186 



Prof. J. C. Bose. 



[Apr. 18, 



manner, so that the important preliminary phase of "molecular nego- 

 tiation " may not be missed. I have in my two previous papers shown 

 how the above ideal requirements may be realised by taking advantage 

 of the conductivity or electromotive variation methods. 



These methods not only enable us to detect extremely minute 

 molecular changes produced by radiation, but also to follow the 

 changes moment after moment in a continuous manner. 



I have described in the two previous papers the various molecular 

 effects produced by light, electric radiation, and mechanical disturb- 

 ance under different conditions. The consideration of these will give 

 a clear insight into various obscure phenomena connected with photo- 

 graphic action, among which may be mentioned the following ; — 



1. Photo-chemical induction. 



2. Relapse of invisible image. 



3. Recurrent reversals. • ... 



4. The development of pressure marks. 



1. " Chemical " and "Physical" Theories of Photographic Action. 



It is an arbitrary distinction to call a phenomenon either physical or 

 chemical when it happens to be on the common borderland. I have 

 shown that when a substance is molecularly strained by light, its 

 chemical activity is modified in consequence of the physical strain. 

 The acted and the unacted portions will therefore be unequally attacked 

 by a developer. In the case of a compound, the strain produced by light 

 may cause a modification which renders it susceptible to decomposition by 

 the action of a reducing agent. The observed evolution of chlorine when 

 moist AgCl is 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 photographic action we have merely the 

 effect of a moderate stress producing the corresponding strain (with 

 concomitant variation of chemical activity), and not the disruptive 

 effect of a breaking stress. 



With reference to photographic action, various facts are known 

 which cannot be well explained from purely chemical considerations. 

 In connection with this the following experiment of Professor Dewar 

 is suggestive. It is found that at the low temperature of -180° C, 

 there is a cessation of all chemical action. Even such an extremely 

 active substance as K does not show any action when immersed in 

 liquid oxygen.* Now at these extremely low temperatures, where 

 the action of such an active substance as K is suspended, an Eastman 

 film was still found fairly sensitive to photographic influence. 



* Dewar. Friday Evening Discourse at Boyal Institution, June 26, 1891. 



