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THE CHEMICAL EFFECT OF THE SPECTROM.
BY
..j<?y%
Dr. ]rM? ^DER,
PROFESSOR AT THE TECHNISCHE HOCHSCHULE, VIENNA.
TRANSLATED AND EDITED BY
CAPTAIN W. de W. ABNEY, R.E., F.R.S.,
Vice-President of the Photographic Society.
{Reprinted from the " Photographic Journal" 1881 and 1882.]
London :
HARRISON AND SONS, 59, PALL MALL.
1883.
Price Two Shillings.
(oO\
HARRISON AND SONS,
PRINTERS IN ORDINARY TO HER MAJESTY,
ST. MARTIN'S LANE, LONDON.
-X5"l<\
TRANSLATOR'S PREFACE.
The original of this small book is in German, but it
subsequently appeared in French, and from a copy of the
latter, annotated by the Author, it was translated for the
pages of the Photographic Journal The author, Dr. Eder,
^ - who is so well known for his practical and scientific acquaint-
]l ance with photography, has in this work shown the depth
:\of his knowledge of the photographic literature of every
{^country; for the size of the compilation can by no means
<*,be taken as a measure of the labour it involved. Its value
^ as a book of reference can scarcely be overrated, and it
is for this reason that it has been deemed advisable to
reprint it from the Photographic Journal. From some of
the conclusions which the Author draws, the Translator and
Editor has ventured to differ, but as to the facts there is
no difference of opinion between them.
W. de W. A.
November ", 1883.
CONTENTS.
PAGE
I
6
13
CHAP.
I. — Introductory
II. — Action of Light on Silver Salts
III. — Photographs in Natural Colours
IV. — Influence of the Developer upon the Sensitiveness to Light
of the Haloid Salts of Silver 21
V. — Sensitizers ... ... ... ... ... ... ... 26
VI. — Dyed Films 35
VII. — Reversed Action of Light 48
VIII. — Action of Light on Metallic Compounds 55
IX. — Action of Light on Iron Salts ... 60
X. — Action of Light on the Salts of Uranium, Vanidium, &c. ... 65
XI. — Action of Light on the Iodides, &c 69
XII. — Action of Light on Organic Compounds 75
XIII. — Action of Light on Plants and Animal Life 81
X I V.~ General Effect of the Spectrum 86
THE CHEMICAL EFFECT OF
THE SPECTRUM.
CHAPTER I.
INTRODUCTORY.
For nearly half a century detailed experiments have been
made on the chemical action of luminous rays. These
experiments are very delicate, and the reactions which pro-
duce them extremely difficult to render useful ; for not only
the quality of the chemical agents used, but also the quality
of light which acts upon them and causes the reaction has to
be taken into account.
It is interesting to see in so many years how much has
produced conclusive results. It is altogether impossible to
deny that modern photo-chemists are those who, by aid of
spectrum analysis, have explained the chemical reactions pro-
duced by light.
Of all the chemical compounds the salts of silver have
been most studied in regard to their behaviour in light. It
is upon these salts the observation was first made, that
all rays of the spectrum have not the same chemical action.
For this reason we shall first occupy ourselves with the action
of the solar spectrum on the different silver-compounds, as
they are, above others, those which occupy the chief position in
photography.
Nearly all the salts of silver take a deep colour when exposed
B
2 THE CHEMICAL EFFECT OF THE SPECTRUM.
to the action of white light, and for nearly all the blue and
violet rays have a more intense reducing action than the red
rays. In 1777 Scheele had already written, in his work "Von
der Luft und der Erde," that of all the rays of the spectrum the
violet most blacken silver chloride.
Sennebier, repeating Scheele's experiments, found that
chloride of silver was darkened by violet light in 15 seconds
in the same manner as if exposed to the yellow rays for
5^ minutes, or during 20 minutes to the red rays.
Berard (Gilbert, " Annales de Physique/' 181 3, vol. vi) and
Seebeck (Goethe, " De la Science des Couleurs," 1820, vol. ii,
fig. 716) confirmed these experiments at the commencement of
this century, and remarked that the extreme limit of the violet
has the most intense chemical action. They impregnated
paper with silver chloride, exposed it to the solar spectrum,
and saw that the violet region strongly blackened it, the blue
less, the yellow very little, or not at all, and the red very
feebly.
Berard made another very interesting experiment. He col-
lected by means of a lens the rays of the spectrum between
the violet and the green, to form a beam of white light, and
he proceeded in the same way to produce a brilliant white
point by collecting the rays between the yellow and the red :*
despite the heat of this beam, it had no action on silver
chloride, although the exposure was prolonged for two hours.
The other beam, although less intense, blackened silver chloride
in ten minutes.
On the 22nd February, 1801, Ritter ("Des Rayons
Lumineux Chimiques," Gilbert, "Ann. de Phys.," vol. vii,
p. 525, vol. xii, p. 409) found that silver chloride was quickly
darkened also by the rays which lay beyond the violet of the
solar spectrum. He deduced from this that there were two
kinds of invisible rays in the solar spectrum, the one below
the red (where is found, as is known, the invisible-heat rayst),
[* There seems to be some mistake regarding the colour of brilliant dots
produced. Yellow and red will never make white, neither will violet, blue,
and green. — Editor .] •
[t This term is that which is usually adopted ; but it must be remem-
bered that rays are neither "heat rays nor "chemical rays." The same
rays may have a heating effect or a chemical effect, according to the kind
of matter on which they falL — Editor •.]
INTRODUCTORY. 3
the others beyond the violet possessing above all chemical
properties. Wollaston (" De certains Effets Chimiques de
la Lumifere," Gilbert, "Ann. de Phys.," vol. xxxix, p. 291)
confirms Ritter's deductions. It is recognised that solar light
contains numberless rays completely invisible, which neverthe
less well show their action on a photographic plate.
It is better to submit the substances to be experimented
upon to the action of each region separately. Experiments
made with pure spectrum colours — that is to say, colours
produced through the decomposition of white light by means
of a prism — are more exact than those made by means of
variously coloured glasses. Light coloured by its passage
through differently coloured glasses, like the spectrum colours,
shows a totally different chemical action according to the
colour of the glass through which it has passed ; and this
Seebeck had already indicated. He found that silver chloride
darkened under the action of light, after having passed through
violet, blue, and green-blue glasses, whilst it preserved its
colour after passing through yellow, green, and red glasses.
Other yellow substances take away the blackening property
of sun-light : thus silver-chloride paper remains unaltered by
light when it passes through chromate of potash,* sulphide
of ammonium, the chlorides of iron, gold, and platinum
(Draper, " PhiL Mag.," vol. xvi, p. 81). Becquerel, in his
interesting Memoir ("Ann. de Chim. et de Phys.," 1843,
vol. ix), made very curious observations upon the use of
coloured glasses. He examined the rays which these glasses
transmitted, and remarked that the light so obtained was not
homogeneous when examined by the spectroscope ; and that
for this reason it was useful to try coloured glasses in the
spectroscope before adopting them for photographic experi-
ments.
As the blue and violet light of the extremity of the spectrum
* In 1855 Hunt recommended a mixture of bichromate of potash and
gum or glue for tinting the windows of the dark-room in which sensitive
plates have to be prepared (Hunt's " Photography "). I cannot recom-
mend this preparation, because mixtures of this kind become brown or
green under the action of light, and are then not so safe. For the action
of other colouring matters, and particularly organic matters, see Burdy,
"Bulletin de la Societe* Francaise," 1879, P» 98; Abney, T6th, and Eder,
" Photographic Journal," 1879.
E 2
4 THE CHEMICAL EFFECT OF THE SPECTRUM.
has the most considerable reducing effect on most compounds,
for a long time, in fact since the discovery of photography,
photographic laboratories have been glazed with yellow or red
glass ; this light, it is true, does not act upon the usual photo-
graphic preparations, and it is for this reason that it is customary
to consider this light as non-actinic.
All rays of the spectrum, then, have not the same chemical
energy; and on different compounds the decomposing action
by various colours is also variable.
The closely allied haloid compounds of silver, under the
influence of coloured rays, exhibit decompositions which are
very diverse in character. This was first indicated by Sir John
Herschel in 1840, and minutely described by Robert Hunt, in
his very complete work (" Researches on Light," 1840), as
regards daguerreotype plates. The influence of the spectrum
upon wet-collodion plates was first investigated by J. Mtiller
(Pogg., vol. lxvii, p. 133). He caused spectra to be photo-
graphed by Haase, at Fribourg. He remarked that it was
only the region of the spectrum above G which was impressed
on the plate. The plate also showed an impression of the
fluorescent spectrum far beyond the ordinary visible spectrum.
A wet-collodion plate, exposed during fifteen seconds, showed
a spectrum with all the Fraunhofer lines from G as far as
the group P, which Stokes had observed by means of
fluorescence. The rays H, L, M, N, and O appeared exactly
as in Stokes's spectrum, observed by means of quinine, which
is a proof that the same rays which can produce fluorescence
can also act upon sensitive salts of silver. In 1857
Helmholtz presented, at a meeting of the Socidte du Bas
Rhin, at Bonn, some very successful photographs of the ultra-
violet end of the spectrum. Since then, the solar spectrum
has often been photographed. Rutherfurd, of New York,
has photographed a spectrum 2 '50 metres long, in fifteen
separate pieces. The spectra of gases, &c, have also been
photographed.
The material of the prism, by means of which the solar
light is decomposed, is not without influence on the separation
of the rays themselves in the spectrum. At first it was con-
sidered as a matter of indifference as to the kind of glass
which formed the prisms, since it was believed that each
coloured region would have the same relative action. Thus it
INTRODUCTORY. 5
was that Herschel, ordinarily so conscientious, has not
noted anywhere the density of the glass of his prism, with
which he discovered the greatest heating intensity of the spec-
trum to be the infra-red rays.
Seebeck had already worked in this direction from 1806 to
1808, and it was only in 181 9, through his works published in the
Journal of the Berlin Academy of Science, that he made known
his experiments as to the differences obtained when using glass
of different compositions. Since then, this question has been
much studied.
Further, the atmospheric constitution of the sky influences
the results obtained. H. W. Vogel has shown that the
chemical intensity of the solar spectrum, on account of the
variable transparency of the atmosphere, is subject to very
considerable fluctuations. Thus, the maximum of intensity for
the reduction of silver chloride is sometimes found in the ultra-
violet and sometimes in the violet, and often even in the blue,
—even when employing pure silver salts.
It is curious that these compounds of silver behave diffe-
rently to light, according to the manner in which it acts, —
that is, according as they are allowed to darken sponta-
neously, or as their colour is developed. The nature of the
development has also a considerable influence (mercury, sul-
phate of iron, pyrogallic acid, &c.).* To produce an image
by development, it is necessary to give a shorter exposure than
if an image is to be obtained without development This is
true for all colours of the spectrum with which it may be
desired to obtain an image on the salts of silver.
* The latent image can be developed by salts of uranium (Van Monck-
hoven), by glucosides, sugar, organic salts of iron, resins, ethereal oils,
organic bases (Carey Lea, "British Journal of Photography," No. 849)
Relatively to the spectrum, these developers have not yet been experi-
mented with.
CHAPTER II.
ACTION OF LIGHT ON SILVER SALTS.
We now give in detail the effect of the action of coloured
light on each salt of silver.
Silver Nitrate, in a pure state, or in an aqueous solution
(without contact with organic matter), is insensitive to light.
With organic substances, such as albumen, gum, paper, &c,
it blackens rapidly in blue light, whilst red light has no effect
on it Paper impregnated with silver nitrate blackens when
exposed to the solar spectrum, from the violet to the yellow.
Wedgwood remarked in 1802, that nitrate of silver paper is
only very feebly attacked by the red rays ; the green and the
yellow rays do possess darkening power, but the most intense
and rapid effects are found in the blue and violet ("Ann. de
Phys.," vol. xiii, p. 113). Under the action of the rays coming
from the blue region of the spectrum the paper takes a blue-
brown tint, whilst the violet rays give a peculiar rose-colour.
The rays of minimum activity are found in the middle of the
yellow ; the maximum, in the middle of the blue. From this
point there is great diminution of activity to the limit of the
violet The ultra violet only darkens it very feebly (Herschel,
"Photographic News," 1859, vol. iii, p. 2).
Silver Iodide. — If a paper impregnated with silver iodide
is exposed to the rays of the solar spectrum, according to
Herschel, an image is obtained from the red to the violet ; the
impressions produced by the violet go further with this com-
pound than with silver bromide (" Photographic News," vol. ii,
p. 279). The silver iodide paper is most sensitive to the ultra-
violet rays of the spectrum, the maximum of intensity depend-
ing on the quantity of free silver nitrate present. The effect
commences at the middle of the red, where the paper takes
ACTION OF LIGHT QN SILVER SALTS. 7
an ashy-grey tint, whilst under the action of the less refrangible
rays it takes an orange tint The action of the ultra-violet rays
is shown further on the silver-iodide paper than on paper im-
pregnated with the chloride or bromide of the same metal
(Hunt's " Researches on Light ").
- The experiments which we have just described give an
image directly visible to the eye without the aid of develop-
ment. The action of light is quite another thing when, after
once being impressed, the image is brought out by a de-
veloper. Daguerreotype plates, with silver iodide developed by
mercury, only show a very feeble sensitiveness to red light;
they are, nevertheless, coloured slightly rose. Herschel and
Hunt did not find any activity in the yellow and orange, and
in the green the action was scarcely distinguishable. The blue
and the violet rays had, on the other hand, a very intense
action on these plates. The maximum action was found
between G and H, extending from the ultra violet as far as F
(Becquerel). When sufficiently long exposure was given
(Draper), or when the silver-iodide plate was exposed to dif-
fused light before exposure, the spectrum from the red to the
ultra-violet was impressed in a very appreciable manner
(Becquerel, " La Lumi&re," p. 891).
Films of silver iodide developed with pyrogallic acid are not
at all, or very little, influenced by the red ; in the orange, the
yellow, and the green they do not change, according to
Crookes, even after a very long exposure ; in the blue, and in
the region beyond, they are strongly affected. If they are
exposed for a longer time than is necessary for the blue and
violet rays to have their proper action, there is " solariza-
tion," * though it is not apparent in the yellow, orange, or
red. The place of maximum action is found in the indigo
and the commencement of the violet. Monckhoven found
that silver iodide was sensitive to the limit of the violet
Schulz-Sellack (" Berliner Berichte," vol. iv, p. 210) has stated
that a plate prepared in the ordinary manner with iodized collo-
* If salts of silver are exposed for a long period to light, their power of
giving an image by a developer increases up to a certain limit. After a
very prolonged exposure this intensity decreases, the image becomes feeble,
and scarcely shows itself on development. This phenomenon goes under
the name of solarization.
8 tHE CHEMICAL EFFECT OF THE SPECTRUM.
dion (viz , by coating a glass plate with iodized collodion, im-
mersing it in the silver-nitrate bath), and developing with ferrous
sulphate, is sensitive to the rays of the spectrum from the ultra
violet as far as G. Becquerel states the same thing ("La
Lumifere," p. 87).
Vogel has remarked that the chemical action of the dif-
ferent colours of the spectrum is totally different. It differs
accordingly as pure salts of silver are applied, with the addi-
tion of silver nitrate or an organic sensitizer. The sensitive-
ness towards the red then increases in a notable degree.
Collodion iodized with cadmium iodide was sensitized in a
silver bath ; after having been washed, the plate was dried ;
the plates thus prepared were exposed, and, after well washing,
dried, and the development carried out by ferrous sulphate
acidified with citric acid to which was added silver nitrate :
silver bromide and chloride were also used, but in that case
the development, was carried out by an ammoniacal solution
of pyrogallic acid with the addition of potassium bromide.
Dry silver iodide of this kind was found sensitive from H
to D, sometimes even to A. The maximum of sensitiveness
was found between G and F. Silver iodide in the wet state
is less sensitive in the green. The manner in which silver
iodide behaves when exposed to light with free silver nitrate
is very remarkable. Its sensitiveness increases from the ultra
violet to G, where it attains its maximum, and there it abruptly
falls. (This is according to Becquerel) Silver iodide thus
shows its maximum of sensitiveness in the blue-indigo part of
the spectrum.
When the exposure is sufficiently prolonged, silver iodide is
sensitive to all the colours of the spectrum as far as A in the
red ("Pogg. Ann.," vol cl, p. 453).
Silver Bromide, — Silver bromide discolours as a general
rule more rapidly than the iodide of the same metal A
small quantity of free silver nitrate aids its decomposition
by light, but nevertheless not so markedly as it does the
iodide.
Silver bromide on paper with an excess of silver nitrate
prepared by double decomposition is acted on by the spec-
trum as far as the extreme visible part of the red. The
maximum intensity is found in the indigo (Herschel). The
ACTION OF LIGHT ON SILVER SALTS. 9
r
*
effect reaches from H to F; if, however, a preliminary exposure
to diffused light is given to the paper for a short time, it blackens
under the influence of the spectrum as far as the red (Becquerel,
" La Lumiere," p. 86).
Daguerreotype plates sensitized with bromine and developed
with mercury are more sensitive to the green than those sensi-
tized with iodine. It is for this reason that, in 1840 and
1842, Hunt recommended the use of the former (" PhiL
Trans."). He said on this subject, " That a new photography
ought to be created, the foundation stone of which would be
bromine." As a matter of fact, daguerreotype plates were
produced with bromo-iodide of silver ; chlorine was also often
employed.
Wet-collodion plates with silver bromide, with excess of
silver nitrate, developed by pyrogallic acid, are infinitely less
sensitive to white light than if prepared with iodide, and the
images obtained have very little vigour ; on the other hand,
.green is admirably reproduced (Herschel). William Crookes
had observed this action, and indicated that it was produced
from E and F as far as H in the violet. Schultz-Sellack
found that the bromide of silver, with excess of silver nitrate
{prepared and developed as we have shown before when
speaking of the iodide), is sensitive to near F in the blue;
Becquerel found as the limit the line E ("La Lumiere,"
p. 86).
According to Dr. H. W. Vogel (" Phot. Corr.," vol. x, p. 212),
silver bromide prepared in a wet-collodion plate (and with
excess of silver nitrate) is particularly sensitive to the more
refracted rays in the blue of the visible spectrum when
development takes place by acid ferrous sulphate; on the
other hand, silver bromide in a collodion dry plate (obtained
by washing and drying ordinary wet plates) shows a greater
range of sensitiveness to colour, and it is impressed by the
rays as far as D in the orange, and even into the infra red.
The presence of silver nitrate intensifies the action of the blue
and the green rays. When sufficiently long exposure is given
to pure dry bromide of silver in a collodion film it is sensitive
to the extreme red, in such a way that the line A is quite
♦distinctly seen ; even a part of the infra-red region is taken —
a part which equals in length the distance of B irom A
{"Berliner Berichte," vol viii, p. 1635). Alkaline develop-
IO THE CHEMICAL EFFECT OF THE SPECTRUM.
ment with pyrogallic acid causes greater sensitiveness to red
light*
Silver bromide prepared with an excess of soluble bromide
with gelatine, with a sufficiently long exposure, according to
Monckhoven, gives an image from the ultra violet into the red.
The blue and the violet rays have the strongest effect; the
red, on the other hand, the least. He has also remarked
.that freshly made silver-bromide emulsion is orange by trans-
mitted light, and that its sensitiveness extends from the ray
F as far as M. Old and ripened emulsion, in which the
silver bromide is molecularly changed, appears blue by trans-
mitted light, and its sensitiveness extends as far as A in the
red. Very old green emulsion is even sensitive as far as the
infra-red rays. (See further,: for the sensitizing influence of
gelatine.)
Silver Chloride. — Pure silver chloride is chiefly changed
and blackened in the regions which extend from the blue to the
ultra-violet. Silver-chloride paper containing an excess of
silver nitrate blackens first in the ultra-violet, then between
the rays H and G, and, lastly, in the blue as far as the ray F.
If silver chloride has been exposed beforehand for a few mo-
ments to the light (without, however, the time of exposure being
enough to allow a visible impression of the light), it blackens
as far as the infra red, according to the experiments of Seebeck,
Herschel, Poitevin, Zenker, and others. On paper treated
like this we find two places where the light has acted most
vigorously — one situated between the rays G and H, the other
between D and E (Becquerel, " La Lumiere," p. 84).
H. W. Vogel gives the following explanation of this pheno-
menon ("Pogg. Ann.," vol. cxxxv, p. 284). He says that
silver chloride, having taken a violet tone by the light,
acquires thereby the faculty of absorbing more strongly the
yellow and orange rays of the spectrum, in such a manner
that the rays which at first appeared inactive become by that
means active. It may be said that in the process with silver
chloride, that photographers daily employ, it is the violet
and ultra-violet rays which are particularly active. The same
thing is observed in Bunsen and Roscoe's actinometer, in
* Upon the influence of the molecular structure of silver bromide as ta
its sensitiveness to colour, see subsequent pages.
ACTION OF LIGHT ON SILVER SALTS. II
which chloride of silver paper is employed for their pendulum
photometer.*
Silver chloride mixed with collodion, with an excess of silver
nitrate (and with development), is sensitive from the ultra
violet rays as far as G. Dry collodio-chloride of silver shows
after development, a blackening from the violet as far as the
red, when the exposure has been sufficiently prolonged ; with
a very prolonged exposure the action extends as far as A.
If it be employed for dry plates, it will be found to be much
less sensitive fo the light than silver bromide, and that it
requires twice as long an exposure (Schultz-Sellack ; H. W.
Vogel; "Pogg. Ann.," vol. cliii, p. 223; Becquerel, "Phot.
Mittheilungen," vol. xi, p. 182).
Silver Fluoride is much less sensitive to the action of light
than the chloride, bromide, or iodide of the same metal.
It does not darken strongly when exposed to direct light, and
does not give good latent images capable of being developed.
According to Monckhoven, it is less sensitive to the ultra-violet
than the iodide and bromide, whilst, on the other hand, it is
more sensitive to the yellow and green regions.
