Conductivity of Gases. 311 



to do with a synthesis of carbon and hydrogen to C 2 H 2 , or 

 whether acetylene is not also formed in a vacuum, and con- 

 sequently we have to imagine a partial decomposition of the 

 hydrocarbons present in the carbons, appears still uncertain. 

 " § 4. The Disintegration of the Electrodes. — Under the usual 

 conditions of research the process of disintegration produced 

 by the glow-discharge is confined to the kathode ; as far as 

 this extends the glass wall of the vacuum tube is covered with 

 an extraordinarily thin reflecting deposit, which forms very 

 quickly when thin platinum wires are used. The surface of 

 the electrode after some use is found to be eaten away into 

 fine points and hairs, as is seen most distinctly with the 

 difficultly volatile aluminium. 



But when Hittorf sent the current of his battery without 

 interposed resistances through a vacuum-tube filled with 

 hydrogen or nitrogen at about 50 millim. pressure, both 

 iridium electrodes became white-hot, and even began to melt ; 

 at the same time the glow-light disappeared from the kathode, 

 and with it the metallic deposit. Hence it appears to me 

 that we must ascribe the metallic mirror formed in normal 

 cases, not to a superficial volatilization produced by the high 

 temperature of the glow-light, but to some peculiar action of 

 the glow-light. An observation of Dewar's would seem to 

 show the same thing, that a metallic deposit formed with 

 magnesium electrodes disappeared again some time after the 

 interruption of the current ; possibly the gaseous molecules, 

 whilst conveying the glow-discharge, possess greater affinities, 

 and form compounds with the metal of which the kathode 

 consists, which under favourable conditions are again gradually 

 absorbed by the electrode. Warren De La Rue and H. Miiller 

 obtained a similar result with palladium electrodes in hydrogen- 

 tubes. Under the experimental conditions, however, where 

 both electrodes are heated to intense white heat and even to 

 the melting-point, both emit metallic vapours, as occurs in 

 a greater degree in the arc-light between metallic electrodes. 



That with this latter mode of discharge the volatilization 

 of the electrodes takes place as well in air as in other cases 

 and in vacuo has long been known. The high temperature 

 of the electrodes is the chief cause of their wasting away, 

 and the loss is only secondarily due to combustion. In air, 

 in consequence of this higher temperature the anode is used up 

 more rapidly than the kathode ; and, further, the electrodes 

 become disintegrated so much the more rapidly the more 

 volatile they are. 



The process is somewhat more complicated with the carbon 

 light. With very powerful currents (400 to 500 Bunsen 



