of the grains. 5

From the crystallographic standpoint only one method of
investigation in this direction is possible without methods of
X-ray crystal analysis. That method is to determine the
directions of most rapid growth relative to the characteristics
of the crystal lattice, an observation which is especially
significant in the case of the silver bromide octahedra. And
since these octahedra occur in three forms, as ordinary
crystals, plates, and needles it is possible to investigate the
directions of greatest growth not only in volume, but even on
the octahedral faces.

1 Lehmann, E., and Knoche, P., Plate-grain and albumen emulsions. B. J. Phot. 61:
759. 1914.

2 The H. and D. interpretation of light sensitivity is intended here.

3 Luppo-Cramer, Phot. Prob., p. 35.

4 The formation of a thermodynamically more stable form of silver bromide does not
indicate that such a form is less sensitive to light, as R. Abegg (Die Silberkeimtheone des
latenten Bildes. Arch. wiss. Phot. 1: 18. 1899) and V. Bellach (1. c., p. 37) assume.

s W. D. Bancroft (The photographic plate, 1. c.) and W. Reinders (1. c.) attribute the
high sensitivity to the presence of gelatine. But it must be borne in mind that emulsions
of unusually varied sensitivities may be prepared from the same gelatine which may be
explained by differences in structure.

106

CHAPTER IX

THE DIRECTIONS OF MOST RAPID GROWTH IN SILVER BROMIDE

CRYSTALS AND THE OCCURRENCE OF

ANOMALOUS FORMS

1. In the ordinary octahedra. According to Lehmann, 1
the directions of most rapid growth are along the line of
greatest acumination. Since in octahedra these directions
coincide with the three principal crystallographic axes of the
crystal, the former are indicated by the latter. Lehmann
has also shown that skeletons grow in these directions, and
silver bromide skeletons which were formed exactly like the
framework of the three co-ordinate axes were repeatedly
found.

2. In octahedral plates. To determine the direction of
most rapid development here, either of two methods may be
followed :

a. By observing the direction in which the skeleton
develops ;

b. By very rapid but suddenly disturbed crystallization.
In addition to the skeletons which coincide with the

crystal axes, there is in plates one form of skeleton the rays
of which coincide with the edges of vicinal faces, and which
may be termed surface skeletons. 2 They must not be regarded
as special needle-combinations, for they always appear in
uniform and entirely regular star-shaped figures from which
three definite rays emanate at angles of usually 120. These
skeletons often form the framework of plates, when they have
three, six, nine, or twelve rays. Four rays usually appear
only in trapezoidal plates, and five in octahedral pentagonal
plates. 3

Structural anomalies occurred so frequently that they
merit special attention. Crystal-development is possible
only when the mother-liquor is slightly supersaturated. The
surplus quantity of the crystalline substance in solution is
deposited and thus the concentration of that part of the
liquid in the vicinity of a crystal is decreased. By the liber-
ation of the latent heat of solution and the decrease in satur-

1 Lehman, O., cited by Groth in Physikalische Krystallographie, p. 284.

2 The ice-skeletons of snow flakes may be classified here.

3 These trapezoidal rays can be crystallographically constructed from a vicinal triakis-
octahedral plate (triangle) in which one of the edges between two vicinal faces is truncated
by a yicinal-ikositetrahedron. Similarly, the five rays of the octahedral pentagon may be
explained by the appearance of two vicinal ikositetrahedra. The octahedral rhomboids
occur so seldom that their rays have not been satisfactorily determined.

107

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

ation, the solution becomes specifically lighter and local
concentration and convection currents set in.

According to Lehmann, 1 these local concentration-currents
are the cause of the formation of structural anomalies and
skeletons, and Wulff 2 considers them the cause of the formation
of vicinal faces. 3

Among the plates obtained from ammoniacal solution
were a large number of structural anomalies, a few of which
are illustrated in Figs. 44 and 45. That such lamellar-
structures actually are produced may be demonstrated by
the variations in the Newton colors. That vicinal faces
having absolutely no connection with the crystal faces may
be simulated in this way is shown, e. g., in Fig. 44a, where
there are indications of an unusually flat ikosi tetrahedron,
while the other plates and those in Fig. 45 show traces of
unusually flat triakisoctahedra. Fig. 46 shows the same
much more sharply defined, and Fig. 46d is especially remark-
able for the combination of a triakisoctahedral skeleton 4
with an octahedral plate. Thus one has here a transition to
the skeleton. That these skeletons may appear isolated is
shown in Fig. 47, where twelve triakisoctahedral silver bromide
skeletons are reproduced. In addition to these, a large
number of ikositetrahedral skeletons of silver bromide were
found, twelve of which are pictured in Fig. 48. Ikositetrahedral
skeletons in octahedral plates are shown 5 in Fig. 49, and
combinations of triakisoctahedral and ikositetrahedral skele-
tons in octahedral plates in Fig. 50, among which Fig. 50f
is noteworthy because of the indications of a right and left
dyakisdodecahedral skeleton in combination with the triakis-
octahedral and the ikositetrahedral skeleton in an octahedral
plate.

The attempted classification of skeletons does not pretend
to be conclusive, because of the difficulties of establishing an

1 Lehmann, O., Molekular Physik. Vol. I., p. 354; and Groth, P., 1. c., p. 284.

2 Wulff, J., 1. c.

3 H. A. Miers (An enquiry into the variations of angles observed in crystals. Phil.
Trans. A. 202: 459. 1903), attempted to destroy the concentration-currents by stirring the
liquid (a solution of alum) but even so obtained vicinal faces. Then he demonstrated by
refractometric measurements that the refractive indices of the solution which is in contact
with the crystals is the same as that of a strongly supersaturated solution. This adsorption-
layer is, of course, quite different from the solution itself, so the real nature of the vicinal
faces is still not entirely clear.

4 So far as we know, crystallographers have not introduced any especial differentiations
nor a nomenclature for skeletons. The silver bromide skeletons are here designated ac-
cording to the theoretical vicinal faces of which they may form the edges, without, however,
considering them capable of being classified crystallographically.

5 Ikositetrahedral skeletons in octahedral laminae are very seldom obtained in the
above-described methods of precipitation. Those reproduced here were obtained by add-
ing a mixture of gum arabic and dextrose in varying quantities to the solution, and heating
it to not higher than 60 C.

108

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

FlG. 44

Lamellae formations on octahedral faces of silver bromide.
Magnification, 800 diameters.

109

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

FlG. 45

Lamellae formation on octahedral faces of silver bromide.
Magnification, 800 diameters

110

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

FlG. 46

Ikositetrahedral skeletons on octahedral faces of silver bromide
crystals. Magnified 800 times

111

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

FlG. 47
Triakisoctahedral skeletons of silver bromide, enlarged 800 diameters

112

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

FIG. 48
Ikositetrahedral skeletons of silver bromide, enlarged 800 diameters

113

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

No. 49

Ikositetrahedral skeletons on octahedral faces of silver bromide
crystals. Magnified 800 times

114

FIG. 50

A-E, Combinations of triakisoctahedral and ikositetrahedral skele-
tons on octahedral faces of silver bromide, magnified 800 times.

F, Diagrammatic combination of triakisoctahedral, ikositetrahe-
dral, r- and 1- dyakisdodecahedral skeleton on an octahedral face of
a silver bromide crystal. Magnification, 800 diameters.

115

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

exact classification; for instance, the differences between
triakisoctahedral and ikositetrahedral skeletons are not
always evident. The ends of the rays vary, being either
pointed or blunt. Skeletons having pointed rays have been
attributed to the triakisoctahedra, those having blunt rays
to the ikositetrahedra. To which of these groups Fig. 47c
belongs is not clear; but, since the writer was able to observe
this skeleton as a plate with an octahedral face, it has been
assigned to the triakisoctahedra.

