Page:Collected Physical Papers.djvu/244

224 substances, e.g., K, Br, I, show a diminution. The effect of molecular conditions in determining the sign of response to electric radiation (i.e., increase or diminution of conductivity) is exhibited by the two molecular varieties of silver, the response being positive in one case and negative in the other.

The conductivity variation is thus the expression of some molecular change. In many cases (potassium, Ag, brominated lead, etc.) we find that the change persists only during the action of radiation. On the cessation of the stimulus the substance at once shows elastic self-recovery. It would thus appear that the conductivity variation is due to molecular strain produced by radiation. This view is further supported by the fact that in working with self-recovering receivers of various types, both positive and negative, I have not only found that for each intensity of radiation there is a corresponding conductivity variation, but that under the continued action of radiation, the conductivity variation attains a maximum value, which remains constant, balanced by a force of restitution; when the radiation is stopped, the substance is found to recover its original conductivity. With an increase of intensity of radiation, the corresponding conductivity variation also increases, and vice versa. I have, in a Paper "On the Continuity of Effect of Light and Electric Radiation on Matter" given an account of self-recovering receiver, of negative type, made of silver particles, in which the conductivity variation is almost exactly proportional to the intensity of radiation. Self-recovery, in general, is merely a question of time; with certain substances it takes place immediately, with others, after a short or after a comparatively