the Electric Discharge in highly Rarefied Gaseous Media. 527 



same electric conductivity as aqueous vapour at 13 millims. 

 Nevertheless an electric discharge of the same intensity made in 

 the former medium 49 turns (with the ring positive) and 25 

 (ring negative) in 15 seconds, whereas in the latter it made only 

 31 and 17 in the same time. This result proves that the rate of 

 rotation does not depend solely upon the intensity of the electric 

 discharge, but also to a great extent upon the molecular consti- 

 tution of the gaseous medium. 



On the other hand, if without paying any attention to differ- 

 ences of conductivity, both jars are placed in succession in the 

 same circuit, so that the same discharge necessarily traverses both, 

 the rate of rotation is found to be the same in air and in aqueous 

 vapour when the pressure is the same. 



Air and Aqueous Vapour at Equal Pressures. 



Pressure. 



Number of turns in ] minute. 



Ring positive. 



Ring negative. 



millims. 

 10 

 13 



50 

 32 



20 

 17 



Each of these experiments was made three times, and gave the 

 same result every time. 



It is therefore equality of elastic force, and not of electrical re- 

 sistance, which determines the equality in the rate of rotation 

 in dry air and aqueous vapour of the same electrical discharge 

 under the influence of two electromagnets of the same strength. 

 Some trials made with alcohol-vapour did not give quite such 

 conclusive results. Thus, at a pressure of 10 millims., the rate of 

 rotation was 17 turns per minute in both media with the ring 

 negative, but with the ring positive the number of turns was 30 

 in air and 20 in the alcohol-vapour. At 6 millims. pressure and 

 with the ring negative, the number of turns was 24 in a minute 

 in both air and alcohol -vapour, and with the ring positive it was 

 30 in both media alike. It therefore seems that, as the pres- 

 sure diminishes, the rates of rotation approach equality. 



In the foregoing experiments, the two jars, exactly similar as 

 to their dimensions and construction, were placed one on each 

 polar surface of a powerful electromagnet, so that it was certain 

 that the intensity of the magnetism of the soft-iron rods in the 

 interior of each was really the same. Subsequently the soft-iron 

 rod in one of the jars was replaced by a rod of brass of like shape 

 and size. The jar thus modified was placed on one of the polar 

 surfaces of the electromagnet ; and rotation took place as before, 



