Samuel A. Forman' DeDartrnent of History and Sociology of Science University of Pennsylvania Philadelphia
The Dynamic Interplay between Photochemistry and Photography
In this oaoer the earlv ohotochemical and ohotomaohic - . researche; oi French, German, and English naiural philosohers will he examined. These investieafions mav be di\,ided into several endeavors, some of and otiers of applied scientific interest 1) Explanation of the photochemical processes of photography. 2) Investigation of ways to increase the sensitivity of photographic
plates. 3) Invention of a practical means of photo-engravingfor mass pro-
duction printing processes. Determination of the laws governing photochemical reactions. 5 ) Application of photography to existing sciences such as mieroscopy and astronomy. 4)
The first three stages had as their aim the improvement of photography. Stage four, photometry, also aimed a t facilitating the taking of photographs, but i t assumed wider significance to all photochemistry. Stage five, applications of photography, was important to the advancement of microscopic and telescopic studies. Since these had no immediate impact on chemistry, I have neglected them in the present study (I).. During the early 1840's photography and photochemistry were indistinguishable. They grew apart, each gaining from the association. Photography gained valuable improvements in photo-engraving and portraiture. ~ h o t o c h e m i s t r ~ gained the labors of many eminent scientists, culminating in its recognition as a separate field and the solution of i& major barrier to advancement, the determination of laws governing photochemical reaction rates. An inventor, Louis Daguerre, is credited with the first practical photographic method. In 1835 while experimenting with iodized silver plates, he accidentally discovered that mercury vapor could develop an image on a plate subsequent to exposure in a camera (2). Daguerre's mercury development was the first exploitation of the latent image, the basis of all useful photographic processes. The latent image is a specific catalytic photo-product. I t is only brought out by secondary manipulation (see figs.). Use of the latent image phenomenon reduced exposure time from many hours to seconds. Daguerre sold his idea to the French government in 1839. It enjoyed a spectacular public release, sponsored by Dominique Arago, president of the Academie de Science. Henry Talbot, an English inventor, experimented with silver chloride paper photography. His methods were not investigated by scientists extensively due to both fuzzy results and rigorous patent enforcement. Nevertheless, Talhot's negatives and paper prints are the ancestors of modern photography. Sir John F. W. Herschel. on hearing of Daeuerre's nrocess and aroused by the great excitement, independently discovered a method of photography on paper before either Daguerre or Talbot released their processes. Herschel was a well resoected astronomer and chemist. His 11 nublished papers on photography included practical improvements as well as theoretical contributions to photochemistry. He coined the words "positive" and "negative" and consciously
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'Author's current address: Cornell University Medical College, New York, New York.
heloed establish ohotochemistrv. - . which he called actinochemistry (3). In a reoort of earlv 1839 to the Roval Societv. Herschel made the first sigiificant contribuiion to photography from the chemists. He suggested the use of sodium thiosulfate to dissolve unexposed silver halides, thereby making oictures insensitive to further exposure (4). He had discovired sodium thiosulfate in 1819. His application of it to photography was an immediate improvement over the fixing methods of Daguerre and Talbot and were immediately adopted. So popular was it for fixing, that it still retains part of its old nomenclature, "hypo," from hyposulfite of soda, in ohotocraohic iargon. ~ifferknceshetween the paper and metal processes held the attention of the sc~entifirrommunity before the release of the daguerreotype in August 1839 (5). A deputation of the Royal Society including