The progress of photography since the year 1879 : a review of the more important discoveries in photography and photographic chemistry, within the last four years, with special consideration of emulsion photography, and an additional chapter on photography for amateurs

Front matter

Entered according to Act of Congress, in the year 1883, by in the office of the Librarian of Congress, at Washington, D. C.

TH£ 6ETTY CEflTER

LIBRARY

Preface To The American Edition

No apology is needed for the issue of another work by Dr. Vogel. For nearly twenty years he has contributed largely to the knowledge of American photographers by means of his monthly correspondence in the Philadelphia Photographer, and three editions of his excellent Handbook. Therefore, a warm welcome is confidently expected for this new contribution to our literature.

By his own request, I have revised and "Americanized" it as much as it would bear without damage, and have added from his correspondence such items of progress as have appeared since the issue of the German edition.

Dr. Wallace has given us an admirable translation, exercising, as he has, the most conscientious care throughout.

I commend the work to the craft with a knowledge of what it contains, and trust it will prove as useful as I anticipate.

Edward L. Wilson.

Philadelphia, July, 1883.

A*

NOTE FROM THE TRANSLATOR.

Believing that the usefulness of this work to the American public would be materially increased by substituting the English weights, measures, and thermometric scale, the translator has done so in almost every instance. The non-correspondence of the gramme or cubic centimetre with an even number of grains or minims has been the cause of the awkward form in which some of the formulae necessarily stand. In some instances, as in the test for soluble haloids in emulsions, fractions of a grain even have been retained in the English, inasmuch as the slightest departure from strict chemical accuracy would render the tests unreliable and worthless.

Ellerslie Wallace, Jr.

(vii)

Preface

At no period of its highly interesting course of development has the Art of Photography been enabled to show such marked advances as within the last four years.

The old Collodion Process, which had been firmly established for more than a quarter of a century without a rival, suddenly found one in the highly sensitive Gelatine Dry Process, which unites certainty and ease of manipulation with good keeping qualities and clean results. By its aid, the photographing of moving and feebly illuminated objects was wonderfully facilitated, instantaneous and amateur photography received a fresh impulse, and results obtained by the employment of photography in art, science, and general industry which in earlier times would have been deemed impossible.

At the same time, the Electric Light offered itself as an important substitute for daylight in photographic studios, and made the portrait photographer independent of changes of weather. New and important Positive Processes, such as the Platinum, and various Direct Copying Processes, went hand-in-hand with the new Dry Process, so that the labor of the amateur, as well as of the eng'ineer and man of science — who are employing the camera more and more — was sensibly lessened. Novel and excellent forms of Lenses, Instantaneous Shutters, and Landscape Apparatus did their part in diffusing and increasing the capacities of the Art in every direction. Many excellent chemists and scientific men devoted themselves to the investigation of

(ix)

X PREFACE.

the problems of photographic chemistry, and by no means the smallest share of the said advances in the Art are owing to them.

The Author has set himself the task of giving a general review of the same, by selecting from among recently discovered facts only the more important ones — those that had been proved in practice, or those capable of further development, endeavoring to present them in brief, clear terms, such as are in ordinary use.

The book may also be regarded as a necessary addition to his already known Lehrbuch (Handbook), to which it bears a resemblance in the general division of the matter. The third edition of the Handbook had already appeared when the new discoveries forming the subject of the present volume could scarcely have been said to claim an existence.

Photographic Processes with the Printing Press, which have lately created a literature of their own, could be but alluded to ; a detailed account of them would have made the book too costly, and lessened its universal circulation.

A supplementary chapter contains special advice to scientific men and amateurs.

The Author hopes that the book may be as kindly received as his Handbook.

Berlin.

CHAPTER II. Chemical Effects of Light

A. Effects of Light upon Metalloids and their Compounds

B. Effects of Light upon Metals and their Compounds, The Double Salts of Iron and Oxalic Acid The Iron Lichtpaus Process, Actinium, a new Metal, .