A Mixture of Silver Iodide, Bromide, and Chloride
ordinarily unite the qualities of each of its elements ; so that
a product of this kind ordinarily gives a qualitative and quan-
titative sensitiveness greater than each of its factors separately.
It is for this reason that a mixture of this kind is used in
photography.
When daguerreotype plates are sensitized by a mixture of
bromine and iodine, or iodine and chlorine, they give the
spectrum from the ultra violet (the same as silver iodide alone)
as far as the infra-red. The maximum intensity is situated
between the rays G and H (Becquerel, " La Lumifere," voL ii.,
p. 91). For this reason in the practice of daguerrotype the use
of iodide alone was soon abandoned, and the plates were sen-
sitized by iodo-bromine or iodo-bromo-chlorine, and much
superior results were obtained to those which were got by the
first method.
* The time necessary to produce an exactly determined discoloration is
proportional to the chemical intensity of the light (see Bunsen and Roscoe,
"Phil. Trans.").
12 THE CHEMICAL EFFECT OF THE SPECTRUM.
In the wet-collodion process silver bromo-iodide is almost
exclusively employed (in presence of an excess of silver nitrate,
and acid development by ferrous sulphate). This mixture is
more sensitive than each of the two elements taken separately
(Carey Lea, " Brit. Journ.," 1875). Mttller and Sdiultz-
Sellack have shown that the sensitiveness of the silver bromo-
iodide extends further from the violet region towards the red
than silver iodide does by itself. The last of these two
savants found that films of bromo-iodide reached in sensi-
tiveness from the ultra-violet as far as the ray E in the
green. Thus the sensitiveness to colour is more considerable
in a mixture of iodide and bromide than in each of the two
salts taken separately. According to H. W. Vogel, silver
bromo-iodide, mixed with collodion in presence of an excess
of silver nitrate, is sensitive as far as the ray D when deve-
loped with acid ferrous sulphate. Its maximum of sensitive-
ness is situated about the line G : under the same circum-
stances, when the iodide is greater in quantity than the
bromide, it is remarked that the ultra-violet and violet regions
are chiefly impressed on the bromo-iodide ; on the other hand,
when the bromide has the preponderance, the light blue and
green regions give the maximum intensity.
Pure and dry silver bromo-iodide is less sensitive in the
ultra violet and in the green than when it is wet (in presence
of silver nitrate), and is in general much less modified by the
action of light.
CHAPTER III.
PHOTOGRAPHS IN NATURAL COLOURS.
Whilst silver iodide or bromide, whether directly printed
upon in the spectrum or if the latent image is developed,
shows an impression by a blackening more or less intense, and
never gives a coloured image, it is nevertheless possible with
silver chloride to reproduce natural colours. Dr. Seebeck, of
Jena, was the first who in 1810 obtained a reproduction of a
spectrum in natural colours on chloride of silver paper. In
the " Farbenlehre " of Goethe, he says (see also Dr. Roth,
" Fortschritte der Photographie," 1868, p. 20), "When I
project a spectrum, produced by a prism correctly constructed,
upon moist chloride of silver paper, and if I continue the
printing for fifteen to twenty minutes, giving a constant
position to the spectrum by any means, I observe as
follows : — In the violet band the chloride is a reddish
brown (sometimes more violet, sometimes more blue), and
this coloration extends well beyond the limit of the violet,
though it is not stronger than in the violet ; in the blue
band the chloride takes a clear blue tint, which fades out,
becoming lighter in the green. In the yellow I usually found
that the chloride was intact ; sometimes, however, it had
a light yellow tint ; in the red, and beyond the red it took a
rose or lilac tint.
"This image of the spectrum shows, beyond the red and
the violet, a region more or less light and uncoloured ; this
is how the decomposition of the silver chloride is seen in
this region. Beyond the brown band, of which we have
written above, and which was produced in the violet region
14 THE CHEMICAL EFFECT OF THE SPECTRUM.
and beyond, the silver chloride was coloured a grey-violet for
a distance of several inches. In proportion as the distance
from the violet increased, the tint became lighter. Beyond
the red, on the contrary, the chloride took a feeble red tint
for a considerable distance. When wet silver chloride, having
received the action of light for an equal time, is exposed, the
blue and violet behave as above. In the yellow and red
regions, on the other hand, it is found that the silver chloride
becomes lighter; and although this phenomenon is not very
intense, it is nevertheless quite visible, and cannot be mistaken.
The same can be remarked as in the preceding case : the part
acted on by the red rays, and by those beyond, take a light
red coloration."
Thus wrote Dr. Seebeck in 1810. There was, therefore,
nothing new in the communication made in 1839 by Sir J.
Herschel ("Athenaeum," No. 621), that is to say, that when
chloride of silver paper, which was preliminarily darkened in
light, was exposed to the spectrum, the paper took tints analo-
gous to red, green, and blue.
In 1848 the French physicist, Edmond Becquerel, succeeded
in reproducing upon a daguerreotype plate not only all the
colours of the spectrum, but also, up to a certain point, the
colours of drawings and of certain objects. Niepce pursued
the subject, and the colours which he obtained were much
more intense and vivid than those obtained by his prede-
cessor.
When a silver plate, properly polished, is covered with a
thin film of chloride of the same metal, this film is altered
under the influence of the spectrum, showing natural colours.
The silver plate may be chlorinated in different ways; but
then the results obtained differ. A silver plate is immersed in a
solution of ferric chloride or cupric chloride (Becquerel), or
in a mixture of two solutions — one potassium chloride and
the other cupric sulphate. It is then washed and dried (Niepce
de St Victor, " Comptes Rend.," vol. xxxi, p. 491). It may
also be immersed in chlorine water till it takes a feeble white
rose tint (Becquerel).
Becquerel preferred voltaic chlorination. He operated
thus : — The silver plate, taking the place of the positive pole,
is immersed in a solution of 12*4 per cent, of hydrochloric
acid in water; the negative pole is formed of a platinum plate.
PHOTOGRAPHS IN NATURAL COLOURS. 15
At the end of a minute it takes successively a grey, yellowish,
violet, and blue tint, which are again repeated in the same
order. The moment that the violet coloration begins to
appear for the second time, the operation is suspended the
plate is washed, and dried over the flame of a spirit lamp.
The silver plate thus prepared reproduces all the colours of
the spectrum. The blue and the violet are reproduced with
the greatest intensity ; the green is only feebly shown. If the
plate is warmed to ioo° C, which gives the film a rose-
colour, the sensitiveness is particularly increased for the yellow
region.
(More precise directions can be found in the account given
by Becquerel in the Seance of the Socie'te Photographique de
Paris on the 18th December, 1857, and in Dingler's "Polytech-
nischen Journal," vol. cxxxiv, p. 123.)
The sensitiveness of the film of silver chloride to coloured
light depends on its thickness and on the concentration of
the chlorinating solution, but still more on the purity of the
silver, which ought not to contain more than 10 per cent of
copper (Niepce, Martin, " Handbuch der Photographic,"
1857, p. 311, 1 st and 2nd parts). The chloride of copper
gives to the colours a greater vividness than chlorine-
water alone. The use of very dilute chlorine is favourable
to the reproduction of the yellow region ; very concen-
trated chlorine gives, on the other hand, the orange and
the red with great intensity. The employment of a mixture
of magnesium chloride and of cupric sulphate is to be
recommended.
At a later date, Nifcpce chlorized by means of calcium
chloride. This alkaline bath did not give such constant
results as the chlorinating bath previously used by Becquerel
(see above) ; this method is, however, very simple. The sensi-
tiveness of a prepared silver plate is strongly exalted if it is
treated, before exposure, with a saturated solution of lead
chloride in dextrine (4th Memoir, " Comp. Rend.," 1862,
vol. liv, p. 281).
Upon films of this description positive pictures are always
obtained; that is to say, the black lines of a copper plate
engraving are reproduced in black (loc. cit, p. 299).
Sometimes coloured images are obtained on collodion films
containing silver chloride. The colours appeared after fixing
l6 THE CHEMICAL EFFECT OF THE SPECTRUM.
with potassium cyanide, and when the vapour of chloride of
iodine is allowed to act ("Bulletin de la Societe Francaise," 1857,
p. 1 1 6) . Chloride of silver emulsion, with a slight excess of silver
nitrate and citric acid, is coloured, as is known, a slate-coloured
grey by the action of light : under a red glass it becomes
red ; under aniline green it becomes green (Simpson, " Photo-
graphic News ").
Pure silver chloride upon paper becomes rapidly violet in
the ultra-violet; but in the visible part of the spectrum it
changes slowly ; if it is given a preliminary exposure to diffused
sunlight, so as to form violet subchloride, it gives the spectrum
colours within the same limits. The yellow and green regions
are, it is true, feeble and scarcely visible.
Silver chloride, in the presence of an excess of silver nitrate,
blackens only in the ultra-violet; but, if exposed first to
diffused light, it is also sensitive in the visible spectrum from
the red to the blue ; the colours appear only feebly, but can,
however, be easily distinguished (Becquerel, " Phot. Archives, ,r
1868, p. 300). By adding to it appropriate substances, e.g., salts
rich in oxygen, the violet silver chloride gives upon paper much
better images.
Poitevin prepares ordinary photographic paper, first with a
layer of silver chloride by floating it upon a solution of sodium
chloride, and after upon a solution of silver nitrate. After
having washed the excess of nitrate, the paper is immersed
in a very dilute solution of stannic chloride; the bath and
its contents are exposed for five or six minutes to diffused
light ; the paper is taken out, and well washed, to increase the
sensitiveness of the silver subchloride thus formed upon the
paper, it is then treated with a solution of potassium
bichromate and cupric sulphate. The sheets of paper, dried
in darkness and exposed under coloured pictures on glass
or in an enlarging apparatus, gave coloured images which
could be fixed by sulphuric acid (De Roth, " Fortschritte
der Phot.," 1868, p. 22; "Comptes Rendus," 1868, vol. lxi,
p. 11).
Later, St. Floretnt specially occupied himself in experiments
tending to reproduce the spectrum colours by means of silver
chloride (" Bulletin de la Socie*te Francaise," 1874). Chloride
of silver paper is prepared by him in the following manner :
PHOTOGRAPHS IN NATURAL COLOURS. 1 7
— He immerses ordinary or albumenized paper in a solution
of silver nitrate; he afterwards plunges it into a solution of
uranium nitrate and zinc chloride in alcohol acidulated by
hydrochloric acid; the paper is then exposed to. light till it
takes a violet-blue or lavender tint. Before exposure, the
paper is sensitized in an acid solution of mercury nitrate, and
it is more or less dried on blotting-paper. Under the action
of light the mercuric chloride which is formed by this im-
mersion is transformed into mercurous chloride and chlorine,
which transforms the subchloride of silver into white-silver
chloride, and at the same time takes up such a particular
•physical constitution as to render it capable of taking impres-
sions of the luminous coloured rays. When the silver chloride
is placed in concentrated nitric acid, it is coloured when
exposed beneath a coloured negative. It is to be noted,
however, that all the colours appear mixed up, from the red
to the violet, which is probably due to the preponderating
influence of the infra-red and ultra-violet rays. If collodion
is used to retain the silver chloride, a helio-chromograph upon
glass can be obtained.
The nature of the chloride, that is to say, of the metal with
which the chlorine is combined, influences so much the quality
of the silver chloride obtained by its action, that the action
of light altogether varies according to the metallic chloride
employed. This influence has already been remarked upon
in speaking of the reproduction of colours by means of the
silver subchloride : the same remarks also hold good when
bromide and iodide of silver are used for photography by
development.
Thus according to the nature of the chloride employed to
precipitate the. silver chloride, and according to the coloration
of the different glasses to the action by which it is sub-
mitted, so does it take different colorations. This is a curious
and remarkable fact, for it cannot be that silver chloride is
affected in its properties by a small quantity of nitrate (formed
from the double decomposition of the soluble chloride and
the silver nitrate) which is indifferent to light, and that it is only
mixed with it by the fact of preparation. Hunt had already
observed this phenomenon : he soaked different papers, im-
pregnated with the chlorides of different metals, in a solution
c
1 8 THE CHEMICAL EFFECT OF THE SPECTRUM.
of silver nitrate, and exposed them to the light under glasses
of different colours.
Chloride employed to
form Silver Chloride.
Coloration produced by the action of Light beneath.
Blue Glass.
Green Glass.
Yellow Glass.
Red Glass.
Ammonium chloride
Potassium ,,
Sodium ' ,,
Barium ,,
Calcium ,,
Manganese ,,
Ferrous , ,
Ferric ,,
Olive brown
Light purple
Purple
Purple
Intense
violet
Intense
brown
Red
Blue
Light brown
Sky blue . . .
Blue
.ullclC • • • • • •
Light blue...
Ruddy
Uncoloured
Yellowish . . .
Brown
Light violet
Violet
Red brown
Blue
Light rose...
Light red ...
Straw colour
Deep
orange.
Red.
Red brown.
Light red.
Ruddy.
Yellow.
Lead colour-
Yellowish
green.
If these papers were darkened in direct solar light, or
under coloured glasses, the following colorations were ob-
tained : —
Coloration produced by the action of Light
White.
Blue.
Deep green.
Yellow.
Red.
Ammonium chloride
Red
Olive
Deep
Dirty
Red.
brown
green
green
yellow
Barium chloride ...
Deep
Deep
Dirty . . .
Green
Red.
brown
brown
brown
Chlorine water
Rich
Blue
Black ...
Deeper
Deep
brown
black
brown
red.
Potassium chloride
Greenish
brown
Brown
black
Blacker
Bluish ...
Ruddy.
By prolonging the exposures, other colorations were
obtained.
Bromide and iodide of silver behave in an analogous
manner. These substances also show, in their decomposi-
PHOTOGRAPHS IN NATURAL COLOURS. 1 9
tion by light, colorations which differ according to the kind
of iodide or bromide which is used in their formation. I have
before me a very old note by Zantedeschi, and by Bortinetto,
dated 1856, "Compt Rend.," vol. xxi., p. 243. This is what
they say : — They made negatives by the ordinary collodion pro-
cess, introducing different iodides, which they developed with
pyrogailic acid. The different tones of the negatives were as
follows : —
Iodide of silver obtained from potassium iodide — black.
„ cadmium „ deep violet.
f colour of
„ zinc „ < light In-
L dian ink.
„ ammonium „ ruddy black.
»5 J»
5> »
„ iodo-hydrate ofl { ±
quinine / F F
Negative collodions containing iodides and bromides give,
according to the metal in which the iodine or bromine is com-
bined, negatives of different aspect — sometimes light, some-
times deep.
The manner of acting on the different bromides in the
collodio-bromide emulsion process has been experimented
upon by Warnerke (" Photographic Journal," February, 1876) ;
his researches were only carried on with regard to white light.
He found that the intensity of the images and the sensitiveness
of the film differed according to the nature of the bromide.
Zinc bromide, for example, gives images more intense than
does copper, and gives a superior sensitiveness to that pro-
duced by other bromides. Collodio-bromide emulsions also
discolour differently according to the kind of bromide used :
silver bromide discolours blue, when bromides of uranium,
ammonium, potassium, and sodium are employed to produce
it ; red, when bromides of cadmium and zinc ; grey, when
bromide of iron and bromine.
Niepce (Martin, "Handbuch der Photog.," 1857, p. 311)
made some very curious observations tending to show the rela-
tionship existing between certain coloured flames and the photo-
graphic images produced by light. He immersed a plate of
silver chloride in chlorine-water, to which he added a chloride
capable of colouring a flame. The red of the solar spectrum
c 2
20 THE CHEMICAL EFFECT OF THE SPECTRUM.
is particularly well reproduced upon plates "of this kind when
the chlorine-water, which is used to chlorinate it, contains
strontium-chloride, which colours the flame purple red. By
adding to the chlorine-water calcium-chloride or uranium-
chloride, the reproduction of the orange region of the spec-
trum is helped for the yellow sodium-chloride. The green
region is reproduced by the addition of boracic acid, nickel
chloride, or a salt of copper. The blue can be obtained by
ammoniacal copper chloride; and, finally, the violet by a
mixture of strontium-chloride and copper sulphate.
Niepce imagined that all the substances producing coloured
flames should also give coloured images by light ; and that all
bodies which do not colour flame, do not give images which
are coloured by light.
I only give these remarks to make this work as com-
plete as possible ; for experiments do not confirm them in any
degree.
CHAPTER IV.
INFLUENCE OF THE DEVELOPER UPON THE SENSITIVENESS TO
LIGHT OF THE HALOID SALTS OF SILVER.
The direct action of light* is shown in its greatest intensity
on silver chloride : it blackens under the influence of the
luminous rays, especially in the presence of organic matter
and silver nitrate with infinitely greater rapidity than do the
bromide and iodide. For this reason it is employed where
photographic images are to be obtained by the direct action
of light, without using development (for example, in printing
negatives with silver-chloride paper) ; on the other hand, the
iodide and bromide are infinitely better for giving, with a
very short exposure, a latent image, which can be developed
by means of reducing agents. The latent action of light
takes place much more rapidly ; and when it is desired to
obtain images in the minimum of time, it is useful to employ
compounds giving a latent image which can be developed
subsequently. For this reason, in modern processes (at least
in processes for the production of negatives) silver iodide
and bromide are sure to have a greater preponderance than
chloride.
The old experiments of Herschel and Hunt showed that
the relative sensitiveness of the compounds of silver vary with
the mode of development adopted. The sensitiveness of
salts of silver to light can be shown in three different ways.
* By the direct action of light, I mean the visible change which light
produces when acting upon a body ; for instance, the blackening of silver
chloride by light, — the property that stuffs have of bleaching, — the oxidation
of iron salts. If, on the other hand, it produces upon a sensitive film an
invisible image, which leaves the film quite intact, but which favours
decomposition by means of other agents, I call that phenomenon a latent
action. Thus a photographic image is developed by mercury vapour, or
the reducing salts of iron.
22 THE CHEMICAL EFFECT OF THE SPECTRUM.
They can blacken visibly in the light (direct action of light), or
give a latent image which can be developed by an acid
developer or by an alkaline developer of pyrogallol.* The
above two methods of development differ from one another
altogether: whilst in the first method the image is formed
almost entirely from free silver nitrate, which precipitates
as reduced and finely divided metallic silver, depositing upon
the parts altered by light, the silver iodide being unre-
duced ; in the second, on the other hand, the silver nitrate
is absent, and it is the bromide alone which, first acted on by
the light, is afterwards reduced. Thus, in the first case, the
parts acted upon by the light attract the nascent precipitated
silver (silver precipitated by ferrous sulphate, for example) ; in
the other, the same parts are attacked with the greatest ease by
certain reducing agents.
We have already spoken of the difference which exists,
i st, between films of wet silver salts, that is to say, those
which are treated with ferrous sulphate and the silver nitrate
which is on the plate, and, 2nd, between the dry plates which
can be developed either by iron and silver, or with alkaline
pyrogallic acid. These two series of experiments cannot be
compared, amongst themselves, with that which is to follow ;
for, to know the effect and power of the developer, pure and
dry films of similar haloid salts of . silver must be operated
upon. These films ought to be developed in different ways,
whilst in the above-described cases the haloid salts of silver
were sometimes exposed pure, and sometimes in the presence
of silver nitrate.
It can be said in general that alkaline development by
pyrogallic acid, with the addition of potassium bromide, gives,
of all known agents, the most intense impressions with the
least exposure. This is quite as true for the violet rays as for
the red rays : a very short exposure to white light can also be
made apparent by the same method. On the other hand,
alkaline development gives also the least active part of the
spectrum, that is to say, the red, with greater intensity than
* To the developing substances indicated and studied by Carey Lea
( " British Journal of Photography," 1877), the ferrous oxalate and lactate,
which act very energetically on dry plates, and develop like the alkaline
pyrogallic acid developer, can be added.
INFLUENCE OF THE DEVELOPER. 23
does development with acid and iron. In relation to this,
I quote the experiments of H. W. Vogel, which, having
been made only with white light, have more importance as
regards the blue and violet rays than for the red and
yellow rays.
H. W. Vogel (" Berliner Berichte," vol. vi, p. 88 ; " Photo.
Corr.," vol. ix, p. 190) prepared emulsions with iodide,
bromide, and chloride of silver, by adding to a collodion
containing silver nitrate an iodide, a bromide, &c. ; he added
iodide in such a way as to have excess of silver nitrate, which
he eliminated by washing in water after coating the plates with
the emulsion : with these he prepared his dry plates. The wet-
collodion plates were prepared in the ordinary manner, and
contained excess of nitrate.
The following is the scale of sensitiveness resulting from his
experiments. The most sensitive salt is named first : —
The wet process, acid development, for strong lights: silver
iodide, bromo-iodide, bromide. For feeble light,
shadows, or those which, by the absence of violet
and indigo rays, are not completely white : bromo-
iodide, iodide, bromide.
It is for this reason that, in practice, where details in the
shadows are desired, a mixture of iodide and bromide of
silver is desirable. Five atoms of iodide for 1 atom of
bromide seem to be the best proportion (Vogel, "Photo.
Mitt.," bk. lix, p. 239).
For alkaline development (pyrogallic acid 1 : 200 and
caustic potash), the nitrate having been eliminated by washing,
either wet or dry : silver bromide, bromo-iodide, iodide.
When developing with neutral pyrogallic acid, which acts,
of course, very slowly : bromo-iodide, bromide, iodide.
When pyrogallic acid, slightly acidulated by acetic acid, is
used alone, it does not develop any image, but does so on the
addition of silver nitrate.
The following is then the scale of sensitiveness : — silver
bromo-iodide, iodide, and bromide.
It is also to be remarked that plates prepared with
bromo-iodide of silver, which is very rich in bromide (1 atom
of bromine to 1 of iodine), are more sensitive to acid deve-
lopment than those which are rich in iodide (1 atom of
24 THE CHEMICAL EFFECT OF THE SPECTRUM.
bromine to 5 atoms of iodine). Development with acid pyro-
gallic acid seems to give, in dry plates, more detail than deve-
lopment with acid iron. I have observed this fact myself on
different occasions.
