Ionization of Platinum by Cathode Rays. 295 



it did not so change, for at least twenty minutes necessarily 

 elapsed between the passing of the discharge and any readings, 

 the time being occupied by producing the necessary vacuum. 

 I think also that the low value of Z was maintained if W 

 was heated while the rays from it were not allowed to 

 reach P; but on this point the record of experiment is not 

 conclusive. 



10. The heating of P in a vacuum and in oxygen, with 

 occasional spluttering in the discharge in oxygen, was con- 

 tinued for a fortnight. The normal value of Z (that is the 

 value which could be quickly restored after Z had been 

 increased by heating W, and after Z had been decreased by 

 the discharge) fell still further. Ultimately a value 0'21 was 

 reached ; the corresponding curve is shown in fig. 2, C ; it 

 will be noticed that there is no evidence that W has increased ; 

 if the curve were drawn on a larger scale it would be seen 

 that W has not decreased notably. At this stage Z could no 

 longer be increased by heating "W ; the supply of gas (?) 

 from W was exhausted. The evolution of gas from P when 

 it was heated had decreased very greatly, but could still be 

 detected ; it is likely that much, if not all, of this gas came 

 not from P but from the brass cylinder surrounding it. 

 Z could be decreased further temporarily to about (J- 15, as 

 before, by passing a discharge with P as cathode ; but it was 

 found (the experiment had not been tried before) that the 

 discharge with P as anode produced exactly the same effect. 

 (There was a valve-tube in the discharge circuit, so that there 

 is not likely to have been any doubt as to the true direction 

 of the current.) 



At this point, when P had been heated in all for some 

 350 hours, the attempts to reduce Z still further were 

 abandoned. There is, of course, no evidence that the limit 

 had been reached, but progress had become so slow that it 

 did not appear desirable to pursue the matter further for the 

 time. Attempts were now made to restore the original value 

 of Z (about 1*4) obtained before the operations were begun. 

 It has been noted that restoration by heating \V had become 

 impossible. Practically no effect was produced by admitting 

 hydrogen and either heating P in it or allowing it to remain 

 cool. Even when P was left for three days in a mixture of 

 hydrogen, air, and the vapours of water and tap-grease 

 scareelv any increase of Z took place: Z remained below 0*3. 

 About 001 mm. of hydrogen was then admitted, AY heated 

 and the rays from it allowed to fall on P with a speed of some 

 80 volts ; no change was produced. No permanent change 

 was noted after P had been made the anode of a discharge in 