Salts of Copper, Salts of Chromic Acid, .

Combinations of Mercury, Platinum Printing, .

The Salts of Silver, The Various Modifications of Bromide of Silver and the

Chemical Principles of the Emulsion Process Photo-chemistry of Chloride of Silver, . Rare Salts of Silver, .....

PAGE

50 74 80

CHAPTER VI. The New Photographic Processes

A. New Negative Processes (Dry Processes),

Preparation of Gelatine Emulsion, .....

a. The Author's Method, ......

b. Eder's Method with Ammonio-nitratc of Silver,

c. Eder's Combination Method with the Author's Modification, ........

d. Eder's New Method with Carbonate of Ammonia, .

e. Precipitation Methods, /. Vogel's Emulsion, .

B. The Manipulations of the Gelatine 1.

Gelatine Plates

a. The Exposure, .

b. The Development,

c. The Fixing,

d. The Intensifying,

e. The Drying and Finishing,

2. Vogel's Emulsion Plates,

3. Failures in Gelatines Plates,

4. Notes on the Gelatine Process, C. New Positive Processes,

1. The Platinum Processes (Platinotype)

2. New Silver Printing Processes, . 3 New Enlargement Processes and Apparatus, 4. New Permanent Printing Processes, .

190 190 194 202 218

Chapter I. Physical Effects Of Light

The Photophone. — Among the non-material (nicht stofflich) effects of light, those upon the element selenium are the most remarkable (Vogel, Lehrbuch, 13). A telephone placed in an electric current, passing also through a selenium plate, will respond,

Fig. 1.

if the latter be subjected to the influence of light and darkness following each other in rapid succession. This may be best effected by causing a cardboard disk (Fig. 1), with holes near its circumference, 2* B

PHYSICAL EFFECTS OF LIGHT.

to revolve before the selenium plate, and allowing light to fall upon the disk.

At the instant when a hole passes the opening, the selenium is illuminated, but is immediately darkened again. This intermitting illumination can also be effected from a distance if light be concentrated or focussed upon the selenium by means of a parabolic reflector. If speech be directed towards a thin mirror which also sends rays of light towards this distant reflector, the vibrations of the mirror set up by the words spoken reproduce themselves at the distant point, and become audible in the telephone, which is there in connection with the selenium plate. This is the principle of Bell's Photophone.

Radiophony. — Bell and Tainter found that not only selenium, but many other bodies in thin sheets, such as hard rubber, ebonite, metals, ivory, and paper, would give sounds under the influence of intermittent light, and this without the aid of an electric current, as in the case of selenium. This phenomenon may be called Radiophony. Many substances are acted upon in this manner, principally by the heat rays. 1

Mercadier enclosed lampblack, cotton, woollen threads, and cork in a small tube, r (Fig. 1), connected with an ear-trumpet, h, and caused light to fall upon them through a revolving disk, P. Distinct sounds were given by all these bodies (Thermophone). Mercadier regards this capability of giving tones under the influence of the heat rays

1 For further details, see Jahrbuch der Erfindungen, xvii. 110.

PHOTO-ELECTRICITY,

.19 as a common property of matter. (Phot. Mitth., 1881, 200.)

Action of Light on Ebonite. — Hoorweg found that ebonite increased in length under the influence of light, and more noticeably so in the blue rays than in the red or yellow. He proved this with Edison's Tasimeter. — This apparatus consists ot a stick of ebonite fastened at one end, while the other is brought into contact with a row of pieces of carbon, through which an electric current passes. If the stick of ebonite is lengthened under any influence, it will correspondingly affect the position of the pieces of carbon, whereby their electrical resistance is lessened, and the needle of the galvanometer included in the circuit deflected. Thus the tasimeter is an exceedingly sensitive thermometer.

