[ 461 ] 



LI. On Fundamental Frequencies in the Spectra of Various 

 Elements. By Professor J. C. McLennan, F.R.S., and 

 H. J. 0. Ieeton*. 



[Plates XIII. -XV.] 



Part I. 

 1. Introduction. 



iii 



IN a series of papers by McLennan and Henderson f and 

 others it has been shown that when the vapours of 

 mercury, zinc, cadmium, and magnesium were subjected to 

 bombardment by electrons whose velocity was gradually 

 increased, these vapours were stimulated to the emission of 

 a monochromatic radiation. With mercury the radiation 

 emitted had the wave-length X = 2536*72 A. U., with zinc 

 X = 3075*99 A.U., with cadmium X = 3260'17 A.U., and with 

 magnesium X = 2852*22 A.U. In order to bring the vapours 

 to the emission of these respective radiations, it was found 

 that the bombarding electrons had to have kinetic energy 

 corresponding to a fall of potential given by the quantum 

 relation Ve = hv, where v is the frequency of the mono- 

 chromatic radiation emitted. 



When the velocity of the electrons was increased beyond 

 that given by the quantum relation for these frequencies, 

 there did not appear to be any radiation emitted of shorter 

 wave-length than those given above by any of the vapours 

 mentioned until the electrons possessed the requisite energy 

 to ionize the vapours. When this occurred arcs were struck 

 and the many-lined spectra were obtained. 



In order to obtain the many-lined spectra it was found 

 that the electrons required to have kinetic energy corre- 

 sponding to a potential fall given by the quantum relation 

 Ye = kv where v was the frequency, v = (l*5, S), namely that 

 of the shortest wave-length of the series v=(l*5_, S) — (m, P). 



It was thought that, in these experiments in which an 

 incandescent limed iplatinum cathode was used, it might be 

 possible, by giving the electrons kinetic energy intermediate 

 between that which would bring on the monochromatic 

 radiation and that which sufficed for striking the arc, to 

 cause the vapours to emit radiations which became shorter 



* Communicated by the Authors. 



t McLeuuan aud Henderson, Proc. Eoy. Soc. A, vol. xci. p. 425 

 (1915). 



Phil. Mag, S. 6. Vol. 3G. No. 21G. Dec. 1918. 2 I 



