Resistance of Vacuum. 7 



it was impossible for the current of the most powerful Ruhm- 

 korff induction-apparatus to leap from one of the electrodes 

 to the other. Instead of this, the current passed from one of 

 the platinum wires to the other along the outside of the tube. 

 We shall, further on, return to Hittorf 's very instructive in- 

 vestigation. The experiments we have just mentioned show 

 that, if the air of a glass tube is rarefied beyond a certain 

 limit, it is impossible for the strongest current to pass through 

 it ; but the experiments of Grassiot and Pliicker prove more- 

 over that, even when the limit of rarefaction is reached, it is 

 possible by induction to excite a current in the rarefied gas. 

 From that time there does not appear to have been any reason 

 for maintaining that it is the lack of conductivity in the rare- 

 fied gas that prevents the transmission of the direct current of 

 a discharge, that lack not bringing any obstacle to the deve- 

 lopment of the assuredly weaker influence-currents. The 

 cause of the dissimilarity of the two currents can hardly be 

 sought elsewhere than in the electrodes, which in some way 

 hinder the passage of the current from them to the rarefied 

 gas or vice versa. 



The influence-current has not such an obstacle to surmount, 

 seeing that it is formed in the gaseous mass itself and has no 

 need to pass, at any point whatever, from a solid conductor to 

 the gas. This deduction is moreover in accordance with the 

 experiment made by Gaugain with the tinfoil ; and we shall 

 have, in the sequel, repeated opportunities of confirming it. 

 Thus the experiments which we have mentioned do not appear 

 to constitute a satisfactory proof of the assumption that highly 

 rarefied gas or an absolute vacuum is by itself a nonconductor 

 of electricity. 



§ 3. 



"We now pass to the researches respecting the electrical 

 resistance of gases at different degrees of density. 



According to the researches of M. B. Becquerel, gases under 

 the ordinary pressure of the atmosphere commence to become 

 conductors if they be heated to redness*. In his experiments he 

 employed an ordinary battery to produce the current. Among 

 the observations which he was able to make in this respect we 

 shall cite two which are of special interest for our subject. He 

 observed that, if the surfaces of the electrodes between which 

 the current passed were of different sizes, the apparent resist- 

 ance of the gas was greater when the current passed from the 

 larger to the smaller than if it circulated in the opposite direc- 



* Comptes Rendus, xxxvii. p. 22 ; TraiU du magnetisme et de V electricity 

 Paris, 1855. 



