July 13, 1900.] 



SCIENCE. 



43 



tides, to react upon the field and exert a 

 force tending to move the magnet to which 

 the field is due ; no such reaction could be 

 detected.* Many other instances might be 

 cited in which the results of observation 

 were apparently in direct contradiction 

 with the Crookes theory. 



Such, in brief, was the condition of the 

 subject at the beginning of the present de- 

 cade. Of the two theories that had been 

 proposed, each possessed strong arguments 

 in its favor, l^either was free from serious 

 objection. 



Previous to this time, very little work of 

 a quantitative nature had been done in 

 connection with the kathode rays, although 

 several estimates had been made of their 

 velocity. Thus, according to Spottiswood 

 and Moultonf the velocity was considerably 

 less than that of light ; whole GoldsteinJ 

 had reached the conclusion that the ve- 

 locity was greater than one four hundredth 

 of the velocity of light. In 1894 a direct 

 determination of the velocity was made by 

 J. J. Thomson§, the method being to ob- 

 serve two fluorescent spots, produced by 

 the kathode rays at different distances from 

 the kathode, by means of a revolving mir- 

 ror. The result obtained was 2x10' cm. 

 per second, or about one thousand times 

 less than the velocity of light. This ve- 

 locity is practically the same as that which 

 would be acquired by a hydrogen ion re- 

 pelled from the kathode. Thomson's re- 

 sult therefore supported the view, pre- 

 viously expressed by Schuster, that the 

 kathode rays were not composed of par- 

 ticles of metal torn loose from the electrode, 

 or of charged molecules of the residual gas, 

 but that they consisted of a stream of ions 

 such as occur in ordinary electrolysis. 



Recent determinations of the velocity of 



* Hertz, 1. c. 



t Phil. Trans., 171, p. 627, 1880. 



X Goldstein, Wied. Ann., 12, p. 101, 1880. 



I Thomson, PTiil. 3Iag., 38, p. 358, 1894. 



the kathode rays have shown that the value 

 obtained by Thomson was too small, so 

 that the conclusions based upon it were in- 

 correct. N'evertheless, I am inclined to 

 think that they served a useful purpose. 

 For by directing attention to the discredited 

 emission theory, and to the probable elec- 

 trolytic nature of gaseous conduction, they 

 stimulated investigation and contributed to 

 the advance of the subject. 



The more modern phase of our subject 

 properly begins in 1892, when it was dis- 

 covered by Hertz* that the kathode rays 

 were able to penetrate thin sheets of gold 

 foil, aluminium, and glass. Taking advan- 

 tage of this discovery, Lenard in 1893t 

 constructed a vacuum tube containing a 

 small opening covered with aluminium foil, 

 through which the rays passed out into the 

 open air, or into a second tube. It was 

 thus possible to study the rays under con- 

 ditions which could be readily varied, while 

 the conditions under which the rays were 

 developed remained unaltered. This form 

 of apparatus not only made possible a more 

 systematic study of the known properties 

 of the kathode rays, but also led to the dis- 

 covery of many new phenomena. Thus, 

 in air at ordinary pressures, the rays were 

 found to discharge electrified bodies, to de- 

 velop ozone, and to give an impression upon 

 a photographic plate. The photographs 

 published by Lenard, showing the opacity 

 of glass and quartz to these rays, and the 

 comparative transparency of the metals, are 

 strikingly similar to those since obtained 

 with the X-rays. In fact, it now seems 

 probable that X-rays were present to some 

 extent in all Lenard's experiments, and 

 that the phenomena observed by him were 

 in part caused hj them. 



One of the first questions investigated by 

 Lenard was the influence of the medium 

 through which the rays passed upon their 



* Hertz, Wied. Ann., 45, p. 28, 1892. 

 t Lenard, Wied. Ann., 51, p. 225, 1894. 



