1901.] Effect of Light and Electric Radiation on Matter. 163 



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 obser- 

 vation described in my paper previously mentioned.* 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 super- 

 posed 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. ext. 



