luminescence.—Another remarkable phenomenon will be noticed in the recovery curve. It will be seen that the complete recovery is effected after a series of minor oscillations. In other words, there seems to be an after-vibration which persists for a time in substances subjected to radiation. This is very suggestive with reference to a not altogether different after-effect of light in the fluorescent and phosphorescent bodies. In the case of Ag′ receiver, owing to its molecular mobility, the recovery is automatic. But in the case of so-called non-recovering substances, the strain persists for a considerable time, but the recovery may be hastened by removing molecular friction through gentle heating. In connection with this, I will quote an interesting observation described in my paper previously mentioned.[1] In an iron receiver strained by radiation there was quick recovery after heating, but "on careful inspection a slight oscillatory movement of the galvanometer spot was noticed during the process." Here the strain produced by radiation which remained latent was released by heat. In connection with this, one cannot help being reminded of the phenomena of thermo-luminescence, where the strain effect of light remains latent till set free by the application of heat.
Effect of Continuous Radiation.—Still more interesting are the superposed effects of a series of flashes of radiation. The first flash produces a certain molecular distortion, attended with conductivity variation, from which it will tend to recover. I may anticipate certain results which will be described later, in saying that the force of restitution increases with increasing distortion. Now if, before the substance has recovered from the first shock, a second flash be superposed, it will produce further distortion; but the effect will not be quite so strong, inasmuch as the force of restitution is increasing. Thus a series of superposed flashes will produce a limiting effect, which is kept balanced by the force of restitution. If the intensity of radiation is increased, the balancing position will be different.
It also appears from the result of other experiments that the after-effect persists for a little longer time when the stimulus is stronger. I shall show that this is the case when interpreting the curves of effect.
In fig. 5 are shown the effects of rapidly succeeding flashes of radiation caused by the spring vibrator of a Ruhmkorff's coil. In (a) the radiator was kept at a distance of 40 cm., and the radiation was continued for 15 seconds, after which 15 seconds was allowed for recovery. A longer time would have allowed a more complete recovery, but this would have entailed a great loss of time in the long series of experiments contemplated. The recovery is thus seen to be partial, the return curves not exactly reaching the original starting position. It will be seen from the effect at 40 cm. that even
- ↑ Loc. cit.