The connection between skeleton forms and structural
anomalies due to lamination is thus very evident in the case
of silver bromide. There is, therefore, no objection to Leh-
mann's theory that these forms are the results of special
growth conditions caused by local concentration currents.
But it must be remembered that under these special growth-
conditions regularities appear which can be conditioned only
by the lattice structure of the crystals, as, for example, the
directions of the rays of the skeleton. Then these rays
probably represent the directions of most rapid development.
The diagram in Fig. 50f contains all the skeletal radiations
thus far verified; from which we conclude that in the octa-
hedral face there are twelve possible directions of most rapid
growth, which form angles of 30 with one another.

The second method for determining the direction of most
rapid growth, by the sudden interruption of unusually rapid
crystallization processes, consists in greatly diluting the
ammoniacal silver bromide solution with water. (See table
on page 86.) The crystals obtained in this way show growth
phenomena at the edges of the octahedral plates, as indicated
in Fig. 51. The directions of growth of the triakisoctahedral
and ikositetrahedral skeletons of the octahedral plates are
represented in the center of gravity of the triangle, but for the
sake of clarity the development directions of both dyakis-
dodecahedral skeletons are omitted. If one transposes the
directions of growth at the corners so that they start from
the center of gravity, it is obvious that they coincide exactly
with those of all skeletal forms.

3. In octahedral needles. In needles the direction of most
rapid growth obviously coincides with the needle axis. In
ordinary needles this direction coincides with one of the
principal crystallographic axes. If a, b, and c are the three
main axes, having development-velocities of V a , Vt>, and V c ,
then to form a needle only one of the three need have a much
greater velocity of growth than the other two, i. e., V a = Vt>,
V b <V c .

116

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

The directions of most rapid growth in laminate needles
(see Fig. 42) may be determined from the octahedral hexagon,
since these needles are distorted hexagons. As is readily
seen, this direction coincides with one of the rays of the
triakisoctahedral or of the ikositetrahedral skeleton.

The octahedral silver bromide needles thus show no other
directions of most rapid growth than have already been found
in the usual octahedra and the octahedral plates.

FIG. 51

Diagram showing directions of most intensive growth
of an octahedral plate

4. In other crystal forms of silver bromide. In addition to
octahedra, prisms and plates of pentagonal dodecahedra and
of rhombic dodecahedra have been found in silver bromide.
That there are rhombic dodecahedral skeletons with rays
which coincide with the sides of the rhombus could not be
proved.

The pentagonal dodecahedra, which appear only in com-
bination with octahedra, are formed in large numbers when
one or two per cent gelatine or about . 005 per cent of agar-
agar is added during the process of precipitation from suddenly
cooled, highly concentrated solutions. Skeletons were often
observed of which the rays coincided with the sides of the
pentagonal pyramids which are so characteristic in these
combinations. The rays of these skeletons formed angles of
about 72 with each other. No dimensions can be given,
as it was impossible to measure the skeletons on account of
their crude formation.

117

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

A large number of cubes and cubic skeletons were observed,
of which fifteen different forms are reproduced in Fig. 52.
Here combinations with the octahedra may be clearly seen
in f and h. Whether these were plates or the ordinary crystals
could not always be determined with certainty. Presumably
a to f are plates. The only ones that could be positively
identified as ordinary crystals are g to p. Cubes could readily
be identified by turning them over, when the different dimen-
sions could be definitely seen. They are approximately
mathematically true cubes.

It is difficult to say in which direction the most rapid
growth of fully developed cubes takes place. Many axial
skeletons were observed which coincided with the main axes
and had cubic faces at the ends. This may be the case with
skeletons which coincide with the diagonals of the cube, but
so far such skeletons have not been observed.

The most rapid growth in cubic faces is clearly shown in
the skeletons b to e in Fig. 52. These coincide with the
diagonals of the rectangle, and this direction is also shown in
the laminated structure on the cube surfaces g to p; so that,
presumably, the skeleton was formed first, and then developed
into the complete crystal. The surfaces k to o (Fig. 52) show
growth also in directions parallel to the sides of the cube.
This is very clearly shown in Fig. 52m. However, these
directions are the same as those already mentioned, which
coincide with the main axes.

It is obvious that the direction of greatest growth of cubic
needles coincides with one of the main axes. A cubic needle
without an octahedral point, although not impossible, has not
yet been observed in silver bromide. But the fact that
these cubic needles may have only one octahedral point, the
other being (e. g.) cubical, leaves unsettled the question
as to whether the growth in one direction is to be attributed
to the greater octahedral acumination.

SUMMARY OF THE OBSERVATIONS CONCERNING THE DIRECTION

OF THE MOST RAPID GROWTH OF SILVER

BROMIDE CRYSTALS

The principle enunciated by Lehmann that the directions
of greatest development are generally coincident with the place
of greatest acumination of the crystals may, therefore, not
apply to all silver bromide crystals.

The principle applies to the following forms:

Octahedra ;

Octahedral axial skeletons;

118

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

FlG. 52

Skeletons and cubes of silver bromide. A-B, 2500 diameters'
magnification; C-P, 800 diameters'

119

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

Octahedral plates with triakisoctahedral skeletons;
Triakisoctahedral skeletons;
Cubic plate skeletons;
Cubic plates.

The principle as definitely does not apply to the following
forms :

Octahedral plates with ikositetrahedral skeletons (see Fig. 48f) ;
Ikositetrahedral skeletons;

Octahedral plates with triakisoctahedral and ikositetrahedral skeletons;
Octahedral plates which have dyakisdodecahedral skeletons.

The principle is doubtful in regard to cubes and cubic
needles.

A law concerning the direction of most rapid growth
can not yet be formulated. It may coincide with the angles of
greatest acumination, or it may coincide with the corners of
more obtuse points, or even with the normals of the surfaces.
That the capillary constant may play a very important part
here has already been intimated. The extreme complexity
of the problem is evident from the fact that, in many cases, a
certain direction of growth may suddenly change during the
process of crystal-formation without it being certain that this
was caused by a modification of the conditions under which
the crystallization proceeded.

120

CHAPTER X

THE BEHAVIOR OF SILVER BROMIDE AND SILVER IODO-BROMIDE
CRYSTALS IN POLARIZED LIGHT

Since the polymorphism of silver bromide is of the greatest
importance in the theory of the photochemical processes in
photographic plates, a large number of the silver bromide
hexagons were microscopically examined in polarized light.

If these hexagons belong to the hexagonal crystal system,
they can not be other than a combination of hexagonal prisms
with the pinacoid as basis. Such a combination crystal
shows simple optical refraction only in the direction of the
principal axis. In every other direction it is said to show
double refraction between crossed nicols. In fact this was
observed by Elsden, and may perhaps be the reason for his
mention of hexagonal silver bromide.

Polarization of light in these crystals is, however, quite
different from that which is caused by anisotropism of the
crystals of other than the regular system. Higson (1. c.) has
obtained only negative results from the examination of crystals
of the regular system in polarized light. In the different
crystal plates, which are of various thicknesses, the polarization
is always approximately the same, and it is difficult to see the
characteristic Newton color series. This phenomenon shows
clearly that here one is dealing with polarization due to
reflection, and not with double refraction produced by the
crystal structure. This reflection is very strong in prepara-
tions of microscopic crystals mounted in Canada balsam on
account of the great differences in the refractive indices. For
silver bromide at X431, n = 2.360; at X656, n = 2.2336; 1
and for Canada balsam, 2 n = 1 . 528 - 1 . 532. 3

But the silver iodo-bromide crystals of photographic
emulsions behave quite differently. Between crossed nicols
they show, beside the unavoidable reflections, a distinct double
refraction, as may be seen in Fig. 22. This figure is a photo-
micrograph of exactly the same area of the same emulsion as
that in Fig. 21, and is taken at the same magnification.

In some emulsions even a colored axial figure was observed,
which, in combination with the cloud-like distribution of the

1 Landolt's Physikalisch-chemische Tabellen, p. 983 (Fourth Edition, 1912).

2 Ibid., p. 981.

3 It would be simplest to examine the axial image in convergent polarized light. How-
ever, on account of the difficulty of mounting a Bertrand lens in a microscope, and of
obtaining a petrographic microscope during the war, these methods were dispensed with.

