the Corpuscular Hypothesis of the y and X Rays. 413 



2--A0, which leads to a value 1G3 in the last column. Schmidt's 

 values of ajD for the ft rays of actinium do not show this 

 irregularity in the case of tin. 



The values of d are clearly less for the smaller atomic 

 weights. The whole track of a ft particle in lead is actually 

 greater, weight for weight, than in aluminium. Yet as is 

 well known a ft particle can penetrate a heavier screen of 

 aluminium than of lead. The reason is that the lead atoms 

 turn back the ft particles so much more than the lighter 

 atoms do. In lead the particle finishes its course much more 

 closely to its starting-point; it is really a longer course, but 

 there are many more turns in it. 



It is easy to see that there will consequently be considerable 

 differences in the " absorption curves " of different materials; 

 i, e. the curves which show the relation between the thickness 

 of a screen placed normally to the path of a stream of ft rays, 

 and the ionization in a chamber on the other side, a chamber 

 which the rays can usually cross. A ft ray going through 

 aluminium behaves rather more like an a particle than when 

 it goes through lead, since it is less liable to deflexion in the 

 former case, and the a. particle has very few departures from 

 a straight line course. The absorption curve of the ft ray in 

 aluminium should, therefore, be more like that of an a particle 

 than the curve of ft ray in lead. Now the a particle actually 

 causes more ionization when screens are placed in its path, 

 unless the screen is too thick, than when it is unimpeded ; 



Fi- 4. 



that is to say, the curve which is plotted with thickness of 

 screen as abscissa and ionization on the other side of the 

 screen as ordinate rises at first ; subsequently it falls rapidly 



