THEORIES OF T H E LATENT IMAGE A S D REVERSAL BY
E. P. WIGHTAIAN
Of all the topics of photographic chemistry which have caused the greatest amount of investigation and discussion that of the nature of the latent image is preeminent. Yet, in spite of the vast quantity of experimental work which has been done, especially in the last ten or twelve years, and in spite of the volumes which have been written, the statement made in Roscoe Schorlemmer’s Treatise 072 Chemistry, “ t h a t the exact change in the sensitive salt is as yet not understood,” is as true to-day as it would have been fifty years ago. It should not be said that we are not any nearer a solution of the problem, because we would not, if we could, discredit the excellent work of Luppo-Cramer, of von Hiibl, of R. Luther, of K. Sichling, of Reinders, of Renwick, of Bancroft, Rlatthews, Mees, and many others. For convenience the theories which have been set forth may all be classified in three groups, one attributing the change to a chemical change, another attributing it t o a physical change, and the third to a mixture of physical and chemical changes, or rather, to a physical-chemical change. The Chemical Theories qt the Late9zt Image. I-Schelle, in 1777, po’nted out the loss of chlorine from silver chloride on exposure to light. He treated the darkened residue with ammonia and found that black flakes of silver remained behind. “The notion that the darkened residue was a subchloride with the composition Ag2C1, was first suggested by Fischer in 1814, and reiterated by Wetzlar in 1834.” I t was reasoned in those early days that there must be either a subchloride or metallic silver formed. But they argued that the latter could not take place very well because the normal chloride will darken in the presence of nitric acid and apparently no NOTE.-In the early history of the theory I have made free use of Meldola’s Chemistry of Photography. * Yon der Luft und dem Feuer: Leipzig (1784), p. 64.
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silver is dissolved. This happens, too-although the darkening is not quite so pronounced-even in a fairly concentrated acid. Meldola states that “one strong argument formerly used in favor of a subchloride was U’ohler’s supposed suboxide of silverj Ag40, obtained in 1839 by reducing silver citrate in a current of hydrogen at 100’ C. But investigations in 1887 and 1888 by hluthmann,’ by Friedheim,? and by Bailey and Fowler3 appear to render the existence of the oxide improbable thereby making statements about the subchloride equally doubtful,”: Meldola points out also that small amounts of moisture may influence photodecomposition because, ‘ ‘ according to Abney, dry silver chloride sealed up in a Sprengel vacuum does not darken when exposed to light for months.”j Certain solvents such as sodium hyposulphite, potassium cyanide and ammonia are known to dissolve normal silver chloride. The darkened chloride, however, leaves a residue of silver. It might be assumed that such reactions as the following take place : ilg Ag2Cl NazSz03 = AgKaSe03 NaCl Ag2C1 zKCN = AgK(CN)z KC1 Ag AgzCl zKH~OH= AgNH4(OH)z WHdC1 Xg
+ + +
+ +
+
+ +
+
But this is merely an assumption for which we have no proof. It is similar to the argument of Riche that after exposure for a year and a half the reduced salt has the composition Ag3C12. He first assumes the existence of the salt, and then attributes the loss of chlorine to its formation. What if we reason from analogy? Mercury, copper, and thorium form lower chlorides. Are their lower chlorides analogous to the subchloride of silver, as some have argued? Ber. deutsch. chem. Ges., 2 0 , 983 (1887). Ibid., 2 0 , 2554 (1887); 21, 307 (1888). See also Pillitz: Zeit. anal. Chem., 21, 27, 496 (1882); Sewberry: Am. Chem. Jour., 8, 196 (1886); Pfordten: Ber. deutsch. chem. Ges., 2 0 , 1458, 3375 (1887); 21, 2288 (1888). Jour. Chem. SOC. Trans., 51, 416 (1887). Bibra: Ber. deutsch. chem. Ges., 8, 741 (1875). Abney: “Recent Advances in Photography,” p. 19 (1882).