121

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

light spots in the crystal, shows that here one is dealing not
with a normal crystallographic anisotropism of silver bromide,
but with an optical anomaly which is caused by tensions in
the crystal structure. This has no connection with the
polymorphism of silver bromide. It shows only that the
crystalline structure of silver bromide in photographic emul-
sions is more complicated than that of silver bromide alone,
not in so far as that one has to do with another lattice struc-
ture, but rather that the crystal lattice contains more or less
regularly distributed foreign bodies which greatly affect the
optical properties of silver bromide and, as Reinders 1 has
demonstrated, probably exert a very great influence on the
light-sensitivity of the bromide.

The above-mentioned examination shows that at present
we know only regular silver bromide in the stable phase at
regular temperatures. This greatly simplified the present
work, as it made it possible to distinguish the different crystals
direct without troublesome goniometric measurements. Vari-
ous necessary measurements, e. g., of the silver bromide skel-
eton are not possible on account of the small size of the
crystals (10-30^) on the one hand, and the impossibility of ob-
serving the same crystal in different positions on the other.
One method, to obtain the crystals suspended in different posi-
tions in gelatine, does not give satisfactory results on account
of the large aberration due to the depth of the necessary opti-
cal system, and the great diversity of the skeletons.

THE ANOMALOUS OPTICAL ACTIVITY OF SILVER IODO-BROMIDE
CRYSTALS IN PHOTOGRAPHIC EMULSIONS

Only two methods are known by which it is possible to
change a mono-refractive medium into a doubly refractive
one: a) by placing the medium in an electric or magnetic
field of high intensity; and b) by subjecting the medium to
an internal or external mechanical strain.

The first method is practically impossible in the case of
the silver halide crystals of photographic emulsions. Even
if there is a contact potential between the silver halide and
the gelatine, it can not be more than a few volts, which is
much too low to produce double refraction between crossed
nicols. For the present, then, we must attribute the double
refraction of the silver halide crystals to mechanical strain.

Brauns 2 has shown that alum crystals are mono-refractive
only when they are chemically pure, and that isomorphic
mixtures are made doubly refractive by even a slight content

1 Reinders, W., 1. c.

2 Brauns, R., cited by Groth in Physikalische Krystallographie, p. 513.

122

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

of another alum. Because of the different intervals between
the alternately superimposed atoms in the crystal structure,
an internal tension is produced which causes double refraction.

Later the same phenomenon was observed in other salts.

In view of the fact that the silver halide crystals of photo-
graphic emulsions may, within certain limits, be considered
as isomorphous mixtures of regular silver iodide and silver
bromide, the anomalous double refraction may be partly
accounted for.

FIG. 53A

Silver iodo-bromide emulsion between crossed nicols, showing the
effect of mechanical strain in the gelatine around the grains

In order to prove this experimentally, silver iodide was
added to an ammoniacal solution of silver bromide. The
solution was shaken frequently until, after 24 hours, it was
supposed that equilibrium between the solid and the liquid
phase was established. 5 cc. of this solution were mixed with
40 cc. of water heated to 95 C., and cooled in a closed bottle
set in ice-water (temperature 4 C.). The crystals formed

123

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

were washed, put on a slide in Canada balsam, and covered
with a cover-glass. The anomalous double refraction between
crossed nicols is very distinct in Fig. 53a, which is a photo-
micrograph of this preparation at 500 diameters' magnification.
However, this does not quite solve the problem, because pure
silver bromide emulsions, specially prepared in this laboratory,
also show polarization phenomena between crossed nicols.

It is possible that in the drying of the plates the surround-
ing gelatine exerts a pressure on the silver halide grains. This

FIG. 53B

Silver iodo-bromide emulsion between crossed nicols, showing the effect
of mechanical strain in the gelatine around the grains. It is possible that
in this case these radial effects are due to scattering of light reflected by
the flat surfaces of the crystals, which is not the case in emulsion grains.

external strain should be present in the gelatine around the
crystals, and should be visible between crossed nicols. Indeed,
faint indications of this were noticed in very coarse-grained
emulsions, such as that shown in Fig. 22 at 1,350 diameters'
magnification. Therefore, to make fhe phenomenon more
visible, some emulsions were dried more rapidly and at a

124

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

higher temperature. The result is shown in Fig. 53b. There
are, therefore, external strain lines which radiate from the
silver halide grains into the colloidal matrix. But the silver
halide grains and even the silver bromide grains of photo-
graphic emulsions still showed anomalous polarization phe-
nomena between crossed nicols when they were freed from
the surrounding gelatine by boiling with dilute sulphuric acid.
So this experiment presented no definite solution of the
problem.

Therefore, the only remaining explanation is in the struc-
ture of the silver halide grains.

That the grains of photographic plates do not consist of
pure silver halide, but of a system of gelatine and silver halide
(plus water plus salts?), is a thesis which has already been
discussed. (Chapter III.) Quincke 1 advanced a theory
concerning the colloid-chemical structure of silver halide
grains in photographic emulsions. Bellach 2 found that
careful drying sometimes reduces the average size of the
grains. When testing one of Eder's emulsions he found a
contraction of 0.65 x 10~ 5 sq. mm. per grain, which indicates
a complex structure. Hodgson, 3 however, could not observe,
even at the highest magnification, any swelling of the grains
w r hen soaked in water. 4 This, however, does not contradict
Bellach 's observation, in view of the fact that for changes
in volume of the silver halide grains, riot only is the presence
of gelatine essential, but the manner in which it is distributed
in the grains is also of importance. If there is a regular
distribution of hermetically sealed gelatine particles in the
grains, their swelling is not to be expected under any circum-
stances. If, on the other hand, there is a continuous network
of gelatine in the grains, it is quite possible to effect a change
of volume if the internal gelatine is in contact with the external
gelatine, and if the elasticity of the layers of the silver bromide
crystals resisting displacement is not too great. The greater
the resistance of the silver bromide to changes in volume,
the greater the internal tension in the grains.

With the recognition of the crystalline structure of silver
halide grains in photographic emulsions, the conception that

1 Quincke, G., Niederschlagmembranen und Zellen in Gallerten oder Losungen von
Leim, Eiweiss, und Starke. Ann. Physik. IV. 11: 449. 1903. Die Bedeutung der
Oberflachenspannung fur die Photographic mit Bromsilbergelatine. Ibid. IV. 11:1100.
1903.

2 Bellach, V., 1. c.

3 Hodgson, M. B., 1. c.

4 We repeated Hodgson's experiment at a magnification of 2,500 diameters, using the
emulsion pictured in Fig. 21. This emulsion was poured out in a very thin layer (of one
grain in thickness) and dried. The emulsion side was turned toward the condenser and a
photomicrograph of the grains taken. If the condenser is removed very carefully, one can
moisten the emulsion and again photograph it. This procedure showed no difference in
the dimensions of the grains in the dry and the wet emulsion.

125

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

these crystals are a binary system disappeared. Luppo-
Cramer 1 expressed himself differently at different times
regarding the structure of the silver halide grains. First,
he assumed that the crystals are free from gelatine. In his
later work, however, he vigorously and often defends the
theory of the heterogeneity of silver halide grains in photo-
graphic emulsions. So he probably differentiates between
two different kinds of silver halide grains in emulsions: crystal-
line grains and colloidal grains. W. D. Bancroft 2 is also of
the opinion that the whole question of the high sensitivity of
photographic plates depends on the system silver halide-
gelatine.

The possibility of gelatine in silver halide crystals was
first demonstrated experimentally by Reinders. 3 He showed
that silver chloride, crystallizing in the presence of various
colloids, such as colloidal silver, gelatine, albumen, casein,
etc., has the capacity of taking up and homogeneously distrib-
uting these colloids in the crystals. With gelatine this effect
was noticeable even in concentrations of 1 mg. gelatine in 10
liters of water. 4

If the anomalous double refraction of silver halide crystals
is caused by this enclosed gelatine, then silver bromide crystals
from ammoniacal silver bromide solutions containing gelatine
should show double refraction, since colloid-free silver bromide
solutions apparently can yield only simply refractive crystals.
But no definite double refraction could be detected
experimentally.

