6o 



SCIENCE. 



[Vol. XV. No. 364 



BOOK-REVIEWS. 



The Chemistry of Photography. By Raphael Meldola, F.R.S. 

 New York, Macmillan & Co. 12°. $2. 



This book consists of nine lectures which were delivered as a 

 special course at the Finsbury Technical College. With the 

 chemistry of photographic materials, their preparation, properties, 

 and re-actions, and with the practical details of photographic 

 manipulation, the author does not deal, but confines his attention 

 to the consideration of the chemical changes which occur in pho- 

 tographic processes, or the chemistry of photography, properly so 

 called. His object is to present the principles involved in these 

 processes, to show what point has been reached in the explanation 

 of them, and to stimulate further investigation. He hopes, too, 

 " that the |)resent work may contribute toward convincing " purely 

 scientific chemists " that there are many important problems still 

 awaiting solution in this field of research." Each lecture is fol- 

 lowed by an appendix containing directions for performing well- 

 selected experiments in illustration of the text. As the lectures 

 were originally addressed to an audience of chemical students and 

 photographers, some elementary knowledge of chemistry is as- 

 sumed. 



The amateur picture-maker who is content " to push the button " 

 and let some professional photographer " do the rest," or who has 

 no ambition beyond the knowledge of the simple manipulative de- 

 tails which enable him to mix his solutions successfully and make 

 passable photographs, will find little to interest him in this book. 

 But all who have felt the real fascination of the " dark room," and 

 desire to know more of the nature of the mysterious action of light 

 and the " developer " on the responsive film, will give it a hearty 

 welcome. The reader must not, however, look to have all his 

 questions satisfactorily answered, or all his difficulties solved ; for 

 the subtile re actions caused by light in the salts of silver are 

 among the most perplexing problems known to chemistry, and 

 photochemical theories are to a large extent still in the speculative 

 stage. Mr. Meldola does not -attempt to conceal this fact. He 

 distinctly and repeatedly points out the insufficiency of certain 

 hypotheses in regard to the nature of photochemical processes, 

 and, as it happens, gives in his own constructive efforts one or two 

 striking illustrations of the difficulties which beset the theorist in 

 this obscure region, and tend to lift his feet from the solid ground 

 of experimental facts. It should be said, however, that his theo- 

 retical suggestions are free from any undue assertiveness, and are 

 advanced chiefly from the motive that they " may serve as a stimu- 

 lus to further experimental inquiry "(p. 214). They will perhaps 

 attain this object quite as much through their evident inadequacy 

 and the criticism they will undoubtedly provoke as in any other and 

 more direct way. 



Lecture II. is devoted to the discussion of the 'composition of the 

 darkened product formed from silver chloride under the influence 

 of light. This is a subject of fundamental importance, for' the 

 identity of the material of the latent image with this darkened sub- 

 stance is universally admitted. Mr. Meldola rejects the generally 

 accepted subchloride theory, and attempts to show that the prod- 

 uct in question is probably an oxy-chloride. The argument 

 against the subchloride is that its existence "is^only inferred from 

 the analogy with the metals of the copper group, aijd is not the 

 result of the analysis of the pure compound " (p. 40). This is 

 hardly a fair statement of the case. It is true that the argument 

 from analogy is flimsy : it does not deserve the attention the au- 

 thor bestows upon it. It is true that no satisfactory direct proof 

 of the existence of the subchloride has been obtained through its 

 isolation and complete analysis ; but it is also true that the loss of 

 chlorine which occurs when silver chloride is exposed to light, and 

 the fact that metallic silver is not the result of the action, as well 

 as the whole mass of observation on the effect of light on this and 

 other salts, indicate very strongly that the darkened substance is a 

 reduction product ; and Cary Lea's brilliant work, two or three 

 years since, on the photo-salts of silver, furnishes weighty evidence 

 that this product is a subchloride united with a larger amount of 

 unchanged normal salt after the manner of a " lake." The most 

 that can reasonably be said against the subchloride theory is that 

 it is not yet absolutely proven by the isolation and analysis of the 



substance. This is no sufficient ground for its rejection, unless a 

 better theory can be formulated. Mr. Meldola thinks that such 

 is found in the hypothesis that the change produced by light is 

 probably due to the formation of an oxy-chloride of the formula 

 Ag4 OCU. This he supports on an experiment of Robert Hunt's 

 in which oxygen was found to disappear during the darkening of 

 silver chloride, some conclusions of Dr. W. R. Hodgkinson the 

 experimental evidence for which does not seem to have been yet 

 published, and an appeal to the analogy supplied by the darkening 

 of thallous and cuprous chlorides on exposure to light ; the change 

 in the case of the latter "being in all probability due to the forma- 

 tion of an oxy-chloride " (p. 57). 