Theories o j the Latent Image and Reversal
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Are they not more nearly similar to the normal chloride of silver? The lower chlorides of mercury, copper, and thorium are insoluble; they are not darkened, but are white, or nearly so, similar to silver chloride, AgC1. Moreover, mercurous chloride is “redissolvable by light, not to a subchloride, but to mercuric chloride.” Besides, ‘‘ these lower chlorides are definite chemical salts easily prepared by chemical methods, which cannot be claimed for the supposed subchloride of silver.” While there are still some chemists who hold to the subchloride theory, it has been practically abandoned since before Meldola’s time ( I 890). Holleman and Cooper’s textbook of Inorganic Chemistry (3rd edition, 1910, p. 362) states that “ a subbromide of the composition AgzBr is formed by the action of light, and not metallic silver, because dilute nitric acid does not destroy the latent image. Moreover, not all the silver bromide is decomposed, but an equilibrium is established. 2XgBr
If AgzBr + Br
which is displaced farther to the right the stronger the illumination.” A number of other such statements might be cited from text-books which have not been recently revised on this subject. Eder,l in 1905, modified the theory to this extent. He suggested that there might be a series of subbromides, depending upon the length of exposure. Trivelli2 holds that the phenomena of solarization, that is, double, or negative and positive image, cannot be accounted for on the assumption of only one compound, no matter what its composition. He gets around the difficulty by supposing four compounds having the composition Ag8Br7, AgsBr6, Ag8Br5, Ag8Bri. There seems to be no grounds for such, however. “ T h a t oxygen was essential to the darkening of the chloride was first announced by Robert Hunt,”3 in 1854. Powdered silver chloride was placed in one end of a bent tube, 1
3
Brit. Jour. Phot., 52, 9j0, 968 (190j). Cf. Luppo-Cramer: Das latente Bild., 23 (1911). “Researches on Light,” 2nd Ed.,p. 80 (1854).
574
E . P . TViightnzan
the other end of which was held under water. “As the chloride darkened, the tube was thoroughly shaken in order to be sure that as much as possible of the chloride was exposed.” The water rose in the other end of the tube.l Of course, this is not conclusive, because an oxide of chlorine might have been formed which is soluble in water. That an oxychloride of the composition Ag40C12 was formed was the explanation of the phenomenon given by 11’. R. Hodgkinson. Meldola was one of those who thought the solution of the problem might be in the direction of the oxychloride as a reduction product though he did not commit himself to any definite statement as to the exact nature of the change. Baker? showed that not only is chlorine lost when the chloride blackens but apparently oxygen is at the same time absorbed, an oxychloride of the formula Ag2C10 being the result. The amount is extremely small even for a large quantity of chloride when exposed for a long time. Abney and Baker both showed that pure, dry silver chloride does not blacken when exposed to light in a vacuum tube, in perfectly dry oxygen, or under carbon tetrachloride in the absence of oxygen. Sonstadt states that hydrogen peroxide is always formed when light acts on silver chloride. He finds that if silver chloride is sealed up in a tube and blackened by exposure to light, a reversion to the white chloride takes place if the tube is kept in the dark. On the other hand, if the products of decomposition-those that are volatile-are removed by having calcium chloride and ammonia in another part of the tube containing the silver chloride no bleaching of the blackened compound takes place. So much for the older and practically discarded theories. Now what have modern research and reasoning to say? “While Luther’s3 earlier work pointed t o the formation of a _ _ ~ ~
1 Cf , “The -4rt of Photography” (English translation) by Halleur, p. 76 (1854. Jour. Chem. SOC., 61, 728 (1892). Cf. Sbney: “Treatise on Photography,’’ 5th Ed., p. 24; also Hunt: “Researches on Light,” 2nd Ed., p. 98. Heyer: Jour. Phys. Chem., 15, 557, 5 6 0 (1911).
