Frequencies in the Spectra of Various Elements. 467 



the first member of the singlet principal series given by 

 x=(l-5, 8)-(m, P). • 



The question naturally arises then as to which of these two 

 series is the more fundamental in character from the point 

 of view of atomic structure. 



It is impossible as yet to decide, but everything goes to 

 show that, while the radiation given by v = (l*5, 8) — (2, p 2 ) 

 is the more easily stimulated by electronic bombardment of 

 mercury, zinc, and cadmium atoms, the series of wave-lengths 

 given by v = (1*5, S) — (m, P) is the one which corresponds 

 to some very simple type of electronic vibrations within the 

 atom. 



It has been shown by McLennan and Edwards * that with 

 the vapours of mercury, zinc, and cadmium absorption is more 

 marked in the region corresponding to ^=(15, S) — (2, P) 

 than it is in the region about j/ = (l*5 S) — (2, p 2 )- With 

 magnesium vapour too, McLennan f has shown that ab- 

 sorption at v = (l'5, S) — (2, P) is very much more marked 

 than it is at v— (1*5, S) — (2, p 2 ). More recently still 

 McLennan and YoungJ have shown that with the vapours of 

 calcium, strontium, and barium, absorption and reversal can 

 be obtained much more readily at y=(l'5, Sj — (2, P) than 

 it can at v-~ (1*5, S) — (2, p 2 ). 



It will also be recalled that McLennan § has shown that 

 when magnesium was bombarded by electrons whose kinetic 

 energy was gradually increased, no radiation of wave-length 

 atX = 4571A.U. was obtained until the arc struck, while the 

 line at A, = 2852'22 A.U., i/ = (l/5, S)— (2, P) came out on 

 the plates as soon as the electrons attained kinetic energy 

 corresponding to the impact voltage given by the quantum 

 relation for the frequency of this wave-length. 



It would appear, then, that of the two series v= (1/5, S) 

 — (m, p 2 ) and v = (l'D } S) — («?, P), the latter is the more 

 fundamental in character, and that through it our attention 

 is directed to vibrations within the atom which are of a 

 principal or main type. 



* McLennan and Edwards, Phil. Mag. vol. xxx. p. 695, Nov. 1915. 

 t McLennan, Proc. Roy. Soc. A, vol. xcii. p. 307. 

 t McLennan and Young-. Communicated to the Royal Soc. Oct. 1918. 

 § McLennan, he. cit. 



