hy Metals under the Influence of Alpha Rays. 021 



however, that the ordinates of the different curves are not 

 exactly adjusted to eacli other, on account of the differences 

 just mentioned. It is indeed impossible to make satisfactory 

 correction for these differences without knowing the relative 

 values of the "incidence" and "emergence"''' S-radiation ; 

 for, between the generation of the two, the a-rays pass 

 through the leaf on the electrode, in the one case aluminium, 

 in the other case gold. It is plain, however, that this 

 correction (if it could be made) would bring closer together 

 the two curves in their decreasing portions, while it would 

 not much affect their increasing portions where they are 

 already in close agreement. 



The results of Taylor, mentioned at the beginning of this 

 paper, would lead us to expect that the metal curves would 

 lie to the left of the air-curve and have a less pronounced 

 knee, as appears to be the case. But they give equal grounds 

 for anticipating that the gold curve should be to the left of 

 that obtained with aluminium, and this is not confirmed by 

 the experiments. Quite apart from these somewhat hypo- 

 thetical considerations it does not appear probable that two 

 metals which differ so much in atomic weight and in other 

 properties should give effects so nearly identical ; the ion- 

 ization curves in different gases are markedly different even 

 when the gases differ much less from each other than gold 

 and aluminium. The close similarity observed with the two 

 metals gives rise to the suspicion that the electrons we have 

 been measuring are emitted, not from the metals themselves, 

 but perhaps from a layer of adsorbed gas which is the same 

 in both cases. I have not yet had an opportunity to test this 

 possibility, or to obtain the " ionization curves " of other 

 metals, but I hope to do so shortly. The observation of 

 Aschkinass (p. 915J, footnote) seems to indicate that copper 

 and aluminium would not give identical results. 



Conclusions. 



1. The emission of electrons by aluminium and gold foils 

 under the influence of a-rays (secondary S-rays) varies with 

 the speed of the a-rays in a manner entirely analogous to the 

 variation in the gaseous ionization produced by a-rays. The 

 emission at first increases and then rapidly decreases as the 

 a-rays near the end of their range, and the curves obtained 

 show all the characteristics of the ionization curves in gases 

 first obtained by Bragg. 



2. The curves lie within (to the left of) the corresponding 

 curves for gases and have a less conspicuous " knee." &o 



