1890.] on the FliotograjpMc Image. 143 



thoroughly — that a shearing stress applied mechanically to a sensitive 

 film leaves an impression which can be developed in just the same 

 way as though it had been produced by the action of light. [Pres- 

 sure marks on Eastman bromide paper developed by ferrous oxalate.] 

 Now that result cannot be produced on a surface of the pure haloid ; 

 some halogen absorbent, such as gelatine, must be associated with the 

 haloid. We are concerned here with a chemical change of that class 

 so ably investigated by Prof. Spring, of Liege, who has shown that 

 by mere mechanical pressure it is possible to bring about chemical 

 reaction between mixtures of finely divided solids.* Then again, 

 mild reducing agents, too feeble to reduce the silver haloids directly 

 to the metallic state, such as alkaline hypophosphites, glucose or 

 lactose, and alkali, &c., form invisible images which can be developed 

 in precisely the same way as the photographic image. All this 

 looks like chemical change, and not physical modification pure and 

 simple. 



I have in this discourse stoically resisted the tempting opportunities 

 for pictorial display which the subject aifords. My aim has been to 

 summarise the position in which we find ourselves with respect to 

 the invisible image after fifty years' practice of the art. This 

 image is, I venture to think, the property of the chemist, and by 

 him must the scientific foundation of photography be laid. We 

 may not be able to give the formula of the photosalt, but if the 

 solution of the problem has hitherto eluded our grasp it is because 

 of the intrinsic difficulties of the investigation. The photographic 

 image brings us face to face — not with an ordinary, but with an 

 extraordinary class of chemical changes due entirely to the peculiar 

 character of the silver salts. The material composing the image 

 is not of that definite nature with which modern chemical methods 

 are in the habit of dealing. The stability of the photosalt is deter- 

 mined by some kind of combination between the sub-haloid or oxy- 

 haloid, or whatever it may be, and the excess of unaltered haloid 

 which enters into its composition. The formation of the coloured 

 product presents certain analogies with the formation of a saturated 

 solution ; the product of photo- chemical decomposition is formed 

 under the influence of light up to a certain percentage of the whole 

 photosalt, beyond which it cannot be increased — in other words the 

 silver haloid is saturated by a very minute percentage of its own 

 product of photo-decomposition. The photosalt belongs to a domain 

 of chemistry — a no-man's land — peopled by so-called "molecular 

 compounds," into which the pure chemist ventures but timidly. But 

 these compounds are more and more urging their claims for considera- 

 tion, and sooner or later they will have to be reckoned with, even if 



* Tlie comiection between the two phenomena was suggested during a course 

 of lectures delivered by me two years ago (' Chemistry of Photography!^' p. 191), 

 I have since learnt that the same conclusion had been arrived at independently, 

 by Mr. C. H. Botharaley, of the Yorksliire College, Leeds. 