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subchloride or bromide, as the case might be, the later work from his laboratory has shown their non-existence. I n his resume of arguments for and against certain theories, he thinks the problem may never be solved, because passage from chemical combination to mixtures and absorbed substances is of the most gradual kind.” Yet, we still have the argument of Perley2 that since the action of light can be reproduced by suitable reducing agents, the latent image cannot be a modification of silver bromide but must be a reduction product. According to Bancroft3 it cannot consist of metallic silver, “because the image does not show the reaction of metallic silver, because it does not show the electrical potential of metallic silver, and, moreover, the hypothesis can not be reconciled with the facts of solarization.” He also discards the modification of the silver bromide and the subbromide views. What he does claim is that “ t h e latent image is a phase of variable composition, presumably due to adsorption of silver by silver bromide, and that it is identical with photo-halides except as to color.” He points out from the work of Liippo-Cramer that the latent image which gives rise to a negative under normal development consists of silver bromide with an excess of about 0.002 to 0.1 percent silver. The view of Bancroft which has just been stated, namely, that the latent image is a phase of variable composition with silver as the end-point, is the view first suggested by Carey Lea’ in 1887, and later brought into prominence by LiippoCramer. It was also adopted by Reindersj as the result of his most recent experiments. The silver, they say, probably exists in the colloidal state, which would explain the color phenomena. In this connection Liippo-Cramer6 has boiled colloidal Phot. Rundschau, 24, 2 2 1 (1910); Brit. Jour. Phot., 57, 6 j 1 (1910) Jour. Phys. Chem., 14, 689 (1910). Ibid., 17, 93-153 (1913). Am. Jour. Sci., ( 3 ) 33, 349 (1887). Zeit. phys. Chem., 77, 213,2j6, 677 (1911). Zeit. Kolloidchemie, 8, 4 2 (1911); Phot. Korr., 48, 188 (1911).
E . P.IVightmaut
5 76
silver with silk, wool, cotton, etc., and obtained colors which seem to confirm the formation of adsorptive combination of the photo-halides with colloidal silver, the color depending upon the size of the adsorbed colloidal particles. C. E. K. Meesl points out that “Abegg considered the latent image to consist of a nucleus of silver forming a solid solution in silver bromide.” He did not state whether or not the silver was supposed to be in the colloidal state. K. Sichling,2 by means of measurements of potential, and electrical determinations of solubility, shows that photochlorides are single phase systems-solid solutions of amorphous silver. He also showed that silver chloride and colloidal silver possess continuous miscibility. Reinders3 prepared crystallized photo-halides colored with colloidal silver and discusses the subhalide as opposed to the silver adsorption theory, and concludes that photohalides are normal salts, colored by small amounts of colloidal silver. The color and other phenomena depends upon the size and distribution of the colloidal particles. Hurter and Driffieldl made certain energy calculations which Bancroft says lose their force when it is assumed that an almost infinitesimal change in the composition of the silver bromide grain is sufficient to make development possible. There seems to be no reason, also, he says, why all the silver bromide, instead of that part affected by the light, should be developable, although the argument has been attacked by Chapman, Jones,j by Namias,6 and by others. With reference to the extremely small quantity of silver bromide affected by light on a photographic plate, Meldola says that the character of the photo-decomposition products of the latent image on the plate are the same as in the case of the darkened silver bromide as ordinarily precipitated, differing Jour. Frank. Inst., 179,141 (1915). Zeit. phys. Chem., 77, I (1911); Phot. Korr., 48, 33 (1911). 3Zeit. phys. Chem., 77, 213,356 (1911). 4 Jour. Phys. Chem., 15, 365 (1911). Sci. and Prac. of Phot., 374, 387 (1902). e Phot. Korr., 43, 155 (1905). 1
2
