Moving Electrified Particles passing through Matter. 25 



helium of a-rays from polonium. Adams finds the absorption 

 in helium a little bigger than in hydrogen ; the ratio being 

 1-15. 



According to Cuthbertson's experiments * and Drude's 

 theory the dispersion in helium can be explained by assuming- 

 two electrons per atom (the calculated value is 2*3) of a 

 frequency n = 3'72 . 10 16 . 



Introducing these values for r and n in the formula (4) , 



p. 19, we get values for — — in helium which are a little 



smaller than those found above for hydrogen, the ratio being 

 0-92 for V = l-75 . 10 9 and 0'90 for V=l/35 . 10 9 . 



If this disagreement is real (Adams states (loc. cit. p. Ill) 

 that the purity of the gases used was not secured very 

 effectively ; a little contamination of the helium with heavier 

 gases will explain the disagreement in question) it suggests 

 that even for helium the neglected corrections will be of 



Oil 



sensible influence. As, however, the quantity ~ is to be 



assumed about twice as large for helium as for hydrogen, we 

 see, on the other hand, that the corrections, being about 

 30 per cent, for helium, will not be more than about 10 per 

 cent, for hydrogen. The following results for oxygen and 

 aluminium seem, however, to indicate that the corrections 

 considered are much smaller. 



The value r=2 for the number of electrons in a helium 

 atom, indicated by experiments on dispersion and on absorption 

 of a-rays, is what we, adopting Rutherford's theory of atoms, 

 necessarily must conclude from the behaviour of a-rays, 

 according to which helium atoms formed from a-particles 

 will only contain 2 electrons outside the central nucleus. 



Oxygen. 



For the ratio between the absorption in oxygen and in 

 air, Adams (loc. cit. p. 113) found 1*03, and according to the 

 table, p. 22, we therefore get for oxygen 



for V = l-35.10 9 , 4^ = -2*07.10 8 , 



ax 



for V=l-75.10 9 , ~=-l'24.10 8 . 



ax 



* C. & M. Cuthbertson, Proc. Roy. Soc. A. lxxxiv. p. 13 (1910). 



