Experiments with Rontgen 3 s RayS. 461 



sensitiveness whether the electrodes were of platinum or of 

 aluminium. The cells were made according to the directions 

 given by Mr. Shelford Bidwell * in his paper read before the 

 (Society last year — the only difference being that purified 

 selenium was employed. 



The result of a good many observations was to show that 

 a certain selenium cell with platinum electrodes was acted on 

 to about the same extent and in the same direction whether it 

 was exposed to the radiation of a u standard " candle at a 

 distance of three metres, or to the Rontgen radiation at 

 a distance of ten centimetres and passing through* 7 mm. of 

 aluminium and about 3 mm. of wood. The tube was 

 working so as to cause visible fluorescence in a barium platino- 

 cyanide screen — not of quite the best quality — at a distance 

 of rather over a metre in a room nearly but not absolutely 

 dark. This is of course a very rough way of stating the 

 degree of activity of the tube, but when the experiments were 

 made Professor Minchin's work had not reached us, and 

 consequently the simple scale of tube intensities which it 

 implies was not available. 



In order to test whether the action of the light differed in 

 kind from that of the Rontgen radiation two experiments 

 were made — in one the rate of resistance-recovery of the 

 selenium cell was carefully studied and compared with the 

 rate of recovery of the cell after exposure to candle-light ; 

 in the other tests were made in the hope of discovering that a 

 permanent electromotive force was established by the radia- 

 tion, and that it persisted after the radiation was cut off. 

 Neither of these experiments led to positive results. The rate 

 of change of resistance during the twenty seconds of exposure 

 to Rontgen's radiation was, so far as could be seen, exactly the 

 same as when the candle-flame was substituted at the proper 

 distance. The recovery curves, extending over about half an 

 hour, were also very similar on the whole (several tests were 

 made), though both curves themselves exhibited great 

 irregularities. 



In order to obtain effects as little complicated as possible 

 by previous history the sensitive cell was kept in circuit on 

 the bridge, and was traversed by the testing current for two 

 or three days before the observations. 



It was for the same reason that exposures were limited to 

 20 seconds, for the rate of recovery of resistance with the 

 cell employed was very slow. On one occasion when an 

 accidental exposure for several minutes to Rontgen's radiation 



* Phil Mag. vui. xi. p. "o3. 



