1896-97.] Prof. John Gibson on Photo -Chemical Action . 303 
On Photo-Chemical Action. By Professor John Gibson. 
(Read February 15,, 1897.) 
A chief difficulty in the study of photo-chemical action lies in its 
variety, for it gives rise not merely to changes of a chemically 
simple kind,, but also' to the innumerable complex changes of 
plant-life and photography. 
The difficulty is further increased by the extremely small pro- 
portion of material substance, chemically altered in a limited 
time by the action of those vibrations of the ether which we call 
light. To this, perhaps, more than to any other cause, must be 
attributed our continued inability to ascertain the true nature of 
photo-chemical changes so constantly observed as the decom- 
position of silver haloids by light. 
Repeated attempts have been made to connect particular chemi- 
cal effects, such as oxidation and reduction, with greater or smaller 
wave-length ; but all such attempts have ended in self-contra- 
diction (1, 2). 
Now, if there be any single characteristic associating all photo- 
chemical actions together, this characteristic must be present 
in the simplest as well as in the most complex cases, and the 
consideration of the former seems, therefore, most likely to lead to 
definite conclusions. 
Perhaps the most striking and the most simple case that presents 
itself is the increased electric conductivity imparted to crystalline 
selenium on exposure to light. 
Monkmann has shown that pure soluble sulphur also acquires 
increased conductivity when exposed to light, though to a less 
degree than selenium (3). 
Ultra-violet light (4), and also the Rontgen rays, impart electric 
conductivity to the air. 
In these cases the change is a temporary one. 
Ordinary phosphorus, under the action of light, is converted into 
red phosphorus — a substance possessing a markedly greater electric 
