THE BECQUEREL RAYS 139 



X-rays discharge electrified bodies equally well, whether 

 they be charged positively or negatively. - 



There is accordingly a certain degree of probability in 

 favour of the view that cathode rays are due to molecular 

 or ionic bombardment ; but they are generally mixed with 

 X-rays, which are apparently independent of matter for 

 their propagation, and are therefore to be considered as 

 due to disturbances of the ether. Ultra-violet rays, on 

 the other hand, must be ethereal waves of very short 

 wave-length ; but they have the power of splitting gaseous 

 molecules into charged atoms or groups of atoms, termed 

 ions. It may be calculated, too, that the atoms conveying 

 cathode-rays have a velocity of 124 miles a second; it 

 would follow that of such atoms a single gram, or about 

 one- thirtieth of an ounce, must have the same energy as 

 a locomotive of 80 tons weight rushing at the rate of 

 50 miles an hour ! No wonder, .then, that they penetrate 

 thin sheets of metal and embed themselves in glass. 



In 1896 M. Poincare, the well-known mathematician, 

 suggested that all fluorescent substances might emit 

 Rontgen rays; being guided to this guess by the hypo- 

 thesis that it is the glass, against which the Rontgen rays 

 strike, which phosphoresces and emits the rays. This 

 suggestion was almost at once verified by M. Charles 

 Henry, when he discovered that sulphide of zinc, a sub- 

 stance which shows marked phosphorescence, greatly 

 increases the effect of X-rays when placed in their path. 

 M. Henri Becquerel, too, in the same year, found that rays 

 were emitted from a compound of the metal uranium, 

 which affected a photographic plate wrapped in black 

 paper, sufficient to exclude rays of direct sunlight. This 

 power to affect a sensitised plate persists long after all 

 visible phosphorescence ceases. Moreover, it is unneces- 

 sary to expose uranium salts to light before they are 

 capable of producing a photographic image, for these 