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only in degree and not in kind. “The amount of silver bromide acted upon is so small as to be infinitesimal, the transverse section having a diameter hardly more than the molecules themselves. As to the color, that of such a thin layer could hardly be seen, besides, only the chloride is strongly colored.” Mention only can be made of other more or less important work on the chemical theories, by Wohler,l Mees and Wratten,? Wohler and R ~ d e w a l d ,Baur,4 ~ Liippo-Cramer,j and others. Ph33sical Theories Coiicerizing the Latent Image.--These are few in number. It was found by Dewar that gelatine emulsions of silver bromide remained sensitive to light-though to a smaller extent to be sure-even at -252.5’ C, a t which temperature nearly all chemical actions proceed ;'cry slowly. This gave rise to the view that no chemical action is brought about on the photographic plate, but that the molecules of silver bromide are altered so as to make them more sensitive to light. So far as we could learn he did not say what the change was, nor whether it was accompanied by some sort of reduction. It is hardly possible that the change is similar to that produced in “ ripening” the sensitive emulsion, because it is likely that the amount of light acting in a normal exposure would be too small to bring about such a change-i. e . , a molecular or allotropic change, as stated by R. J. ‘‘ It is very generally understood that silver bromide (2AgBr) is the chief substance employed in the making of gelatine dry plates, but that silver bromide exists in several different allotropic forms has long been known-the first, formed by the admixture of the gelatine and bromide and silver salts, is of slow sensitiveness, but in the process of ripening passes gradually through several modifications, finally ending in a state which is capable of reduction by a developer without Osterr. Chem. Ztg., 14,2 6 8 (1911). Brit. Jour. Phot., 5 5 , 831 (1908). Zeit. anorg. Chem., 61, 54 (1909). Zeit. phys. Chem., 77, 58 (1911). Phot. Rundschau, 24, 2 2 6 (1910). Astrophys. Jour., ( 2 ) 20, 114 (1904).
E . P . 11;ightman
578
the previous action of light, ciz., the blue allotrope of silver bromide (blue to transmitted light). If the ripening of the emulsion be stopped prior to the formation of this last form the result is still another allotrope, which is green by transmitted light, and of high sensitiveness.” Another theory, that of Renwick,l and known as the “explosion or pulverization theory” is based on an observation by Scheffer that the silver bromide grain on exposure violently throws off part of itself. It ruptures the surrounding gelatine, which, according to Renwick, encloses the bromide in a tangled meshwork. Where the bromide grains are surrounded by gelatine, the developer does not penetrate to any extent, but where the explosion has opened up a channel of relatively large size giving access to the developer it does attack the bromide. That there is a certain amount of pulverization seems to be confirmed by an experiment made by Liippo-Cramer,2 in. which a silver mirror is, exposed to iodine vapors, becoming coated with a film of potassium iodide. The sensitive plate thus formed, or part of it a t least, is exposed to the light for ten to fifteen minutes, and when brushed with cotton, silver iodide powder is removed from only the exposed portion. It was J. C. Bose3 who said, ‘‘With reference to photographic action, various facts are known which cannot be explained from purely chemical considerations. It will be shown that when a substance is molecularly strained, its chemical activity is modified in consequence of a physical strain. The acted and unacted portions will, therefore, be unequally attacked by developer. I n the case of a compound the strain produced may cause a modification which renders it susceptible to decomposition by the action of a reducing agent. The observed evolution of chlorine when moist silver chloride is .~
. _
Brit. Jour. Phot., 58, 48 (1911). * Phot. Korr., 47, 226 (1910). 3 Phot. Jour, Trans., June, 1902; see also Proc. Roy. SOC., 1899; Ibid., 1900; Ibid., 1902; Report, B. A , , 1900 (Electrician); Travaux du Cong. Intern. de Pliys., Paris, 1900; Friday Evening Discourse, Royal Inst., May, 1901; “Response in the Living and Non-living,” J. C. Bose, Longmans, Green & Co. (1902).
