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 proportion 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 decomposition of silver haloids by light.

Repeated attempts have been, made to connect particular chemical effects, such as oxidation and reduction, with greater or smaller wave-length; but all such attempts have ended in self-contradiction (1, 2).

Now, if there be any single characteristic associating all photochemical 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 Röntgen rays, impart electric conductivity to the air.

In these eases the change is a temporary one.