will also be found characteristic of other receivers responding to the stimulus of light or of mechanical vibration.
7. Physical Nature of the Change.
From the fact that the conductivity variation above described takes place in platinum and other noble metals, and the further fact that the action goes on even when the substance is kept immersed in a protecting medium such as naphtha, it would appear that the observed effect is primarily due to physical strain. By chemical action we generally understand an irreversible operation. But in the cases of complete self-recovery exhibited by various substances, we have an automatic return to the original condition. It should, however, be borne in mind that, as a result of the strain, the chemical activity of the substance may be changed, and if the strained substance happen to be immersed in a medium for which under strain it has a relatively stronger attraction, there may then be a chemical action. But this would be merely a secondary effect.
8. Electromotive Variation produced by Electric Radiation.
I said that if radiation causes molecular strain, there might be produced a difference of potential between the acted and unacted portions of a substance. A voltaic cell could be made with two plates of similar substance; there would then be no P.D. between the two. But on exposing one to the effect of radiation, a difference of potential may be established between the acted and unacted plates. The differential effect, if it exists, could then be detected by a galvanometer