Herschel visited Paris to check on t h e daguerreotype. They were extremely pleased with its clarity, a fact which helped direct much of the chemists' photographic researches on the French metallic process. The first controversy was over the fundamental change lieht induced on the exoosed daeuerreotvoe ?. .date. Araeo maintained that light caused a chemical change, but was unable to carrv out exoeriments himself. He did. however. stimulate several young members of the ~ c a d e m i edd Science to oursue ohotoeraohic research. includine" Jean Foucault, ~ i m a n ~izeau;ancl d Edmond ~ & u e r e l . Alfred Donn6, teacher of clinical microscopy a t the EEole de MBdicine, took the position that daguerreotypy was a ohvsical process (6). He noted that lieht chanced the silver that the iodide surface to a powdery state. H; mercuw. vawors could amalgamate with the metallic silver . surface by penetrating thevpowdered halide. A chemical change . was not induced, the silver iodide merely changed . its state during exposure. Ludwig Moser, professor of physics a t Konigsberg University, produced evidence for the physical theory (7). He observed that two bodies placed close together would impress their images upon one another. The images appeared when one breathed on the picture. He believed that the daguerreotype developed physically because the pictures appeared hy the action of condensing mercury vapors as did those of his Hauchbilder (breath pictures) by water vapor. He further supported his views by noting that starch covered daeuerreotvoes . exoosed in sunlieht did not indicate the of free ioiine (i.e. implicated the absence of the chemical reduction of silver iodide into metallic silver and iodine, as Arago assumed). The physical theory enioved . some oooularitv in England (8) . . in addition to France and the ~ e r k a n i e s . " The comoetine . - chemical hvoothesis won acceotance in 1843. Choiselet and Ratel (9) ;;cognized the importance of the observation that exposed d a t e s subiected to chlorine. iodine, or bromine vapors d i d n o t deveiop an image (10): The physical theory could not account for the incorporation of halide vapors after exposure, since the silver iodide was only to have changed its physical state. The Frenchmen proposed a photochemical reduction to a silver subhalide. It remained the predominant theorv of the latent image for many years. The recognition that light reduced silver salts was valuable in that i t allowed the later use of a
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Figure 2. Schematic view of sensitized Plate.
Figure 3. Chamber for exposure to iodme.
Figure 1. Dadusrreotype plate holder and plate.
Figure 4. An early camera.
Figure 5. Alcohol lamp-heated mercury development apparatus
Daguerreotype Rocess
me daguerreotype material was based an a silver-plated copper sheet. It was labor~uslypolished with rouge, pumice, and dilute nitric acid. h order to avoid unwanted surface contact, special plate holders were devised (Fig. 1) for use during cleaning. The silver surface was exposed to dry iodine, forming a light sensitive surface (Fig. 2). 2Ag
+
I*
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2AgI
The chemists later suggested the use of chlorine and bromine f w faster plates. Formation of the silver helide was completed in a closed, light tight box (Fig. 3). Sensitized plates were transferred to the camera far exposure (Fig. 4). Light effected Ule formation of a latent image. Darkness left the silver halide unchanged.
Ad
hr
AgLpiiilrralllvliinh","pmd"",
The latent image was developed in a chamber of heated mercury vapor (Fig. 5).
The mercury condensed on lhe latent hage. mmplethg the reduction of the silver h a m . Unexposad silver salt was dissolved in aqueous sodium thbsuifate. The Silver and mercury amalgam was stabilized with gold chloride saUion, as suggested by the French scientist Fizeau. The daguerreotype was a direct positive image on metal, in contrast to modern cellulose negatives and paper positives. Light areas were depicted by the daguerreotype in whaish amalgam and dark areas in metallic silver. &couldbe seen as a negative image in unfavorable light.
