﻿from the Cathode in Helium and Argon. 483 



aluminium and magnesium give off hydrogen much more 

 freely than the denser and more electropositive metals, these 

 were chosen for making the comparative study of the two 

 gases. 



Observations with Aluminium Cathodes. 



Six similar cathodes of aluminium (circular disks, dis- 

 charge area, 1"8 sq.cm.) were prepared and mounted, as 

 already described. These were then successively tested 

 during an uninterrupted series of experiments. AVith a fresh 

 sample each time as cathode — an aluminium wire serving 

 throughout as anode — a definite current was passed through 

 the discharge-tube and the gas pressure observed at given 

 time intervals. The test of one being completed, the system 

 was thereupon immediately evacuated, filled with fresh gas, 

 and the next tested. In this manner three samples were 

 tested in helium under currents of 1*0, 2*0, and 3*0 milli- 

 amperes respectively : and three others in argon. 



The results of these tests are incorporated in Tables I., II., 

 and III. ; the observed values being recorded in the first, 

 second, and fourth columns : while in the third and fifth is 

 given the pressure of the evolved gas ; and in the sixth the 

 calculated value of the latter in case Faraday's law controls 

 rhe rate of evolution from the start. At the top of each of 

 the four medial columns is indicated the possible error of 

 observation, though the probable error was not more than 

 half as great. 



For the sake of clearness these results are also represented 

 graphically in figs. 1,2, and 3 ; the pressure of the evolved 



Fig. 1. 



(7.2- 



.<*/ 



u 



3 



$ to 15 



Duration of Current (Minutes) 



gas by the ordinates and the corresponding duration of current 

 by the abscissas. The values obtained in helium are indi- 

 cated by circles, those in argon by crosses. The line F in 

 each figure represents the value required by Faraday's law. 

 Referring to fig. 1, we find for a current of one milliampere 



212 