Hoorweg caused a spectrum to fall upon the tasimeter in such a manner that the ebonite stick was exposed first to the red rays and afterwards to the blue, obtaining with the former a deflection of 10°, and with the latter 11° or 12°.

If these data should be verified, Edison's tasimeter would become a chemical photometer for the estimation of the intensity of the more highly refrangible rays, or, in other words, those chiefly effective in photography; and such an instrument also would be simple enough to offer a prospect of being employed in practice.

Photo-electricity. — Electricity is generated by light. Hankel recognized the electrical disturbances produced in crystals of fluorspar by light. (Phot, Mitth., xvii. 311). In crystals of the dark-green tint, the middle is negative, and the edges positive. The

PHYSICAL EFFECTS OF LIGHT.

electrical condition ceases in the dark within twenty-four hours. Chastaing recognized the same electrical conditions in guaiacum resin under the influence of light. (Ibid., xv. 29.) The action of light upon selenium also gives rise to an electric current. (Adams and Day, tfaturf., 1876, 446.)

Chapter II. Chemical Effects Of Light

A. — EFFECTS OF LIGHT UPON METALLOIDS AND THEIR COMPOUNDS.

Formation of Ozone in Light. — According to Boettcher, the action of light upon oil of turpentine in presence of oxygen does not generate ozone, but peroxide of hydrogen. Schone also declares that the formation of the latter takes place in very many cases where it had been supposed that the former was produced. It is worthy of remark that, according to Downes and Blunt, the peroxide of hydrogen becomes slowly decomposed by the action of light. (Naturf., 20, 251.)

Amorphous Selenium becomes crystalline under the influence of light. As its electrical conducting power becomes better under these circumstances, we can thus explain the action of the selenium photometer.

Nitroprusside of Sodium is sensitive to light ; still more striking is the behavior of a mixture of this substance with the alkaline sulphides. This mixture has a beautiful violet color, and by means of it we can detect very minute traces of sulphur. It is known that this color is unstable, and rapidly so; the author proved that this disappearance of color was caused chiefly by light.

If this fine violet solution be examined spectro-

CHEMICAL EFFECTS OF LICHT.

scopically, a very distinct absorption band may be seen in the yellow and orange. If the examination be conducted by daylight, the band becomes gradually fainter and finally disappears more or less quickly, according to the chemical power of the light. If an instrument with VogePs reflection apparatus, which shows two spectra side by side, be employed, the most delicate variations caused by absorption may be noted, and the disappearance of the band observed with a watch and the time required fixed upon.

Thus we are enabled to fix the chemical power of daylight in a very simple manner. The quicker the bands disappear with a given strength of solution, the more chemically powerful the light. It is not, however, the strength of the photographically effective blue rays that is thus determined, but that of the yellow, for these alone are absorbed by the violetcolored solution. This may be easily seen if a test-tube full of the solution be placed in a glass containing a solution of bichromate of potash, which allows only the yellow and red rays to pass. The sulphurous nitroprusside solution bleaches almost as quickly in orange light as in full open daylight. The blue rays have no action on the solution; therefore, in a blue glass, the solution bleaches but slowly. The solution must consequently be prepared in dark rooms glazed with blue glass, the precise opposite of the ordinary photographic dark-room.

In view of the fact that the effect of light upon dye-stuffs is always a bleaching one, it will be of interest to mention that one of the most famous of the dyes of antiquity — the purple of the Purpuracca —

PHOTO -POLYMERISM.

was produced through the agency of light. According to Schunk, the purple snail contains a yellow substance soluble in ether and in alcohol, and which even without the presence cf oxygen, under the influence of light, passes through green and blue to purple, giving off a garlicky odor. This dye is insoluble in alcohol or ether. If it were possible to obtain larger quantities of this substance, we might easily make purple photographic images by saturating paper with the original substance and exposing to light under a negative. (Phot. Mitth., xvi. 100.) Cole and Reaumur made similar observations in the last century. (Eder, Geschichte der Photochem.)

