152 



Mr. WE. Pound on the Absorption of the 



collected in Table XXI., and in fig. 18 curves are drawn with 

 the thicknesses of the absorbing layers as abscissae and the 



Table XXI. 



Lead. 



Tin. 



Aluminium. 



Thickness required to \ 

 absorb /3 and j3 se- I 

 condary radiation. J 



mm. 

 09 



mm. 

 2-5 



mm. 

 7-0 



Thickness giving maxi- 1 

 mum reflected se- | 

 condary radiation f 

 due to /3 rays J 



0-16 



0-24 



0-4 



Density 



113 



7-3 



2-6 





densities o£ the absorbing substances as ordinates. The 

 curve A is plotted from the results of the transmitted radiation 

 experiment, while the curve B corresponds to the measure- 

 ments on the reflected rays. It will be noticed that the scale 

 of abscissae used for the latter curve is only one-tenth that 

 adopted in laying out the former. From the results in the 

 table and from the form of the curve, it will be seen that the 

 thicknesses of the absorbing material required to stop the ft 

 and (5 secondary rays were not directly proportional to the 

 densities, but that as the densities decreased it required a 

 greater thickness to stop the rays than should have been 

 expected from density considerations alone. 



It is highly probable that the maximum depth from which 

 the secondary rays come on the front side of a metal plate 

 when primary ft rays impinge on it represents the thickness 

 that the secondary rays excited by the primary ones will 

 penetrate in that metal. Now if the secondary rays excited 

 by the primary in the three metals are all of the same 

 penetrability, one should expect, on the assumption that they 

 are ft rays, that numbers representing the maximum pene- 

 trability found for these secondary rays would follow the 

 same absorption-law with reference to the density that the 

 numbers representing the maximum penetrabilities of the 

 primary radiation followed. In other words, the two curves 

 A and B should be similar in form if the secondary rays 

 excited in the three metals possess the same penetrability. 

 But it is clear from the manner in which the two curves 