These observations are still more interesting, as it has been
observed that a bromo-iodized collodion containing a large
proportion of iodide is better for the ordinary wet process.
For this reason Toth and myself recommended, in 1876
(" Photo. Corr.," vol. xiii, p. 97), a collodion containing
plenty of iodide for portraits and reproductions of drawings.
This collodion has given excellent results. Vogel, Kleffel r
and other German authors add to a negative collodion 3 or 4
parts of iodide to 1 part of bromide. It is necessary to add,
however, that many foreign photographers, and particularly
the English, use a collodion richer in bromide, as I remarked
at the time (" Photo. Corr.," vol. * xv, p. 204). Thus
William Heighway adds 3 parts of bromide to the 5 of
iodide which the collodion contains. In dark or cloudy da\s,
when the sunlight is feeble and does not radiate violet rays,
.t is more probable that collodions rich in bromide will give
better results than those which contain a large proportion of
iodide. Bromo-iodized collodion for use in landscape-photo-
graphy, according to experiment, ought to contain a much
larger proportion of bromide than collodion for use in por-
traiture. It ought to contain twice as much bromide as iodide ;
at least, that is the rule for the wet process.
Silver, chloride has not been employed, either in the wet
process or in the dry process ; at least, it has not been gene-
rally used. 1
In the dry process the presence of silver iodide with /
bromide is almost totally without effect, more particularly with
alkaline development, which is now generally used. On the
other hand, a small quantity of silver chloride added to the
silver bromide is not hurtful for dry plates developed by' a
weak alkaline developer. The images are intense, but not
always free from fog, and without doubt silver bromide ought to
form the staple of dry plates.
The sensitiveness to light of the haloid salts of silver is not
only influenced by the mode of preparation, but also by the
mode of development. The alkaline development with pyro-
gallic acid is much preferable to the acid development ; but
1
INFLUENCE OF THE DEVELOPER. 2$
the superiority of the new concentrated pyrogallic-acid deve-
lopers, made strongly alkaline, over the old feebler developers
that were formerly employed, is incontestable. This new
mode of development was introduced by Wortley. When
using very strong alkaline developers the smallest impact of
light causes a reduction of silver bromide. Even the impact
of the yellow and green rays, which are usually very feeble in
their effect, are perfectly shown. Red light, however, affects
the plate the least, and for this reason we are obliged to
prepare very sensitive dry bromide plates (and particularly
gelatino-bromide plates, of which we shall speak further on)
in places lighted only with very deep ruby glass. Without
this precaution the plates are spoilt T<5th and myself found
that a plate coated with a mixture of coralline and fuschine,
dissolved in varnish, answered perfectly that object, and only
allowed red rays to pass. Unfortunately these aniline colours
bleach, and then allow blue rays to pass.
The manner in which light acts on the salts of silver is
complicated, and becomes still more difficult when foreign
substances are added to these salts. By adding certain
substances to the haloid salts of silver, the action of light is
diminished ; whilst by adding others it is increased. The latter
bodies are called sensitizers. H. W. Vogel has studied these
sensitizers, in all their details and all the effects, from two points
of view. The study of these bodies is not only of capital
importance for theoretical photo-chemistry, but also opens out
researches of the greatest interest in practical photography and
modern chromo-photography.
CHAPTER V.
SENSITIZERS.
By adding colouring-matter to a (silver bromide) dry plate
the sensitiveness of the silver bromide is augmented by the
•rays which the colouring-matter absorbs. In this manner
the bromide is rendered sensitive to colours by which they
are little if at all impressed. Such bodies which so act
are called optical sensitizers. Certain combinations, however,
have the property of aiding the decomposition of silver
bromide by light. In this category are the bodies which
combine with iodine and bromine, and which thus favour
the decomposition of silver iodide into sub -iodide and
iodine. Compounds of this kind are called chemical sensi-
tizers. According to Poitevin and Vogel, the bodies which
readily absorb iodine and bromine are good chemical sen-
sitizers. Captain Abney is of the same opinion ; but Carey Lea
is of the opinion that the affinity of the body for iodine has less
importance than has its affinity for oxygen, and that chemical sen-
sitizers ought to be reducing bodies. (Poggendorf, " Bulletin,"
1877, vol. i, p. 563.) H. W. Vogel has energetically com-
bated this opinion of Carey Lea. It is easy to imagine a
body which unites together the property of being a chemical
sensitizer and also an optical sensitizer, and the division of
these two classes of sensitizers has been combated at dif-
ferent times. The arguments, however, which have been
brought forward against this kind of division do not appear
to be sufficiently weighty for it to be abandoned ; for it
cannot be denied that there is a great deal to be said for it.
Many substances destroy or dimirish the sensitiveness of salts
of silver to light. Nitric acid particularly impedes the deve-
lopment of iodide or bromide of silver when exposed to the
light, even when the solution is but little concentrated
(SG. = i*2). When the acid is more dilute it does not
destroy the sensitiveness of the salts of silver ; it only makes
it less. Hydrochloric acid has the same effect. Sulphuric
SENSITIZERS. 2J
acid and acetic acid in small quantities also diminish sensitive-
ness. Dilute acid and acid vapours, more particularly nitrous
acid, destroy, little by little, the latent image on a washed
film of iodide of silver in collodion. Silver chloride darkens
in the light, even when exposed to concentrated nitric acid ;
but, on the other hand, it does not blacken when it contains
even a trace of mercuric chloride. Hydriodic acid prevents
the darkening by light of silver iodide (Rodwell, "Proa
Roy. Soc," 1877).
Silver iodide containing an excess of potassium iodide is
almost entirely insensitive to light. Even when the excess of
potassium iodide has been almost completely eliminated by
washing, the insensitiveness is still present, although by pro-
longing the exposure for a very long time an image is obtained
by using an iron-developer with silver nitrate (Carey Lea,
Vogel, " Phot. Corr.," vol. viii, p. 181). The anti-photographic
properties of potassium iodide are also to be remarked by
applying it to the latent image. The impression made by
light disappears. It must, however, be remembered that potas-
sium iodide does not act in the same manner on all latent
images. Silver iodide impressed in the presence of silver
nitrate, and treated with potassium iodide, loses all trace of
an impression, whilst with bromo-iodide of silver the image
is only enfeebled; the image on silver iodide produced in
the presence of organic matter is not destroyed by potassium
iodide (Vogel, "Phot. Corr., ,, vol. viii, p. 243). By treating
plates prepared with silver bromide emulsion with a dilute
solution of nitric acid and potassium bromide all lumi-
nous impression is effaced, and the plate can be re-used.
Chlorine, bromine, or cupric chloride can also be substituted.
Silver iodide precipitated in an excess of silver nitrate
and washed is, according to Schultz-Sellack, from ten to
twenty times more sensitive than that prepared with an excess
of potassium iodide, although the two can be considered
as pure.
Potassium iodide acts in a singular manner upon silver-
chloride paper : it destroys even the direct impression. If a
chloride of silver paper blackened in the light be brushed
over with a solution of potassium iodide, and be then exposed
to the light, it soon bleaches, afterwards becoming quite
yellow; it then darkens when the lummous action is con-
28 THE CHEMICAL EFFECT OF THE SPECTRUM.
tinued. (It appears that in darkness it disengages hydrogen.)*
The paper bleaches more rapidly in the violet rays than in the
red or green rays (Hunt, "Researches on Light"). Potassium
iodide destroys the latent image equally as well as the direct
image. <
Similarly an excess of potassium, bromide also reduces the
luminous impression quite as well when it acts upon silver
bromide as on the iodide, even when the bromide has been
carefully eliminated by washing. Silver bromide and iodide
produced in this manner are less sensitive than those precipi-
tated by an excess of silver nitrate and then washed (according
to Vogel, four times) in pure water. The difference is not,
however, so visible as in silver iodide in presence of potas-
sium iodide. In the emulsion process, in which the bromide
is precipitated in an excess of potassium bromide, it is im-
portant to know the effects produced. In general, emulsions
precipitated in excess of silver nitrate are more sensitive than
the others, although the sensitiveness of those precipitated in
an excess of bromide can be increased by using an organic
sensitizer, such as tannin, resin, or gum.
An excess of chloride does not act so anti-photographically
as the iodide and bromide of potassium. Bromide of silver
emulsions, in which the excess of silver nitrate has been
precipitated by an excess of chloride and afterwards washed,
are only slightly less sensitive with alkaline development than
emulsions containing an excess of nitrate which is subsequently
removed by washing.
It is interesting to remark, that it is possible to deter-
mine, by the colour of a negative developed by the alkaline
method, whether the silver bromide has been prepared with
an excess of nitrate or bromide. In the first case, the
negative has a greenish colour ; in the second, a ruddy or
brown tint (Wortley, " Photographic News," vol. xv r
p. 291).*
The sensitiveness to coloured rays, as it is with white light
is diminished by the addition of potassium bromide to the
emulsion. The spectrum appears altogether more feeble ;
[* This is hardly exact. The green or brown colour can be given to a
collodion-emulsion negative, according to the length of exposure (see
" Photo, News," vol. xv, pp. 322 and 479). — Edtior.j
SENSITIZERS. 29
"but the yellow region more particularly resents this addition.
Moreover, if dry silver bromide is treated with a slight excess
of soluble chloride, a decrease in sensitiveness for white light,
as well as for the coloured rays, is to be remarked, but this
diminution is much less strongly marked than in the preceding
•case.
I conclude, from this, that it is best either to prepare
emulsions with an excess of silver nitrate (though they are,
it is true, more sensitive, yet they spoil by keeping, and more
•easily give fog than others), or to use a soluble chloride in
excess, when it is desired to have the most stable emul-
sions and, at the same time, the most sensitive ; a soluble
bromide in excess diminishes the sensitiveness in the highest
degree.
The advice which certain photographers give, to add a
little nitric acid, or nitro-muriatic acid, to the emulsion to
prevent fog, ought not to be followed, except with great
caution ; for emulsions of this kind easily lose every trace of
•the latent image, if too long a time elapses between exposure
and development
Amongst sensitizers, silver nitrate especially acts with great
energy. A concentrated solution of silver nitrate (10 per
cent.) increases the sensitiveness of the chloride, the bromide,
and more particularly the iodide of silver. The latent
action of light on silver iodide is enormously increased by
the addition of silver nitrate. It is less strongly shown on
silver bromide ; but with silver chloride the direct action of
light is much increased. In the wet-negative process this
method is adopted when the bromo-iodide of silver is exposed
in the presence of silver nitrate. By washing off the nitrate the
•sensitiveness of the plate is greatly diminished, although
even small quantities of nitrate enormously increase the action
that light exercises on the plate.
Vogel explains this phenomenon by the great affinity that
silver nitrate has for bromine and iodine, . which greatly aids
the splitting up of the molecule of haloid silver by light.
Silver nitrate is a greater sensitizer than all the organic sensi-
tizers. Modern alkaline development allows the preparation
of instantaneous plates without excess of silver nitrate. Silver
nitrate, although colourless, exercises an influence on the
sensitiveness to colours, as I have already remarked above;
30 THE CHEMICAL EFFECT OF THE SPECTRUM.
it augments more particularly the sensitiveness to the most
refrangible rays (blue-violet). The following bodies act as
sensitizers : —
Mercurous nitrate, stannous chloride, sodium sulphite, sodium
nitrite, soda, caustic potash, sodium-arsenite, tannin, pyrogallic
acid, potassium, ferrocyanide, which not only accelerate the
blackening by light when they are in direct contact with silver
iodide, but also increase the darkening in development Dry
plates made with silver iodide, when coated with these sub-
stances, give stronger and more vigorous negatives. As Vogel
has shown ("Pogg. Ann.," vol. 195, p. 518), all these bodies
have a great affinity for iodine.
A body which especially increases the sensitiveness of silver
bromide is gelatine, which Poitevin (Martin, "Manuel de
Photographie," 1857, p. 145) had already used to replace col-
lodion in the production of films of silver iodide, and which
Norris later, in 1856, brought forward as a preservative for
washed silver iodide plates. Norris used a mixture of gelatine
and gum, to which he added a minute quantity of glycerine
(" Photographic Journal," 1856, p. 257).*
These facts are all the more interesting, as at the present
time gelatine emulsions (bromide held in suspension in gelatine)-
are so much in vogue, as were collodion emulsions sensi-
tized by gelatine. Wilde has proposed to precipitate collodio-
bromide emulsions by means of an aqueous solution of
gelatine. Obernetter coats his collodio-bromide emulsions
with an alcoholized solution of gelatine and water (" Phot.
Mittheilungen," vol. xv, p. 266), which is Hill Norris's old
plan. The sensitiveness of these collodio-gelatino-bromide
plates greatly surpasses in rapidity the old iodide of silver
plates.
The sensitiveness of gelatino-bromide plates is such that,
in using alkaline development, portraits can be taken with
much shorter exposure than that necessary for the production
of images by wet collodion. On account of their extreme
* It is curious to see the notice made in 1840 by Hunt, who said that
gum arrests the decoloration in iodide of silver chloride blackened by light.
-Gum arabic and gum tragacanth make the image much less stable.
SENSITIZERS. 3 1
sensitiveness even for the red rays, the infra-red region can
easily be photographed.*
Even photographs have been taken of the Geissler-tube
spectra, which could be taken by no other plates (Monck-
hoven ; Poggendorf, " Beiblatter," vol. i, p. 286). Again,
photographs of Cazin's sparks (Pogg., "Beibl.," vol. i, p. 287)
have been recently made by Vogel with success (" Phot.
Mitt," vol. xv, p. 279). He discovered in the spectrum
of oxygen, after an exposure of two hours, a new series
of rays in the blue, the violet, and the ultra-violet. In the
spectrum of hydrogen he also discovered a quantity of
unknown rays, and could distinctly see the rays in the red.
Gelatine emulsion is singularly sensitive to the red,f which
must not be lost sight of in preparing it In practice its sensi-
tiveness for the yellow and green are also proportionately
increased, and thus more perfect images can be obtained, in
the sense that the lights and shades are more in relation to
those which are to be found on the object itself.
Collodio-bromide emulsion covered with gelatine is not so
sensitive as gelatino-bromide, although the sensitizing film of
gelatine accelerates the action of the luminous rays. The great
sensitiveness of gelatino-bromide of silver appears, then, to
be due not to a particularly intense sensitiveness produced
by the gelatine, but rather by a physical modification in the
nature of the silver bromide. The experiments of Bennett and
Monckhoven bear on this point
Tannin is often employed as a preservative to collodion
dry plates prepared with silver bromide or iodide. The tannin
dry-plate process discovered by Russell has everywhere been
a success, and is still employed with some few modifications.
Tannin has also been employed as a preservative for emul-
sions ; it increases the sensitiveness of plates for the violet,
but above all for the green and yellow. Silver bromide plates,
[* There is a mistake here. The ultra-red rays cannot be photographed
by ordinary gelatine plates : the gelatine itself in a measure prevents this
being done. — Editor. J
[f The sensitiveness of gelatine* plates to the red is not -rfayth of the
same plates to the violet, and perhaps to the green they have -^th of the
sensitiveness they have to the violet ; hence the statement must be taken
cum grano salts. — Editor J\
%2 THE CHEMICAL EFFECT OF THE SPECTRUM.
■
with an excess of silver nitrate, do not give greater sensitive-
ness by the introduction of tannin ; but, in the presence of
a slight excess of bromide, the influence of this body upon
the silver bromide gives a very considerable increase of sensi-
tiveness.
Morphia has no action on silver bromide when it is in
the presence of an excess of nitrate : it even appears that
it slows the action of light. On the other hand, it acts as an
energetic sensitizer upon silver bromide formed with a slight
excess of bromide. Silver bromo-iodide in a dry state, to
which a little silver nitrate is added, is made more sensitive,
not only to the blue and violet, but also to the green rays
<H. W. Vogel).
Pyrogallic acid behaves in an analogous manner. (See
further on.)
Salicine increases the sensitiveness of dry silver bromide for
the red and yellow (Carey Lea, "Photographic News," 1874).
It has been employed as a preservative for dry plates.
A small addition of resin to the collodion sensitizes washed-
and-dried iodide and bromide of silver, all excess of silver
nitrate being thus eliminated. For this reason, resins were
employed, twenty years ago, as preservatives in dry plates.
Abney has recently remarked that the addition of resin in-
creases the sensitiveness of silver iodide and bromide to the
red rays (" Monthly Notices of the Royal Astr. Soc," 1875).
H. W. Vogel and Lohse ("Pogg. Ann.," vol. cxix, p. 297)
have obtained upon plates of this description a reproduction
of the spectrum as far as the extreme red, — according to
Vogel, even to the ultra-red. For this reason resin has
been recommended for dry plates, in order to photograph
the yellow and red. Abney finds that these plates are ten
times more sensitive to the blue than to the red and yellow
regions (" Phot. News," 1876). Nevertheless, as H. W. Vogel
has shown, the increase in sensitiveness to the red rays
obtained by these plates is only a minimum. Abney himself
has abandoned these resin plates, and now prefers another
preparation of bromide sensitive to the red rays. (See further
on.)
It follows from this that all rays have a chemical action
on silver salts ; and that, from the red to the extreme violet of
the spectrum, all the rays act in a reducing manner.
SENSITIZERS. 33
The action of each particular ray depends not only on the
nature of the salt of silver, but it also has an intimate con-
nection with the sensitizer. The sensitizer acts especially
upon the latent image, — that is to say, upon that which
ought to appear by action of reducing agents ; for, in speaking*
of sensitizers, we have always had in view the latent* action
of light.
The manner in which the substances exposed to light
behave optically towards coloured light is of the greatest
influence. // is only those rays which are stopped or absorbed by
the substance sensitive to light which act chemically on it This
law is important for the explanation of the chemical action of
coloured rays.
Draper showed, in 1841, that all the coloured rays which
act chemically on a substance are absorbed by them (Draper's
Memoirs, " Phil. Mag.," vol. ix, p. 195 ; vol. li, p. 161).
Later, Schultz-Sellack produced silver iodide, bromide, and
chloride in transparent slabs, and showed that the haloid
salts of silver are chemically changed with a certain energy
by all the rays they absorb. This difference of colour can
be seen in the precipitates, which indicates different degrees
of sensitiveness to light ; thus pure silver iodide and bromide
are feebly yellow. The precipitate of a mixture of these
bodies with metallic bromo-iodides and chloro-iodides is an
intense yellow.
H. W. Vogel has made the most conscientious studies upon
the alteration of silver bromide by light With nearly every
sensitizer the optical absorbing action comes into play. A
solution of silver nitrate sensitizes vigorously, only because the
wet films of silver iodide and bromide absorb the blue rays
better than dry plates of the same substances. Pyrogalfic
acid, on the other hand, acts better in a dry state, because
when in solution it allows the chemical rays to pass. It is,
therefore, necessary here to have a further action, that the
chemical action should combine with the optical (" Phot. Mitt.,"
voL viii, p. 133 ; "Pogg. Ann.," vol. cliii, p. 288).
[* We demur to the term, " the latent action of light." The effect of the
action of light is visible or invisible : invisible with a short exposure ; visible
usually when the action is prolonged. It is a mere case of quantity of par-
ticles acted upon. — Editor.]
D
34. THE CHEMICAL EFFECT OF THE SPECTRUM.
If all the rays which are absorbed by a body are elimi-
nated from the spectrum, the remaining rays are not capable
of affecting a body. Thus, for instance, a luminous ray which
traverses a solution of the double citrate of iron and am-
monium has no more action on that salt; a luminous ray
which passes through a mixture of hydrogen and chlorine does
not act on a second mixture (Draper) ; a ray which passes
through a solution of bichromate of potash will not decom-
pose a mixture of this salt when in contact with organic matter;
in the same way, chrysoidin is not bleached by a ray which has
first passed through a sheet of gelatine coloured with chrysoidin.
CHAPTER VI.
DYED FILMS.
The integration of the luminous rays acts upon many sub-
stances, and particularly upon silver compounds; but the
rays which are the most actively absorbed are those which
act in the most energetic manner. The decomposing action
of the solar spectrum upon these compounds, e.g., silver
bromide, can be notably increased by the addition of colour-
ing matters to this salt. The sensitiveness of the silver
bromide is thus augmented, and especially for those rays which
are absorbed by the matter which is added to it (It is not
necessary that the colouring matter should undergo any
chemical* change.) Thanks to this method, it is possible to
greatly augment the sensitiveness of the salts of silver to the
red, yellow, or green rays.
In order to increase the sensitiveness of silver bromide to
certain colours, an addition is made of colouring matter which
absorbs precisely the rays of the spectrum to which it is
desired to render the plate sensitive. The colouring matter
can be added by dissolving it in collodio-bromide emulsion,
or by flowing a solution of the colouring matter over a plate
prepared with uncoloured emulsion. It is well to remark,
that the maximum photographic activity does not coincide
exactly with the locality of absorption, but is found slightly
shifted towards the red extremity of the spectrum. There is
nothing extraordinary in this, if the absorbing regions remain
perfectly constant. This want of coincidence is explained by
[* We differ from the author here. It appears to us, any colouring
matter 1 , which, is effective in the manner indicated above, is affected by
light.— Editor.]
D 2
36 THE CHEMICAL EFFECT OF THE SPECTRUM.
the law of Kundt* In effect the zone of absorption recedes
towards the red when the refractive index of the medium in-
creases. In the preceding experiment the colouring matter,
with the bromide suspended in the collodion, is enclosed in a
more strongly refracting substance, and causes the accelerating
action to shift towards the red (H. W. Vogel).
Colouring matter which is to be used to sensitize salts of
silver, making allowance for solubility, should fulfil the follow-
ing conditions : —
i st It ought optically to absorb the light for which
its collodion is to be sensitized.
snd. It ought to absorb free iodine or bromine. In
the contrary case, it is necessary to add a chemical
sensitizer.
3rd. It should not decompose silver nitrate, for without
this qualification it would hinder the preparation of
the plate. (It is evident that for collodio-bromide
of silver emulsions which do not contain pure silver
nitrate this condition is unnecessary.)
