386 THE PHOTOGRAPHIC IMAGE. 



know tliat gelatine is a splenilid sensitizer — /. c, bromine absorbent. 

 There is another proof wliich Las been in our hantls for nearly 30 years 

 but ] do not thinlv it has been viewed in this light before. It has beeu 

 shown by Carey Lea, Edcr, and especially by Abney, Mho has investi- 

 gated the matter most thoroughly, that a shearing stress a[)i)lied me- 

 chanically to a sensitive tilm leaves an impression which can be devel- 

 oped in just the same way as though it had been produced by the 

 action of light. [Pressure marks on Eastman bromide paper developed 

 by ferrous oxalate.] Now that result can not 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 Professor 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 hal- 

 oids directly to the metallic state, such as alkaline hypophosphites, glu- 

 cose or lactose and alkali, etc., form invisible images which can be de- 

 veloi)ed in i)recisely the same way as the i)hotographic image. All 

 this looks like chemical change, and not physical modification pure and 

 simple. 



1 have in this discourse stoically resisted the tempting opportunities 

 for pictorial display which the subject affords. My aim has been to sum- 

 marize the position in which we find ourselves with respect to the in- 

 visible image after fifty years' ijractice 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 photo-salt, 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 determined 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 colored 

 product presents certain analogies with the formation of a saturated 

 solution ; the product of photochemical decomposition is formed under 

 the influence of light up to a certain percentage of the whole photo-salt, 

 beyond which it can not be increased, — in other words, the silver haloid 

 is saturated by a very minute percentage of its own product of photo- 

 decomposition. The photo-salt belongs to a domain of chemistry — a uo- 



* The connection between the two phenomena was suggested during a course of 

 lectures delivered by mo two years ago (" Chemistry of Pliotograi)hy," p. 191). I 

 have since loarnt that the same conclusiou had been arrived at independently by Mr, 

 C. U. Bottamley, of the Yorkshire College, Leeds. 



