136 



ME. G. W. C. KAYE ON THE 



The curve presents some resemblances to those obtained by Prof. J. J. THOMSON* 

 when working on secondary Rontgen rays. He also found less difference between the 

 amounts of secondary radiation from a metal of low atomic weight and one of high 

 when the incident rays were soft than when they were hard. 



To regard the intensity of radiation as proportional to the atomic weight of the 

 antieathode is a good rough working rule. The curve has been used to obtain the 

 intensities of the radiation for intermediate elements. 



o -50 100 iso 



ATOMIC WEIGHT or RADIATOR . . 



Fig. 6. 



The list below gives the radia-tion values and the latest determinations of the 

 melting points (where known) of those elements which by reason of their refractoriness 

 may be regarded as suitable for the antieathode of a focus tube. The physical 

 properties of some of them are not convenient, and to others the scarcity and 

 consequent price are at present an insurmountable objection. Tantalum is now being 

 used in Rontgen bulbs. Its radiation is rather richer than platinum radiation in soft 

 rays, and it emits about 10 per cent, less hard rays than platinum. It has the 

 advantage of a much higher melting point, and appears not to harden the tubes so 

 much as platinum on continued running. All the metals of this group would make 

 excellent anticathodes. Generally speaking, the lower the atomic weight the larger 

 the proportion of soft rays (or, rather, rays not transparent to aluminium) in the 

 radiation. This is especially the case with the metals of the iron group. 



* J. J. THOMSON, ' Proc. Camb. Phil. Soc.,' XIV., 1, p. 109, Nov., 1906. 





