294 



Coohsey — Corjiniseula/' Bay* i>fu<luc< <l hi 



In all the curves there is a very sudden drop up to the first 

 three or four sheets of aluminium, and then the curves fall off 

 more slowly. The first part of the curve up to the first three 

 or four sheets of aluminium is undoubtedly due, for the most 

 part, to the corpuscular secondary rays, while beyond this point 

 the corpuscular rays are practically all absorbed and there 

 remain only the secondary rays of the Rontgen type. As has 

 been said before, these corpuscles come off with velocities com- 

 parable with the velocities of the cathode particles in a highly 

 exhausted tube; say between 10 9 and 10 10cms /sec. Seitz- 



.001 jOOZ X)03 .004- 



008 



'mm. of- ALuminium. 



.0/2 



has investigated the absorption of cathode rays driven by 20,000 

 volts, corresponding to about 9xt0 9cms /sec, and finds that they 

 will penetrate aluminium about -002 mm thick, the fraction 

 which gets through being about 0'16. A little further .on we 

 shall show that this is about the order of absorption for the rays 

 constituting the first part of the curves under discussion. It 

 is this agreement which leads us to believe that the first part 

 of the curves is largely due to the corpuscular secondary rays. 

 With the exception" of zinc and nickel the first part of all 

 the curves due to the secondary rays produced by the hard 

 primary rays lies above those produced by the soft primary rays. 

 This would' be expected from the results of Bestelmeyer, before 

 mentioned, where the corpuscles produced by harder primary 

 rays had higher velocities than those produced by the softer 

 *Annalen derPhysik, xii, p. 860, 1903. 



