194 Mr. J. (Ihadwick on the 



glass tube, and consequently was also a strong source of /9 

 and 7 rays. The presence of the primary 7 rays renders 

 very difficult the detection o£ a small amount of excited 

 radiation, since this is observed only as an increase in the 

 total 7 radiation. Care must also be taken to eliminate the 

 effect of any 7 rays excited by the /3 rays. 



The method finally adopted was as follows. The a ray 

 tube was surrounded by a tube of some material of low 

 atomic weight, say aluminium, just thick enough to stop all 

 a rays. Then round this was placed a similar tube of a 

 metal of high atomic weight, say gold. The ionization 

 produced by the 7 rays emitted by the emanation tube under 

 these conditions, was measured by the balance method of 

 Rutherford and Chadwick*. The positions of the tubes 

 were now reversed, so that the gold tube was the inner one, 

 and was bombarded by the a rays. The ionization was again 

 measured. 



It was found that when the face of the ionization-chamber 

 was an aluminium plate 1 cm. thick there was no detectable 

 difference in the ionizations in the two cases. With a plate 

 1 mm. thick, however, a slight difference was observed, the 

 ionization being greater when the inner tube was of gold. 

 The difference was about 1 part in 300. 



It was evident that, to establish definitely the excitation 

 of 7 rays by a rays, some method was necessary which would 

 increase the ionization produced by the excited 7 rays re- 

 lative to that produced by the primary rays. It is well 

 known that the amount of ionization produced by X rays in 

 a gas relative to that produced in air depends upon the 

 penetrating power of the rays. For example, Struttf and 

 Eve J have shown that the ratio between the ionizations 

 produced in methyl iodide and in air is 12 for soft X rays, 13 '5 

 for hard X rays, and 5 for 7 rays. As it was to be expected 

 that 7 rays excited by a rays would resemble X ra} r s in their 

 penetrating power, advantage could be taken of this pheno- 

 menon to increase the ionization of the excited rays relative 

 to that of the primary rays. 



Accordingly, an airtight ionization-chamber was made, 

 which could be quickly evacuated and filled with the vapour 

 of methyl iodide or carbon bisulphide. The construction 

 will be clearly seen from the figure. The ionization-chamber 

 consisted of a brass cylinder about 8^ cm. long and 6^ cm. 

 diameter. The electrode was insulated from the cylinder by 



* Rutherford and Chadwick, Proc. Phys. Soc. April 1912. 

 t Strutt, Proc. Roy. Soc. A. vol. lxxiv. (1903). 

 + Eve, Phil. Mag. viii. p. 010 (1904). 