However, this experiment is not conclusive, because the
conditions of crystallization in photographic emulsions are
entirely different from those in ammoniacal solution. In
photographic emulsions there is, first, a suspension of colloidal
silver halide which consists of more or less fine flakes of silver
halide of varying size and structure. The diameters vary
from submicroscopic to dr I/*. In the presence of a silver
halide solvent (such as potassium bromide or ammonia), the
colloidal silver halide passes over into the more stable crystal-
line silver halide. Gelatine is continually absorbed during
the formation of the crystal, throughout which it is homo-
geneously distributed (Reinders).

But the silver halide flakes also contain some microhetero-
geneously distributed particles of gelatine which remain in

1 Luppo-Cramer, Photographische Probleme, 1. c.

2 Bancroft, W. D., The photographic plate, 1. c.

3 Reinders, W., Studien iiber die Photohaloide, 1. c.

4 This visible decomposition is distinct from the latent effect of light. Therefore, the
laws concerning visible photochemical decomposition can not be applied to the latent effect
of light.

126

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

the crystal and form a structure different from that described
by Reinders. The grains of sensitive plates have a diameter
of 2 to 3/^ and more. Hence there must be another factor
concerned than the simple conversion of flakes into crystals.
The increase in size of the grains may be due either to the
combining of several single flakes during the crystallization
process, or to the crystallization of smaller flakes on larger.
However this development-process is regarded, the gelatine
structure of the flakes is not necessarily lost in crystallization,
but may pass over into the crystals. There may be a change
in structure, depending upon the treatment of the emulsion.
In fact, the gelatine structure depends upon the conditions
of the precipitation and the subsequent treatment of the
emulsion. (Hodgson vs. Bellach.) This microheterogeneous
structure, which does not occur when silver bromide is precip-
itated from ammoniacal gelatine-containing solutions, may be
the cause of the tensions in the emulsion crystals, to which
the anomalous double refraction is to be ascribed.

If, however, a mono-refractive medium contains micro-
scopic or submicroscopic suspensions of dielectrics, the double
refractivity may not be the only cause of the abnormal phe-
nomenon seen between crossed nicols. Diffuse reflections
can, under these conditions, illuminate the dark field. But
it is hardly conceivable that this may produce polarization
phenomena which resemble the axial images sometimes found
in emulsions. However, these images are possible if strains
are present.

Thus we must conclude that the anomalous optical activity
of silver-iodo-bromide crystals in photographic plates may
be the result of any of several factors :

a. The tensions resultant from the isomorphic mixture of silver iodide
and silver bromide (cf . Chapter V) ;

b. The mechanical strain exerted to a small degree by the gelatine sur-
rounding the silver iodo-bromide crystals;

c. The probable mechanical strain of microheterogeneously distributed
gelatine particles in the silver halide crystals;

d. The probable diffuse reflections in microheterogeneously suspended
gelatine in the silver halide crystals.

Hence we must regard sensitive photographic emulsions
not only as suspensions of silver halide crystals in gelatine,
but also as a probably very much more complex suspension
of gelatine in crystalline silver halide. A fuller discussion
of this aspect of the question has been given in Chapter IV.

The probable complexity of this suspension has been
indicated by a distinction between a homogeneous and a

127

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

microheterogeneous suspension, which refers only to a different
degree of distribution. Therefore, these terms should be
interpreted only in a relative sense. 1 As a matter of fact,
both types of distribution lie beyond the limits of microscopic
resolving power. Therefore, both could be interpreted
equally well as homogeneously distributed gelatine. Gibbs
has emphasized the fact that the conceptions "homogeneous"
and "heterogeneous" are relative. That is to say, one can not
make a sharp distinction between the two because there is a
continuous transition between homogeneous and hetero-
geneous, according to the point of view. Take, for example,
the photographic emulsion. Microscopically considered, it
is heterogeneous; but macroscopically it is homogeneous.
The purest crystal medium is homogeneous in ordinary light
but heterogeneous in the X-ray. Hence the grades of distrib-
ution of gelatine in silver halide crystals, as mentioned above,
can be interpreted as both homogeneous and heterogeneous,
or one as homogeneously, and the other as heterogeneously
distributed, according to one's point of view.

THE GELATINE ENCLOSING THE SILVER IODO-BROMIDE
CRYSTALS OF PHOTOGRAPHIC EMULSIONS

The radial distribution of the tension exerted by the
gelatine around the crystals in photographic emulsions indi-
cates a complex effect. This appears not only in gelatine,
but also, as Fig. 53B shows, in Canada balsam. This balsam
is a very viscous liquid, which makes the phenomenon appear
even more complex. The cause is unknown. Possibly the
following observations may suggest a solution.

In some preliminary experiments on the adsorption of
colloids by silver bromide crystals, colloidal dyes were used.
If the entire process of crystallization takes place in the
presence of such dyes, then the Reinders' distribution of
colloid in crystal can be directly (microscopically) proven. 2
An analogous phenomenon was noticed, especially with the
sodium salt of l-naphthol-4-sulphonic acid-azo-/3-napthol.
An excess of this dye coagulates around the silver bromide
crystal in the form of rays which may vary greatly under
different conditions.

For gelatine, therefore, the phenomenon may be explained
as follows: By a ray-like coagulation of the gelatine around

1 Where differences in space distribution of the same kind of material are concerned,
the terms "iso-psegmatic" (equal-grained) and "allo-psegmatic" or poly-psegmatic
(vari-grained) are more suitable.

2 Cf. also Marc's experiments, cited on page 52.

128

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

the grains, spots of greater or less density occur in the gelatine.
The drying of the gelatine has a different effect in different
places because of this structure, as the rays will tend to dry
more quickly than the rest of the gelatine. All strain will be
neutralized by internal displacements if the drying is slow.
But if the drying is rapid, there is not sufficient time to com-
pensate the resultant strain, and a double refraction of gelatine
may be seen through crossed nicols.

This presupposes that gelatine possesses a solidification
structure which persists in dry gelatine, and which originates
in a manner analogous to that of the solidification of liquids
to crystalline aggregates in which radially arranged structures
(spherolites) may be observed. Still this leaves unexplained
the same phenomenon in the viscous Canada balsam. Here,
however, one is entering the province of pure colloid-chemical
investigation, which is outside the scope of the present
discussion.

GENERAL SUMMARY OF THE CRYSTALLOGRAPHIC STUDY OF
SILVER BROMIDE CRYSTALS

1. It has been shown that silver iodide is precipitated from
ammoniacal solutions in the metastable regular form, which is
isomorphous with silver bromide, and that the crystallographic
classification of silver bromide may apply also to the silver
iodo-bromide crystals of photographic emulsions.

2. The occurrence of silver bromide pseudomorphs was
investigated, and methods for recognizing silver bromide-am-
monia complexes described.

3. All the crystalline forms of silver bromide obtained
were identified.

4. The faces of needles and plates of silver bromide were
studied, and shown to be growth-modifications of octahedra
and cubes.

5. Etch-figures were obtained on the octahedral faces of
silver bromide.

6. Silver bromide was assigned to the dyakisdodecahedral
class of the regular system.

7. Because of their isomorphism, silver chloride and regular
silver iodide were placed in the same class as silver bromide.

8. The possibilities of modifications of silver bromide
were discussed.

9. The occurrence of needles, plates, and ordinary crystals
of silver iodo-bromide in photographic emulsions was
demonstrated.

129

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

10. It was shown that the Gibbs-Curie-Wulff law is
applicable to photographic emulsions, and that thereby all
octahedral forms appearing in these emulsions may be
explained.

11. The directions of most rapid growth were determined
for the various crystalline forms of silver bromide.

12. It was shown that Lehmann's theory concerning the
direction of most rapid development of crystals does not
always apply to silver bromide crystals.

