PRODUCED BY HOT PLATINUM IN DIFFERENT OASES. 



49 



The rate of evolution of hydrogen from the wire decreased considerably during the 

 above experiment. During the first half hour the pressure in the apparatus rose by 

 0-0033 millim., and during the last half hour by 0*00016 millim. The volume of the 

 apparatus was about 2000 cub. centims., that of the wire being - 0033 cub. ceutim. 

 The amount of hydrogen left in the wire would probably be comparable with that 

 given out in the last half hour, so that the pressure of the hydrogen inside the 

 wire would, according to the above numbers, still be considerable. The current 

 21 x 10" l3 ampere at the end of the experiment might be due to the residual gas left 

 in the wire, so that these experiments are not contrary to the view that the positive 

 leak in hydrogen, such as it is, is due to absorbed gas. 



The effect of letting in hydrogen to a pressure of 38 millims. was now tried, and 

 the leak at 900 C. measured at various times, with the results shown. 



The big leak obtained on letting in the fresh gas is a somewhat surprising result, 

 but might possibly be due to impurities which are gradually destroyed or removed. 

 The point which seems most important is the small value, 5'2x 10~ 13 ampere, of the 

 steady leak at this pressure. This was only two and a-half times the value of the 

 positive leak obtained after the wire had been heated for 3 hours in a good vacuum. 

 The value of the positive leak in oxygen at this pressure and temperature would have 

 been about 10~"' ampere, or nearly twenty times the above number. 



Owing to the smallness of the positive leak in hydrogen, together with other 

 difficulties which arose, few other satisfactory measurements were made on it. 



Measurements of the variation with pressure of the negative ionisation from hot 

 platinum in hydrogen have l>een made by H. A. WILSON.* WILSON'S method 

 consisted in measuring the leak when hydrogen at successively increasing pressumi 



VOL. ccvn. A. 



* ' Phil. Trans.,' A, vol. 202, p. 243. 

 H 



