46 



SCIENCE. 



[N. S. Vol. XII. No. 289. 



elapsed after the discovery of the Lenard 

 rays before this modification of the Crookes 

 theory was proposed. 



In 1S95, about a year after the publication 

 of Lenard's results, came the discovery of 

 the X-rays by Eontgen. The widespread 

 interest which this discovery aroused is fresh 

 in the minds of all of us, and is probably 

 without a parallel in the whole history 

 of physics. Apart from their importance 

 from a purely scientific standpoint, and 

 from their sensational features, the X-rays 

 occupy a unique position among the phe- 

 nomena connected with the electrical dis- 

 charge in vacuum tubes ; for they aflPord the 

 first instance in which the scientific results 

 obtained in this branch of physics have 

 been made directly useful in everyday life. 

 Although it is not the purpose of the pure 

 scientist to seek directly such applications, 

 yet every instance of this kind is always a 

 source of gratification. Each new case 

 serves to strengthen that belief which forms 

 the real basis of scientific investigation ; the 

 belief that every advance in our knowledge 

 of natural law, be it ever so small, or ever 

 so removed in appearance from the affairs 

 of everyday life, must ultimately contribute 

 to the increase of human happiness and the 

 progress of mankind. 



The discovery of the X-rays served to 

 stimulate investigation along all related 

 lines. Interest in the phenomena of the 

 electrical discharge through gases, and espe- 

 cially in the kathode rays, became stronger 

 than ever before ; for it was natural to ex- 

 pect that the puzzling problem of determin- 

 ing the nature of the Eontgen rays might 

 be simplified by a better understanding of 

 the kathode rays, that produced them. 



The numerous difficulties and apparent 

 contradictions which had stood in the way 

 of the adoption of the Crookes theory have 

 already been referred to. These may be 

 said to have culminated with the discovery 

 of the Lenard rays, and the theory in its 



earlier form was of necessity abandoned. 

 But since that time the difiBculties have been 

 one by one removed. Thus, in 1896, it was 

 shown by Perrin* that the kathode rays 

 really do carry a negative charge ; this 

 conclusion was confirmed by J. J. Thom- 

 sonf in 1897. That a negative charge is 

 also carried by the Lenard rays was after- 

 wards shown by McClelland, | Wien,§ and 

 Lenard. 1 1 By passing the rays through an 

 aluminium window in a completely closed 

 metal box, Lenard was able to give a nega- 

 tive charge to an insulated conductor within. 

 Certainly a more conclusive proof that the 

 kathode rays are electrified can hardly be 

 demanded. 



The deflection of the kathode rays in 

 passing through an electrostatic field, which 

 the Crookes theory required, and which 

 Hertz had looked for in vain, was proved 

 to exist by Jaumann^ in 1896, and much 

 more conclusively by J. J. Thomson** in 

 1897 . A year later it was shown by Wien f f 

 and Lenard J J that a similar electrostatic 

 deflection occurred in the case of the Lenard 

 rays. 



Not only were the earlier experiments 

 shown to be in error in both these cases, 

 but the reasons for their failure are now 

 pretty well understood. Probably the most 

 important sources of error were due to the 

 fact that the residual gas in a vacuum tube 

 is rendered conducting by the discharge. 

 The kathode rays also exert a special ion- 

 izing influence of their own, so that in 

 those parts of the tube which are traversed 

 by these rays, the gas becomes temporarily 

 a good conductor. In consequence it acts 



* Perrin, Nature, 53, p. 298, 1896. 



t Thomson, Phil. 3Iag., 44, p. 293, 1897. 



; McClelland, Land. Elect, 39, p, 74, 1897. 



gWien, Wied. Ann., 65, p, 440, 1898. 



II Lenard, Wied. Ann., 64, p. 279, 1898. 



II Jaumann, Wiener Berichte, 105, 2a, p. 291, 1896. 



** Thomson, Phil. Mag., 44, p. 293, 1897. 



tt Wien, Wied. Ann., 65, p. 440, 1898. 



H Lenard, Wied. Ann., 64, p. 279, 1898. 



