Page:Collected Physical Papers.djvu/235

Rh electromotive variation curve for nickel (fig. 49), which was subjected for a long time to continuous mechanical vibration.

The first part of the curve, or the latent period, is very suggestive as regards the obscure phenomenon of photo-electric induction. Thus "Quantitative measurements have shown that the action of light is not instantaneous. On the contrary, it gradually develops, and requires a considerable time before it attains its full strength. When a mixture of chlorine and hydrogen, which has been kept in the dark, is exposed to the light, there is either no hydrochloric acid or only a very small quantity formed in the first moment; but the rate of formation increases so that the quantity formed in a given time, e.g., a minute, continues to increase until it attains a maximum value. Bunsen terms the gradual increase in the action induction. if the gaseous mixture has been once exposed to the light, it will retain in the dark, for about half an hour, its capacity for forming HCl in the light. If the gas has remained in the dark for a short period and is again brought into the light, it requires a very short period of induction; but the period of induction will be lengthened by keeping the mixture in the dark for a long time. [This is evidently due to self-recovery. J. C. B.]. Exposure to the light renders the