Optical sensitizers require the presence of a chemical sen-
sitizer. Certain colouring matters only show their sensitizing
properties when they are in the presence of chemical sensi-
tizers, e.g. 9 a small quantity of silver nitrate, of tannin, of
morphia, &c ; in presence of an excess of bromide or chlo-
ride, on the other hand, they are completely inert (H. W.
Vogel, "Photo. Mitt," vol. xiii, p. 31). Regions of sharply
increased intensity in plates of silver bromide, coloured by
different matters, are only found when the corresponding
colour of the spectrum in the absorbing regions is very
strongly shown. Colours which do not possess the power of
absorbing certain rays of the spectrum will never be seen,
according to Waterhouse (it would be more correct to say,
will never be distinctly seen), to give an increase of intensity,
even when these substances markedly lend themselves as sensi-
tizers of silver-bromide dry plates. Fluorescent colours seem
[* A law not established like that of gravitation. Kundt's experiments
may be explained on other grounds. — Editor.]
DYED FILMS. 37
also to be particularly apt to produce this effect. All colouring
matter does not augment the sensitiveness of the different salts
of silver in the same manner; thus fuchsine, for example,
renders silver bromide very sensitive to the yellow rays, whilst
it only influences the chloride very slightly.
Coralline and aurine in solution absorb the yellow and
green rays of the spectrum. If silver bromide is coloured by
means of these two substances, it becomes as sensitive to
the indigo as to the yellow — a colour to which, by itself, it
is but little sensitive. In the blue region the action shows
itself less strongly, for the silver bromide is already in itself
sensitive to yellow and blue rays. The addition of a colouring
matter augments the sensitiveness, in the place where coralline
shows an absorption band. There is, however, a great diffe-
rence between the different kinds of coralline found in
commerce. They act also in quite a different manner. Accord-
ing to Carey Lea, by the addition of coralline the sensitiveness
of silver bromide is greatly increased, more especially for the
red rays ; the action of the yellow rays is slightly intensified ;
while that of the green rays is unchanged. The same sub-
stance diminishes the sensitiveness of silver iodide in the red
(" Jahresbericht Chem.," 1875, p. 147). The experiments of
H. W. Vogel in regard to the action of coralline upon silver
iodide dq not give the same results. Vogel has found that
the action of coralline on silver iodide is the same as on the
bromide.*
* According to the experiments of Carey Lea (Silliman, "Amer, Journ."
(3), vol. vii, p. 200), the following substances are subject to very varied
changes when they are exposed to coloured light, and in presence of
colouring matters indicated further on. These substances are — ammoniacal
oxide of iron, potassium bichromate, potassium ferrocyanide, uranic nitrate,
silver chloride, bromide and iodide of the same metal. The colouring
matters are aurin, aniline blue, and green, tincture of saffron, coralline,
aniline red, and mauvine. It is to be remarked that Carey Lea operated
with coloured glasses, and not with the solar spectrum. Quite recently,
Gros ("Compt. Rend.," vol. clxxxviii, p. 79) has treated of the action of
differently coloured rays on a film of silver bromide impregnated with
different colouring matters. Becquerel remarks, to complete the research,
that collodio-bromide of silver plates with an excess of potassium bromide
become more sensitive to the rays absorbed by the following substances : —
Alcoholic chlorophyl, carthainine curcuma, extract of red currants, and
mauve, and finally haemaglobin. I have received his work too late to be
able to take it into consideration.
*J8 THE CHEMICAL EFFECT OF THE SPECTRUM.
Coralline acts most markedly in the yellow region on
silver chloride plates near D, and as far as G and C. They
are thus more generally sensitive to light than pure silver
chloride. They render yellow the best; red and green less
well ; whilst blue is very badly rendered. A mixture of
bromide and chloride of silver in collodion gives, when their
relative sensitiveness to colours is taken into account, a mean
between the two. Such plates are very sensitive to the yellow
near D ; the second maximum of sensitiveness is found at £ ;
between the violet and yellow regions they only show a very
feeble action.*
Silver bromo-iodide containing a certain quantity of coral-
line has its maximum of sensitiveness at G. The sensitive-
ness then falls rapidly, to ascend again at F. This substance
shows a greater sensitiveness for deep blue than for light blue.
Exposed in the wet state, with free silver nitrate, the maximum
intensity is at F, and afterwards at G. The blue produces the
strongest action; the violet a less action (H. W. Vogel and
Becquerel)
Ducros du Hauron found that aurine (orange coralline) also
increases the sensitiveness of silver bromide for the green
rays. But it was necessary that the bromide should be pro-
duced in the presence of an excess of silver nitrate. He
wished to utilize the quality to produce negatives for use in
his chromo-photography.
All the above statements refer to the latent action (see Note,
P« 33) of light, which is only made visible by the action of
reducing agents.
Naphthaline red increases the sensitiveness of silver bromide
for the yellow region (from near D to C), as also for the
violet region. It is only necessary that the bromide be pro-
duced in the presence of a slight excess of silver nitrate.
A great quantity of colouring matter — this applies generally
— does not increase the sensitiveness for that part of the spec-
trum, but, on the contrary, rather diminishes it. With a
redder coloration of the film of silver bromide all sensitive-
ness to the yellow is destroyed. H. W. Vogel explains this
apparent paradox in the following manner : — A strong film
* To study absorption spectra of different colouring matters, see H. "W.
Vogel's "Analyse Spectrale Pratique." Nordlingen, 1877.
DYED FILMS. 39
of colour absorbs the light before it can reach the silver
bromide, and the luminous rays cannot make the coloured
molecule of silver bromide vibrate rapidly enough. It is alto-
gether otherwise when the density of the colouring matter is
diminished. Thus the luminous rays, after having traversed
the top film, have still force enough to put the neighbouring
molecules energetically into vibration ("Berliner Berichte,"
Bd. vii, p. 976; "Chem. Centralb.," 1874, p. 561). If the
silver bromide contains an excess of potassium bromide, no
increase of sensitiveness for any particular colour is to be
remarked; it is only when a little silver nitrate, tannin, morphia,
or other chemical sensitizer is added that the sensitiveness to
the yellow is increased.*
Fuchsine (aniline red) does not give sensitiveness to silver
bromide to any colour of the spectrum. It renders silver
bromide, when it contains a trace of free silver nitrate, com-
pletely insensitive. At the same time, the sensitiveness to the
blue is diminished, probably because aniline red makes the
silver bromide more translucent to the blue rays, and, conse-
quently, only slightly augments its sensitiveness.
According to Waterhouse, naphthaline red increases the
sensitiveness for the yellow, but diminishes it for the green
* * .. ._■.
[* The above is Dr. Vogel's explanation of the matter, and of the reason
of coloured films acting when the colouring matter is small in quantity,
and not acting when the colouring matter is large in quantity. I must
enter a protest at once against all theories of the existence of optical sensi-
tizers : it is against all known laws of molecular physics. If the theory
of optical , sensitizers were correct, we should have to believe that a ray
of some particular refiangibility, which was absorbed by a dye, was, in its
partially absorbed state, capable of acting more vigorously on the salt of
silver than the same ray totally unabsorbed could do. If a ray which is
absorbed is capable of setting up phosphorescence, then the change in
refrangibility of the ray might account for it ; but this seems improbable,
since in that case the refrangibility of the impinging ray would have to be
greater than the phosphorescent ray, which is contrary to Stokes's law.
A combination between the dye and free silver nitrate, when the emul-
sion is prepared with an excess of silver, or the action of light on the dye
itself (thus forming an oxidized nucleus for the depositing silver during
development), will account for the phenomenon without doing violence to
known physical laws. Later experiments have shown that, without any
•chemical sensitizer being present, when the emulsion has been prepared
with excess of bromide the same action occurs, which eliminates the only
objection to the simple explanation offered, as opposed to Vogel's. —
Editor.}
40 THE CHEMICAL EFFECT OF THE SPECTRUM.
and the red. Violet and purple colours augment the sensi-
tiveness for the yellow and orange, but diminish that -of the
green.
The addition of fuchsine to silver chloride only produces
a feeble increase of sensitiveness for the yellow. On the other
hand, sensitiveness for the violet is exceptionally increased;
it is greater than with pure silver chloride. In the presence
of free silver nitrate and of fuchsine the sensitiveness for the
indigo and blue rays is much increased (H. W. Vogel, " Phot
Corn," vol. ii, p. 202).
Aldehyde green and picrate of methyl-rosaniline greatly
increase the sensitiveness of silver bromide for the yellow-
orange (between C and D), and, at the same time, for the
blue. Aldehyde green is, at the same time, an optical and
chemical sensitizer; in fact, it absorbs iodine and bromine,
and gives sensitiveness for the yellow-orange rays. It is not
necessary, therefore, with silver bromide, to use with it a
chemical sensitizer, as it is one in itself. It has almost the
same action on silver iodide as on silver bromide (H. W.
Vogel; Becquerel, "Phot. Mitt," 1874, p. 139).
Eosin in solution shows a band from E to F. Added to
the silver bromide emulsion, it sensitizes it for the green rays.
Films of this nature are infinitely more sensitive to the green
and yellow than to the blue and violet of the spectrum.*
The maximum of action is found from G to F. From the
blue to the violet the action is very feeble. Wet plates pre-
pared with silver bromo-iodide and an excess of silver nitrate,
and coloured with eosin, are sensitive to the green and yellow,
although their maximum sensitiveness is situated at the violet
extremity of the spectrum. Small quantities of acid retard
the sensitizing action (H. W. Vogel, "Phot. Mitt," vol. xiv,
p. 19 ; Waterhouse, " Pogg. Ann./' vol. clix, p. 616). When
Waterhouse tried collodion stained with eosin for dry plates,
and for wet plates prepared with silver bromide and iodide on
landscapes, bouquets of flowers, &c, he did not observe any
very favourable results. He had, it is true, a slight increase
of sensitiveness for the yellow rays; but he did not observe
any intensification in the green of trees. The exposure given
[ * The blue and violet are still far the most active, according to our
experience. — Editor.]
DYED FILMS. 41
was three times that which would have been required for wet
collodion, and this although the collodion was only slightly
coloured.
Chlorophyl strongly sensitizes silver bromide and iodide
chiefly in the red, about the regions where the rays are
absorbed. In the red and yellow regions of the spectrum we
see several maxima of intensity, which correspond to the
absorption-bands of chlorophyl, the one between B and C,
and the other between C and E. With a short exposure, the
films are sensitive to E in the green, and it is only after a
more prolonged exposure that the action in the red is
seen. The sensitiveness in the red is five to ten times
less than in the violet (Becquerel, " Compt. Rend.," vol lxxix,
p. 189).
The colouring matter of turmeric increases the sensitiveness
of silver bromide to the less refrangible rays. If to collodio-
bromide of silver an alcoholic solution of turmeric (seeds) is
added, till a brilliant yellow colour is obtained, this collodion
becomes more sensitive to the yellow and red than if it is
sensitized with anything else. The plates show a diminution
of intensity from G to F; after that an increase of intensity
from B in the green as far as D in the yellow, from which
point it gradually diminishes to A (Waterhouse).
Methyl violet makes silver bromide very sensitive to the
yellow, near D.
Cyanin blue, itself sensitive to the light, renders silver
bromide sensitive to the colours which itself absorbs ; it sen-
sitizes for the yellow and orange, and this in such a manner
that a plate is more sensitive to these colours than to the
violet.* The maximum effect is at D and C (Waterhouse).
If silver bromide contains only a trace of soluble bromide,
Vogel finds that it does not become sensitive to the yellow ;
cyanin blue only is decomposed by yellow light. If normal
collodion stained with cyanin is flowed over a plate, and
exposed to the solar spectrum, and it is afterwards covered
with silver bromide emulsion and developed without further
exposure, a deposit is found in the part of the plates where
the yellow light has acted, J in the same place where the
[* See previous Note. — Editor.] ' f " Photographic Journal."
42 THE CHEMICAL . EFFECT OF THE SPECTRUM.
absorption bands of cyanin are found. By adding colouring
matter to the emulsion, and afterwards exposing, the same
results are obtained. Abney concludes from this that the
reduction of colouring matter precedes the decomposition of
the silver bromide, and that the good effect obtained by the
addition of colouring matter should be always referred to a
purely chemical action ; and that the more readily the
colouring matter combines with the salt of silver, the more
readily would it be acted upon (see above). He does not,
therefore, agree to Vogel's theory as to the chemical sensitizers
and optical sensitizers.
H. W. Vogel has successfully refuted Abney's opinion
("Phot. Mitt," vol. xv, p. 91). He considers that some-
times a combination between the colouring matter and the
silver nitrate takes place when the latter is in excess (e.g., with
aniline red) ; and yet, when the excess of silver nitrate does
not exist, the aniline red acts also very strongly as a sensi-
tizer for the yellow rays when a chemical sensitizer, such as
tannin or morphia, is added.* It cannot then be a question
here of a combination sensitive to light formed with the
morphia and colouring matter. There are optical sensitizers
which at the same time absorb bromine, which are also
chemical sensitizers (e\g., aldehyde green). Abney's supposi-
tion, which he bases on an experiment with cyanin blue,
which appears to prove that the colouring matter undergoes
first of all an alteration and a reduction, and that it is this
reduction which is the cause of the chromatic sensitiveness,
is not admitted by Vogel, who says that on this hypothesis
chemical sensitizers — whose presence, besides the colouring
matter, is necessary — would be superfluous. Abney's propo-
sition (who says that colouring matter in presence of silver
bromide is more sensitive than when it is alone) is opposed to
the hypothesis of a primary reduction of the colouring matter
and its subsequent action on the silver bromide, f
* See note to page 39.
[t We say, " not proven." The dye on a film may be only 10 ^ flfl of a
grain, and that distributed round each particle of the silver salt, thus expos-
ing an enormous surface on which light can act. The minutest alteration
in the dye would cause a nucleus sufficient for developing purposes, exactly
in the same way that the minute portion of subsalt of silver forms a nucleus.
—Editor.']
DYED FILMS. 43
Abney's remarks, then, only apply to special cases of
colouring matter of strong sensitiveness to the light. This
rare case was present only with very long exposures ; but it is
not possible to apply it when latent images are obtained on
coloured silver bromide and with a short exposure. I say short
exposure, for we have, in the foregoing experiments, spoken of
exposures of short duration.
Many other colouring matters do not increase the sensitive-
ness, according to Vogel, of the silver salts for the spectrum
colours, although they themselves absorb certain luminous rays.
Thus, for instance, picric acid, aniline blue, ultra-marine, lake,
indigo, and purpurin do not increase the sensitiveness of silver
bromide coloured with them.
Aniline blue, however, produces irregularity of action in its
behaviour with silver bromide. It does not increase the sen-
sitiveness of that salt for the colours it absorbs. According
to Waterhouse, it increases the sensitiveness of the bromide
for the blue rays, but very little for the green or orange rays.
Although it is possible to make photographic plates sensi-
tive to all the colours of the spectrum, from the red to the
violet, by adding colouring matter to them, it is not possible,
however, to reproduce the yellow in nature with the same
facility and the same exposure as the violet. In pictures,
natural flowers, &c, the yellow and red develop, in spite of
the bromide sensitized for the yellow and red, much worse than
the blue and violet.
The reason of this is, that the relative brightness of yellow,
on the one hand, and of the blue on the other, is not so
sensibly different, since the feeble chemical action of the
yellow and red arrests the action of the blue (the yellow is
often optically lighter than the blue). On the other hand, the
yellow of the spectrum is one hundred times brighter than the
blue of the spectrum, and acts also in these very favourable
circumstances as energetically as the blue. In nature the
difference of brightness is less sensible.*
[* We cannot agree with the author in this. The yellow of nature is
reflected light. It might be said that light from the sky exhibits the same
deficiency of yellow as sun -light. This is not the case, however. No dried
film, in our experience, is nearly so sensitive to the yellow as to the blue
of the spectrum. — Editor, ]
44 THE CHEMICAL EFFECT OF THE SPECTRUM.
If it is desired, to reproduce natural objects, so as to have a
proper rendering of blue and yellow, the blue must be toned
down by yellow glasses.
It was upon such considerations that the discovery of pho-
tography in natural colours by indirect means was thought of and
put in practice at the same time by Cros and Ducos du Hauron
in 1869, and another anonymous person. Whilst Cros was
contented to make known the theory of the process, Ducos
du Hauron had tried to carry it out practically.
After long researches, Ducos du Hauron produced coloured
photographs in the following manner (see "Photo. News,"
vol. xx, and " Traite Pratique de Photographie des Couleurs,"
1878, Ducos du Hauron): — He produces in the camera three
different images, that is to say, one by green, another by
orange, and another by violet light, and the model is photo-
graphed through glasses thus coloured. From these negatives
three coloured positives are made, in red, green, and blue,
called monochromes, and these three positives are superposed,
and by the mixture of tints an image in natural colours is
obtained.
The red monochrome is obtained from the negative pro-
duced with green light ; the blue monochrome from the
negative given by orange light ; and the yellow from a negative
taken with violet light Why are there three different nega*
tives ? and why also this diversity— this chass'e croise of colours ?
We give the words of the inventor : —
" Chacun de ces trois monochromes &ant constitue par une
preparation douee de transparence, et susceptible d'&re fixee
par la lumiere, et, proportionnellement a cette action, chacun
d'eux, par Peffet de l'interversion de couleurs qui vient d'etre
sp^cifiee, donnera necessairement, soit comme gradation des
clairs et des ombres, soit comme repartition de la couleur
speciale des objects, Pimage voulue.
"Et d'abord, comme gradation des clairs et des ombres,
abstraction faite de la couleur spe*ciale des objets a representee
chacun des trois monochromes sera eVidemment exact. Chacun
d'eux, en effet, e*tant fourni par un cliche* negatif qui traduit les
noirs du modele par du blanc et les blancs du modele par du
noir, traduira a son tour les noirs du modele par la preparation
rouge, bleue ou jaune propre a ce monochrome ; et ce rouge,
DYED FILMS. , 45
ou ce bleu, ou ce jaune sera d'autant plus intense, que le noir
«du modele sera plus noir; et reciproquement,. chacun d'eux
traduira les blancs du modele par l'absence de mati&re
colorante, cette mati&re colorante etant 61imin^e sous les noirs
du negatif, et d'autant plus eliminee que le blanc du module
sera plus blanc
" En second lieu, comme repartition de la couleur sp^ciale
aux objets qu'il s'agit de representee chacun des trois mono-
chromes sera non moins exact, et c'est cette repartition qui
-engendrera, d'un. monochrome a l'autre, les differences qu'ils
doivent offrir, et qui ne naitraient pas de la simple traduction
des clairs et des ombres. Prenons pour exemple le mono-
chrome rouge, fourni par le negatif qui est du a la lumiere
verte. Comme le verre ou milieu transparent de couleur
verte qui filtre cette lumiere laisse passer presque exclusive-
ment les rayons verts, et qu'il intercepte d'autant plus d'autres
rayon? que leur totality se rapproche davantage du rouge, et
comme, d'autre part, les objets de la nature qui £mettent
abondamment les rayons verts sont les objets jaunes, les verts
et les bleus, il en rdsulte que le negatif en question traduira
par du noir les surfaces jaunes, les vertes et les bleues ; que
la preparation^ rouge du monochrome fourni par ce negatif
traduira les rouges du modele par du rouge, et par un rouge
d'autant plus intense que le rouge du module sera plus
prononc^ ; qu'enfin cette m§me preparation rouge sera eliminee
sous les noirs du negatif, c'est-a-dire dans les parties du susdit
monochrome qui correspondent aux surfaces jaunes, aux vertes
et aux bleues, et que cette elimination sera d'autant plus forte
que ce jaune, ce vert et ce bleu seront plus prononces. Un
raisonnement analogue s'applique a chacun des deux autres
monochromes; contiendront Tun et l'autre une fidele repartition
de la couleur sp^ciale, soit simple, soit melang^e, qu'ils sunt
tenus de representer."
These three monochromes, placed the one upon the other,
and coinciding in all parts, produce upon a white background
a mixture of colours, — that is to say, a polychromatic image.
Plates of coloured glass are placed before the lens or
before the sensitive plate. They are prepared by covering white
glass with varnish, coloured in the usual manner. ■ The plates
must«have the necessary depth of colour.
46 THE CHEMICAL EFFECT OF THE SPECTRUM.
The three negatives of the different coloured images are
obtained upon collodio-bromide of silver containing eosine,
either dry (sensitized with tannin) or wet (with excess of silver
nitrate). The development is done with an alkaline solution
of pyrogallic acid. This collodion gives with an equal fidelity
the image after passing through violet, orange, or green
glass. The same preparation is thus used for the three
colours.
The negatives are printed on coloured carbon tissue (bichro-
mate process). For red, carmine; for blue, prussian blue;,
and yellow, saffron-colour. These prints are carefully super-
posed one on another, so as to give a faithful representation
of the natural colour.
Through the possibility of obtaining images of objects
coloured by means of negatives obtained through different
coloured glasses, Husnik brought out a new process in 1870
("Photo. News," 1870) : he proposed to print over each other
coloured Lichtdrucks produced by the three negatives.
Albert of Munich has, after numerous and long trials,,
realised the idea, and produced by phototype coloured photo-
graphic images. He thus obtained the same effects as Ducos
du Hauron, though more rapidly,- and made it possible to
obtain more proofs in a given time ; besides which the exact
superposition of the three impressions is more easily carried
out. The production of negatives through green and orange
glasses, according to the experiments of Albert of Munich
and Marion of Paris, presents great difficulties. Husnik
believes that a short preliminary exposure of the silver
bromide to violet light really increases the sensitiveness to
the yellow and red (above all, with the chloride and fluoride
of silver), and the united experiments of several years on the
action of a short preliminary exposure give weight to his
opinion,
I cain certify that Albert's experiments have resulted in
very beautiful photographs in natural colours. I have seen
very successful phototypes in colour due to him at the Photo-
graphic Society of Vienna. (As to the difficulties which he
encountered in his trials, see Zaff£, "Photo. Corr.," 1878,.
p. 139.)
