Page:Collected Physical Papers.djvu/230

210 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 satisfactorily explained by 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 action.

In the above case, it is difficult to see how light could have produced any chemical action upon the relatively inactive silver salt. It is more likely that the effect produced was of the nature of some physical strain. That light does produce molecular strain even at such low temperatures—a strain which may remain latent—is shown from Professor Dewar's experiments on phosphorescence. Ammon. Pt. Cyanide cooled to