176 Mr Kleeman, The nature of the ionisation 



before, is proportional to the square of the pressure. Now, Strutt* 

 has shown that the ionisation in a gas by /3 rays is proportional 

 to the pressure over a wide range of pressures. The second part 

 of the ionisation is therefore small in comparison with the first. 

 Thus the /3 rays also produce directly rays which do not themselves 

 possess sufficient velocity to make any further ions. 



Thus the ionisation of a gas by 7, /3, or X rays, is a measure 

 of the number of 8 rays produced directly by the ionising agent. 



Since these rays are not able to produce any further ions 

 themselves they must have a velocity less than 2'7 x 10* cm./sec, 

 for at higher velocities, according to Townsend, ions are produced 

 by collision. 



The writerf has shown that the 7 rays of radium produce 

 cathode rays of very different velocities, the maximum velocity 

 being equal to that of the penetrating y8 rays from radium. It 

 appears therefore that the 7 rays produce cathode rays ranging in 

 velocity from 2'7 x 10* to 2*9 x 10" cm./sec. — the velocity of the 

 penetrating /3 rays from radium. In the case of X rays cathode 

 rays are produced probably ranging in velocity from 27 x 10* to 

 8'3 X 10^ cm./sec, the latter velocity being that obtained by Innes 

 for the penetrating cathode rays from a plate exposed to X rays. 



Since the most penetrating cathode rays ejected by X rays 

 have a range in air at atmospheric pressure equal to a fraction of 

 a mm. only, the rays produced in the gas contained in an ionisation 

 chamber are entirely absorbed by the gas. The ionisation in 

 a gas by X rays is therefore a measure of the energy converted 

 directly into cathode rays. Now, according to Crowther, the 

 ionisation produced by the secondary cathode rays which are able 

 to produce ions by collision is small in comparison with the 

 ionisation produced directly by the X rays. It follows therefore 

 that the energy expended in the production of penetrating cathode 

 rays is small in comparison with that expended in the production 

 of 8 rays. And since the energy of one of the most penetrating 

 cathode rays ejected by X rays is about 900 times that of a S ray, 

 the number of penetrating cathode rays ejected must be very 

 small indeed in comparison with the number of S rays ejected. 



Whether this is also true for the cathode rays ejected by 

 7 rays cannot yet be determined since there are not sufficient 

 experimental data. The penetrating cathode rays ejected by 

 7 rays are not absorbed in a distance of a fraction of a mm. in 

 a gas like those ejected by X rays, but are able to cross an 

 ionisation chamber without any appreciable absorption. And 

 since the ionisation they produce in a chamber is small in 

 comparison with the total ionisation, this quantity is, provided 



* Phil. Trans., A, vol. 196, p. 507, 1901. 

 + loc. cit. 



