1896 - 97 .] Prof. John Gibson on Photo-Chemical Action. 307 
It will be sufficient for the present to give the following equations, 
taken from standard works on photography. To all familiar with 
recent theories of solution, the increase of electrolytic conductivity 
consequent on the action of light will be at once apparent. 
2FeCl 3 + C 2 H g O = 2FeCl 2 + C 2 H 4 0 + 2HC1 
2 FeCl 3 + H 2 C 2 0 4 - 2FeCl 2 + 2C0 2 + 2HC1 
4HgCl 2 + 2H 2 0 = 4IIgCl + 0 2 + 4HC1 
2HgCl 2 + (NH 4 ) 2 C 2 0 4 = 2HgCl + 2C0 2 + 2NH 4 C1 (19). 
The examples cited so far seem sufficiently numerous and varied 
in character to justify the expectation that further investigation 
will prove the rule generally. This idea is further supported by a 
consideration of the means whereby we may increase or diminish 
sensitiveness to light in particular cases. 
Photographically speaking, sensitizers are essentially halogen 
