Conductivity of the ^Either. 379 



Theory of Light, a vacuum cannot be a conductor or it 

 would be opaque, and we should not receive any light from 

 the sun or stars." 



The experiments which have been made hitherto on this 

 subject have been conducted with comparatively feeble 

 electrostatic forces. By means of a storage- battery of 10,000 

 cells in connexion with a Plante rheostatic machine* I have 

 studied the resistance of highly rarefied media under dis- 

 ruptive discharges, and I am led to the conclusion that with 

 a sufficiently powerful electrical stress, what we term a 

 vacuum can be broken down, and that the disruptive discharge 

 during its oscillations encounters very little resistance. In 

 the case of a highly exhausted Crookes tube I have measured 

 this resistance and find it in the special case I considered less 

 than three ohms. 



My experiments lead me to the conclusion that the chief 

 resistance is encountered at the surface of the electrodes, 

 and that when this is overcome the aether offers little resist- 

 ance. The method I have employed seems to me to be 

 a very useful one for the study of electrical discharges. It 

 may be termed the damping of the additional Spark Method, 

 or the comparison of resistances by the estimation of the 

 damping of electrical oscillations f. The electrical circuit is 

 provided with two spark-gaps. One of these is placed in 

 a gas, or under the conditions which are to be examined, 

 while the other is photographed according to Feddersen's 

 method by a revolving mirror. With cadmium terminals 

 this method enables one to estimate the resistance of a spark 

 in air or in rarefied media to one half an ohm. 



Having at my command a battery giving a voltage of 

 twenty thousand, with an internal resistance of only one 

 quarter of an ohm per cell, and capable therefore of giving a 

 very powerful current, I first studied the behaviour of 

 Crookes tubes which were connected to the terminals of this 

 battery. I found that no Rontgen rays could be obtained 

 with a voltage of twenty thousand. On heating the Crookes 

 tubes, they were filled with a pale white light, which showed 

 very faint bands in the green when examined by the spectro- 

 scope. Then the entire strength of the battery appeared to 

 be manifested in the tubes, the electrodes became red-hot — 

 the medium broke down and offered no resistance to the 

 current of the battery. This white discharge showed even 

 at its culminating point no Rontgen rays. I then employed 



* Comptes. Rend. t. lxxxv. p. 794, Oct. 1877. 



t " Damping of Electrical Oscillations on Iron Wires " (Phil. Ma** 

 Dec. 1891). 