Oleococca Oil is described as sensitive to light by Fleury Ilermagis, becoming hard, opaque, and waxlike. (Bulletin de la Soc. Francaise, 1881.)

Solutions of India-rubber change very quickly in the light (Eder and Toth), become thin, and give films partially soluble in alcohol and ether. Negatives made on an India-rubber substratum do not keep, the film easily lifting from the glass. At all events, this may be classed among the effects of light.

Photo-polymerism. — Numerous organic bodies under the influence of light become changed in their properties without experiencing a similar change in their ultimate composition. We may explain this as a change in the grouping of their molecular structure, and have chosen the above name by which to designate it. A change of this nature occurs in the case of bromide of vinyl, CJ1^>y 2 . According to D. M. Loof, water, air, and carbonic acid have no influence upon this process, but sulphuric acid accelerates

CHEMICAL EFFECTS OF LIGHT.

it considerably. A slight addition of iodine colors bromide of vinyl rosy-violet, if it is free from alcohol, and stops the polymerization completely until the color struck by the addition of the iodine has disappeared by exposure to light; then the liquid becomes dark again, owing to separation of the polymeric body. If a slight excess of iodine be present, the liquid will not change in daylight, but remains clear. The iodides of ethyl and methyl will impede the action of light because of their own decomposition, iodine being set free, and the solution of bromide of vinyl then remains unchanged in the light for a long time. Aniline likewise hinders decomposition by light, being acted upon itself, and assuming a reddish-brown color.

That variety of anthracene (mother-substance of alizarine) which is soluble in benzole is changed into insoluble paranthracene under the influence of light, but will regain its solubility if melted.

Chloral behaves in a similar manner, and styrol still more markedly so, becoming thick and insoluble in alcohol under the action of light ; it regains its former properties, however, when heated to 570° F. (Lemoine, Phot. Mitth., xviii. 244.)

Action of Light on Asphaltum. — Kayser's recent researches upon asphaltum are of particular interest for practical heliography. He pointed out that the Syrian and Trinidad varieties can be separated into three principal constituent parts by the action of alcohol and ether — one soluble in alcohol, one in ether, and one insoluble in either menstruum. Thus, the Syrian variety contains about four per cent, of the matter soluble in alcohol, which appears as a

ACTIOX OF LIGHT ON ASFHALTUM yellow oily mass, ot a strong bituminous odor; also forty-four per cent, of the matters soluble in ether, which form a brownish-black, resinous, brittle mass, almost odorless, and melting at 150°; besides fifty-two per cent, of insoluble matter. The latter is a black, highly brittle, odorless mass, whose melting-point is 313°, easily soluble in chloroform and oil of turpentine, and with difficulty in benzole and petroleum. Its solutions possess a brownish-yellow color, and green fluorescence. The Trinidad variety is very similar, containing about Ave per cent, of matters soluble in alcohol, fifty-seven per cent, of those in ether, and thirty-eight per cent, of those insoluble in both. All three of these matters contain sulphur; in Syrian asphaltum the first contains about seven per cent., the second ten per cent., the third about thirteen per cent. According to Kayser's analysis, however, the soluble principles of both varieties of asphaltum have different compositions; he fixes upon the formula : , 2 H. 1B S for the matter soluble in alcohol in the Syrian variety, but C 20 II., S for that in the Trinidad variety ; for the matters soluble in ether in the Syrian, C W H 92 S 3 ; and in the Trinidad, C^HjjS. The insoluble constituents of both varieties of asphaltum, however, have the same composition, C 32 H 42 S.

Other varieties of asphaltum show different reactions, as, for instance, the Bechelbronn, which consists principally of carburetted hydrogen. On the other hand, the Maracaibo samples acted like the Trinidad and Syrian varieties.