13. The examination of the characteristic optical properties
of silver bromide crystals in polarized light showed that the
so-called hexagonal silver bromide is really regular.

14. The possible causes of the anomalous optical activity
of silver halide crystals of photographic emulsions in polarized
light were studied, and it was suggested that these crystals
are the center of various mechanical strains.

15. It was suggested that there are two different gelatine
structures in the crystals of photographic emulsions.

16. The structure of the gelatine surrounding the grains
in photographic emulsions was more closely studied and a
radial coagulation of gelatine around the grains observed.

In conclusion we wish to express our great appreciation
of the co-operation of Messrs. W. H. Davis, F. A. Elliott,
G. H. Norris and L. Schneider, who assisted in the experi-
mental part of this investigation, and of Professor A. C. Gill,
of Cornell University, who read the manuscript and made help-
ful criticisms and valuable suggestions.

130

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

CONCORDANCE OF POSSIBLE SYMMETRY CLASSES
OF SILVER BROMIDE

Inasmuch as the forms included in the regular system, to
which silver bromide belongs, are variously named by different
authorities, a correlation of the terminology is given below:

Authorities
Hilton
Symbol Schonfliess

T Tetartrohedry

Th Paramorphic
hemihedry

Central

Td Hemimorphic XXXI

hemihedry Ditesseral

Polar

O Enantiomorphous XXIX
hemihedry Tesseral

holoaxial

H. A.
Miers

E. S.
Dana

Number of
faces in
Other general
Names form

XX-VIII

Tesseral
Polar

5 Tetartro-
hedral

Pentagonal
dodecahedral

XXX

Tesseral

2 Pyrito-
hedral

Parallel faced
hemihedry

Pentagonal
hemihedry
Dyakisdodecahedral

Oh Holohedry

XXXII

Ditesseral
Central

3 Tetrahedral

4 Plagihedral

1 Normal

Hexakis-
tetrahedral
Inclined faced
hemihedry

Gyroidal
hemihedry
Pentagon-
ikositetrahedral

Hexakis-
octahedral

131

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

The Silver Bromide Grain

Abbreviations adopted in citations of serial publications.

Astrophys. J The Astrophysical Journal

Arch. wiss. Phot. . . . . Archiv fiir wissenschaftliches Photographic

Ann. Physik Annalen der Physik

Ann. chim. phys Annales de chimie et de physique

Ber. chem. Gesell. . . . Berichte der deutschen chemischen

Gesellschaft
Bull. soc. franc., mineral. . Bulletin de la societe francaise de

mineralogie

Brit. J. Phot The British Journal of Photography

Brit. J. Phot. Almanac . . The British Journal of Photography

Almanac

Fortschr. Mineral. . . . Fortschritte der Mineralogie, Krystal-

lographie, und Petrographie

J. Amer. Chem. Soc. . . . Journal of the American Chemical Society
J. Amer. Leather Chem. Assoc. Journal of the American Leather Chemists'

Association
J. Chem. Soc. (Trans.) . . Journal of the Chemical Society

(Transactions)
J. chim. phys. .... Journal de chimie physique

J. Frankl. Inst Journal of the Franklin Institute

J. Phys. Chem The Journal of Physical Chemistry

J. Soc. Chem. Ind. . . . Journal of the Society of Chemical

Industry
J. Wash. Acad. Sci. . . . Journal of the Washington Academy of

Science
Jahrb. Phot Jahrbuch fiir Photographic und Repro-

ductionstechnik (Eder's)
Jahrb. Rad. u. Elektr. . . Jahrbuch der Radioaktivitat und

Elektronik

Koll.-Zeits Kolloid-Zeitschrift

Neues Jahrb. Mineral. Geol. Neues Jahrbuch fiir Mineralogie, Geologic

und Palaontologie
Phil. Mag The London, Edinburgh and Dublin

Philosophical Magazine and Journal of

Science
Phil. Trans Philosophical Transactions of the Royal

Society of London

Phot. J The Photographic Journal

Phot. News Photographic News

Physik. Zeits. .... Physikalische Zeitschrift

Sitzungsber. Akad. Wiss. Wien Sitzungberichte der kaiserlichen

Akademie der Wissenschaften, Wien

Zeits. anorg. Chem. . . . Zeitschrift fiir anorganische und

allgemeine Chemie
Zeits. Kryst. u. Mineral. . Zeitschrift fiir Krystallographie und

Mineralogie
Zeits. physik. Chem. . . Zeitschrift fur physikalische Chemie,

Stochiometrie und Verwandtschaftslehre
Zeits. wiss. Phot. . . . Zeitschrift fiir wissenschaftliches

Photographic

132

Bibliography

ABEGG, R., Die Silberkeimtheorie des latenten Bildes. Arch. wiss. Phot.

1: 18. 1899.

BAKHUIS-ROOZEBOOM, Heterogene Gleichgewichte. (Vieweg, Braun-
schweig, 1901.)
BANCROFT, W. D., The electrochemistry of light. J. Phys. Chem. 12: 209,

318 and 417. 1908: 13: 1, 181, 449 and 538. 1909.
, . ., The photographic plate. J. Phys. Chem. 14: 12,

97, 201 and 620. 1910: 15: 313, 551. 1911.
BANKS, Remarks in discussion of Hurter and Driffield's paper on "The

latent image and its development." Phot. J. 22: 159. 1898.
BARMHAUER, H., Die neuere Entwicklung der Krystallographie. (Vieweg,

Braunschweig, 1905.)
BAUR, E., Silber, in Abegg's Handbuch der anorganischen Chemie, Vol.

II, Part I, p. 684. (Hirzel, Leipzig, 1908.)
BECKE, F., Ueber die Ausbildung der Zwillingskristalle. Fortschr. Mineral.

1:68. 1911.
BECKENKAMP, J., Der tetrakishexagonale oder oktaedrische Typus der

Kristalle. Ann. Physik. IV. 39: 346. 1912.
BELLACH, V., Die Struktur der photographischen Negative. (Knapp,

Halle a. S., 1903.)

BEMMELEN, J. M. van, Die Absorption. (Steinkopf, Dresden, 1910.)
BODLANDER, G., and FITTIG, R., Das Verhalten von Molekularverbin-

dungen bei der Auflosung. Zeits. physik. Chem. 39: 597. 1902.
BOWMAN, J. H., A study in crystallization. J. Soc. Chem. Ind. 25: 143.

1906.

BRAUNS, R., Chemische Mineralogie. (Tauchnitz, Leipzig, 1896.)
BROTHER, G. H., Suggestions on some common precipitations. J. Amer.

Leather Chem. Assoc. 13: 159. 1918.
BURGESS, C. H., and CHAPMAN, D. L., The interaction of chlorine and

hydrogen. J. Chem. Soc. (Trans.) 89 2 : 1399. 1906.
CHAPMAN, D. L., CHADWICK, S., and RAMSBOTTOM, J. E., The chemical

changes induced in gases submitted to the action of ultra-violet light.

J. Chem. Soc. (Trans.) 91 1 : 942. 1907.
, . ., and MACMAHON, P. S., The interaction of hydrogen

and chlorine. J. Chem. Soc. (Trans.) 95 1 : 135. 1909.
, . ., , . ., The retarding effect of oxygen

on the rate of interaction of chlorine and hydrogen. J. Chem. Soc.

(Trans.) 9^:959. 1909.
CURIE, P., Sur la formation des cristaux et sur les constantes capillaires

de leurs differentes faces. Bull. soc. frang. mineral. 8: 145. 1885.
DUHEM, P., Traite elementaire de mecanique chimique, fondle sur la

thermodynamique. 4 vols. (Hermann, Paris, 1897-99.)

DYER, Note under Emulsionsbereitung. Jahrb. Phot. 18: 437. 1904.
EDER, J. M., Ausfiihrliches Handbuch der Photographic. (Knapp, Halle
a. S., 1905.)

-, . ., Beitrage zur Photochemie des Bromsilbers. Sitzungsber.