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exposed to the long continued action of intense light is often adduced in support of the chemical nature of photographic action. This extreme case of dissociation cannot, however, be regarded as representative of the action of light in the formation of latent images. In ordinary photograhic action we have nearly the effect of a moderate stress producing the corresponding strain (with concomitant variations of chemical activity, and not the disruptive effect of a breaking stress).”l These remarks by Bose furnish the introduction to a very interesting paper on the subject of what he calls the “molecular strain theory.” It would not be worth while to take up the subject, as he treats it, in detail, but it would be well, no doubt, to give the following conclusions with which he sums up his paper. “It is thus seen: ( I ) That molecular strain is produced by the action of light. “ ( 2 ) That both in the artificial and real retinae, the molecular strain produced by light gives rise to similar electric impulses. ‘‘ ( 3 ) That as the physico-chemical properties of a substance are changed by strain, it is possible to develop the latent image, through the difference in the following properties between the exposed and unexposed portion, produced by light: (a)difference in adhesive power, e. g . , development of daguerreotype by mercury vapor; ( b ) difference in chemical stability, e. g., development by reducing agents. “ ( 4 ) That molecular strain may not only be produced by visible or invisible radiation, but also by ( a ) electric, and ( b ) mechanical stress. Latent images produced by such means may be developed, e. g., inductoscripts, development by pressure marks. “ ( 5 ) That nearly all substances are sensitive to radiation, but the effect cannot in all cases be rendered visible, (a)owing to want of suitable chemical developers; ( b ) owing I‘
Cf. above R. J. ‘A’allace (Loc. cit.) and preceding paragraph.
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to quick self-recovery. The effect may be rendered more or less permanent by overstrain or by ‘ arrestors.’ “ ( 6 ) That the molecular effect due to radiation can be demonstrated by the conductivity or E. M. F. variation method. “ (7) That the latent period of overcoming inertia corresponds to the photographic induction period. “(8) That the relapse of image is due to self recovery. (9) That owing to the tendency toward self-recovery, the radiation effect does not solely depend on the total quantity of light, but depends also on the time rate of illumination. Hence, the photographic effects of intermittent and continuous illuminations are not the same. “ (IO) That the continuous action of radiation produces recurrent reversals.” This molecular strain theory of Bose is interesting, but does not go far enough. It could be stated, probably a little more effectively in other words, as the theory of the change in internal equilibrium forces of the molecules due to the influence of light. Tire know that with change of molecular structure (such as white to red phosphorous, orthorhombic to monoclinic sulphur, etc., as cited by Bose) there is an accompanying loss or gain of intrinsic energy of the molecules, observable as heat given off or heat absorbed, respectively. This change of internal energy which is also accompanied b y change of chemical activity does not necessarily mean a strain, or, in other words, a lack of internal equilibrium, it merely means that the chemical potential of the substance in question has been lowered or raised as the case may be, with respect to other chemical substances not so easily affected by light. Even this modification of Bose’s idea is not sufficient t o explain all the facts; we are forced to go farther. Phj’sico-Chei.nica1 Theories o j the Latewt Inzage.--It is well known from Hertz and others that light has the power t o cause a discharge of negative electrification from the surface of many bodies. Joly’ considers that light causes an elec1
Kature, 72, 3 0 8 (1905).
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581
tronization of silver bromide, since the silver haloids are known to be photo-electric. An interesting thing about it is that they are photo-electric in the descending order, bromide, chloride, and iodide, the same order as their photo-sensitiveness. lloreover. an image capable of development is also formed by cathode rays or electronic discharges from radium. Joly thinks, therefore, that the latent image is made up of atoms or molecules electronized by the action of light, and that it is upon these that the developer acts. I n a recent paper. by C. E. K. h!Iees,l this theory is upheld as to the initial action of light. “An objection to all these theories,” says liees, referring to those which have t o do with reduction either to subbromide or t o silver, that is, the chemical theories, “is that the energy available during exposure is not sufficient to produce an appreciable amount either of metallic silver or silver subbromide from normal silver bromide. This was first pointed out by Hurter, who, from consideration of the amount of energy which was liberated in the burning of a candle, came to the conclusion that the available energy was not one percent of that necessary to reduce to silver subbromide the silver grains (he evidently means, the silver bromide grains ”) which eventually proved to be developable. ” Mees gives the following calculations made by Dr. P. G. Xutting for a high speed plate: “Consider the exposure to light which is sufficient, after full development, to produce a deposit of unit density; that is, one which will transmit one-tenth of the incident light. A deposit which has this density contains ten milligrams of metallic silver per square decimeter, or one-tenth of a milligram per square centimeter, which represents roughly 1 0 1 9 moles of silver, or 10’grains 3 p in diameter. Now, the energy of the amount of violet light required to give an exposure necessary to make an emulsion film developable to this density, ‘ I
Jour. Frank. Inst., 179,146 (1915).