The matter soluble in alcohol remains so in daylight, but the others lose their solubility when 3

CHEMICAL EFFECTS OF LIGHT.

exposed, the one insoluble in alcohol and ether being most strongly acted upon. This accordingly is the valuable constituent of asphaltum for heliographic purposes, the Syrian gum having fifty-two per cent, and the West Indian thirty-two per cent. Lately, purified samples have appeared in the market — i. e., such as have had the soluble matters removed by extraction with alcohol and ether, thus being specially adapted to heliographic purposes.

Kayser holds the view that, when asphaltum is exposed to light, no oxidation takes place, as is commonly supposed, but merely a polymeric modification (see above). The reasons are that a plate prepared with asphaltum does not gain in weight upon exposure ; solutions of the gum in closed bottles are also acted upon by light, becoming insoluble. Asphaltum which has become insoluble under the influence of light, returns to its former condition when melted.

Eder, nevertheless, believes that oxygen possibly plays some part in the photo-chemical changes of asphaltum. (Phot. Mitth., xvi. 228.) See Kayser's brochure for spectral reactions of the same.

Effect of Light upon Animals. — Moleschott and Fubini give the following results of experiments: Light is an active agent in the breaking-down of tissue; it increases the excretion of carbonic acid and the appropriation of oxygen. The effect of light in starting breath is seen in the case of the amphibia?, birds, mammalia, and insects.

Chemical intensity of light increases the excretion of carbonic acid in the case of all warm- and coldblooded animals, whether blind or with sight. The

POTASSIO-FERRIC OXALATE, same effect is produced upon birds and the mammalia by red and violet light, the latter having almost the power of pure white light, but the red being much weaker. The same relative power of violet light obtains in the case of frogs, red being without action (iSTaturf., 1.880.)

B. EFFECTS OF LIGHT UPON METALS AND THEIR

COMPOUNDS.

The Double Salts of Iron and Oxalic Acid.

Ferric Oxalate, (according to Eder, Fe. 2 (C 2 4 ) 3 ) is soluble in water and alcohol, and uncrystalHzable. Eder prepares it by dissolving Fe,(OH) 6 in oxalic acid. It forms faintly basic salts — for instance, Fe 2 (C 2 4 ) 3 Fe. 2 (OH) 6 + 4H. 2 0, which is insoluble in water.

The Double Salt, Potassio-ferric Oxalate (together with the corresponding salts of soda and ammonia), has been lately re-investigated by Eder. The emerald-green potassio-ferric oxalate (Fe 2 (C 2 4 ) 6 K 6 + 6H 2 0) dissolves at a temperature of 54° in fifteen times its volume of water, strikes a blue tint with ferrocyanide of potassium (the precipitate, however, rapidly redissolving), and, when made to react with ferric oxalate, gives a brown double salt that crystallizes badly, easily breaks down, and contains two equivalents less of oxalate of potash.

The double salt of sodium reacts in a similar manner, but is extremely soluble in water (at 63° in 1.7 parts of water). Formula : Fe 3 (C 2 4 ) 6 Na 6 +llIT 2 0. Commercial samples of the corresponding double salt of ammonium (Fe 2 (C 2 4 ) 6 (NH 4 ) 6 + 8H 2 0) dissolved at 63° in 2.1 parts of water, and were easily decomposed by heat. All the ferric oxalates are directly sensitive to light.

Ferrous oxalate, on the other hand, is not decomposed by light; it combines with K 2 C 2 4 (neutral oxalate of potash) to form a very soluble double salt which is a powerful reducing agent (the well-known oxalate developer for dry plates). Formula: Fe(C 2 4 ) 2 K 2 + H 2 0. The pure double salt easily decomposes, with the loss of FeC 2 4 ; it keeps better if an excess of K 2 2 4 be present. A 40 per cent, cold solution of neutral oxalate of potash dissolves 9.37 per cent, of oxalate of iron; a 20 per cent, solution, only 4.9 per cent.