variety of mild reducing agents as developers. Daguerre's iodized silver plates were impractical for taking pictures of people in 1839. Exposure times were too long. Experimenters endeavored to increase the sensitivity of the plates to reduce exposures to reasonable intervals. Many of the same chemists who attempted to elucidate the chemical basis of the latent image, a purely scientific matter, also pursued this and other technical improvements to photography concurrently. Among them were several young French scientists, who began their careers with photographic studies in cooperation with their elder colleagues. Jean Foucault, known now for his classic pendulum experiments, began his research career in photography. His first paper concerned an improved process for using hromine to accelerate daguerreotype plates. Photographic use of silver bromide was independently discovered in England hy Goddard and in America by Draper. I t decreased exposure time enough to make portraiture practical (11). Armand Fizeau was another young French natural philosopher who did his first work in photography. Along with Donn6, he attempted to create a method of converting daguerreotypes into printing plates. In 1840 he introduced 630 / Journal of Chemical Education
a method of gold toning daguerreotypes (12) which, when combined with the increased sensitivity achieved with hromine, laid the basis for a new portraiture industrv. In collaboration with Foucault he in;estigated the effects of red light on daguerreotypes (13). The conversion of daguerreotypes into printing plates was a major object of scientists in photography, as announced by Donne in 1839 (14). In 1841 Fizeau produced a method of electroplating the daguerreotype to make i t strong enough to withstand the printing press (15). He was moderately successful, hut could only draw off several hundred copies. In the 1850's a photo-engraving process for steel made mass production photographs possible. The foremost problem which photography and photochemistry shared was that of determining light intensity. For photographers, determination of luminous intensity was essential for the efficient use of plates and papers. As with the other technical problems of photography, scientists hastened to develop answers. The difficult question remained unresolved until the late 1850's. Youthful Edmond Becquerel devised a galvanic photometer, consisting of silver iodide coated plates immersed in
acidic solution and connected to a galvanometer (16). I t proved too insensitive. Malaguti, Pelouze's assistant in Guy-Lussac's laboratory, attempted to measure the intensity of daylight with silver chloride paper (17). The object was to measure darkening to a standard tone. The failing of i t and similar methods was over dependence on sight judgment of tones. Malaguti hypothesized but did not prove the law governing the blackening of silver salts, known as the law of reciprocity I X t = h
where I = light intensity, t = time, and h = constant, a given endpoint of the photochemical reaction. The error involved in the methods of Malaguti, and certain inexplicable photographic observations not only led to better photometers hut to additions t o light theory. By 1840 differences between luminous intensity and effects on silver salts were noted. Herschel related (18)
. .. that the intensity of chemical action of different rays in the solar spectrum appears to he in great measure disconnected with their colorific impressions on the eye.. . William Draper, professor of chemistry a t New York University attempted to explain the differences (19) "It is very probable, that there exist in the sunlight, rays having particular chemical powers." All such observations were made by the chemists from problems arising in photography. They quickly crystallized into a theory of light encompassing several types of rays. The first were the luminous or colorific rays seen by man. The second were heat or calorific rays. The third were chemically active or actinic rays. Herschel first proposed the classification in 1840. The fact that Edmond Becquerel held the competing theory, that all effects were the result of a unified light, spurred on research for more exacting methods of photometry. Draper devised a photometer utilizing the reaction in light between hydrogen and chlorine gases (20). The instrument was an upright U-shaped tube with water filling the bottom curve and the reactants in one arm. The other arm was a pressure gauge. The rate of reaction was followed by a decrease in pressure. Draper confirmed the law of reciprocity under all conditions, but hecause the pressure of the reactants was not held constant, his results were questionable. In 1844 Fizeau and Foucault demonstrated that daguerreotype plates did not obey the reciprocity law (21). A better photometer was needed t o