Akad. Wiss. Wien 81: 679. 1880.

133

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

ELSDEN, J. V., On the formation of a chemical compound of ammonia with

silver bromide. Phot. News 25: 174. 1881.
*NGLISCH, E., Zeits. wiss. Phot. 2: 416. 1905.
EPHRAIM, F., Ueber die Natur der Nebenvalenzen. Ber. chem. Gesell.

51: 706. 1918.
FREUNDLICH, H., Kapillarchemie. (Akademische Verlagsgesellschaft,

Leipzig, 1909.)
FRIEDEL, G., Examen critique de la theorie de Curie-Wulff sur les formes

crystallines. J. chim. phys. 11:478. 1913.
GAEDICKE, J., Ammoniakraucherung bei Trockenplatten. Jahrb. Phot.

27: 62. 1913.
GIBBS, J. W., Elementary principles in statistical mechanics. (Scribner,

New York, 1902.)

, . ., Scientific Papers. 2 vols. (Longmans, 1906.)
GMELIN-KRAUT, Handbuch der anorganischen Chemie. (Winter, Heidel-
berg, 1914.)
GROSS, R., Sammelkristallization in Beziehung zum Atomfeld der Kristalle.

Jahrb. Rad. u. Elektr. 15: 270. 1918.

GROTH, P., Chemische Krystallographie. (Engelmann, Leipzig, 1906-10.)
, ., Physikalische Krystallographie. Third edition. (Engel-
mann, Leipzig, 1895.)
GROTTHUS, T. F. VON, Physisch-chemische Forschungen. (Numbers:,

1820.)
HIGSON, G. I., Photomicrography in photographic research. Phot. J.

60: 140. 1920.
HILTON, H., Mathematical crystallography. (Clarendon Press, Oxford,

1903.)
HODGSON, M. B., The physical characteristics of the elementary grains of a

photographic plate. J. Frankl. Inst. 184: 705. 1917.
HULETT, G. A., Beziehungen zwischen Oberflachenspannung und

Loslichkeit. Zeits. physik. Chem. 37: 385. 1901.
JOHNSEN, A., Untersuchungen liber Kristallzwillinge und deren Zusam-

menhang mit anderen Erscheinungen. Neues Jahrb. Mineral. Geol.

23: 237. 1907.
JOHNSTON, J., Gelatine-bromide of silver emulsions treated with ammonia.

Brit. J. Phot. Almanac 1877:95. 1877.
KOCH, P. P., and DU PREL, G., Ueber das Korn der photographischen

Platte und eine Methode zu einer Untersuchung. Physik. Zeits.

17: 536. 1916.

KROHN, F. W. T., The mechanism of development of the image in a dry-
plate negative. Phot. J. 58: 193. 1918.
KUESSNER, H., Ueber Loslichkeitsunterschiede an Kristalloberflachen.

Zeits. physik. Chem. 84: 313. 1913.
KUSTER, F. W., Ueber Gleichgewichtserscheinungen bei Fallungsreaktionen.

Zeits. anorg. Chem. 19: 81. 1898.
LANDOLT, H., BORNSTEIN, R., and MEYERHOFFER, W., Physikalisch-

chemische Tabellen. (Springer, Berlin, 1905.)
LANGMUIR, I., The constitution and fundamental properties of solids and

liquids. J. Amer. Chem. Soc. 38: 2221. 1916; and 39: 1848. 1917.
LEHMANN, E., and KNOCHE, P., Plate-grain and albumen emulsions-

Brit. J. Phot. 61: 759. 1914.
*Title not available.

134

SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

LEHMANN, O., Molekularphysik. (Engelmann, Leipzig, 1888.)

LEWIS, W. C. M., Studies in catalysis. J. Chem. Soc. (Trans.) 109: 796.

1916.
LORENZ, R., Kolloidchemie und Photographie. Koll. Zeits. 22: 103.

1918.

LUPPO-CRAMER, Photographische Probleme. (Knapp, Halle a. S., 1907.)
, Kolloidchemie und Photographie. Koll. Zeits. 9:240.

1911; 10; 182. 1912.
MARC ROBERT, Ueber Adsorption und gesattigte Oberflachen. Zeits.

physik. Chem. 81:641. 1912.
, , Ueber die Kristallization aus wasserigen Losungen.

Zeits. physik. Chem. 79: 71. 1912.

-, and RITZEL, A., Ueber die Faktoren, die den Kristall-

habitus bedingen. Zeits. physik. Chem. 76: 584. 1911.
MEES, C. E. K., The physics of the photographic process. J. Frankl.

Inst. 179: 141. 1915.
MIERS, H. A., An enquiry into the variation of angles observed in crystals.

Phil. Trans. A. 202: 459. 1904.
MONKEMEYER, K., Ueber die Bildung von Mischkrystallen der. Blei-,

Silber-, Thallo-, und Cupro-halogene aus Schmelzfluss. Neues Jahrb.

Mineral. Geol. 22: 1. 1906.
MUGGE, O., Ueber die Zwillingsbildung der Kristalle. Fortschr. Mineral.

1:38. 1911.
NERNST, W., Theoretische Chemie. Seventh Edition, 1913. (Enke,

Stuttgart.)
NIGGLI, P., Kolloidchemie und Zwillingskristalle. Koll. Zeits. 10: 268.

1912.
OSTWALD, W., Ueber die vermeintliche Isomerie des roten und gelben

Quecksilberoxyds und die Oberflachenspannung fester Korper. Zeits.

physik. Chem. 34: 495. 1900.
PAWLOW, P., Ueber die Bildung, das Gleichgewicht, und die Verander-

ungen des Kristalles im isothermen Medium. Zeits. physik. Chem.

72: 385. 1910.

QUINCKE, G., Die Bedeutung der Oberflachenspannung fur die Photo-
graphie mit Bromsilbergelatine und eine Theorie des Reifungsprozesses

der Bromsilbergelatine. Jahrb. Phot. 19: 3. 1905; Ann. Physik.

IV. 11: 1100. 1903.

, ., Niederschlagmembranen und Zellen in Gallerten oder

Losungen von Leim, Eiweiss, und Starke. Ann. Physik. IV. 11: 449.

1903.
REINDERS, W., Studien liber die Photohaloide. Zeits. physik. Chem.

77: 213, 357, 677. 1911.

, ., Ueber die Bildung und Umwandlung der Mischkrystalle

von Quecksilberbromid und Quecksilberjodid. Zeits. physik. Chem.

32: 494. 1900.

, ., und NIEUWENBURG, J. VAN, Gelatine and andere Kolloide

als Verzogerer bei der Reduktion von Chlorsilber. Koll. Zeits. 10: 36.

1912.

RENWICK, F. F., Discussion of Krohn's paper. Phot. J. 58: 195. 1918.
RETGERS, J. W., Beitrage zur Kenntnis des Isomorphismus. Zeits. physik.

Chem. 9: 267. 1892.

135

MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

RIECKE, E., Ueber Wechselwirkung und Gleichgewicht trigonaler Poly-.

systeme, ein Beitrag zur Theorie der Krystallstructur. Ann. Physik.

IV. 3: 543. 1900.
RITZEL, A., Die Kristalltracht des Chlornatriums in ihrer Abhangigkeit

vom Losungsmittel. Zeits. Kryst. u. Mineral. 49:152. 1911.
ROSCOE, H. E., and SCHORLEMMER, C., Treatise on Chemistry. (Apple-
ton, New York, 1890.)
SCHEFFER, W., Microscopical researches on the size and distribution of

the plate grains. Brit. J. Phot. 54: 116. 1907.
SHEPPARD, S. E., Photochemistry. (Longmans, 1914.)

, . ., and MEES, C. E. K., Investigations on the theory of

the photographic process. (Longmans, 1907.)

, . ., and MEYER, G., Chemical induction and photographic

development. Phot. J. 60: 12. 1920; J. Amer. Chem. Soc. 42: 689

1920.
SMITS, A., Eine neue Theorie der Erscheinung Allotropie. Zeits. physik

Chem. 76:421. 1911.
STARK, J., Zur Energetik und Chemie der Bandenspektra. Physik. Zeits.

