TROWHRIIXIE. — RONTGEN RAYS. 265 



than one of two inches in length. In popular language, it can be main- 

 tained that a discharge of lightning a mile long encounters no more 

 resistance than one of a foot in length. Ohm's law does not hold for 

 electrical discharges in air and rarefied gases. It is well known that a 

 voltaic arc can be started in a vacuum. My experiments lead me to 

 believe that in every case the arc is started by a spark which breaks down 

 the medium, and the arc follows. I am led to believe that electrical 

 oscillations are of the nature of voltaic arcs, and that the discharges in 

 Crookes tubes are voltaic arcs. I am thus forced to the conclusion that 

 under high electrical stress the ether breaks down and becomes a good 

 conductor. 



Jeffeuso.v Physical LAnoRAxoRT, 

 Harvard University. 



