L. Page — A Century's Progress in Physics. 319 



first two are certainly to be attributed largely to the 

 great advances which had been made in obtaining high 

 vacua, and the last two might not have occurred so soon 

 had it not been for the photographic plate. 



The Electron. — The atomic theory of electricity dates 

 from the time of Faraday. His experiments on electroly- 

 sis showed that each monovalent atom or radical, what- 

 ever its nature, carries the same charge, each bivalent ion 

 a charge twice as great. Only a lack of knowledge of the 

 number of atoms in a gram of the dissociated salt pre- 

 vented him from calculating the value of the elementary 

 charge. As the discharge of electricity through gases at 

 low pressures became a subject for experimental inves- 

 tigation, another line of approach to the study of the 

 atom of electricity was opened up. As early as the sev- 

 enties Hittorf and Goldstein had observed that a shadow 

 is cast by a screen placed in front of the cathode of a 

 Crookes tube. Yarley suggested that the cathode rays 

 producing the shadow consist of i ' attenuated particles of 

 matter, projected from the negative pole by electricity. ' ' 

 The discovery that these rays are deflected by a magnetic 

 field led English physicists to the conclusion that they 

 must be composed of charged particles, and the direction 

 of the deflection was such as to require the charge to be 

 negative. Hertz contested this view on the ground that 

 his experiments showed the rays to be unaffected by an 

 electrostatic field, and suggested that they consist of 

 etherial disturbances. Finally Perrin succeeded in pass- 

 ing the rays into a metal cylinder which received from 

 them a negative charge, and Lenard showed how exces- 

 sively minute these negatively charged particles must be 

 by actually passing them through a thin sheet of alumi- 

 nium in the wall of a vacuum tube, and detecting their 

 presence in the air outside. Conclusive information as 

 to the nature of the electron, as it was named by John- 

 stone Stoney, was supplied by the classic experiments 

 of J. J. Thomson. 5 First he showed that Hertz's failure 

 to find -a deflection when a stream of electrons passes 

 between the plates of a charged condenser was due to the 

 screening effect of the gaseous ions produced by the dis- 

 charge. With a much more highly evacuated tube he 

 found no difficulty in obtaining a deflection in an electro- 

 static field. By using crossed electric and magnetic 



3 J. J. Thomson, Phil. Mag., 44, 293, 1897. 