Ammonio-ferrous oxalate (Fe(C 2 4 ) 2 (NH 4 ) 2 -f 3H 2 0) reacts in a manner similar to the potassium salt. Under like circumstances, neutral oxalate of ammonia will dissolve almost as much oxalate of iron as the neutral oxalate of potash (see above).

Neutral oxalate of soda also forms a double salt with oxalate of iron. Its dissolving power, however, is less.

Sensitiveness of Iron Salts. — Eder publishes the following : Mixtures of chloride of iron with acetic and formic acids, as well as the iron salts of the same acids, are not sensitive to light. The double salts of iron and oxalic acid decompose most quickly in solution. In the case of the potassium compound, the reaction is as follows :

Eder has fixed the sensitiveness of the various iron salts in solution by the quantity of suooxide of iron

NEGATIVE REPRODUCTIONS.

formed by a certain exposure to light. From this it appears that a mixture of chloride of iron and oxalic acid is the most sensitive. If we fix upon the number 100 to represent the quantity of suboxide of iron formed (with respect to subchloride of iron), we will have the following figures for the different solutions containing from one to five per cent, at temperatures varying from 63° to 68° :

Chloride of iron -\- oxalic acid,

Feme oxalate,

Ammonium ferric oxalate, .

Potassium ferric oxalate,

Ferric tartrate, .

Ammonium ferric tartrate,

Ammonium ferric citrate, .

Chloride of iron -(-citric acid,

Chloride of iron -f- tartaric aeid,

With stronger solutions the sensitiveness is greater, and the difference between their behaviors becomes less. Papers treated with these solutions behave in a surprisingly (liferent manner. Chloride of iron + oxalic acid is here also the most sensitive ; next follows ferric oxalate ; then the double salts of ammonium, sodium, and potassium. But the differences in sensitiveness are not as great as those given in the above table.

Eder recommends the sodium ferric oxalate for the iron printing processes. (Not yet in the market.)

The Iron Lichtpaus Process. 1

Negative Reproductions from positive drawings are best obtained, according to Pizzighelli, with the fol-

1 The term " lichtpaus" is applied by the Germans to the direct photographic reproduction of a drawing without first making a negative. — Tr. 3* lowing solution : 100 parts of water, 8 parts of red prussiate of potash, 10 parts of citrate of iron and ammonia. The mixture, according to the author's experience, keeps eight days. Bnt it is better to dissolve each of the salts separately in 50 parts of water, and mix them as required. Paper brushed over with this and dried, gives, after exposure under a positive drawing, a negative copy, white on a blue ground. (Yogel, Lehrbuch, 432.)

Pizzighellf s Positive Blue Process. — As the foregoing iron process gives negatives, the following is given for obtaining positives 1 from positive oiiginals. Mix together 1360 minims of a solution of gum Arabic (1 part of gum, 5 parts of water), with 51 minims of a solution of citrate of iron and ammonia, (1 part of the salt, 2 parts of water), and 85 minims of a solution of chloride of iron (1 part of chloride of iron, 2 parts of water).

This mixture gradually thickens, and therefore must be used soon. It is to be evenh r spread over well-sized paper with a brush, and dried in the dark. It may be exposed for a few minutes under a drawing (longer in dull weather), until the fine lines of the original, which are white upon a black ground, become faintly visible ; it is then to be brushed over with a solution of 1 part of yellow prussiate of potash to 5 parts of water. The picture quickly appears as a deep-blue positive. When all the details are out, the picture is quickly washed with water, and then laid in a pan containing dilute hydrochloric acid (1 part of acid, 10 parts of water).

cola's positive lichtpaus process. 31

The action of this is to clear up the lights and remove what is left of the gum and iron. Another washing is given, and the picture then dried.

