136 ESSAYS BIOGRAPHICAL AND CHEMICAL 



travellers. An important argument in favour of this 

 contention is, that the heat developed in such tubes is 

 proportional to the intensity of the current, and not to the 

 square of the intensity, as would be the case were the 

 passage of electricity one of ordinary conduction. Thomson 

 attributes the heat to the recombination of the ions to 

 molecules, after discharge ; and the number of ions would 

 obviously be proportional to the intensity of the current 

 and not to its square. 



Goldstein and Crookes both thought that ordinary 

 matter, such as glass or metal, was opaque to such 

 cathode-discharges ; but Lenard, following a suggestion of 

 Hertz's, carried out at Bonn a beautiful series of experi- 

 ments, which showed that the cathode rays could pass 

 through a thin piece of aluminium foil, and be prolonged 

 outside of the exhausted tube. Not only could they pass 

 through ordinary air, although not to such a distance as 

 through rarefied gases, but they also passed through a 

 vacuum as perfect as could be produced by a mercury- 

 pump, aided by intense cold to condense mercury- vapour 

 out of the empty space. It appeared that the absorbing 

 power of different gases is proportional to their specific 

 mass. 



The velocity of propagation of such cathode rays has 

 been measured by an ingenious process by Professor J. J. 

 Thomson. It is found to be approximately 200 kilo- 

 metres, or about 124 miles a second. This, however, is 

 enormously less than the velocity of light or of electric 

 waves through the ether, which approximates to 180,000 

 miles a second. The accidental discovery by Professor 

 Eontgen in 1896 that some flakes of platinocyanide of 

 barium, placed near a Hittorf tube which was wrapped up 

 in black paper, emitted a phosphorescent light, led to a 

 great development of the subject. It was soon discovered 

 that even behind a book of 1000 pages, or a plate of 



