49 



produced is nearly equal to the speed of the )3 rays 

 emitted by radium itself. In the case of uranium, 

 thorium, and actinium there are indications that 

 the law holds good also, but no exact measurements 

 have yet been made. 



(5) When very hard y rays traverse matter the absorp- 

 tion and the consequent production of )8 rays are 

 almost independent of the atomic structure of the 

 matter, and a density law follows. Softer rays are 

 affected by atomic structure, in that the heavier 

 atoms are more absorbent than the lighter, weight 

 for weight : and the softer the rays are the more pro- 

 minent this effect becomes. 



(6) If there are secondary y rays, the ionization which 

 they produce is negligible compared with that pro- 

 duced by the secondary /3 radiation, at least within a 

 moderate distance (say 100 cm. of air) of the 

 radiator. 



§ III. 

 Various hypotheses have been suggested as to the mode in 

 which the secondary ^8 radiation is produced by the interac- 

 tion of the y (or X-) ray and the atom. It is convenient to 

 divide them into three classes, and to consider to what extent 

 each class is able to furnish an explanation of the properties 

 set out in the summary just given. 



To the first class belong those hypotheses which suppose 

 both the energy and the material of the ^ radiation to be 

 furnished by the atom alone ; the y ray is a pulse which merely 

 pulls the trigger. In its most recent form this idea is put for- 

 ward by W. Wien (Gottingen Nachrichten, 1907, p. 598). 



Secondly, it may be supposed that the energy of the j8 

 radiation comes from the y ray, but the material from the 

 atom. The y ray is a bundle of electric energy, possessing 

 mass, which impinges on the atom and drives out the elec- 

 tron before it. This is the view now held by J. J. Thomson 

 (Camb. Phil. Soc. Proc, vol. xiv., pt. iv., p. 417). 



Thirdly, both the energy and the material of the ^ ray 

 may be supposed to be derived from the y ray. The latter is 

 not a pulse but a neutral pair, positive and negative ; in pass- 

 ing through some atom the bonds are dissolved, and the nega- 

 tive flies on. (Trans. Roy. Soc. of S.A., May, 1907 : Phil. 

 Mag., Oct., 1907.) 



Consider first the theory which considers that the whole 

 of the energy of the expelled electron comes from the internal 

 energy of the atom. It is to be supposed that the pulse as it 

 widens finds an atom here and there which is in an explosive 