9: 85. 1908.
STAS, J. S., Recherches de statique chimique au sujet du chlorure et du

bromure d'argent. Ann. chim. phys. V. 3: 145. 1874.
STEUBING, W., Fluoreszenze und lichtelektrische Empfindlichkeit anor-

ganischer Substanzen. Physik. Zeits. 9:493.1908.
STOLTZENBERG, H., and HUTH, M. E., Ueber kristallinisch-fliissige Phasen

bei den Monohalogeniden des Thalliums und Silbers. Zeits. physik.

Chem. 71: 641. 1910.
TAMMANN, G., Das Zustandsdiagramm des Jodsilbers. Zeits. physik.

Chem. 75: 733. 1911.

, ., Krystallizieren und Schmelzen. (Barth, Leipzig, 1903.)

THIEL, A., Umkehrbare Elektroden zweiter Art mit gemischten Depo-

larisatoren. Zeits. anorg. Chem. 24: 1. 1900.
TUBANDT, C., and LORENZ, F., Das elektrische Leitvermogen als Methode

zur Bestimmung des Zustandsdiagramms binarer Salzgemische. Zeits

physik. Chem. 87: 543. 1914.

VOGEL, H. W., Handbuch der Photographie. (Oppenheim, Berlin, 1890.)
WALLACE, R., The silver "grain" in photography. Astrophys. J.

20: 113. 1904.
WEIMARN, P. VON, Zur Lehre von den Zustanden der Materie. 2 vols.

(Steinkopf, Dresden.)
WILDERMAN, M., On the velocity of reaction before complete equilibrium

and before the point of transition, etc. Phil. Mag. VI. 2 2 : 50. 1901.
WOOD, R. W., Researches in physical optics. (Columbia University Press,

New York, 1913.)
WULFF, G., Zur Frage der Geschwindigkeit des Wachstums und der

Auflosung der Krystallflachen. Zeits. Kryst. u. Mineral. 34: 449.

1901.
WYCKOFF, R. W. G., The nature of the forces between atoms and solids.

J. Wash. Acad. Sci. 9: 564. 1919.

136

Index of Authors

ABEGG, R., 106

BAKHUIS-ROOZEBOOM, 79

BANCROFT, W. D., 45, 48, 56, 61,

106, 126

BANKS, E., 61, 75
BARMHAUER, H., 70
BAUR, E., 80
BECKE, F., 72
BECKENKAMP, J., 50
BELLACH, V., 61, 76, 80, 125, 127
BEMMELEN, J. M. VAN, 29
BODLANDER, G., and FITTIG, R., 87
BORNSTEIN, R., LANDOLT, H. and

MEYERHOFFER, W., 121
BOWERMAN, 40
BOWMAN, J. H., 39
BRAUNS, R., 41, 122
BROTHER, G. H., 39
BURGESS, C. H., and

CHAPMAN, D. L., 48
CHADWICK, S., CHAPMAN, D. L.,

and - , 48
CHAPMAN, D. L., BURGESS, C. H.,

and - , 48

, CHADWICK, S., and

RAMSBOTTOM, J. E., 48

, and MACMAHON, P. S., 48
CURIE, P., 57
DUHEM, P., 43
DYER, 61
EDER, J. M., 11, 13, 17, 25, 28,

37, 45

ELSDEN, J. V., 80, 89, 121
ENGLISCH, E., 12
EPHRAIM, F., 25, 88
FITTIG, R., BODLANDER, G.,

and - , 87
FREUNDLICH, H., 66
FRIEDEL, G., 58
GAEDICKE, J., 11
GIBBS, J. W., 41, 57, 128
GMELIN-KRAUT, 79
GROSS, R., 59

GROTH, P., 81, 97, 98, 107, 108, 122
GROTTHUS, T. F. VON, 48
HIGSON, G. I., 81, 100, 121
HILTON, H., 58, 66
HODGSON, M. B., 84, 90, 125, 127
HULETT, G. A., 59
HUTH, M. E., STOLTZENBERG, H.,

and , 46
JOHNSEN, A., 68, 71
JOHNSTON, J., 11
KNOCHE, P., LEHMANN, E.,

and - , 106

KOCH, P. P., and DU PREL, G.,

93, 104

KROHN, F. W. T., 75, 81, 93
KUESSNER, H., 66, 67
KUSTER, F. W., 43
LANDOLT, H., BORNSTEIN, R., and

MEYERHOFFER, W., 121
LANGMUIR, I., 30, 49
LEHMANN, E., and KNOCHE, P., 106
LEHMANN, O., 41, 107, 108, 118
LEWIS, W. C. M., 48
LORENZ, F., TUBANDT, C., and

, 46, 47
LORENZ, R., 84
LUPPO-CRAMER, 12, 13, 17, 20, 22,

23, 25, 27, 35, 48, 62, 76, 78, 91,

106, 126

LUTHER, R., 106
MACMAHON, P. S., CHAPMAN, D. L.,

and - , 48
MARBACH, 98
MARC, R., 49, 52, 53

, and RITZEL, A., 59
MEES, C. E. K., 27, 33;

SHEPPARD, S. E., and

-i , 11, 35, 53
MEYER, G., SHEPPARD, S. E., and

, 66
MEYERHOFFER, W., LANDOLT, H.,

BORNSTEIN, R., and ,121
MIERS, H. A., 108

M6NKEMEYER, K., 46

MUGGE, O., 69, 72
NERNST, W., 31

NlEUWENBURG, J. VAN, REINDERS,

W., and - , 35
NIGGLI, P., 68
OSTWALD, W., 59, 66, 68
PAWLOW, P., 68
PREL, G. DU, KOCH, P. P., and

, 93, 104
QUINCKE, G., 27, 125
RAMSBOTTOM, J. E., CHAPMAN,

D. L., CHADWICK, S., and

, ^O

REINDERS, W., 49, 53

and VAN NIEUWENBURG,
J., 35

RENWICK, F. F., 79, 81
RETGERS, J. W., 49, 52
RIECKE, E., 41
RITZEL, A., 66
ROSCOE, H. E., and

SCHORLEMMER, C., 80

137

SCHEFFER, W., 84 TAMMANN, G., 28, 47
SCHORLEMMER, C.,' RoscoE, H. E. TniEL, A., 44, 45 47 79 97

S^^^TA 51 T T z C F 46' 47'

-3717 and MEES ' C E * K " "' VALETO'J J 59

'- and MEYER, G., 66 WALLACE, R., 91

SMITS, A., 72 WEIMARN, P. VON, 28, 29, 30, 32

STARK, J., 48 33, 68

STAS, J. S., 28 WILDERMAN, M., 31

STEUBING, W., 48 WOOD, R. W., 49

STOLTZENBERG, H., and WYCKOFF, R/W. G 50

HUTH, M. E., 46 WULFF, G., 57, 58, 66 108

138

Index of Subjects

Allotropy, Smits' theory of, 72

Ammonia, complexes of silver halides and , 16, 25, 86, 87; in

reversal, 21
development, effect of moisture in, 18, 19

- of latent image by, 11, 12, 18
- of visible image by, 13

, rate of, 22
, theory of, 22

exposure necessary for development with, 18
fuming of exposed plates, 18

unexposed layers with, 13

with, in absence of light, 22

with, after destruction of the image, 23
use of , in ripening emulsions, 11

, in preparation of silver bromide crystals, 82, 85
Anisotropism, of silver bromide crystals, 121, 122
Apparatus used in microscopic study of silver bromide, 82

Bancroft's theory of ripening, 56
Beckenkamp's theory of crystallization, 50
Bellach's study of ripening, 61
Birefringence in crystals of emulsions, 47

in uniaxial crystals, 69
Bowerman's principle of crystal growth, 40

Capillarity, and crystal growth, 42, 57, 101, 103
constants, calculation of, 58
influence of, on crystal form, 42, 57, 100, 103
Capstan effect, 53, 54, 55