The printing and finishing are very quickly done; in good weather the process may be ended within one or two hours. It is one of the most sensitive of the "paus" processes, (five times as sensitive as the carbon process). It is remarkable that red prussiate of potash develops the picture in the same way as the yellow. It is difficult to give a theory of the process. It has been supposed that those parts unacted on by light remain easily penetrable by the developer, so that they are acted on by the latter down into the body of the paper; while the parts that have been acted upon lose their penetrability, so that the developer can only work on the surface. Therefore, care is to be taken not to allow the developer to touch the back of the paper. A drawback to the process is that the acid wash-waters roughen the paper.

As in the case of all positive "pans" processes, under-printed proofs come out too dark, and become discolored where they should remain white, while over-printed ones are too light.

Cola's Positive Lichtpaus Process (Tintenprocess). — This process, also based upon the use of iron salts, and for which a patent has been granted in Germany (from July 29, 1880), gives positives from positives, and black lines on a white ground. Paper is coated with the following solution.

Sulphate of Iron, .

Chloride of Iron, .

Gelatine, Tartaric Acid, Distilled Water, .

which must be warm :

340 minims.

77 drachms.

CHEMICAL EFFECTS OF LIGHT.

Endless band paper is put upon a properly arranged system of rollers, and then drawn over the surface of the solution placed underneath in a tray; afterwards, it is hung up to dry. After the paper is dried, it must be kept away from the air, and it is accordingly placed in an apparatus resembling that shown in the accompanying figure, which

C

Fig. 2.

Id

P*

S i r~

R r-

R

A r. mtm

A

INS

B'h— is a simple press, with two plates, B B, having India-rubber facings, C C. The sensitive paper is laid between the plates, and the screw, D, tightened. The India-rubber parts are forced out against the sides of the box, _R, by the pressure, and so effectually protect the paper within from light and moisture.

The drawing to be copied must be drawn in deepblack lines either upon thin tracing paper, "paus" linen, or some other translucent material. It is then laid in an ordinary printing-frame. A piece of the sensitive paper, prepared side down, is then laid on the drawing, and the exposure conducted as usual. The greenish-yellow color of the prepared paper disappears on exposure to light, and only those

ACTINIUM. 66

parts protected by the black lines of the drawing preserve their color. As soon as the ground-work of the paper lias fully lost its color, it is immersed in a bath composed of

Gallic Acid,

Alcohol.

Afterward mixed with thirty -two ounces of water. The greenish-yellow lines immediately become black by the action of this bath; all that now need be done, is to wash the print well with clean water, which renders it permanent.

Actinium, a new Metal.

Actinium. — Phipson, in the Chem. News, 43, 283, 44, 33, gives an account of this metal, discovered by photo-chemical means. Its existence is still doubtful, however. If a solution of sulphate of zinc be mixed with one of sulphide of barium, a white precipitate falls. If any object be painted over with this, and exposed to the direct rays of the sun, the white color becomes transformed to brown, and finally into slate color at the end of about twenty minutes. If the object be now brought back to a dark room, its color will become white again in two and a half or three hours. If the sun's rays be made to pass through a glass before reaching the white precipitate, no darkening takes place. The composition of the precipitate is: Sulphate of baryta, 41.50 parts; sulphide of barium, 1.02 part; oxide of iron, 2.12 parts; oxide of zinc, 16.30 parts; sulphide of zinc, 37.44 parts; loss from calcination, 1.62 part; with traces of lead, arsenic, and manganese.

C

CHEMICAL EFFECTS OF LIGHT.

The change of color which takes place in sunlight is a reduction process, while the reappearance of the white will only take place where there is free access of air accompanied by oxidation. The sensitive element is sulphide of actinium.

The dark matter will dissolve in acetic acid, and may he precipitated by sulphide of ammonium as a dark-green powder, which becomes white by gradual oxidation, and shows reactions similar to those of zinc and iron. If the original white precipitate be treated for a long time with excess of acetic acid, the washed residuum on the filter will retain its actinic properties, so that the acid is proved not to have the power of dissolving this actinic substance before its exposure to light.