190 Prof, Millikan and Mr. Winchester on Influence of 



of like value to that possessed by the atoms of the metal at 

 the given temperature. If this assumption is correct, and if 

 the electrons which escape under the influence of the ultra- 

 violet light are these free electrons of the metal, it is obvious 

 that their rate of escape must bean increasing function of the 

 temperature. Indeed, whether the Maxwell-Boltzinann law 

 can be applied to such corpuscles or not, it is difficult to see 

 how their rate of escape from the surface of a negatively- 

 charged body could be any other than an increasing function of 

 the temperature ; for in view of their mutual repulsions these 

 free corpuscles would be distributed over the surface of the 

 body, and the forces which hold them to it might be expected, 

 from the behaviour of molecular forces generally, to decrease 

 as temperature increases. 



The other view, namely that the emission of corpuscles is 

 due to atomic disintegration effected through resonance, was 

 suggested by Elster and Geitel *, and is very strongly sup- 

 ported by Lenard f . The most convincing evidence adduced 

 by the latter is found in the fact that the aluminium, carbon, 

 and platinum plates upon which he experimented acquired 

 in a vacuum, under the influence of ultra-violet light, posi- 

 tive potentials which were wholly independent of the intensity 

 of the source. As Lenard points out, this means that the 

 kinetic energy of projection of the corpuscles — a quantity 

 which is measured by the positive potential assumed in vacuo 

 — cannot have been acquired to any appreciable extent by 

 the absorption of the ultra-violet light, but must rather have 

 been possessed by the corpuscles before they were set free by 

 the light, which could in that case have acted only as a 

 " detonating '" agent. 



Furthermore, the independence of the quantity of discharge 

 upon the potential of the charged body f constitutes an indica- 

 tion, at least, in favour of Lenard's view. 



Again, Elster and Geitel's % discovery that absorption and 

 emission go hand in hand, whether it be wave-length or 

 plane of polarization of the incident light which is made to 

 vary §, is also more readily reconcilable with the theory of 



* Elster & Geitel, Wied. Ann. xli. p. 175 (1890). 



t Lenard, Ann, d. Phys. ii. p. 359 (1900) ; and viii. p. 149 (1902). In 

 the new edition of ' Conduction of Electricity through Gases ' J. J. 

 Thomson recognizes the strength of Lenard's argument and adopts in the 

 main his viewpoint regarding the mechanism of emission. 



% Elster & Geitel, Wied. Ann. lii. p. 433 (1894) ; lv. p. 682 (1895) ; 

 lxi. p. 445 (1897). 



§ This discovery of Elster and Geitel's as regards the effect of the plane 

 of polarization of the incident light was made hy allowing yellow light 

 to fall upon a sodium surface. It does not hold when ultra-violet rays 

 are employed (see Lenard, Ann. d. Phys. viii. p. 168 (1902) ; and 

 Ladenburg, Ann. d. Phys. xii. p, 558 (.1903)). 



