DISCHARGE OF ELECTRICITY FROM HOT PLATINUM. 
263 
hydrogen. It is well known that it is almost impossiljle to entirely get rid of the 
last traces of hydrogen from a vacuum tube. 
This view is confirmed by the effect of nitric acid on the negative leak, for nitric 
acid oxidises hydrogen. It is also in agreement with the fact that tlie negative leak 
in air at low pressures falls off during long-continued lieating of the wire. The rate 
at which the leak falls off diminishes as the leak gets smaller, so that with wii-es well 
cleaned with nitric acid the rate of fall is not very great. 
Platinum wires not specially cleaned evolve gas when heated in a vacuum, and 
since hydrogen is the only gas which platinum is known to occlude in any quantity, 
this gas is no doubt hydrogen. It is practically impossible to get rid of this evolu¬ 
tion of gas merely by long continued heating and pumping out. Thus Wien {loc. cit.) 
states that his wires continued to evolve gas after weeks of heating in an extremel}/ 
good vacuum. My wires, which had been treated with nitric acid and heated in air 
which was repeatedly changed, did not evolve anything like as much gas as those 
not so treated. 
(4.) The Leak in Hydrogen. 
In the experiments in hydrogen at low pressures the hydrogen was obtained 
sufficiently pure by allowing it to dilfiuse through platinum. A spiral of fine platinum 
tubing, 0'5 millim. external and 0'25 millim. internal diameter, was sealed on the 
apparatus at one end and the other end fused up. On heating this spiral in the 
flame of a spirit lamp, the hydrogen present in the flame diftused through into the 
apparatus. In this way the pressure could he increased several tenths of a millimetre 
in one minute when the total capacity of the apparatus was about 300 cub. centims. 
The spectrum of a discharge in hydrogen let in in this way was examined, and 
appeared to consist almost entirely of the hydrogen lines. It was thonglit that faint 
traces of the CO spectrum could sometimes he seen, hut it was not certain that any 
CO entered through the platinum spiral. For experiments at pressures above 
10 millims. the hydrogen was olffained from pure zinc and liydrochloric acid, and 
passed into the apparatus through tubes containing soda-lime. 
In some experiments the hydrogen was further purified by means of sodium let 
into the apparatus by electrolysis through hot glass, in the manner described by 
E. Warburg.'^ This method was found to work very well, and a deposit of metallic 
sodium could easily be obtained. The sodium, of course, absorbed some hydrogen as 
well as the oxygen and other impurities possibly present. 
The apparatus for this experiment is shown in fig. 6. A is the tulie containing the 
hot wire, B a tiilie tightly packed witli gold leaf, and C tlie sodium tube. Ibis lattei 
consisted of a thin glass tube aliout 15 centims. long and 1'5 centim. in diainetei, 
with an aluminium wire electrode along its axis, as shown. It was immersed in 
^ ‘ Wied. Ann.,’ vol. 40, p.. 1 (1890). 