is of the order of IO-’ ergs per square centimeter. Therefore, each grain (which contains on the average, IO^* moles) receives 10-l~ ergs to make it developable. We know that in order to detach one electron from a mole, 5 X 1 0 - l ~ergs are required in the separate moles of a gas . . . . Clearly, then, the energy incident on a grain during exposure may be sufficient to affect only one mole in the grain, and the latent image, may be composed of grains in each of which, on the average, only one mole has lost an electron by the action of light.” Beyond the initial stages of the action of light Mees thinks the changes taking place are more likely purely chemical, since, considering the facts from the chemical standpoint, he is able to explain with greater satisfaction the phenomenon of reversal. He is not in sympathy with Allen’s1 purely physical explanation on the basis of the electronic discharge. But more of this presently. It has been said, with truth, that no theory of the action of light on the photographic film can be considered satisfactory until it explains the phenomenon known as rmersal or solarization. What is reversal? It is just this: with short exposure the latent image develops a negative, with longer exposure, a positive. How can this be explained on the basis of the theories which we have studied? As late as 1912, Bancroft’ states that “discussions up to the present on solarization have been merely qualitative. Liippo-Cramer3 has obtained data making silver bromide emulsions containing known amounts of colloidal silver adsorbed by the silver bromide.” With 0 . 0 0 2 percent silver in the bromide emulsion there was a distinct fogging on development. “With increasing amounts of silver the rate of blackening increased and the sensitiveness to light also increased to a maximum a t about 0.1percent silver, the latter ~
~~
~~
H S. Allen “Photoelectricity,” Longmans, Green & Co. (1913). 2 Eighth Int.Congr. of Applied Chem., 20, 51 (1912); Jour. Phys. Chem., 17, 93-153 ( 1 9 1 3 ) . 3 Phot. Korr., 46,526 (1909). 1
Theories o j the Lateut Image aTzd ReLtersal
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being about twenty-five times as sensitive as one containing 0.4 percent silver. From 0 . 0 0 2 to 0.01percent we are dealing with the first negative. Somewhere beyond 0 .I percent silver we have the first positive.” Stating it in other words, Bancroft says, “The latent image which gives rise to a negative under normal development consists of silver bromide with an excess of about 0 . 0 0 2 to 0.1percent colloidal silver”-adsorbed or in solid solution in the bromide. The reversed negative or positive image contains an excess of about 0.4 percent silver. I n another paper published by the 8th International Congress of Applied Chemistry, Bancroft drew the following conclusions as to a further reversal: ’ ‘ ( I ) If a second positive exists, it requires a very long exposure even with a very bright light. ( 2 ) I n many cases a false first positive or false mongrel may be obtained. ‘‘ (3) Since the emulsion on an ordinary plate is never homogeneous one really observes a combination solarization curve. ‘‘ (4) The inhomogeneity of the emulsion may easily account for the false positive or false mongrel. “ ( 5 ) With long exposures we find great differences between different boxes of the same make of plates and we even find some differences between plates in the same box. ” I n the light of Mees’ paper,l one can obtain a clearer idea of the whole process. He points out that if we plot a curve of the density (that is, of a fully developed plate) against a logarithmic scale of exposure-density increases with exposure to a maximum, as shown in the figure,?then decreases to a minimum and increases again to another maximum, and so on, each time the maximum point being smaller than the one preceding, the minimum points representing the positives. Such change of image and of density is known as reversal. ‘ I
Jour. Frank. Inst., 179, 141 (1915). This curve is not drawn to scale, but is merely an approximation somewhat similar to that given by >lees: Jour. Frank. Inst., 179, 146 (1915). 2
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To return to his explanation of this phenomenon, Mees says that “under the continual influence of light (i. e., after the initial exposure, which he accepts as photo-electric) the silver bromide is broken up into metallic silver and bromine, as it is well known t o do, the bromine given off from a much over-exposed plate being easily smelt.’ Xow, bromine actually attacks exposed silver bromide, preventing it from being developed, so that it would seem likely that grains which have been completely decomposed by light simply suffered photoelectric change to the latent image. The gain in density observed with still more extended exposure may be ascribed to the actual production of metallic silver by long continued action of light, and indeed, such reduced silver can be shown to exist by fixing the exposed but undeveloped plate.”
