PHYSICAL RESEARCHES CONTINUED 83 



communications by J. Chunder Bose published in the 

 " Proceedings of the Royal Society/' 1902. It is supposed 

 on good grounds that " the effect of radiation is to produce 

 a state of molecular strain." Experimental evidence is 

 adduced which shows that the molecular strain caused 



FIG. 2. Photography without light. 



by the action of light, changes the physico-chemical 

 properties of substances, so that it becomes possible to 

 develop a latent image through differences in chemical 

 stability as, for instance, by reducing agents.' Dr. Hartley's 

 own experiments lent strong support to this, for the spectra 

 obtained by him showed ' that the solutions of metallic 

 nitrates are in a state of molecular strain.' * 



1 Journal of the Chemical Society, 1903. 