Catalysis, 48; crystallization , 51, 52

photochemical , 51

substances useful in, 52
Chlorine, photochemical induction of, 48
Colloidal gold, 24, 53
Colloidal silver, adsorption of, by silver halides, 53

panchromatizing effect of, 53, 54, 55
Crystals, birefringence in silver halide, 47

classification of silver bromide, 81, 95, 97
evolution of form of, 68, 69

growth of, 39, 40, 42, 52, 69, 100, 101, 103, 107, 116, 117, 118, 120
skeletons, 108, 109, 116, 118

Valeton's equation for energy in one mol of, 59, 60
Crystallization and chemical affinity, 50
at rest, 39

Beckenkamp's theory of, 50
defined, 51
effect of dyes on, 52
effect of mixed silver halides in, 47
importance of, in emulsion making, 49
in presence of colloids, 39
influence of additions on, 49
nuclei, 12, 13
of silver bromide, 75
regulation of, 50
Tammann's theory of, 28
von Weimarn's theory of, 28
Wilderman's expression for second stage of, 31

139

Cubes, 118

Degelatinization, 56

Development by ammonia, after destruction of the image, 23
of latent image, 18
of visible image, 13
rate of, 22
theory of, 22
Disintegration, Luppo-Cramer's theory of, 23

of silver halide by light, 12, 83, 84
Dispersity, and the surface energy principle, 57
and twinning, 68
average degree of, 70
composition as function of, 29
effect of colloid medium on, 35, 39
, in different emulsions, 28

method of mixing emulsion on, 33

- mixture of silver halides on, 42, 45
, on color of photohalides, 53

, on sensitivity of emulsions, 104

solubilizing agents on, 47

- viscosity on, 36
factors influencing, 35
regulation of, by gelatine, 36
variation of, 33, 34

Dyes, effect of, on crystallization, 52
sensitizing action of, 48, 49

Emulsions, birefringence in crystals of, 47

forms of - grains, 93

mercuric iodide, 62

preparation of, 27

ripening of, by ammonia, 11

used in crystallographic study, 76, 77
Equilibrium, conditions of, 43
false, 43, 44

of heterogeneous substances, Gibbs' work on, 57
Etching of silver bromide crystals, 94
Exposed plates, fuming of, 18
Exposure necessary for ammonia development, 18

Ferrous oxalate, ammonia fuming with, 11

Filter used in preparing photomicrographs, 83

Fluorescence, 49

Fogging after ammonia fuming, 14

Fuming with ammonia, 11; effect of moisture in, 18, 19

, with ferrous oxalate, 1 1
exposed plates, 18
in absence of light, 22
method of, 13, 14
unexposed layers. 13
Gelatine, changes in affinity of, for water, 37

combinations of, with silver halides, 37, 38, 125, 127
distribution of, in silver halide grains, 125
double refraction of, 128, 129
effect of, on dispersity, 35, 39

, on taking up colloidal silver, 53
, on mercuric iodide, 63

, on optical activity of silver bromide crystals, 126

filter theory of, 35, 53
protective effect of, 35, 56

140

Gibbs' study of heterogeneous substances, 57
Growth of crystals, Bowerman's relay principle of, 40

directions of most rapid, 107, 116, 117, 118, 120

effect of colloid on, 39

- consolute substances on, 52

influence of capillarity on, 42, 57, 101, 103

suppression of, 39
Gypsum, Hulett's work on, 59

Hemihedrism of silver bromide, 97
Hulett's work on gypsum, 59

Image, destruction of, 23

latent, ammonia-fuming of, 11, 12, 18

visible, ammonia development of, 13, 24
Inertia, effect of ammonia-fuming on, 11
Isomorphism of silver halides, 45, 46

Johnsen's study of sodium uranyl acetate, 71

Latent image, ammonia development of, 11, 12, 18
destruction of, 12

Mercuric iodide, Luppo-Cramer's study of, 62

ripening of, 62, 63

transition temperature of, 62
Mercuric oxide, Ostwald's work on, 59
Mohr's salt, Wulff's work on, 58
Moisture, effect of, in ammonia-fuming, 18, 19

Nernst's theory of heterogeneous chemical reactions, 31
Nuclei, colloidal gold, 24, 53

colloidal silver, 23

crystallization, 12, 13, 24

effect of gelatine on, 56

from silver halides, 23

gelatine as filter against, 35, 53

ripening, 13

Octahedra, 86, 100, 106, 108

directions of most rapid growth in, 107, 116, 117
Opacity, 14

Optical anomaly, 69, 122-127
Ostwald ripening, 12, 27, 61, 62, 63, 67, 72,^99
Ostwald's law of stages, 68, 69

study of mercuric oxide, 59

Panchromatizing effect of colloidal silver, 53
Pentagonal dodecahedra, 95, 97
Photohalides, cause of color of, 53
Photomicrographs, preparation of, 83
Plates, exposed, fuming of, 18

unexposed, fuming of, 13

used in fuming experiments, 14
Precipitation, of silver bromide crystals, 81, 85
of silver halide pairs, 44
von Weimarn's theory of, 30
Pseudomorphs, 21, 72, 86, 87

Reaggregation, initiation of, by nuclei, 22
Relay principle of crystal growth, 40

141

Reversal, 19, 20, 21, 23

Ripening, 27; as thermodynamic process, 106

Bancroft's theory of, 56

Bellach's study of, 61

of emulsions by ammonia, 11

of mercuric iodide, 62

Ostwald, 12, 27, 61, 62, 63, 67, 99

Sensitivity, and surface energy, 104, 106

effect of ammonia-fuming on, 11
effect of gelatine on, 56
effect of method of mixing emulsion on, 33
Sensitizers, 48; operation of, 50

panchromatic, colloidal silver as, 50
photochemical conception of, 48
Silver, colloidal, and silver halides, 53

as panchromatic sensitizer, 50
effect of gelatine on adsorption of, 53
Silver bromide, classification of, 81, 95, 97
crystal forms of, 89-94
crystallization of, 75
crystallographic investigations of, 80
directions of most rapid growth of crystals of, 107, 116,

117, 118, 120
hemihedrism of, 97
in polarized light, 121
modifications of, 97
plates, 92

polymorphism of, 121

preparation of, for microscopic study, 81, 85
sensitivity and surface energy of, 104
structural anomalies in crystals of, 107, 108
Silver chloride, classification of, 97
Silver halides, and colloid silver, 53

birefringence in crystals of, 47
capacity of, for solid solution, 49, 79
color of mixtures of, 44, 47
crystalline form of mixtures of, 79, 80
double compounds of, with ammonia, 16, 25, 86, 87
effect of gelatine on, .35
effect of, on dispersity, 42
homogeneity of mixtures of, 44, 46
miscibility of, 45, 46
mixtures of, 44 45, 46, 97
photochemical sensitizing of, 47
precipitation relations of, 44
preparation of, for emulsions, 27
solid solutions of, 49, 79
structure of, in photographic emulsions, 125
Thiel's study of, 44
Silver iodide, buffer effect of, 66
classification of, 97
polymorphism of, 63
transition temperature of, 79
Silver iodo-bromide, crystalline form of, 79
in polarized light, 121

Sodium uranyl acetate, Johnsen's study of, 71
Solubility and surface tension, 66
Stability of crystal forms, 67

142

Stark's theory of fluorescence, 49
Structural anomalies, 107, 108
Supersaturation, absolute, 22

specific, 22
Surface tension and solubility, 66

Tammann's theory of crystallization, 28
Temperature, effect of, on dispersity, 38

transition of silver bromide, 40
of silver iodide, 79

Tetrahedra, 97

Thiel's investigation of silver halide pairs, 44
Transparency on fuming, 14, 15
Twinning, 68, 69, 70, 71, 104

Unexposed plates, fuming of, with ammonia, 13

Viscosity, effect of, on dispersity, 36
von Weimarn's theory, 27 et seq.

Wulff 's study of Mohr's salt, 58

143

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