This manner of settling the question of photography in
DYED FILMS. 47
colours, which is as old as photography itself, is extremely
remarkable. It is true it does not respond to the first and
fundamental idea that the coloured image should be directly
produced in the camera by the exposure of a sensitive plate to
light. Direct photography in natural colours, in which a
coloured image is produced on a sensitive plate, will be,
like some other things, a long time before being accom-
plished.
CHAPTER VII.
REVERSED ACTION OF LIGHT.
The colours of the directly coloured prints thus obtained
leave much to be desired ; and, notwithstanding the long
years of toil of Niepce, Poitevin, Zenker, and Flourens, the
fixing of the image is not yet accomplished. (S& the mono-
graph of Zenker on photography in natural colours.) By
this plan satisfactory results have not yet been obtained, and
the future will scarcely furnish them. The last method
described is better for obtaining large coloured photographs.
And, after what has been written, it is scarcely easy to
solve the question as to how the different rays of the spec-
trum act on the haloid salts of silver. The sensitiveness to
the reducing action of light can extend from the infra-red
to the ultra-violet. Sometimes the photo-chemical sensitizers
strongly influence the manner in which this action takes
place. But, on the other hand, we believe that light can
also produce an oxidizing action on the same compounds
of silver ; red light mostly favouring oxidation, and violet light
reduction.
In many cases the red and the violet have the same action
upon the bromide and iodide of silver : thus there is reduction
from the ultra-violet to the infra-red, as has been said before.
This is as applicable to Daguerreotype plates developed
with mercury as to dry plates which are covered with such
or such a preservative, and developed by the alkaline or acid
methods. The course of the reduction is always identically the
same, even if the sensitiveness of the film varies with the
surroundings. The numerous researches before mentioned
prove this. It can be concluded, then, that all the rays
of the spectrum possess a reducing action on the salts of
silver.
On the other hand, it has frequently been remarked that
red light produces an action altogether contrary to that of
REVERSED ACTION OF LIGHT. 49
violet light The action of white light on a Daguerreotype
plate is found to be strongly impeded by rays which have
traversed a red, orange, or yellow glass, and the plate only
slightly retains the property of taking the mercury (Claudet,
" Comp. Rend.," vol. xxv, p. 554). The action of violet light
on Daguerreotype plates is found to be strongly neutralized
by red light (Herschel, Lerebours, Draper, Fizeau, Foucault).
In the collodion process, the salts of silver behave in exactly
the same manner, whether with alkaline or acid development.
If an image of the spectrum is received on a slightly
veiled gelatino-bromide plate (having a slight excess of silver),
the red region destroys the metallic silver, while in the violet
the reduction takes place rapidly. Consequently, in fogged
plates the photographic spectrum, after development, is posi-
tive at the red end ; in other words, where the red light has
acted, there is no metallic silver : the Fraunhofer lines alone
are in metallic silver, appearing as positive. This reversal
of the action of light is not only found in the red, but also
in the violet (Waterhouse, "Photo. News," 1875). A gelatino-
bromide plate is exposed for an instant to daylight, and then
an image of the spectrum is allowed to fall upon it; in develop-
ment (alkaline) not only the portions lighted by the spectrum,
which are most coloured, but also those which have been
impressed by the darkest rays, and where, consequently, light
has not acted, are reversed.
This reversal of the action of light extends, according to
Waterhouse, from the infra-red to the violet, and is produced in
any kind of light (perhaps, but the fact is not yet proved, it may
be found due to oxidation).
Later, Abney has tried to prove that the red and infra-red
rays favour oxidation, and that in collodio-bromide they have
an action directly opposite to those of the violet rays. He
explains the variable action of the spectrum on collodio-
bromide by the presence in the silver iodide and bromide of
two kinds of molecules, one of which absorbs the red, and the
other the blue rays.
Abney ("Bulletin de PAssociation Beige de Phot.," 1878,
vol. v, p. 115) takes up the idea thrown out by Lockyer, in a
note published in the " Proceedings of the Royal Society of
London," 1874, treating of the molecular theory applied to
spectroscopic researcheSr He said, in a few words, that it
£
50 THE CHEMICAL EFFECT OF THE SPECTRUM.
can be admitted that all matter is constituted of two different
kinds of molecular groupings, — one absorbs in the red region,
and the other in the blue region, of the spectrum ; and that a
combination of these two kinds of molecules would explain all
the other phenomena.
This was the point of departure in Abne/s researches. It
can be admitted in all security that, when there is no absorption
of light, no work can be done by light, whilst, when there is an
absorption, work of some kind must be done.
The ordinary state of silver bromide and iodide is yellow,
and consequently the maximum of absorption is in the blue
end of the spectrum, and is more sensitive for it Abney
found that it is possible so to modify the molecular form of
silver bromide as to render it sensitive for the red and infra-
red ; because it absorbed those rays. At first he tried to
augment the weights and the volume of the molecule by the
addition of resin to the emulsion ; but in this way he did not
get any very satisfactory results. The increase of weight gave
more the idea of the formation of a new compound of silver
than the existence of a new molecular combination.
Later, Abney obtained silver bromide in a new form, which
absorbed red light, transmitting consequently blue, whilst in
ordinary silver bromide the reverse took place. The identity
of the composition of the two forms of bromide was proved
by the fact that friction transformed the first into the second.
There was thus only a molecular difference between the two
forms of bromide.
To obtain a collodio-bromide emulsion absorbing the red,
a particular operation was carried out. A collodio-bromide
emulsion* is evaporated by heat to the consistency of jelly.
Such an emulsion will give a film sensitive to the red rays ; at
least it will, in all probability, f
Abney obtained spectrum photographs — the red and the
violet appearing well, whilst the yellow was completely missing.
* It is to be remarked that, by a long digestion with heat, gelatine
emulsions become, in general, more sensitive to light, and probably also
in the same circumstances more sensitive to the red.
t A horny collodion allows fewer red rays to pass than does a powdery
collodion. A cotton for collodion more or less powdery also gives better
results for emulsions which ought to be sensitive to the red (Abney, " Bull.
Assoc Belg.," vol v, p. 116).
REVERSED ACTION OF LIGHT. 5 1
Thus the presence of two kinds of molecules was found
(and up to a certain point this is probably the case in all
emulsions).
Abney also photographed a spectrum, cutting off the blue
by a red glass. He also was able to photograph the infra-red.*
He also found that the red accelerated oxidation, and that
from this fact was to be found an explanation of the pheno-
menon of solarization, and perhaps also of the photography
in colours of Becquerel and Nifepce de St. Victor. Abney
showed, by the following experiment, that in reality the red
and infra-red accelerate oxidation. He prepared a plate with
an emulsion sensitive to the red, exposed it two or three
seconds to diffused light, after which he plunged it in a
bath containing an oxidizing substance (permanganate or
bichromate of potash, nitric acid, or peroxide of hydrogen).
The solutions were very weak, e.g., five or six drops of nitric
acid in an ounce of water, f
Thus immersed, the plate was submitted to the spectrum
for seven minutes, and, on development, not only had the
red rays stopped the reduction, but had cleared away the fog ;
whilst on a transparent background the Fraunhofer lines had
been left opaque.
Silver iodide behaves in the same manner as the bromide.
The film was washed and exposed in a solution, and then
developed by pyrogallic acid. Even by itself, and without an
oxidizing solution, a film of bromo-iodide of silver was oxidized
by red light in the same way as indicated above. On a film,
on which the red exercised no reducing action, the oxidizing
action acted without the use of oxidizing solutions. We
have thus in photography rays which have a strong tendency
to oxidize; and if one has not at his disposal a salt more
sensitive to reduction than to oxidation in the air and red 1
light, a reversed image on that part of the spectrum cannot \
* By the aid of a Rutherford diffraction grating. In this way the wave-
lengths of the ultra-red could be determined.
t Abney thinks that with weaker solutions the oxidation and reduction
mutually destroy each other. [Not quite so. He thinks that with weaker
solutions the reducing action overpowers the oxidizing action, and with
stronger solutions the image is destroyed without the helping action of red
light— Editor.]
£ 2
52 THE CHEMICAL EFFECT OF THE SPECTRUM.
be obtained. Ordinary wet plates of silver bromide are
insensitive to the red, on account of the nitric acid necessarily
present with them for the avoidance of fog.
If the red rays are favourable for oxidation, it is not said
that on that account this property is completely absent in
the blue rays. With these latter it is the deoxidizing action,
on the whole, which predominates. When all the silver is
reduced by the action of light, then the oxidizing action
commences; the image disappears, and the phenomenon of
solarization shows itself. The following experiment proves that
the action is an oxidizing one: — A plate of silver bromide is
exposed till it darkens; after which it is treated for some
minutes with drops of peroxide of hydrogen, permanganate or
bichromate of potash, and a difference in colour is scarcely
perceived.
Sodium hyposulphite fixes those parts alone with clearness
and without veil — the parts treated with the oxidizing sub-
stances ; whilst the rest is covered with a slight veil of silver.
It can be concluded from these experiments that each part
of the spectrum exercises an oxidizing action upon a silver
compound in a feeble degree of concentration.
Abney undertook a new series of experiments as to the
way in which silver iodide behaved. To effect this, he used
a dark slide, so constructed as to allow the plates to be
immersed in different gases or liquids. A plate of silver
iodide was washed and plunged into a bath of nitrite or
sulphite of sodium, and exposed whilst in the solution. As
a result, an image was obtained as far as A; whilst under
ordinary circumstances the film was only sensitive as far as
B : which, Abney thinks, proves that there exists in the
silver iodide some molecular groupings which are greater, and
which absorb and are reduced by red light. He sought to
multiply these groups.
Certain silver bromide emulsions, absorbing the red and
consequently sensitive to that light, are without any sensitive-
ness to ordinary modes of exposure. If they are immersed
in a bath absorbing oxygen and halogen, they become as
sensitive as the other. If the exposing cell contain hydrogen
or nitrogen, the same results are obtained. When exposed in
these conditions it is impossible to obtain a reversed image
of the spectrum. This reversal, as has already been said, is
REVERSED ACTION OF LIGHT. 53
an explanation of the phenomenon of serialization, which is
otherwise so difficultly explicable, and depends in all probability
on the oxidation of the -photographic image. And since an
image on silver iodide is more easily oxidized than one on
bromide, this gives a sufficient explanation of the fact that
silver iodide solarizes more easily than does the bromide.
Abney prepares his emulsion by yellow light (gas light with
a deep yellow glass). The emulsion is less sensitive to the
yellow than to the red light.
Abney had previously found and published,* that by expos-
ing an ordinary emulsion of silver bromide in an atmosphere
of hydrogen or nitrogen, no trace of solarization (or oxidation,
as he called it) of the image ; consequently, by their absorption
of oxygen, preservatives for dry plates impede solarization.
In plates prepared with ordinary silver bromide, and exposed
in solutions of pyrogallic acid, gallic acid, sulphate of
iron, and ferrous oxalate in a solution of potassium oxalate,
and with an alkaline developer, an image is obtained of the
red and infra-red rays. In solutions of sodium sulphite, and
potassium nitrite, films of silver iodide and bromide (which
are without sensitiveness to the red in ordinary conditions)
give, with ferrous oxalate development, an image well into
the red Certain compounds of silver thus become sensitive
to certain rays through the eliminationf of oxygen. J
In terminating the examination of Abney's researches, we call
to mind, that in 1857 Zantedeschi and Borlinetto (Kreutzer,
* "Photographic Journal"
t It seems, by this, that in certain circumstances red light can also exercise
a reducing action, and that by means of this elimination the sensitiveness to
the red can be increased.
X [In reviewing these last paragraphs which apply to our researches,
we may say that they have been most succinctly described. There is one
point which we wish to impress a little more than it has been, which is
this : — It is not the silver bromide or iodide, &c, which is amenable to
oxidation ; it is the sub-bromide or the sub-iodide which can be oxidized.
Hence, before oxidation takes place there must have been a reduction to
the state of sub-salt. It may happen, and probably often happens, that
in certain rays, as fast as a sub-salt is formed by them, the same rays oxidize
the reduced salt, in which case the rays would, to all appearance, be said
lo have no effect on the silver compound. If all chance of oxidation be
taken away by removal of all oxygen, then the reducing effect of these rays
will be seen on development, since the oxidizing effect prevents develop-
ment. — Editor.]
54 THE CHEMICAL EFFECT OF THE SPECTRUM.
"Jahrb. Photo.," 1857, p. 460) made analogous researches.
They remarked that silver iodide (by itself as well as in collo-
dion) exposed to the light in a cell of nitrogen blackened more
rapidly than in the air. In oxygen or in carbonic acid it
coloured very rapidly (more rapidly than in nitrogen). Hydro-
gen also causes a reduction even in the dark.
Sahler obtained, in a remarkable manner, the same results,
which he published in a very detailed work, in which, amongst
other things, he put forth very curious ideas. According to
him, silver chloride blackens more slowly in pure nitrogen
than in oxygen, showing that oxygen aids the blackening of
silver chloride by light (according to Sahler, blackened silver
chloride will be an oxy-chloride (?).
According to Sahler, not only exposure in oxygen, but in
solutions of chromic acid and of permanganate of potash, aids
the blackening of the silver film during development (Com-
pare these statements with those of Abney !)
In the preceding I have principally spoken of the manner
in which silver compounds behave in light, mostly the reduc-
tion being most rapidly made by violet light. Nevertheless
chemical or optical sensitizers have a considerable influence
upon the progress of a process, and on the sensitiveness to
light of the silver compounds.
CHAPTER VIII.
ACTION OF LIGHT ON METALLIC COMPOUNDS.
Mercury. — The combinations of mercury are, like those of
silver, chiefly sensitive to violet light. Oxide of mercury in
the light decomposes into oxygen and mercury; and this
takes place markedly under an ancoloured glass, less under
a violet glass, and feebly under a red glass. The separation
of oxygen under a red glass is five times less than under a
violet glass (Dalk, 1834). Yellow oxide of mercury blackens
strongly in violet light, less in white light, and still less in green
light (Chastaing). Mercurous oxide, obtained by calomel and
potash, takes a red colour in red light, with the absorption of
oxygen; on the other hand, violet light decolorizes the red
oxide of mercury.
Iodide of mercury blackens (reduction) principally in blue
and violet light, and also in green light (Chastaing). It is the
same with organic and inorganic salts of mercury.
Chloride of mercury, in solution in water, is decomposed in
sunlight, forming mercurous chloride, hydrochloric acid and
oxygen (Boullay), but, truth to say, very slowly. The presence
of organic matter notably aids this reduction of the chloride.
Mercuric chloride in solution in ether or alcohol evolves
chlorine behind white or blue glass, but not at all behind
red glass. The volatile oils distilled with water act in the
same manner.
Mercuric chloride in a solution of oxalic acid is rapidly
decomposed in the light. This mixture particularly absorbs
the ultra-violet rays, and is decomposed preferably by them
with the formation of mercurous chloride. Light, having
passed through a mixture of mercurous chloride and oxalic
acid, is without any action on such a mixture. Basing his
ideas on these properties, Becquerel ("La Lumi&re," p. 151)
designed his actinometer. At ioo° C. there was only a feeble
reduction. Unfortunately, according to my researches, the
56 THE CHEMICAL EFFECT OF THE SPECTRUM.
decomposition does not go hand in hand with the light. It
thus follows that this mixture cannot be utilized in an actino-
meter. On the other hand, a mixture of mercuric chloride
and ammonium oxalate can be employed for photometry. The
method to be adopted I described to the Academy of Sciences
at Vienna, and notified the fact in the " Bulletin de TAssoci-
ation Beige de Photographic "
Mercuric and mercurous oxides in oxalic acid blacken,
being reduced in blue or violet light, but do not change in
red or yellow light. A solution of mercuric chloride or cf
mercury nitrate in ferrocyanide of potassium rapidly gives in
the light (principally violet light) a greenish-blue precipitate,
containing mercury. Herschel made cyanotypes in this
manner; that is to say, blue prints on paper ("Phil. Trans.,"
1843).
Cinnabar (particularly that obtained by the wet method)
rapidly assumes a brown-black colour in alkaline solutions,
and principally in ammoniacal solutions. In the presence of
nitric acid, it is scarcely modified, and with water very slowly.
In these mercury is not liberated, but only a modification of
the sulphide of mercury is produced.
Gold. — The oxide of gold liberates oxygen (Scheele) in the
light, even in vacuo (Chevreul; Dingier, "Polytech. Journ.,"
vol. cli, p. 440). A solution of auric chloride in sunlight
gives gold spangles (Scheele). Paper impregnated with auric
chloride, and submitted to the action of the spectrum, is
gradually decomposed. The action is seen from E to G, that
is to say, from the green to the violet; the action sponta-
neously increases in the obscurity. The maximum action is
found near G or H (Becquerel, " La Lumi&re," vol. ii, p. 95).
Calico, silk, linen, skin, ivory, &c, behave in the same way
as paper (Creutzberg, "Journ. of Pract. Chem.," vol. x, p. 880).
A solution of auric chloride in ether or alcohol is bleached
in the light, precipitating gold ; the blue rays acting with the
greatest vigour (Gehlen). The same action is found in solu-
tions with sugar, and with gum or starch (Fischer ; Kastner,
" Arch, fttr Naturlehre," vol. ix, p. 349). Auric chloride with
ammonium oxalate gives purple-red images on paper (Halleur).
This mixture is also reduced little by little ir the dark (rapidly
by heating).
ACTION OF LIGHT ON METALLIC COMPOUNDS. 57
Platinum.— The salts of platinum behave in the light in
the same manner as the salts of gold. Platinic chloride in
contact with organic matter, for instance in solution in ether
(Gehlen), or applied to paper, decomposes completely into
metallic platinum. The bromide and iodide act in the same
manner (Herschel).
Platinic chloride, mixed in lime or baryta water, gives a
colourless or violet precipitate in the light, but none in red
or yellow light Johannsen carefully examined the compo-
sition of this precipitate ("Ann. Chem. Pharm.," vol. civ,
p. 204), and* found that in all probability it is decomposed
into an oxide of platinum and calcium and calcium chloride.
The reduction of platinic chloride to the metallic state is
effected with difficulty by light, in the presence of oxalic acid ;
and the most energetic of spectral rays are the violet. A
mixture of ferric chloride, oxalic acid, and platinic chloride
remains unaltered, except that the ferric oxalate is reduced
to ferrous oxalate, and the chloride of platinum remains
unchanged. The addition of an excess of sodic oxalate dis-
solves the ferrous oxalate, and consequently the platinum
chloride is reduced. It is this reaction on which Willis has
based his new process of printing with salts of platinum
("Photo. Journal," 1878). The spectrum acts on this mix-
ture in the same manner as upon ferric chloride in a solution
•of oxalic acid.
.Copper. — Cuprous chloride, which has been obtained by
the action of sulphurous acid on cupric chloride, or by
means of zinc chloride, especially when moist, is easily
decomposed in the light; the white tint passes to a dirty
violet or a brownish-black (Gmelin). The body formed is
probably an oxychloride of copper. If a copper plate is
treated with vapour of chlorine, iodine, or bromine in the
same way as a Daguerreotype plate, after exposure in the
camera, and developing with mercury, an image is obtained
(Schultz-Sellack ; Becquerel, " La Lumiere," p. 68). The
iodide of copper is, as a rule, less sensitive than the bromide
and chloride. A polished copper plate exposed to hydro-
chloric acid gas, bromine, or iodine is covered respectively
with cuprous chloride, bromide, or iodide, which in the light
changes colour, taking a dark tint. By this means Becquerel
58 THE CHEMICAL EFFECT OF THE SPECTRUM.
observed the action of the spectrum on the iodide of copper,,
from P as far as between H and G, but not in the less
refrangible rays. The bromide of copper has two maxima :
the action lies from the ultra-violet to between H and G, and
attains a maximum there : remains constant as far as G, and
ascends again to a new maximum about D. The action
shows itself as far as A. The chloride of copper, behaves in
the same manner, only the first maximum is found in the
ultra-violet, whilst the second one is reached about D. The
iodide is much less sensitive to the green, yellow, and red
than the bromide or chloride. A solution of cupric chloride
in alcohol or ether is decolorized by light (Gehlen, Neumann),
and is decomposed into cuprous chloride by the addition of
water. I found red or yellow light almost without aGtion on
this mixture. The double oxalate of sodium and copper
(cupricum) is sensitive above all others to light in .the
presence of a ferric oxysalt. Similarly the double salt of
ammonium (Weiske, " Phot. Arch.," 1864, p. 262), and the
double tartrate of potassium and iron (ferricum), in the pre-
sence of a copper salt (Ehrmann), deposits metallic copper in
the light.
It appears rather doubtful if the violet rays play the
greatest part in the action. A mixture of cupric chloride,
ferric chloride, and hydrochloric acid applied to paper gives
in the light, firstly, ferrous chloride, and afterwards cuprous
chloride.*
Lead. — The carbonate of lead, heated to redness, gives a
yellow oxide of lead.. Spread in the form of a powder on
paper, this oxide darkens under blue glass, and remains yellow
under red or yellow glass ; whilst, on the other hand, a paper
somewhat darkened by light is decolorized under yellow or
red glass (Becquerel, " La Lumiere," p. 56). The plumbous
oxide, in the presence of an alkali and water, becomes oxi-
dized and converted to minium (Levol, " Ann. Chem. Phys.,"
vol. xlvii, p. 196). The binoxide is reduced by light to
minium. In this reduction red light acts more rapidly than
* Obernetter (" Phot. Arch.," 1864, p. 77) has based a printing process
on this reaction. The spectrum seems to act upon the ferric chloride alone ;
at the same time the violet end is particularly active.