Fig.
I
(a) Period of underexposure. ( b ) Maximum density of first negative ( c , Maximum brightness of first positive ( d ) hIaximum density of second negative. ( e ) Maximum brightness of second positive
Going back to Meldola’s time, we find that he says,’ “We must look upon the sensitive plate-say gelatinobromide-as a film of silver bromide imbedded in a bromineabsorbing substance (gelatine) and bathed on its surface by atmospheric oxygen. When exposed to light the vehicle (i. e . , gelatine, which may be considered as a sensitizer) becomes brominated up to a certain degree of saturation; complex bromo-derivatives, or additive compounds, or oxidation 1 2
NOTE-The writer doubts this. Meldola: “Chern. of Photog.,” p.
225.
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products are formed, and these a t length begin to react with the reduction product aided by the external oxygen. The process may be regarded as analogous to the destruction of the latent image by the haloid hydracids, and indeed it is not improbable that some hydrobromic acid is actually formed b y the bromination of the gelatine, and takes part in the process of reversal . I t is obvious that a highly sensitive .plate would halogenize its sensitizer more rapidly than a less sensitive plate; that i s lithj~solarizatiopz occurs more readily in ilze most seizsitize processes. ’ ’ The main difference between Meldola’s idea and that of Mees (leaving out the initial photo-electric effect) lies in the rBle played by oxygen, yet the former admits that the phenomenon of reversal is still existent in the absence of oxygen.’ According to hlees’ idea, the most important distinction between the p oduction of a normal negative image and a reversed or posi ive image lies in the fact that in the first case we have simp y a photo-electric effect, or driving off by light of one electro 1 from each grain of the silver bromide, while in the latter case actual decomposition of the silver bromide particles takes place until the negative action of bromine, which is set free in the neighborhood of the grains, is sufficiently great to prevent the developer from attacking those grains. At the same time the grains which were less acted upon by the light are open to attack by the developer, thus producing, a positive, by reversing the image. The above energy calculation of Kutting, which Mees cites, is, however, a little ambiguous. He says that there are approximately 1 0 1 9 moles of silver per square centimeter of the emulsion, corresponding to loT grains 3 p in diameter. He assumes, evidently, that the silver grains have the same size as the bromide particles from which they are derived, and that these are of the maximum size and are all uniform. If his ‘‘ IO’ grains 3 p in diameter” refers to the silver bromide grains, then, after development instead of IO^? moles of silver per grain there would be 0.5 X lol*moles of silver. More-
i
________ See Abney’s “Treatise on Photog.,” 5th Ed., p. 309.
E . P. J17ightman over, Mees assumes that it takes five hundred times more energy to drive off an electron from a mole of a gas than that which will split off an electron from a mole of silver; however, i t may be considered as a fairly reasonable assumption. Let us calculate the approximate amount of energy necessary t o decompose one molecule in each grain of silver bromide and see how it compares with the energy as calculated by Mees necessary to split off an electron from that molecule. We have the reaction ;Ig Br -1gBr 2 2 7 0 0 calories. For each gram-mole of silver formed, therefore, 45,400 calories are required to decompose its equivalence of silver bromide and for 0.1milligram of silver 0.042 calories or 1.76 X 106 ergs are necessary. Dividing this by 1019 moles in 0.1 milligram, gives 1.76 X 1 0 - l ~ergs per mole. In other words, about seventeen and a half times as much violet light would be necessary to decompose a mole of silver bromide as to split off an electron from it. If, instead of employing the exposure suggested by Mees for his hypothetical experiment we should begin to over-expose the plate, then as we increase the exposure there is no reason to suppose that a greater number of moles in each grain of silver bromide would not throw off electrons. Soon those moles which have been exposed the longest would begin t o decompose. Mees does not give the time of exposure upon which his calculations were based, but supposing it were as small as 0.001 of a second, and that only one mole in each grain could be acted on for each instant of exposure (this is obvious from the above consideration) it would require about sixty-six years to drive off one electron each from the 1oI2 moles of bromide in one grain of the latter. To completely decompose all the moles each grain contains would require about seventeen and a half times this long, or over one thousand years. Even though these figures may not come within a hundred percent, even within a thousand percent of being correct, we can see why silver bromide when exposed to the light even for a comparatively long time apparently does not all decompose.
