Natures of various Electric Radiations. 433 



the metal which the)' strike, and ionize so few of the mole- 

 cules through which they pass, that very few of the slow- 

 speed, highly-absorbable electrons can be discharged from 

 the surface of the plate. Even in the case of the a. particle 

 these electrons are not readily observed ; in the case of the 

 (3 particle the difficulty must be much greater. 



As regards X rays, we have no such accurate measure- 

 ments of the velocities of the electrons which are ejected 

 from the molecules of a gas traversed by the rays, as we 

 have in the case of the cathode rays, so far as I am aware. 

 But a very large amount of labour has been spent on the 

 investigation of the secondary radiation caused by the X-rays, 

 from which we may gather much indirect evidence on 

 the point. Perrin (Ann. Chim. Pliys. xi. p. 496, 1897), has 

 showm that the rate of production of ions per cc. by rays of 

 given intensity is proportional to the pressure of the gas. 

 Again, we know r from the investigations of Curie and Sagnac,. 

 Townsend, and Barkla, that metals struck by X rays return 

 a secondary radiation, which, in the case of the low atomic 

 weights, may be considered to consist principally of scattered 

 primary radiation, and in the case of the high atomic weights 

 to contain both X rays more absorbable than the primary 

 and cathode rays. l)orn has shown that the latter have 

 speeds averaging about 5 x 10 9 cm., so that they must produce 

 considerable ionization, consisting of 8 rays, in the few 

 millimetres of air close to the metal. The free path of 

 electrons having this speed is about one millimetre in air at 

 atmospheric pressure. Since the X rays do not appear to 

 produce cathode rays of any speed from the air molecules, 

 which they traverse, or from the molecules of any gas con- 

 sisting of atoms of small weight, and since they produce 

 much ionization in some way or other, we may conclude 

 fairly that they produce slow-speed ions themselves. Thus, 

 wmether they act directly or indirectly through cathode rays, 

 the result is the same. The principal effect appears to be 

 due rather to secondary than primary. As Sagnac remarks 

 (Ann. CJiim. PJiys. xxiii. p. 196) : " The transformation of 

 X rays, by increasing the activity at any point, permits the 

 detection there of very penetrating X rays, which would 

 otherwise have passed unperceived.'' 



In the case of the 7 rays, such evidence as we have is also 

 in favour of the existence of slow-speed ions, as the result of 

 their action. It is known that /3 rays of high speed originate 

 where they strike the molecules of a solid body (Eve, Phil. 

 Mag., December 1904) ; such an action may, therefore, be 

 expected in the case of gas molecules also. It is possible, 



