634 Miss J. M. W. Slater on the 



This conclusion is strengthened by the fact that when the 

 surface made active was mica instead of aluminium, the 

 results obtained were very similar. 



The other possible agents which remain to be considered 

 are (a) low pressure, (b) strong local heating caused by the 

 incidence of cathode rays, (c) ultra-violet light, (d) chemical 

 action of the gases in the tube on the surface made active^ 

 (e) chemical action on the active deposit itself, (/) action of 

 free ions formed during the discharge, {g) a secondary radiation 

 of the nature of soft Rontgen rays. 



(a) Low pressure alone, without the passage of a discharge, 

 has been found by previous investigators to have no effect 

 on the excited activity, and this was confirmed with the present 

 apparatus. 



(b) The heating effect may also be safely excluded, for the 

 temperature o£ volatilization o£ thorium-excited activity has 

 been found to be over 600° C, while the temperature in the 

 present case was probably always under 100°, and usually 

 the tube only became slightly warm. The surface of the 

 aluminium remained bright, and showed no sign of any 

 approach to the melting-point. 



(c) The effect of ultra-violet light was tested by placing 

 an active disk about 1 cm. from aluminium points, between 

 which a discharge from a coil and leyden-jar passed, giving 

 a strong blue light rich in ultra-violet rays. A quartz plate 

 was placed in front of the disk, to prevent any spurious effect 

 due to sputtering from the points, and the disk was con- 

 nected to the negative pole of a battery giving a potential of 

 — 280 volts. No loss of activity occurred after an hour, and 

 the experiment was therefore repeated with the disk in air 

 at a low pressure. This was obtained by placing it in a tube 

 with a quartz window, which was made air-tight and exhausted 

 by means of a water-pump to a pressure of about 1-J cm. 

 The disk was connected to a potential of —500 volts, the 

 positive pole o£ the battery and a metal ring in front of the 

 disk being earthed. Exposure to the discharge still caused 

 no change of activity, and it may therefore be concluded 

 that the ultra-violet light present during the cathode-ray 

 discharge is without effect. 



(d) The mica disks mentioned above were used primarily 

 in order to eliminate any chemical or other effect due to the 

 nature of the active surface. The activity lost was just about 

 the same as when aluminium disks were used, varying from 

 30 to 60 per cent.; and here again the effect was quite as 

 large when the inactive side was exposed to the rays. 



(<?) A direct action of the gas on the active deposit itself, 



