17B Mr. W. Makower on the Number and Absorption 



of the primary rays, then, with the apparatus described above, 

 the quantity of electricity reaching the outer brass cylinder 

 should decrease little, if at all, when screens of glass are 

 interposed between it and the tube containing the emanation. 

 For after scattering, the quantity of radiation reaching the 

 brass cylinder will be the same as before. On the other hand, 

 if the rays are actually stopped on passing into the glass, 

 leaving their charge on the glass by which they are absorbed, 

 then the charge reaching the brass cylinder will be diminished 

 by interposing screens of glass. This was found to be the 

 case ; so that the absorption of the ft rays by glass cannot, 

 at any rate, be entirely accounted for by considering the rays 

 to be scattered. If, further, the law of absorption as found 

 by measuring the charge transmitted through different thick- 

 nesses of glass is found to be identical with that as measured 

 by the ionization produced by the ft rays after absorption by 

 different thicknesses of glass, then the falling off of the 

 ionization due to the rays must be entirely attributed to the 

 stopping of the negatively charged particles of which the rays 

 are known to consist. It will be seen from the experiments 

 now to be described that such is the case *. 



The method of experiment was the same as that already 

 described, the thickness of glass through which the radiation 

 passed before reaching the brass cylinder being increased by 

 covering the glass tube containing the emanation successively 

 by a series of thin glass tubes of gradually increasing dia- 

 meter. The glass tubes so interposed were always coated with 

 thin aluminium-leaf to render them conducting. 



The law of absorption of the ft rays from radium B and C 

 by aluminium, as measured by the ionization produced by the 

 rays after transmission through different thicknesses of that 

 metal, have been very carefully investigated by Schmidt |. 

 As the densities of glass and aluminium are nearly the same, 

 it might be expected that the absorption of the ft rays by 

 these two substances would be not very different. It was, 

 however, necessary to find out whether this supposition was 

 correct or not, and a series of experiments was carried out to 

 test the question. 



* It has been shown by Crowther (Proc. Roy. Soc. A. lxxx. 1908) that 

 the /3 rays are scattered after traversing very small thicknesses of matter. 

 The present experiments are, however, quite consistent with this pheno- 

 menon, since the rays from the emanation had always to traverse the 

 walls of the containing tube. But it seems clear that the diminution of 

 intensity of the /3 rays on traversing matter is ultimately effected by an 

 actual stoppage of the particles of which the rays consist. 



f H. W. Schmidt, Ann. der Phys. xxi. p. 609 (1906) and Phys. Zeit. 

 vii. p. 764 (1906). 



