﻿Evolution of Gas from the Cathode in Helium, 481 



that the tableland formula is inapplicable, owing to displace- 

 ments in the density of the underlying strata; but this 

 would seem not to be the case with the great South African 

 tableland, as the formula 



^=980-62-2-6co3 2*-§. 3 A. 



gives ^ = 978*70 in agreement with the observations. 



Since there are only two really large tablelands in the 

 world, South Africa and Tibet, some geophysical interest 

 attaches to the results, and it is hoped they may be repeated 

 with better apparatus. 



Transvaal Universitv College, Johannesburg, 

 July 1906. 



LIU. Comparative Obsemations on the Evolution of Gas from 

 the Cathode in Helium and Argon. By Clarence A. 

 Skinner, Ph.D., Professor of Physics in the University of 

 Nebraska*. 



IN previous reports t of investigations on the subject of 

 evolution and absorption of gases by the electrodes in 

 vacuum-tubes, the writer has shown that during the first 

 few minutes after starting a glow-current through helium, 

 hydrogen is given off by a metallic cathode at a rate deter- 

 mined by Faraday's law lor electrolytes. Further, if a carbon 

 cathode be used in helium, nitrogen is evolved by it, within 

 certain time limits, according to the same law. But if a 

 current be passed between metallic electrodes in hydrogen, 

 or between carbon electrodes in nitrogen, the increase in gas 

 pressure in the discharge-tube (by which the evolution is 

 detected) is relatively small ; in fact, under certain condi- 

 tions it decreases with duration of current. The difference 

 between the observations with helium on the one hand, and 

 with hydrogen or nitrogen on the other, was traced to anode 

 absorption of the last two, that of helium being inappreciable. 

 Under the assumption that the rate of evolution of gas from 

 a given cathode is determined by the current alone, irrespec- 

 tive of the kind of gas in the discharge-tube, the experimental 

 results indicate that the anode absorbs hydrogen and nitro- 

 gen, when these are used as filling, at a rate also determined 

 by Faraday's law. 



To verify the foregoing assumption is important. This of 

 course can be done only in those gases which are not appre- 

 ciably absorbed under the action of the current. In addition 



* Communicated by the Author. 



f Phys. Rev. xxi. pp. 1 & 169 ; Physik. Zeitschr. 6 Jahrg. p. 610. 



Phil Mag. S. 6. Vol. 12. No. 71. Nov. 1906. 2 I 