Now, not only is direct proof of the existence of the alleged oxy- 

 chloride wanting, but its formation during the action of light is- 

 opposed to all the evidence which points to the reducing nature of 

 that action ; for the oxy-chloride is in no sense a reduction product, 

 oxygen simply taking the place of chlorine in a complex molecule. 



The hypothesis is further in direct contradiction to certain well- 

 known facts which the author has apparently overlooked in his 

 study of the matter, though he gives them place in the discussion 

 of other points. Thus on p. 75 it is stated that hydrogen acts as a 

 sensitizer, accelerating the photo-decomposition of silver chloride; 

 on p. 227, that action goes on under a film of benzene even to the 

 point of reversal ; and again on p. 197, that the invisible image is- 

 destroyed by oxidizing agents. An action which takes place in 

 hydrogen, or under a liquid destitute of oxygen, and which is un- 

 done by oxidizing agents, can hardly consist in formation of an. 

 oxy-chloride. It is, in fact, a weak and untenable hypothesis. Not 

 only does it offer the same difficulty which Mr. Meldola urges as 

 a chief argument against the subchloride theory, but it breaks 

 down completely when confronted with facts which the latter 

 readily explains. It is interesting to note that since the appear- 

 ance of the book, Mr. Lea has published in the American yournat 

 of Science a clever bit of experimental work which disposes of the 

 oxy-chloride hypothesis in the most final manner. Mr. Lea found 

 that silver chloride, poured in the molten condition into naphtha^ 

 blackened instantly in sunlight, and that a black iodine product 

 was formed by the action of light on metallic silver covered with 

 naphtha containing iodine in solution ; that is, the darkened sub- 

 stance is produced under conditions which rigorously exclude all 

 possibility of the presence of moisture or of oxygen in any shape. 



In his discussion of the action involved in the reversal of the 

 image on the photographic plate under prolonged exposure to 

 light, or " solarization," as it is often called, the author again shows 

 his lack of that comprehensive grasp of facts and principles which. 

 is an essential qualification for all sound theorizing. 



The explanation which he proposes for this most perplexing phe- 

 nomenon is, that in a gelatino-bromide plate, for instance, the bro- 

 mine lost at first by the silver salt under the influence of light is 

 taken up by the gelatine in which the salt is embedded, until " the 

 vehicle becomes brominated up to a certain degree of saturation ;. 

 complex bromo-derivatives, or additive compounds, or oxidized 

 products, are formed, and these at length begin to re-act with the 

 reduction product aided by the external oxygen" (p. 225). His 

 conception of the mechanism of the process is clearly given in the 

 closing sentences of Lecture VI. : " A ray of light falling upon a 

 sensitive plate is like the motive power driving a dynamo- machine 

 which is feeding a storage-cell. When the charge of the latter has 

 reached a certain point, it is capable of reversing the motion of the 

 system, and of converting the dynamo into a motor. The sensi- 

 tizer plays the part of such a storage-cell. When it becomes charged, 

 i.e., halogenized, to a certain amount, the chemical energy stored 

 up in it begins to run down, and reversal takes place." Or, to take 

 an equally pertinent but simpler illustration, the ray of light is like 

 a weight resting on a piston which works in a cylinder full of air. 

 The piston sinks under the weight ; but when the compression of 

 the air has reached a certain point, it is capable of reversing the 

 motion of the piston and raising the weight ! It does not require a 

 scientific training to see that this is absurd. It is a scheme for per- 

 petual motion. We have every reason for believing that the law 

 of the conser\-ation of energy applies to chemical as well as to 

 mechanical action, and it is obvious that under this law Mr. Mel- 

 dola's explanation is preposterous. 