explain the discrepancy. As new properties were attributed t o light, various new rays were postulated. The dissimilarity of exposure times required in different parts of the world pointed to a fourth property in addition to the colorific, calorific, and actinic rays (22). Draper called these the tithonic rays, while Hunt named them energia (23). Hunt was a professor of physical science at the English Government School of Mines. Draper and Hunt engaged in a hot dispute over the supposed fourth property of light (24). They expanded their claims to maintain that these new properties were in fact new imponderable elements. These theories seem strange to modern observers in that we view the different effects of light not as separate elements, hut as effects of differing energies of rays depending on wavelength. The elusive fourth imponderable remained until Roscoe and Bunsen constructed a precise photometer. Bunsen was professor of chemistry a t Heidelberg University and Roscoe his young English graduate student. They too used hydrogen and chlorine, hot eliminated Draper's variables. In
a series of exhaustive studies (25) they firmly estahlished the law of reciprocity and its limits. The relationship held only after a period of induction, in which light had no effect. They showed t!~at photochemical change followed the inverse square law when a flame was held a t various distances, demonstrating that the tithonic rays and energia were superfluous constructs and merely manifestations of a sinele effect of light. Roscoe and Bunsen fully realized the importance of their work to photography andbhotochemistrv. The last DaDer of their classic series deals with the ahsolute intensity'of light for different parts of the world on the basis of time of day and latitude. They explicated the previously mysterious worldwide differences in photographic exposures. Their work was the basis of all subsequent photographic exposure tahles. They did not contradict the prevailing three property theory of light. None of the photographic investigations ever affected the prevalent wave theory. The creation of precise photometry was central to the development of both photography and photochemistry. In the earlv 1840's Dhotometrv arose directlv from ~ h o t o graphic observations, and was pursued by scientist-photograohers. The disciolines had divereed - bv. the time of Roscoe and Bunsen. Three factors made ohotoeraohv - . -an attractive and fertile field for scientific endeavors. First, Arago and Herschel, two of the most eminent scientists of the age, enthusiastically welcomed photography. Second, photography enjoyed a soectacular introduction which gained the attention of theworld. Third, its original description by the inventors left open important questions of theory and practical applications to be solved. Once photography was made useful, initial interest waned. By the late 1840's most photographic improvements were discovered by commercial practitioners of the art, unlike a few years before when professional scientists made the important contrihutions. Only at a later period did innovations come from the photographic chemists (26). Acknowledsment -
The author acknowledges Professor A. Thackray, Department of the Historv and Socioloev of Science. University of Pennsylvania, fbr assistance in preparatioh of this manuscript.
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Literature Cited I11 For applications in aslronomysee Norman, D. T., Oairis, 5,560 (19381. (21 Nerhall. B.. "Tho Latent Image:' Doubleday and Co., Inc, Garden City, N.Y..
(81 (91 (101 (111 (121 (131 (14) 115) (161 (17) (18) (191 (201 (211 (221 (231
Roherta,M..Phil. Mag., 18.301 (18411. C h o i w l e t s n d R m l . Compt.Rend.. 16.1436 (1843); 17.173 (18431. Gaudin. Comnt. Rend., 12.1187 (1841). Draper.J..~hil.Mag, 17.217 (18401. Fizeau.H., Campr. Rand., 11,287 11~401. Fovesvlt andFizeau. Campi Rend.. 23,8W (18461. Dann6,A.Compt. Rand., 9.485l18391. 12,401,509 118411. ~ i r c a u , ~ . , C o m p rend.. . Becquerel. E.. Compt. Rend., 9,145 (L8391. Malaguti,F.,Ann. Chim., 72.5 (19391. Hersehe1,J.F W..Phil. Trans., 130.1i18401. Draper. J.,Phil. Mog, 16.81 (1840). Draper, J.,Phil. Mug, 23.401 (18431. Fimauand Foucault, Compl. Rend., 18,746 (1844). Draper. J..Phil. Mog, 21,348 (18421. Hunt. R., ''Researches on Light." Longmsn, Bmwn, Green,and Longmans, London. 1 8 4 4 , 271-274. ~ (241 Hunf,R..Phil. Mag. 25,119 (18441. (251 Bunncn, R.. and Rweoc, H., Ann. Physik, 96. 373 (1855); 100. 43 (1856): 100, 481 (1856): 101. 235 (1856); 107. 193 118591. Phil. Trans., 147, 355 (1857); 147. 381 (1857); 147,601 (1857): 149.879(18591;and 153.139 i18631. (26) For s modern view of photographic chemistry see Mees. C. E., "Principles of Photographic Chemistry," MacMillan and Co.. New York. 1912.
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