ACTION OF LIGHT ON METALLIC COMPOUNDS. 59
violet light (Davy). Iodide of lead, exposed for a long time
to green or violet light, preserves* its yellow tint, and is not
visibly altered. It has, nevertheless, a tendency to reduction - r
for instance, mixed with starch, it becomes blue more rapidly
than that which has been kept in the dark or in red light
(Chastaing). According to Schmitt ("Phot. Mitt," vol. iii,
p. 238), iodide of lead in the dry state is not acted on by
light ; it only decomposes in the moist state, and in contact
with air; there is then an elimination of iodine, and the
production of oxide and carbonate of lead. Chloride of lead
remains unaltered by light. Sulphide of lead, precipitated on
paper, gives an image in the light. The illuminated portions
oxidize rapidly, and become decolorized. It is not yet ascer-
tained if the red rays have the most action.
CHAPTER IX.
ACTION OF LIGHT ON IRON SALTS.
1 i
Ferrous Salts. — The ferrous salts are reduced as rapidly
by light as by heat. Ferrous sulphate oxidizes more rapidly
in contact with air and red light than in the dark, and the
purely chemical action, when shaded from light, is greater
than oxidation in presence of violet light, which has a reduc-
ing action. Chastaing examined the oxidation of ferrous
sulphate during periods of from one to five days in light of
different colours. The oxide formed was ascertained volu-
metrically, the oxidation in darkness being i. The following
figures show the other relative rates of oxidation : —
A
B
C
D
£
In darkness
... I'OO
I'OO
I'OO
I'OO
I'OO
Red light
... i'48
I# 55,
i-8o
I2 5
I '2 1
Violet light
... -15
•39
—
•45
•58
Green light
» » « """" ™
—
•66
•90
•86
The ferrous hydrate does not give very exact or definite
numbers. Here, again, the reducing action of the violet
exceeds that of the red (Chastaing).
Ferric Salts. — The ferric salts, in presence of organic
matter, are reduced by light. A solution of ferric chloride,
brushed on paper, is sensitive to the light : it is reduced to
ferrous chloride.
A solution of ferric chloride in ether passes into the ferrous
state when exposed behind blue or white glass, and not
behind yellow or red (A. Vogel). According to Chastaing, the
violet rays are those which reduce solutions of ferric chloride in
ether to the ferrous chloride with the greatest rapidity. In
the dark, as well as in yellow or red light, a little ferric oxide
separates. Dobereiner found, in 1831, that an aqueous solu-
ACTION OF LIGHT ON IRON SALTS. 6l
tion of ferric oxalate was decomposed into carbonic acid and
ferrous oxalate by sunlight and by blue or violet light, but
not by red or yellow light .(Schweigger, "Journ.," .vol. lxii,
p. 92).
Suckow, in 1832, found that the action of light on ferric
oxalate, after passing through a violet glass, was the same as
that of white light; that the action was retarded by a blue
glass, and still more by a green. In yellow or red light he
remarked no change. Applied to paper, ferric oxalate is sensi-
tive to the green to a notable degree beyond the visible violet
of the spectrum.
Reynolds ("Brit. Journ. of Phot," 1861, p. 9) compares
the relative action of the solar spectrum on paper impreg-
nated with ferric oxalate with that of silver chloride. He
found that the action of light was nearly the same as that on
silver chloride. According to my researches, ferric oxalate is
relatively more sensitive to the green than silver chloride.
A mixture of ferric chloride and oxalic acid, or of oxalates,
behaves in the same way. It is also the same when double
oxalates of iron (ferricum) and ammonium, sodium, or potas-
sium are used. The presence of free hydrochloric acid, does
not hinder the reduction by light According to my researches,
such mixtures can scarcely be surpassed by any other in their
sensibility to direct light. Reynolds ("Brit Journ. Phot,"
1861) and Phipson ("Phot News," 1862) published, inde-
pendently of each other, printing processes with oxalates of
iron.*
The image is formed by insoluble ferrous oxalate, and can
be intensified by gallic acid (black image), potassium, ferricya-
nide (blue image), auric chloride (ruddy-brown image), or by
means of potassium permanganate (brown image).
Actinometers made by means of ferric oxalate give a
measure of the ultra-violet, violet, blue, and blue-green
rays. The same rays are measured by means of ferric chloride
and oxalic acid. Marchand (" Etude sur la Force Chimique,"
1875) examined the action of the spectrum on a mixture
of ferric chloride and oxalic acid, with which he had filled
his photometer. He measured the action of the different
[* It must not be forgotten that Dr. J. W, Draper was the first who
produced a print with ferric oxalate. — Editor, ,]
62 THE CHEMICAL EFFECT OF THE SPECTRUM.
rays by the quantities of carbonic acid eliminated during equal
times. He found as follows : —
In the red rays
•-. 57
„ orange rays
... 9*9
„ yellow rays
„ green rays...
... 134-1
„ blue rays ...
... 615*8
„ indigo rays
... 370*0
„ violet rays...
... 321*0
„ ultra-violet rays
... 52*1
The work of Marchand showed — 1st, that oxalic acid allows
all coloured rays to pass; 2nd, that the ferric chloride only
allows the red, orange, yellow, green, and a very little blue to
pass; 3rd, that a. mixture of the two only allows the red,
orange, yellow, and green to pass, without a trace of blue.
The intensity of luminous action was determined by a quan-
titative determination of the ferrous oxide produced,* or by
the quantity of carbonic anhydride disengaged during oxidation
of the oxalic aci&t
In this method the absorption of the carbonic acid by the
liquid must be taken into account, the absorption being
variable with temperature and pressure; besides which, for
small quantities, the process does not give regular results.
In all actinometers hitherto constructed, this has not been
sufficiently taken into account ; and, in spite of all the favour-
able notices, their indications must be considered doubtful.
As long ago as 1842, Herschel utilized ferric citrate to
obtain images by the action of light. Ferric chloride, mixed
with citric acid, is much less sensitive than the mixture with
* Draper (1857) decomposes the solution, after exposure to light, by
chloride of gold. The ferrous oxide thus formed precipitates metallic gold,
which is weighed. He prefers this method to the measurement of carbonic
acid ("Dingier," vol. cxlvi, p. 29).
f According to Lepowitzand Woods ("Phot. Arch.," 1864), employed
later by Marchand, as an actinometer, in a long series of researches
("Phot. Mitt," vol. xi, p. 142). Becquerel ("Fort. Phys.," 1874) con-
tests the accuracy of Marchand's process, who, in his turn (1874) defends
his method against the criticisms of Becquerel, and says that he did not
wish to determine the chemical intensity of light, except so far as it had
acted on the liquid (" Ann. Chem. Phys.," [5], vol. ii, p. 160).
ACTION OF LIGHT ON IRON SALTS. 63
oxalic acid, and the process does not proceed so well as with
the oxalic acid ; at the same t;ime, whilst a little carbonic acid
is formed, variable quantities of acetic and oxalic acids are
also formed. • Ammonio citrate of iron (ferricum), brushed on
paper, is reduced under the action of the solar spectrum, from
-the violet as far as blue-green to near F (Draper). A mixture
of this salt and potassium ferricyanide rapidly becomes blue
in the blue, in the violet, and in the ultra-violet; but by
prolonged action these rays destroy the colour. The blue rays,
above all others, produce this decoloration with great rapidity
(Herschel).
The reduction of the ferric citrate, and its double salt,
with ammonium citrate form the basis of the chrysotype, or
aurotype,* and the cyanotype.f These salts behave like
the oxalate compounds. Images in carbon can also be
obtained.
A mixture of ferric chloride and tartaric acid exposed to white
light and to coloured light, in the same manner as the organic
salts of iron already described, is less sensitive than the oxalate.
In the light, ferrous tartrate and ferrous chloride are formed
with difficulty, and can be employed in photography like the
oxalate. Poitevin made use of it for photographic purposes
{"Compt. Rend.," vol. lii, p. 94). The portions not acted
upon by light, for instance on paper, possess a variety of
properties; they are not crystallizable, and not hygroscopic,
whilst in sunlight they dissolve and become hygrometric from
ferrous chloride. By the application of coloured powder an
image is formed, by its adhering only to the hygrometric
portions. Gum, glue, and albumen are rendered insoluble by
the ferric oxide becoming soluble in the light, by the formation
of ferrous salts. On these facts Poitevin based different pho-
* Paper sensitized with ammonio-citrate of iron, after prolonged expo-
sure, is plunged in a bath of auric chloride or a solution of silver. On most
parts reduced by light, metallic gold or silver will be deposited (Herschel,
" Athenaeum," 1842, p. 748).
f The ferrous oxide produced is rendered visible by an application of
red prussiate of potash (potassium ferricyanide). The image appears blue
where the light has acted. The ferricyanide can also be added, to begin
with. The yellow prussiate (potassium ferrocyanide) makes the unexposed
portions visible.
64 THE CHEMICAL EFFECT OF THE SPECTRUM.
tographic processes.* The portions altered by light also con-
dense mercury vapour (Merget).t
Ferric sulphocyanide in aqueous alcoholic or etheric solu-
tion, which is of a beautiful red colour, is reduced to the
ferrous sulphocyanide, according to Grottus, principally by the
blue-green rays. In light, in a closed flask, the decoloration
proceeds rapidly ; but the solution becomes red when in con*
tact with the air.
* Coloured gelatine is altogether insoluble in a mixture of tartaric acid
and ferric chloride, but becomes soluble on exposure to light. Thus, it is
possible to work with this in the carbon process, as it is with the chromates.
The exposed portions repel greasy printing ink, whilst it takes on the non-
exposed portions. This is the basis of Poitevin's new phototype process
(" Bull. Soci&e Franchise," T878).
f A mixture of ferric chloride, tartaric acid, and platinum chloride,
exposed to light, produces ferrous chloride ; the portions exposed condense
mercury vapour, which reduces the salt of platinum ("Bull. Soc. Fran.,"'
1872).
CHAPTER X.
ACTION OF LIGHT ON THE SALTS OF URANIUM, VANADIUM, &C.
Uranium. — The salts of uranium behave like the salts of
iron : they are deoxidized in the light ; but to be so they
require the presence of organic substances — alcohol, ether,
glycerine, collodion, &c. (Ni&pce, 1858). A solution of
uranium nitrate in alcohol diluted with water is only reduced
by the blue and violet rays ; the red and yellow are without
action. Aldehyde and acetic acid are formed as secondary
products (Chastaing). The reduction of uranic nitrate to the
uranous salt also takes place with other organic matter, such
as paper. On this fact Burnett, in 1858, based a photographic
printing process. The tartrate and oxalate are particularly
sensitive to light. The photographic image, amongst other
ways, can be intensified by silver nitrate or auric chloride.
With potassium ferricyanide they become red ; and green with
salts of cobalt. Niepce (Dingier, 1859) has studied this pro-
cess from a photographic, and he, Corvoisart, and Lukamp
from a chemical, point of view ("Ann. Chem.," vol. cxiii,
p. 114, and vol. cxxii, p. 113). In 1851 Burnett constructed
an actinometer of oxalate of uranium ("Phil. Mag.," 1859),
and perfected it in i860. Recently, Monckhoven published a
description of a photometer made with ammoniacal oxalate of
uranium. He utilizes Wood's apparatus, and endeavouis to
determine the action of light by the gaseous products. In
this method he did not take possible errors sufficiently into
account (See the criticism of Dr. Eder at the meeting of
the Photographic Association of Vienna, 8th October, 1879.)
In this case also the blue and violet rays are principally
active.
Chromium.— Chromic chloride is decolorized in the light,
and gives images with less oxidizable metallic salts (Monck
66 THE CHEMICAL EFFECT OF THE SPECTRUM.
hoven). The salts of chromium are by themselves stable in
the light ; but, in the presence of organic matter, a reduction
to the state of chromo-chromic oxide, and finally to chromic
oxide, takes place. The alkaline bichromates (and also the
chromates of copper, lead, and silver) are the most sensitive
to light in the presence of gelatine, dextrine, gum, alcohol, &c.
I have found that similar mixtures darken in the violet, blue,
or green of the spectrum, and do not change in the red
and the yellow. Paper prepared with potassium bichromate,
according to Becquerel, changes colours most about the ray
F, at the extremity of the green; near b and E the action
is suddenly stopped. By a prolonged exposure it is possible
to register the Fraunhoferic lines ("La Lumi£re," vol. ii,
p. 95). According to Draper, the action goes from the violet
to the yellow, near the line F. The bichromate of copper,
or a mixture of bichromate of potash and sulphate of copper
on paper, by a partial reduction of the salts of chromium,
forms a neutral insoluble chromate of copper (Burnett, " Phot.
Journal," 1858). The action is due to the blue and violet
rays. A mixture of bichromate and ferrocyanide of potassium
changes, in the light, to an oxide of chromium and ferricyanide
of potassium. Hunt utilized such a mixture on paper for a
chromo-cyanotype process ("Phil. Mag.," 1844, p. 435).
Mixtures of the chromates and organic substances are more
sensitive to the direct action of light than silver-chloride
paper. If, for instance, on the one hand, silver-chloride
paper is exposed till the usual black is obtained, and, on the
other, carbon paper till a good image is produced, it will be
found that the chromated paper with gelatine only requires
from one-third to one-sixth of the exposure necessary for the
silver-chloride paper. It follows that the chromate processes
can be used in bad light, such as in the dark winter days.
Above all, this shows that this process is less dependent on
the ultra-violet rays than is the chloride-of-silver process. On
dark days there are very few ultra-violet rays. Watery vapour
blocks them out, and in the morning, or when the sun is
covered with clouds, only a feeble quantity of the violet end
of the spectrum can pass through; whilst, under the same
conditions, red, yellow, and even green light is but little
diminished in intensity (Janssen, Roscoe, H. W. Vqgel). In
such a case chloride-of-silver paper prints less rapidly than
ACTION OF LIGHT ON URANIUM SALTS. 67
chromated paper ; the violet rays, which have the most ener-
getic action on it, are in defect, whilst the blue, which are the
rays which act most powerfully on the chromate, are less so.
On this account, actinometers of silver chloride paper, for
the control of printing on carbon tissue, are not to be recom-
mended.
All the known practical chromate processes so far rest on
the property that bichromate of potash possesses of rendering
organic substances, such as gelatine, gum, albumen, dextrine,
sugar, &c, insoluble in light, with the production of chromic
oxide. These processes are represented by the carbon, photo-
lithographic, photo-galvanic, and the powder processes. At the
present time bichromate of potash is frequently used for the
manufacture of actinometric paper (H. W. Vogel, " Lehrbuch
der Photo.").*
Vanadium. — Vanadic acid behaves like chromic acid
(Gibbons, "Phot Corr.," 1876, p. 159).
Molybdenum. — Molybdic acid is coloured blue in the
light, when in the presence of organic matter. A solution
of molybdic acid, in sulphuric acid, becomes blue in sunlight,
but only in the presence of organic matter (Phipson, " Phot.
Arch.," 1863, p. 249). Later ("Chem. News," 1874, p. 33),
Phipson again recommended this as an actinometer.
Manganese. — The manganate of potash is sensitive to light,
and gives positive images on paper, as does a solution of the
permanganate in cyanide of potassium (Monckhoven). The
red aqueous solution of oxalate of manganese is decolorized
by sunlight, but more gently in violet and blue light, with
the formation of manganous oxalate and carbonic acid
(Dorbereiner).
Manganous hydroxide oxidizes most rapidly in the red light
* For fuller details as to the reduction of the chromates, and also as to
the scientific principles of modern chromate photography, see my memoir
recently honoured by the Photographic Association of Vienna, "Les
reactions de l'acide chyromique et des chromates sur les substances
organiques dans leurs rapports avec la chromo-photographie," Vienne,
1570.
F 2
68 THE CHEMICAL EFFECT OF THE SPECTRUM.
and in the air. In darkness, or in green light, the oxidation
goes on less rapidly than in red light; more gently in the
violet light, whose action is a reducing action. If the value
of oxidation in the dark be equal to i, according to Chastaing,
in different coloured lights the following rates are obtained : —
A
B
In darkness
I '00
I'OO
„ red light
... i # i8
r 3 8
„ green light . .
• • •
•98
„ violet light ...
••• '57
*57
CHAPTER XL
ACTION OF LIGHT ON THE IODIDES, &C.
Potassium Iodide splits up into potassium hydroxide and
iodine in the light (Loew., " Fortschr. der Phys.," 1869,
p. 413). The presence of carbonic acid and air favours the
decomposition (Battandier). In the presence of sugar,
potassium iodide rapidly becomes yellow (Durewell, "Chem.
Centralblatt," 1876).
Ammonium Iodide is sensitive in the same degree. Blue
light acts strongly on the iodides, and yellow light but very
feebly.
Hydriodic Acid is split up into iodine and hydrogen under
the influence of the most refrangible part of the spectrum
(Lemoine, "Pog. Beib.," vol. i, p. 510). In 1879 Leeds con-
structed a photometer with potassium iodide and weak
sulphuric acid. He calculated the amount of iodine set at
liberty by the action of light, by testing with sodium hypo-
sulphite.
An aqueous solution of iodine does not change in the light
(Vogel, "Phot Cor.," 1866, p. 62). The blue solution of
iodide of starch in water is completely decolorised by white
light, and by the yellow and green rays of the spectrum. The
red and blue rays act feebly, and the violet rays are without
action ; so much so, that the reverse to this, the decoloring
action, progresses in daylight.
Bromine and Chlorine. — Bromine water behaves like
chlorine water. The gaseous chlorine preferably combines
with another body in die presence of light, and principally in
the blue and violet, whilst the red and yellow do scarcely, if at
all, influence it
This is notably the case in a mixture of chlorine and
70 THE CHEMICAL EFFECT OF THE SPECTRUM.
hydrogen, which in the light (in bright light, and with explo-
sion) combine to form hydrochloric acid. The maximum
action of the solar spectrum is between G and H (Fabre and
Silbermann, "Ann. de Chem.," vol. xxxvii, p. 297). In the
indigo, according to Draper, the action is the most intense,
the light then acting 700 times more energetically than in
the ultra-red. Bunsen and Roscoe have found two maxima
in the spectrum, the first between G and H, whence the action
diminishes to about H, then increases as far as the ray I in
the commencement of the ultra-violet, and then is nil in the
ultra-violet; thus, before arriving at the spectrum, rendered
visible by fluorescence, the action becomes inappreciable
("Pogg. Ann.," vol. cviii, p. 267). The results obtained by
Bunsen and Roscoe are to be found in the accompanying
table. The figures giving the action in the different regions
of the spectrum are compared to the quantity of hydrochloric
acid formed in one minute, which are quantities measured
according to the unit of the instrument The indication of
the region gives the limits of the rays passing through the
slit. Thus C — 4DE means that the slit allows a bundle of
rays limited on one side by the C line, and on the other by a
iine midway between D and E; £DE — F means rays com-
mencing at \ of the distance between E and D towards D,,
and ending near F.
C — iDE
0-5
H — |IM ...
55*i
iDE — E
1 '3
±OM — N ...
3**
|DE — F
1 *4
N — |QR ...
18-9
+FG— G
... 28*4
£NQ — |RS...
I2 '5
C — *GH
- 54*5
Irs — ist...
2*1
iGH~H
... 60 *5
|st — fuv
1*2
*GH - I
••• 527
Bunsen and Roscoe, in measuring the absorption of
chlorine, showed that the chemical action of light followed
in the wake of the absorption, that is to say, of the passage
of a certain quantity of energy in the movement of the ether
on the molecules of the body, although this absorption took
place without chemical action; and was comparable to the
mixture of chlorine and hydrogen, when the light causes, at
the same time, the combination of the gases to the state of
hydrochloric acid.
ACTION OF LIGHT ON THE IODIDES. 7 1
The coefficient of extinction of a mixture of equal volumes
of gaseous chlotine and air = 6 . 6 > The co-efficient of ex-
tinction of equal volumes of gaseous chlorine and hydrogen is
-g-. This latter, which is much larger, proves that the
chemical action requires a quantity of light proportional to
its density. The difference between the two gives the co-
efficient of chemical extinction == - — . This proves that
723
whereas in the detonating mixture of chlorine and hydrogen
the light is simply utilized by the chemical action, and that it
produces no other movement, the light of the source utilized
for the experiment was diminished by one-tenth in its chemical
action by passage through a layer of a thickness of 723
millimeters ; for other luminous sources, giving light otherwise
composed, this number would be evidently something else.'
Chlorine decomposes water in the light, with the formation
of hydrochloric acid, oxygen being disengaged. In the same
way chlorine more readily decqmposes organic hydrocarbons in
the light
Marsh-gas and ethylene do not unite with chlorine in the
dark, but rapidly in diffused light In sunlight the combina-
tion is so violent that it is explosive (Regnault and Laurent,
"Ann. Chem. et Phys.," (2) pp. 60 to 71). Chlorine behaves
in the same way with benzine and naphthaline in the presence
of light. This is also the case with other substances, such
as ether, alcohol, acetic and prussic acids, all of which combine
with chlorine more readily under the action of light Bromine
and iodine present the same phenomena, but in a more feeble
degree.
Nitric Acid. — Concentrated nitric acid is decomposed by
light into oxygen and nitrous acid, and at the same time is
coloured yellow : this takes place under white, blue, or violet
glass, but not under red glass (Seebeck).
Phosphorus.— White phosphorus in water or in different
gases is rapidly changed in sunlight or violet light into red
phosphorus. In red light this modification takes place but
slowly (A. Vogel ; Sweigger, "Journ. fur Chem. und Phys.,"
72 THE CHEMICAL EFFECT OF THE SPECTRUM.
voL vii, p. 95, and vol ix, pp. 23 etseq). According to Schrotter,
heat produces the same effect Draper demonstrated in a very
simple manner the sensitiveness to light of white phosphorus
in its passage into the red modification. He flowed a thin film
of phosphorus between two pieces of white glass, and exposed
it to the spectrum. He afterwards dissolved away the white
by carbon disulphide, which left the red intact Draper thus
obtained a photographic image of the most refrangible parts of
the spectrum.
Sulphur. — A concentrated solution of sulphur in carbon
disulphide, exposed to light, deposits yellow insoluble sulphur.
