68 ROYAL SOCIETY OF CANADA 



an exceptionally high co-efficient of absorption for the reflected 

 secondary rays excited by ft and y rays in this metal. 



VII. A Coviparison of the Secondary Radiations excited in different 

 Metals by fi Rays. 



Some conclusions of interest can also be drawn from the results 

 of the experiments of the present investigation regarding the secondary 

 rays excited in different metals by ft rays. For the purpose of making 

 a comparison, the thicknesses of the limiting absorbing layers of the 

 three metals studied with both reflected and transmitted rays are 

 collected in Table XXI, and in Fig. 18 curves are drawn with the thick- 

 nesses of the absorbing layers as abscissee and the densities of 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 measurements 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 materials required to stop the ft and fi secondary 

 rays were not directly proportional to the densities, but that as the 

 densities decreased it required greater thicknesses to stop the rays 

 than should have been expected from density consideration 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 ab- 

 sorption law with reference to the density that the numbers repre- 

 senting 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 

 intersect in the figure that they do not typify the same absorption 

 law. It will be seen from the curve B that the maximum penetra- 

 bilities of the secondary ft rays, as determined by the reflection ex- 

 periments, approximate very closely to a linear relation which exhibits 

 in a striking manner the important result that secondary rays excited 

 in plates of different metals when ft rays are allowed to fall on them 

 are the more penetrative the greater the density of the metal of which 

 the reflector is made. 