+
+
Theories o j the Latent Image and Rezersal
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Of course, with the whole spectrum (including the ultraviolet light) the time for decomposition would be much shortened, and still further so since many grains are smaller than 3 p . Allen’s view of reversal,’ as stated by Mees,? is that “electrons are expelled and become attached to the surrounding gelatine moles; external stimulus, such as radiations of longer wave length, enable the electrons to enter again into combination with the atoms from which they are liberated] due to electrostatic forces between oppositely charged particles. If recombination is not assisted, the electrostatic field increases under the influence of light, and self-neutralization takes place as suggested by Joly.” Assembling all these ideas, we might say that when the light begins to act, electrons are driven off from one mole after another until the point is reached when sufficient light has acted to start decomposition of the bromide. As the negative electrons are expelled from the bromide particles, the latter become charged positively. On decomposition, the bromine being likewise expelled, or split off (and possibly absorbed, as Yeldola said, by the surrounding gelatine) leaves the residue of silver, which may be considered as dissolved (as a solid solution) in the surrounding bromide particle, charged positively. This may account for the latent image not being destroyed] or greatly affected, when it is formed, on exposure of the sensitive film in the presence of nitric acid. It should be noted that this idea does not necessarily imply the initial formation of colloidal silver, in fact, the colloidal state could not be reached until a sufficient number of molecules of the electrically charged silver had been formed to produce a colloidal particle, which, according to Ostwald’s classification3 has a diameter not exceeding IOO ,up and is not smaller than 1 P,u*
As the particles become more and more decomposed by A. H. Allen: “Photoelectricity,” Longmans, Green & Co. (1913).
* LOC.cit. Wo. Ostwald: Grundriss der allgemeinen Chernie, j 4 8 (1909);also von Weimarn and Wo. Ostwald, Zeit Kolloidchernie, 3, 26 (1908).
the light we might consider three things as happening. First, the positive electrification of the silver bromide becomes greater and greater in consequence of more and more electrons being expelled, and this would sooner or later result in a self-neutralization of the electrostatic forces, as explained by Allen. Secondly, the gelatine immediately surrounding the bromide particle would become saturated with bromine and this would then tend to be given off as the gas, thus leaving a residue of silver behind-this would only happen on very long exposure, of course. Finally, since all molecules possess the property of rapid motion, some, if not all of the charged silver particles resulting from decomposition might find their way to the surrounding gelatine, and combine with it. This, too, would help to explain the lack of action of nitric acid above spoken of. When the plate is developed an image will result provided the exposure has not been less than enough to produce the electronic discharge, or provided i t has not been given an exposure just equal to that which will produce self-neutralization. Just over this exposure electronization may set in again, but wherever sufficient free bromine is in the vicinity of the particles of silrer bromide the action of the developer may be prevented as explained by Mees. It has been pointed out to me recently that the presence of gelatine or other material surrounding the grain is not necessary in order to produce a picture. This seems perfectly obvious, the old daguerreotype process being a sufficient proof of it. Our theories of the nature of the photographic change must, therefore, be independent of the presence of the gelatine on the plate, however much that presence may assist the process. The author of the present paper is about to undertake some experimental work on reversal and this paper may be considered as a preliminary introduction to the work. Chemical Laboratory Washtpzgton T&erszty Si L o u i s , M O . Aprd, IgIj