The light, having passed through the solution, is found to have
been deprived of the rays between H and G, and the ultra-violet
of the spectrum is completely absorbed (Lallemand).
Potassium Ferricyanide, in an aqueous solution, is com-
pletely reduced by light to the state of ferrocyanide (together
with a blue body). The decomposition is due to the blue or
violet rays, and not to the yellow (Herschel, "Phil. Trans.,"
1842). The presence of orgatiic substances, such as paper,
helps the reduction. For an aqueous solution the reduction
is found to be twice as strong in the violet as in the yellow
(Chastaing). The presence of gelatine (as is the case with
bichromate of potash) aids enormously in the decomposition
by light of ferricyanide (Gintl, "Chem. CentralbL," 187 1,
p. 591). Ferricyanide of potassium applied to paper is also
sensitive; it changes principally to ferrocyanide. The salt
reduced by the light can be rendered visible by the application
of ferric chloride, of nitrate of uranium (" Lexicon," p. 154), ot
silver nitrate, and of the salts of cobalt (Burnet, " Phot Not,"
1858^. I have found that a mixture of ferricyanide and of a
solution of ammonium oxalate is more sensitive than the first-
named body by itself; it forms a bluish precipitate. Ferricyanide
.and ferric chloride give, gradually, in the solution a precipitate
of prussian blue ; according to my researches, the sensitiveness
to green or blue light does not appear to differ so much as for
silver chloride, or even as the oxalate of iron : both of these,
and principally the latter, decompose much more rapidly in
the light than the above mixture. Red or yellow light only
act very feebly on it. According to Herschel, all the visible
ACTION OF LIGHT ON THE IODIDES. Tl
spectrum acts on it, but principally the blue and the violet ;
from the yellow to the ultra-red the action is only feeble.
A solution of potassium, ferricyanide, and of corrosive
sublimate rapidly becomes turbid in white light, but not under
a red or yellow glass, giving a bluish-green precipitate ; in the
first place the mercuric chloride is reduced to mercurous
chloride, or calomel Mixtures of solutions of potassium,
ferricyanide and of the nitrates of lead or uranium become
slowly turbid in the light ; the first give a light blue precipitate,
the second a brown precipitate. The reduction of the ferri-
cyanide continues; coloured light acts on such mixtures as
indicated above.
Prussian Blue is decolorized in sunlight with the formation
of cyanogen, and also in vacuo (Chevreuil). In the dark the
blue colour reappears, oxygen at the same time being absorbed
<Graham-Otto, " Chem.," 1872, p. 1203). Red light acts
principally on prussian blue (Baudremont, "Phot Corr.," 1878,
P- *73)- A solution of prussian blue in oxalic acid, according
to Scheras, gives, in the light, insoluble prussian blue; but
Bottger ("Chem. Central.," p. 182) denies this.
Sodium Nitro-prussiate. — Aqueous solutions of sodium
Tutro-prussiate give in the light a blue precipitate ; upon paper a
very feeble blue image is obtained, which can be intensified by
means of ferrous sulphate. A very peculiar action is that of
coloured light on a paper prepared with silver chloride and
an excess of silver nitrate, and then plunged into a solution
of nitro-prussiate. Such papers give natural colours with
3. vigour dependent on the excess of silver nitrate. When
free silver nitrate is absent, merely a black and white image is
obtained, and no reproduction of colour (Brakenbridge,
"Zeitsch. Phot," 1861, p. 128). In the light, and most pro-
bably under the influence of the violet end of the spectrum,
a mixture of sodium nitro-prussiate and ferric chloride gives
prussian blue ; at ioo° C. there is no decomposition. According
to Roussin ("Phot Arch.," 1865, p. 342), sunlight precipitates
a quantity of prussian blue proportional to the exposed surface
and to the intensity of the light. This quantity can be collected,
dried at ioo° C, and weighed. This mixture can be employed
as an actinometer ; according to my experiments, the truth is,
this is more sensitive to light than a mixture of potassium
74 THE CHEMICAL EFFECT OF THE SPECTRUM.
ferricyanide and ferric chloride, but less so than the ferric
oxalate. In the absence of free acid the sensitiveness is a
little greater. A. Vogel has established (" Phot Arch.," 1871,
p. 92) that a petroleum lamp — a strong light, but deficient in
violet and ultra-violet rays — gives, after twenty-four hours, a blue
coloration, but not an appreciable precipitate ; while a mag-
nesium light, rich in ultra-violet rays, causes the decomposition
in a very short time.
Arsenic. — Metallic arsenic, in a concentrated solution of
caustic potash, is oxidized in the air, giving potassium arsenite.
These experiments were made with gaslight — a light rich in
yellow and red rays, poor in blue and violet, and giving in
consequence results more pronounced for the red. The oxidation
to the state of arseneous acid is shown in the following table : —
a b c D
In the dark ... roo ... roo ... i*oo ... roo
„ violet ... roo ... 0*98 ... 0*92 ... 0*93
„ red ... 1*04 ... ro6 ... no ... 1*20
In the same way as with the salts of iron, the violet rays have
a reducing action, — that is to say, retard oxidation. The red
oxidizes ; the green is indifferent Arseneous acid in a dilute
solution of caustic potash is oxidized, and becomes arsenic
acid. It may be stated, that after several days' action the
minimum oxidation' takes place in the violet, and that the
maximum is due to the red, although the difference is small.
Sulphides. — A very dilute solution of sulphuretted hydrogen
in water is scarcely more decomposed in violet light than in the
dark, whilst red light decomposes it more rapidly by oxidation^
The mono- and poly-sulphides of sodium give, in different
coloured lights, a variable quantity of sulphur and hyposulphite :
the decomposition is almost equally active in the green, in the
violet, and in the dark; red light is most favourable for
oxidation.
Sulphurous Acid, under the influence of violet light, is
changed to sulphuric acid (thus an oxidation in the violet appears
contrary to the laws laid down by Chastaing). But if this reaction
takes place in a vacuum and in the presence of free sulphur,
it is seen that the light first commences to reduce the sulphurous
acid to the state of sulphur and oxygen, which afterwards oxidizes
the sulphurous acid.
CHAPTER XII.
ACTION OF LIGHT ON ORGANIC COMPOUNDS.
Organic Matter. — This is decomposed, and generally com-
bines with the greater facility under the influence of light than
in its absence. Light principally favours oxidation. Many
colouring matters bleach in the light by oxidation. It is thus
that the bleaching of such organic matters as saffron, indigo,
logwood, turmeric, &c, are explained. Colours are ordinarily
distinguished as permanent or fugitive colours according to their
resistance to exposure to the air. There is not in reality any
absolutely fixed organic colour ; for all undergo change in the
light, but some more slowly and feebly than others. Certain
pigments bleach in the light, and take a deeper tint in the
dark, as very exact comparative experiments have shown (A.
Vogel). In 1857 Chevreuil read to the Acad&nie de Science
of France a work in which he proved that the bleaching of
organic colouring matter by light required not only the inter-
vention of oxygen, but also that of light ; the colours resisted
change equally as well in the light in a vacuum as in the air
in darkness ("Compt. Rend," 1854 and 1858).
Light particularly acts on vegetable colours, and in a
manner varying with each colour. Nearly every ray can
act Thus the rays from the green, as far as the violet,
bleaches the yellow stain of the yellow crocus pressed on paper.
In the same way the red and yellow change the rose-red colour
of the stock ; the rich blue of the scented violet, which car-
bonate of soda makes green, is bleached by the same group
of rays. The green colour of the leaves of the elder is changed
by the ultra-red. These examples show that every part of the
spectrum is active in effecting changes, and that certain vegetable
colours are influenced by certain rays, and other colours by
other rays (Herschel, "Phil. Trans.," 1844). The leaves of the
Papaver Rhceas flower decolorize more rapidly under a blue
?6 THE CHEMICAL EFFECT OF THE SPECTRUM.
than under a white glass (A. Vogel). According to Herschel,
the colours of flowers bleach most rapidly in the colours com-
plementary to them; thus the colouring matter of yellow
flowers fades in blue light, that of violet in the green, of blue
in the yellow-red, of the rose and purple in the yellow and in
the green. There are very few exceptions to this rule.
Colours derived from coal tar (aniline colours) are very
sensitive to the light, in which they rapidly bleach. According
to Toth and myself, we have found that with chrysoidin and
anilin red this bleaching is principally due to blue and violet
light ; for if paper dyed with these colours be exposed under
a yellow-green glass, allowing neither blue nor violet to pass,
or under a red glass, this remains unaltered for a year, whilst
the colour rapidly disappears in daylight The colours of the
wings of butterflies principally bleach in white or violet light :
of all the spectrum rays these last act most energetically
(Capronnier, " Phot. Mit," vol. xiv, p. 50).
Cyanine-blue is sensitive to yellow light. It absorbs strongly
the part of the spectrum — that is to say, the yellow — to which
it is most sensitive. {Vide ante.) Turnsol oxidizes strongly
in the violet, less in the red, and very little in the dark
(Chastaing). According to other observations, yellow should
have the strongest action on turnsol.
Oils oxidize slowly in the air ; this oxidation is accelerated
by heat and light. The blue and violet rays act with greater
energy than the less refrangible rays (Cloez, " Compt. Rend,"
1865, PP- 3 21 an d 981).
Linseed oil, prepared with litharge, oxidizes in the light, and
becomes insoluble in ether and oils (Labord, "Bull Soc.
Phot.," 1858).
Amongst the essential oils are hydrocarbons, the oxidation
of which is strongest in violet light, next in the red or green ;
the oxidation is very feeble in obscurity (Chastaing).
The Oil of Terebenthin gives by oxidation the following
results : —
a b c o
In the dark ... 1 *oo ... 1 *oo ... 1 *oo ... 1 *oo
„ red ... 2*36 ... 1*94 ... 2*30 ... 2*00
„ violet ... 2*57 2*98 ... 2*66 ... 3*00
ACTION OF LIGHT ON ORGANIC COMPOUNDS. TJ
According to these experiments, violet light acts with the
greatest energy:
The Oil of Lemon gives the same results as terebenthin.
The oxidation is —
A
In the dark i *oo
red light 2*23
green light .. . ... 2*43
»
violet light ...
3*i4
B
I'OO
I'4I
i*44
2-05
The oil of lemon absorbs oxygen more slowly than the
oil of terebenthin, though the rate of absorption under the
influence of coloured light is the same.
Xylene. — In this case the maximum absorption is also in
the violet ; least in the dark. Exposure was given from two to
eight days.
Aldehydes. — The oxidation of the aldehydes progresses
more slowly in the dark than in the light. In summer ; in the
dark, about one ; in the yellow, two ; in the violet, three.
Chastaing found the following numbers : —
ABC
In the dark ... ... roo ... i*oo ... i*oo
red ... ... 1*28 ... i*88 ... 1*50
»
»
violet
1 '53
272
2*33
The Oil of Cinnamon oxidizes more slowly than aldehydes.
It oxidizes in about eight days.
A B
In the dark ... ... ... ... i*oo ... i*oo
red ... ... ... ... 1 10 ... 1 1
violet... ... ... ... 2*17 ... 2 45
99
if
Oil of Bitter Almonds. — In from three to twelve days the
oxidation was —
A
B
c
th*
j dark
• •
I'OO ..
. I 'OO . .
. I'OO
»
yellow . . .
• • •
1-44 ..
• • «
•
»
violet
• • •
3-00 ..
. 1*50 -
. i-66
»>
red
• • •
— .
. i'i3 •■
. 1*30
»
green
• • •
- —
• i*33 •■
• 1 '35
78 THE CHEMICAL EFFECT OF THE SPECTRUM.
Ether. —Ordinary ethylic ether oxidizes rapidly in the violet,
a little in the red, and very slowly in the dark
In the dark ... ... ... ... roo
„ red i*20 to 1*40
„ violet 2*50 to 3*50
Phenol becomes red in the light, particularly in violet light,
on account of the oxidation of its impurities.
Mineral Oils, under the influence of light, absorb oxygen
and transform it into ozone, which easily oxidizes the bodies
with which they are in contact (Grotkowsky, "Bull. Soc. Chim.,"
1869 [II], p. 75; "Dingier," vol. cxci, p. 173). It may here
also be supposed that violet light has the predominant effect.
The Tincture of Aloes, originally a yellowish red, becomes
gradually blood-red by oxidation under the influence of violet
or blue light, whilst the red rays produce no alteration in it
(Marbach, "Phys. Lexicon," vol iv ? p. 847).
Asphaltum and Resin. — If a thin film of a solution of these
substances in alcohol, ether, or essential oils be exposed to
light, the parts so exposed, according to old theories, become
oxidized and insoluble. According to the researches of Kayser
(" Phot. Cor.," 1879, Sept), the insolubility is not only caused
by oxidation, but also by an alteration in molecular condition.
Asphaltum gives a spectrum image from A to H, or through
the whole visible spectrum. Caoutchouc (according to Swan,
"Phot News," 1872) and santonine (Vogel, "Lehrb. Phot,"
p. 21) equally change in the light
Gum Guiacum powdered, or spread by an alcoholic solution
on paper, is coloured green in white or violet light (oxidation),
and in yellow or red light regains its original colour (Wollaston;
Herschel). Without the presence of air, for instance, in an
atmosphere of hydrogen, guiacum does not colour green in the
light The green coloration arises from the formation of a blue
insoluble body in alcohol, in the substances still undecomposed
(Biot, " Compt Rend," vol. viii, p. 598). Paper impregnated
with an alcoholic solution of guiacum is coloured a yellow-
green in a mixture of chlorine and air ; the spectrum
prints the natural colours, except the green. The colours
ACTION OF LIGHT ON ORGANIC COMPOUNDS. 79
rapidly disappear (Herschel, Photochromie, "Phil. Trans.,"
1868, vol. xvii, 1842). The maximum action is about M. A
guiacum paper coloured blue by chlorine water, and afterwards
receiving the image of the spectrum, is decolorized from the
red to the violet, with a maximum action about F. This
phenomenon is a deoxidation. Guiacum paper is not coloured,
if the ultra-violet light is cut off from H, by allowing light to
pass through a solution of quinine. In the spectrum it becomes
blue from H to P (Becquerel).
Sulphate of Quinine cuts off those rays which produce
fluorescence,* particularly the ultra-violet, producing a mole-
cular change, and is transformed into sulphate of quinidine.
Turmeric similarly is most acted upon by the rays which
produce fluorescence. Turmeric paper is decolorized by
oxidation under a violet or green glass, and remains unaltered
under a red glass (Chastaing). Turmeric paper is modified
in the light, even when its colour is ndt actually changed.
Dilute lime-water does not darken paper which has been
exposed to light, although this same solution gives an apparent
reaction with this paper when shaded from light (A. Vogel,
41 Phot. Arch.," 1871, p. 97).
Amyl-nitrite, according to Tyndall, is decomposed in the
light, especially when a beam of light is allowed to pass through
its vapour. In the interior of the beam a thick cloud of
amyl-nitrate, hyponitrite, and other products are formed. The
blue rays have the greatest action. The light which has passed
through the vapour of amyl-nitrite is without any effect on this
body. Ethyl-nitrite behaves similarly.
Light is almost without any action on the formation of ether.
This feeble action becomes nil with time, even before the
etherification is complete.
Chlorophyll is the green colouring-matter of leaves. Solu-
tions of chlorophyll are oxidized in the light, and decolorize, and
this in a varying degree, according to the nature of the solvent
[* It would perhaps be more accurate to say, which usually produce
fluorescence, since there are bodies which fluoresce when excited by rays in
the visible spectrum. — Editor.]
80 THE CHEMICAL EFFECT OF THE SPECTRUM.
Chlorophyll in the following solvents is decolorized in the-
light, thus —
Chlorophyll dissolved in alcohol at 75° in '05 hour.
benzene „ # i 1
ether „ '20 „
olive oil „ 3*50 hours.
Chlorophyll is most rapidly oxidized by the rays which it
absorbs ; that is to say, by the orange-red rays. Dilute solutions-
decompose more rapidly than concentrated ones (Gerland,
" Fortschr. der Phys.," 1871, p. 468 ; 1872, p. 433. Wiesner r
loc. ctt., 1874, p. 609). As a rule, an alcoholic solution of
chlorophyll behaves in the spectrum almost inversely to that of
silver chloride. Cossa ("Deutsch. Chem. Gesell.," 1874, p. 358)
exposed such a solution to one part, and chloride of silver to-
another part, of the spectrum — in one case under a solution
of potassium bichromate, and in another beneath an ammoniacal
solution of cupric sulphate. In the first case the chlorophyll
was bleached, and the paper scarcely changed. In the second
case, the chlorophyll remained unaltered, and the silver chloride
became black.
Xanthophyll. — According to Wiesner, is rapidly decolorised
by the rays which pass through ammoniacal cupric sulphate -
green, yellow, or red light has no action.
' *
CHAPTER XIII.
ACTION OF LIGHT ON PLANTS AND ANIMAL LIFE.
Plants. — It was known in 1778 that the bleaching of plants
was due to the absence of light (Bonnet ; Duhamel ; Meese,
"Journ. de Phys."). Chlorophyll is chiefly acted upon by
yellow or red light In 1854 Gardner recognized that the
green leaves of plants decompose carbonic acid, and liberate
oxygen, more readily under a yellow glass than under a blue
glass. Deherain, and more especially Pfeffer (" Pogg. Ann.,"
1872, p. 442), examined the photo-chemical action in question
by means of leaves immersed in water. Measuring the oxygen
set free, Pfeffer found that the following volumes of gas were
liberated : —
• •• ••• ••• ••• J ^T
• • • ••• • • * ••• \J \ \J
• • • ••• ••• ••• X ww \J
• •■ ••• • • • • • • X J mw
• •• ••• • • • • ■ • «£ ^ X
• • • ••• ••• ••■ JL X J
«»<• • • • • • • • • • MM
•* The development of gas does not seem to give altogether
the amount of carbonic acid decomposed. TimivirazefF
("Compt. Rend.," vol. xciv, p. 1236), in his very exact spec-
trum researches, and by analysis, having insolated the leaves
placed in an atmosphere charged with carbonic acid, found
that the greatest decomposition of carbonic acid took place
in the red between B and C ; these rays are directly and very
markedly absorbed by chlorophyll. In the orange, yellow, and
green the quantity of carbonic acid decomposed diminishes,
in the ultra-red there is no decomposition ; on the other hand,
carbonic acid is developed by respiration. Previously, Draper,
Lommel (Pogg., vol. cxliii, p. 581), and Muller ("Fortsch.
In the red ...
99
orange
yellow
5>
green ...
blue ...
5>
indigo
'violet ...
82 THE CHEMICAL EFFECT OF THE SPECTRUM.
Phys.," 1872, p. 146) confirmed the observations of Timivirazeff.
The latter, however, first investigated the question. The green-
ing of the mono- and decotyledons can be effected by all parts
of the spectrum, The action of the red rays is the most
energetic ; the blue, the violet, and ultra-violet act but slowly.
The rays so light, as well as the dark rays, reinforce the respi-
ration of the plants. According to Wiesner ("Vienna Academie
Ansieg.," 1876, p. 137), the ultra-violet appears without effect
in this work. The rays belonging to the region of those ab-
sorbed by chlorophyll favour respiration. According to Krauss
(" Fortsch. Phys.," 1870, p. 406) and Brillenin (" Comp.
Rend.," 1870, p. 511), chlorophyll makes starch in plants
merely by the action of light, and this much more in the
red than in the blue. Famintzin had already shown experi-
mentally that the production of starch in chlorophyll only took
place in diffused yellow light (behind bichromate of potash),
and not in diffused blue light (behind cupric oxide dissolved
in ammonia), in which indeed the starch disappears.
Light is absolutely necessary for organic life, especially for
the growth of plants (Radan, "La Lumifere et les Climats;"
Becquerel, "La Lumifere," vol. ii, p. 235). In the light plants
decompose carbonic acid from the air, giving up oxygen. It
has long been known that the quantity of oxygen liberated by
plants increases with the intensity of the light ; that a cloudy
sky diminishes it; and that darkness renders it nil. In
the dark, plants breathe like animals; that is to say, they
absorb oxygen, and emit carbonic acid. Fruits also breathe
like animals. Light makes leaves green, and colours flowers.
Without light no chlorophyll is formed ; the green cones of
the conifers, however, do not require light for their deve-
lopment
The Seeds of Plants are influenced by different kinds of
light. According to Hunt, yellow, red, and green light are
hurtful to germination ; whilst blue light favours it ; whilst in
the after-development blue light acts less favourably than does
yellow, red, and white light. Zantedeschi (" CompL Rend.,"
181 7, p. 349) obtained more rapid germination under a green
glass ; then under violet. Yellow, orange, and red glasses had
a less favourable action.
To these inexact and contradictory statements (because they
ACTION OF LIGHT ON PLANTS AND ANIMAL LIFE. 83
were obtained by means of coloured films) Poggiale, in 181 7,
opposed his researches with the pure spectrum. He found
that the germination of plants was more rapidly effected in red
than in green light; and more rapidly in the green than in
the violet
Many stems of flowers and trunks of trees bend towards
the light : daily experience teaches that plants turn towards the
light. Dr. Gardner, in 1814, was the first to study the details
of this phenomenon by means of the spectrum. If seeds are
allowed to germinate and grow for some days, they develop
vertical stems several inches in length ; if then they are exposed
'in such a manner as to receive the influence of the spectrum,
they begin to curve. When placed in the other parts of the
spectrum the stems turn towards the indigo; in the indigo
they curve towards the incident rays. If taken into the dark
they return to their vertical position ("Phot Corr.," 1873,
p. 71). The. principal action takes place in the indigo, near G.
Poggiale also remarked this (" Compt Rend.," .842 and
1844). The blue, violet, and ultra-violet rays alter the curva-
ture of the stems of plants in retarding their growth.*
According to Bohm ("Wiener Anzeig.," 1874, p. 47), a
light too feeble to give rise to chlorophyll produces heliotropic
curvature. These observations are in favour of positive helio-
tropism — a fight for light.
Many plants have a sort of sleep ; that is to say, the flowers
close. They wake, according to Hoffmann (1850), more
readily in blue or yellow light than they do in red or white
light. The scent of some kinds of flowers is much more
intense at night than in the day ; similarly, as many animals
rest during the day to wander at night, so many scented plants
are inactive daring the day. In the movements of leaves the
least refrangible rays act as darkness, whilst the direction of
* See the action of light on plants, Sachs, "Botanique," 1874, p. 727. v
In particular, for the action of coloured light on plants, " Botanik Zeit., ,?
1865, where will be found publications on this subject. Bert ("Compt.
Rend.," vol. lxxiii) thinks that all colours are hurtful to vegetation, and
that white light alone is really favourable. According to this, horticul-
turists should do away with the employment of coloured glass in their
houses for young plants. As to the influence of light on organic matter
(vegetable or animal), see Foissac, " Meteorologie," 1859, p. 65 ; and
Wiesner, « Bull, de l'Acad. de Vienne."
G2
84 THE CHEMICAL EFFECT OF THE SPECTRUM.
the movement is determined by the blue, violet, and ultra-
violet rays.
The action of the different coloured rays of the solar spec-
trum can be formulated as follows (Sachs, "Lehrbuch der
Botanik," 1874, p. 709) : — A mixture of rays of different
refrangibility, of white solar light, and which appears to our
eyes as variously coloured bands of the spectrum, has a phy-
siological action on vegetation of such a character, that the
chemical action depends principally on the mean or less
refrangible rays (red, orange, yellow, and green) ; it is thus in
the greening of chlorophyll, in the decomposition of carbonic
acid, and in the formation of starch (sugar . or fat) in
chlorophyll.
On the other hand, the most refrangible rays, blue, violet,
ultra-violet, have a preponderating influence on mechanical
changes, so far as they depend completely on the light. It is
these rays which have influence on the rapidity of growth,
change the movement of protoplasm, give a determinate direc-
tion, and make the cellular tissue in organs of movement of
many leaves to vary, as well as their composition.
Influence of Light on Animal Life. — Light has also
considerable influence on animal life. Lavoisier, at the end
of the eighteenth century ("Trait& Elementaire de Chimie"),
wrote: — "Without light nature would be without life — dead,
inanimate." I do not mention the modern fantastic writing,
"Use in Medicine of the curative power of the Violet and
Blue Light." On the contrary, the remark made by William
Edwards in 1824 is important, that frogs' spawn placed in
an opaque glass perish, whilst those placed in a transparent
glass were regularly hatched The hatching of perch is re-
tarded in the dark. Many illnesses are due to too violent
an action of light, — for example, sun-strokes; whilst others,
from a want of light, — for example, scrofulous diseases.
B&hard found ("Compt. Rend.," 1858, p. 441) that flies'
eggs were hatched more rapidly under a blue or violet glass
than under a white, red, yellow, or orange glass. Guarinoni
believed he found a favourable influence of violet light on
the silkworm. According to Salmi and Piacentini (" Fortsch.
der Phys.," 187 1, p. 463), animals give out most carbonic acid
under yellow or green glass ; these colours should favour
ACTION OF LIGHT ON* PLANTS AND ANIMAL LIFE. 85
respiration. Moleschott stated in 1855 that frogs breathed
more freely in daylight than they did in the dark. The sun
browns the skin, reddens the blood, whilst the Esquimaux
become pallid in their long winters ; most probably blue or violet
light is most effective in this case.
Visual Purple. — The visual purple discovered by Kuhne
in the retina is bleached by light, but is coloured afterwards
It is very sensitive to the yellow-green, and little to the red
rays; the sensitiveness is analogous to the absorption of
colours. In the light this purple becomes yellow, absorbing
principally the blue or violet, and thus decolorizing very
rapidly by it.
It is known that different colours have a different effect on
men and animals. Usually blue or violet rays appear to act
the most on animal organisms, and the yellow and red rays on
plants. Nevertheless, this grave question has been but little
studied as yet.
CHAPTER XIV.
GENERAL EFFECT OF THE SPECTRUM.
We have seen that light and the pure spectrum colours act
sometimes by oxidation and sometimes by a reducing action.
This variable phenomenon of oxidation and reduction, is it sub-
ject to fixed laws ? Must one attribute to a certain kind of light
of the spectrum an oxidizing action, and to another kind a redu-
cing action? It is difficult to reply, in spite of all the preceding
observations, to these questions. Chastaing has made the
most complete study on the influence of coloured light on
chemical phenomena, and more especially on those of oxida-
tion. Since this interesting work is only found succinctly given
in German extracts (Pogg., "Beib.," 1877, p. 517), I have taken
the liberty to give some original details beyond those already
described above.
Most of the experiments lasted days, weeks, and even
months before Chastaing undertook to give the relative instead
of the absolute numbers. A great number of experiments
were made under coloured glasses, first examined by the
spectroscope; in other cases he worked with the spectrum
only.
White Light — When white light acts on a binary metallic
compound, its action is mostly reducing, like that of violet
light ; rarely oxidizing. In general it can be said that white
light acts by reduction. The phenomena of oxidation are prin-
cipally to be attributed to a secondary chemical action, or to a
particular absorption of light.
Violet Light reduces the metallic combinations, and in less
time than white light.
Green Light does not exercise any important action; it is
feebly reducing. Green light is almost always mixed with a
little yellow.
>
GENERAL EFFECT OF THE SPECTRUM. 87
Red and Yellow Light — At first sight these rays do not
seem to act on metallic salts. In reality they have an inverse
action to that of violet light.
To fix more exactly the oxidizing or reducing action of
light, a comparative experiment was every time made in the
dark. The value of the oxidation was determined by mea-
suring the oxygen or by titration. During the determination
of the absorption the substance was not in direct contact with
mercury; it was also placed in a small flask with a flat
bottom, and the air which was in the flask isolated by means
of mercury. A quantity of mercury, proportional to the
oxygen absorbed, entered the tube : by weighing the mercury,
the volume of the gas was easily found. In other cases the
quantity of oxidized matter was determined volumetrically.
The temperature should remain constant during these ex-
periments, to prevent phenomena interfering with the pro-
ceedings. At a high temperature the oxidation is largely
increased, and the influence of light unrecognizable; even
variations of 5 to 6° C. have their effect. This was remarked
above all for ferrous sulphate. In all experiments, then, the
influence of temperature should be avoided.
Chastaing concluded from his researches on the combina-
tions of manganese, chromium, and mercury (see above), —
1 st. The chemical action of the spectrum on binary
combinations and upon salts is double — reducing
for one part of the spectrum and oxidizing for
another. He established the photo-chemical action
by opposite curves, — the first the curve of reduc-
tion as generally known, the second that of oxida-
tion, whose existence is not attributable to a single
peroxide or perchloride, &c, but also to all readily
oxidizable bodies.
2nd. The reducing chemical action is much more pro-
nounced than the oxidizing chemical action. The
state of the sky causes oscillations in the intensity.
3rd. The green rays have a feeble chemical force, and
act as the blue rays.
4th. It follows from the presence of these two oppo-
site chemical actions that there ought to be a point
in the spectrum where the chemical action of light
88 THE CHEMICAL EFFECT OF THE SPECTRUM.
is nil. This neutral point is found between D
and £.
Chastaing considers the formation of hydrochloric acid as
a phenomenon of reduction (combination with hydrogen), and
attributes the anomalies presented by silver iodide and sul-
phurous acid as due to a secondary chemical action.
For organic substances he arrives at the following con-
clusions : —
i st. The photo-chemical action exercised on organic
bodies is oxidizing.
2nd. Its intensity varies with each body, but remains
the same if oxidation in the dark is called i, in the
red 2, in the blue-violet = about 3.
3rd. At the commencement, the oxidizing action of
the green is feeble; after that it increases, to
become finally more intense than that shown in
the red.
Mixtures of organic bodies, and of inorganic salts sub-
mitted to light, show a complicated decomposition. The
effect is due to a conglomeration of simple actions. But as
all reducing action furnishes oxygen in the nascent state, which
in its turn acts by oxidation in other combinations, it follows
that the oxidation provoked by light reduces itself by the
presence of organic combinations which oxidize themselves
strongly.
Chastaing deduces from his researches on mixtures of
uranium nitrate or of ferric chloride with alcohol or ether,
that several cases can present themselves.
1 st. The red and yellow rays oxidize the organic sub-
stances less than the salts : the result is a reduction
of the salt.
2nd. The red and yellow rays oxidize the organic sub-
stances as much as the salt ; or
3rd. They oxidize more the first — that which causes a
reduction of the salt.
In the study of fluorescent substances, very complicated
facts are arrived at. Some of these, such as an alcoholic
solution of tournsol, modify themselves possibly even under
the influence of all the rays (see higher) ; others, on the con-
GENERAL EFFECT OF THE SPECTRUM. 89
trary, like sulphate of quinine and turmeric, only under the
influence of the rays provoking fluorescence. There is here
a new molecular modification, as with quinine, or else an
oxidation, as with turmeric. The total energy of the absorbed
rays exciting fluorescence, do not serve further for fluorescence,
but a portion of these same rays is utilized to realize a chemical
action.
H. W. Vogel objects to the theoretical conclusions ot
Chastaing (Pogg, "Beib.," 1877, p. 681). First of all, he
remarks that the facts of Chastaing are not exact in the case
of chlorine and hydrogen. This, in effect, transforms itself
into chemical combination under the influence of violet
rays, and is not to be considered, with Chastaing, as a
reducing action. The properties of silver salts are very
contradictory. From the violet to the red, there is a
reducing action (see ante\ and only rarely oxidation takes
place.
Again, the strongest decomposition of carbonic acid by
plants (still a reducing action) takes place in the red (see ante).
To the capital fact that certain rays impede the action of
others — a fact which agrees with Chastaing's theory — VogePs
opposes others taken from the photography with silver salts.
Thus, paper dyed with ultramarine or cobalt blue, which is
less affected than white paper by the so-called oxidizing red
rays, acts less energetically than this latter, — that is to say, in
hindering oxidation. The same fact is presented by madder
lake, which reflects many more red rays than cobalt paper,
and besides presents as strong an action as ultramarine. Vogel
concludes from the principal known facts on the action of light
on organic bodies, that each kind of ray can also induce
reducing as well as oxidizing action, according to the nature
of the body absorbing these rays.
Colouring matters are most bleached by oxidation by the
rays they absorb : thus not always by the violet rays.
In the actual state of our knowledge on the chemical action
of coloured light, it is not possible to make exact general
conclusions from a relatively few experimental researches.
Many phenomena cannot be considered from one point of
view, as the controversy between H. W. Vogel and Chastaing
shows; and only when a large number of combinations are
experimented with will it be possible to avoid contradictions.
g 3
90 THE CHEMICAL EFFECT OF THE SPECTRUM.
But if it is not possible to find fixed rules ; at the same time
the greater part of the observations show —
i st. Every colour of light, from the violet to the red,
as well as the invisible ultra-violet and infra-red
rays, can exercise a chemical action.
2nd> The rays having a chemical effect on a body
ought to be absorbed by that body ; the chemical
action of light and optical absorptions are inti-
mately combined.
3rd. Every kind of light can act by oxidation or reduc-
tion, according to the nature of the body which is
sensitive to the light
4th. Although the oxidizing action of the red rays and
the reducing action of the violet rays cannot be
sharply separated, it can be said that in general
red light acts ordinarily by oxidation in metallic
combinations, and violet light by reduction.
Red light is seen sometimes to act by reduction
on metallic combinations, notably in the latent
action of light on salts of silver. Up till now. no
positive oxidizing action has been met with by
violet light on metallic compounds.* In the com-
binations of metalloids with them, violet or blue
light seems to act most energetically, as, for in-
stance, on chlorine and hydrogen, nitric acid,
sulphurous acid, hydroiodic acid : sulphuretted
hydrogen in water is, however, decomposed most
rapidly by red light
The action of light is in part oxidizing, in par
reducing, according to the nature of the sub-
stance. In the most cases violet light has a very
energetic and oxidizing action on organic com-
pounds, chiefly when colourless. Colouring matter
is strongly oxidized by the luminous rays which
they absorb.
Altogether, in all cases the chemical action of
coloured light obeys the law, that the rays having
[* We do not hold to this : the reversal of the violet end of the spectrum,
&c, or Agl, by prolonged exposure is a case in point. — Editor J\
GENERAL EFFECT OF THE SPECTRUM. 91
the most energetic " action are also those which
are most absorbed by the substances sensitive to
light.
5th. Not only does the absorption of the luminous
rays by the bodies themselves, but also the absorp-
tion of light by the substances mixed with them,
play a considerable rdk in the chemical action of
light. The sensitiveness td light of the first is found
strongly augmented by the luminous rays which the
latter absorb (optical sensitizers *).
6th. A substance mixed with a body sensitive to light,
and which can combine chemically with the matter
eliminated by light (oxygen, iodine, bromine, &c),
favours the decomposition by light. These bodies
are given the name of chemical sensitizers.
7th. The manner in which it acts in presence of a
coloured light varies considerably with the purity
of the compound, according to its molecular state,
eventually with the nature of the development of
the latent image.
8th. The direct decomposition of a compound by the
luminous rays do not go exactly with the luminous
latent image.
9th. The action of the solar spectrum varies con-
siderably with the state of the atmosphere, in such
a manner that for the same state of the sun and
sky, apparently pure, the chemical effect is rarely
the same. It is also difficult to give the absolute
figures relative to the chemical action of the colours
of the spectrum.
A great part of the changes induced by the action of light
are also produced by elevation of temperature. It is thus that
many chlorides and metallic oxalates, in solution in alcohol
or ether, are more or less reduced by the temperature of
boiling ; and amongst them, in the first place, are the organic
salts of silver. Ferrous sulphate rapidly oxidizes in red light
as in heat. Colouring matter bleaches not only in the air
and by light, but also by heat. A temperature of 150 to
[* The reader will have gathered our objection to this term. — Editor.]
92 THE CHEMICAL EFFECT OF THE SPECTRUM.
2bo° C. induces a combination of chlorine and hydrogen.
In many cases, however, a high temperature cannot replace
that of light. Silver chloride does not decompose even at red
heat; it is the same for the iodide and bromide of silver. The
great resistance of this body, so easily decomposed by light,
shows clearly that heat and light can act separately. Light,
again, acts when the calorific rays are absorbed by means
of proper appliances absorbing the heat,* as with a solution
of alum, and not the light. It is to be remarked that it is
principally the cold parts of the spectrum which give rise
to chemical action, and not the warm parts. Thus the attempts
of Rumford, Gay-Lussac, and Th£nard (Gmelin, " Handbuch
der Chem.," vol. v, p. 169) to explain the action of light by
an elevation of temperature are not admissible. In many
cases, it is true, an elevation of temperature increases the action
of light.
t* By heat the author evidently means what is popularly and incor-
rectly called "radiant heat." It would have been better to have said in
the line above, — "Light again acts when the rays of long- wave length
are eliminated ; " and again, — " It is to be remarked that it is principally
in the parts of the spectrum which possess least energy which give rise
to chemical action, and not those parts which possess the most." — Editor.]
INDEX.
A.
Abney, on the Molecular Theory applied to Spectroscopic Researches, 49
Spectrum Photographs, 50
on Silver Iodide, 52
on Preservatives impeding Solarization, 53
Action of Light on Silver Salts, 6
on Metallic Compounds, 55
on Iron Salts, 60
on Uranium Salts, 65
on the Iodides, 69
on Organic Compounds, 75
on Plants and Animal Life, 81
Actinometers, Ferric Oxalate, 61
Albert of Munich. Phototype coloured Photographs, 46
Aldehyde-green, effect on Silver Bromide, 40
Aldehydes, action of Light on, 77
Ammonium Iodide, action of Light on, 69
Amyl-nitrate, action of Light on, 79
Aniline Blue, effect of, on Silver Bromide, 41
Colours, action of Light on, 76
Arsenic, action of Light on, 74
Asphaltum and Resin, action of Light on, 78
Atmosphere, influence on Intensity of Solar Spectrum, 5
Amine as a Colouring-matter, 37
B.
Becquerel, on Photographing Natural Colours, 14
Berard and Seebeck, on Silver Salts, 2
Bromine and Chlorine, action of Light on, 69
C.
•
Carbonic Acid, decomposition of, by Plants, 89
Chloride of Mercury, action of Light on, 55
Chlorophyll, action of Light on, 79
effect on Silver Bromide, 40
Chromium, action of Light on, 65
Cinnabar, action of Light on, 56
94 INDEX,
Collodio-bromide Emulsion and Gelatine, 31
Coloured Glasses as affecting Light, 3
Copper, action of Light on, 57
Coraline as a Colouring-matter, 37
Cyanine Blue, action of Light on, 76
effect of Colouring-matter on Silver Bromide, 41 |
Density of Glass Prism, as affecting Decomposition of Light, 5
Developer, influence of, upon the Sensitiveness to Light of the Haloid
Salts of Silver, 21
Ducos du Hauron, Process of, for Coloured Photographs, 44
Dyed Films, 35
Eosin, effect on Silver Bromide, 40
Ether, action of Light on, 78
F.
Ferric Salts, action of Light on, 60
Chloride, mixture with Alcohol or Ether, 88
Ferrous Salts, action of Light on, 60
Fluorescent Substances, action of Light on, 88
Fuchsine (Aniline Red), effect on Silver Bromide, 39
G.
Gelatine-bromide Plates, 30
as increasing Sensitiveness of Silver Bromide, 30
General effect of the Spectrum, 86
Gold, action of Light on, 56
Green Light, Spectrum-effect, 86
Gum Guiacum, action of Light on, 78
H.
Haloid-compound of Silver, action of Spectrum on, 4
Heighway, on Bromide in Collodion, 24
Helmholz, on Photographs of Ultra-violet end of Spectrum, 4
Herschel on the Action of Light on Ferric Citrate, 62
and Hunt, on Development, 21
Hunt, on Photography in Natural Colours, 17
Hydriodic Acid, action of Light on, 69
INDEX. 95
I.
Influence of the Developer upon the Sensitiveness to Light of the Haloid
Salts of Silver, 21
of Light on Animal Life, 8j
Iodide in Collodion, 24
Iodide of Mercury, action of Light on, 55
L.
Lead, action of Light on, 58
M.
Manganese, action of Light on, 67
Mercury, action of Light on, 55
Methyl Violet, effect of Colouring-matter on Silver Bromide, 41
Mineral Oils, action of Light on, 78
Mixture of Silver Iodide, Bromide, and Chloride, action of Light on, 1 1
Molybdenum, action of Light on, 67
Morphia, action of, on Silver Bromide, 32
Miiller, J., influence of Spectrum on Wet Collodion, 4
N.
Naphthaline, Red, and Silver Bromide, 38
Niepce, on Coloured Flames and Images produced by Light, 19
Nitric Acid, action of Light on, 71
O.
Oil of Terebenthin, action of Light on, 76
Lemon, action of Light on, 77
Cinnamon, action of Light on, 77
Bitter Almonds, action of Light on, 77
Oils, action of Light on, 76
Optical, action ofSubstances towards Coloured Light, 33
Organic Matter, action of Light on, 75
' Bodies and Inorganic Salts, Mixture of, 88
P.
Picrate of Methyl-rosaniline, effect on Silver Bromide, 40
Phenol, action of Light on, 78
Photographs in Natural Colours, 13
Phosphorus, action of Light on, 71
96 INDEX.
Plants, action of Light on, 81
Platinum, action of Light on, 57
Poitevin, on Photography in Natural Colours, 16
Potassium Iodide, excess of, in Silver Iodide, 27
action on Silver Chloride paper, 31
action of Light on, 69
Potassium Ferrycyanide, action of Light on, 72
Prism, Material of, 4
Prussian Blue, action of Light on, 73
R.
Red and Yellow Light, Spectrum effect, 36
Resin added to Collodion Emulsion, 31
added to Iodide and Bromide of Silver, 32
Reversed action of Light, 48
Ritter, on Silver Chloride, 2
Russell, Tannin Dry Plate Process, 31
Rutherford, Photograph of Spectrum, 4
S.
Salicene, action on Dry Silver Bromide, 32
Scent of Flowers, action of Light on, 82
Scheele, on Silver Chloride, 2
Seeds of Plants, action of Light on, 82
Sennebier, on Silver Chloride, 2
Sensitizers, 26
Silver Salts, action of Solar Spectrum on, 2
Nitrate, action of Light on, 6
Iodide, 6
Bromide, 8-
Chloride, 10
Fluoride, - 11
Sleep of Flowers, action of Light on, 82
Sodium (Nitro-prussiate), action of Light on, 73
Stems of Flowers, action of Light on, 82
St. Florent, on Photography in Natural Colours, 16
Sulphur, action of Light on, 72
Sulphides, 74
Sulphurous Acid, action of Light on, 74
Sulphate of Quinine, action of Light on, 79
T.
Tartaric Acid and Ferric Chloride, action of Light on, 63
Temperature, as affecting the aqtion of Light,
Tincture of Aloes, action of Light on, 7& -\
INDEX. 97
T6th and Eder, on mixture of Coraline and Fuschine for coating Glass, 25
Turmeric, effect of Colouring-matter on Silver Bromide, 41
action of Light on, 78
U.
Uranium, action of Light on, 65
Nitrate, mixture with Alcohol, 88
V.
Vanadium, action of Light on, 67
Vegetable Colours, action of Light on, 75
Violet Light, Spectrum effect of; 86
Visual Purple, Spectrum effect of, 85
Vogel, H. W., experiments in Developing, 23
on Silver Nitrate, Bromine, and Iodine, 29
on Alteration of Silver Bromide by Light, 33
W.
Warnerke, action of Light on Collodio-bromide Emulsion, 19
Waterhouse, Major, on Reversal of the action of Light, 49 .
White Light, Spectrum effect of, 86
Wortley, Col. H. Stuart, on strong Alkaline Developers, 25
Zanthophyll, action of Light on, 80.
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