the Method of its Transmission. 113 



■of the surrounding gas. The size o£ the cluster is probably 

 iibout the same for the positive ion, since the size is mainly 

 determined by the electric charge which is the same for 

 both. The velocities in an electric field are thus the same for 

 the carriers of excited activity and for the gaseous ion. Since 

 the size of the cluster is large compared with the original 

 nucleus, the velocities of the carriers of thorium and radium 

 excited activity would be about the same, even if the original 

 nuclei were of different masses. 



§ 10. Nature of the Radiations . 

 In considering the question of the size of the body expelled 

 from the molecule of the emanation, and of the nature of the 

 radiation from the emanation, it is necessary to take into 

 account the nature of the emanations emitted from all the 

 known radioactive bodies ; for there is no reason to suppose 

 that the processes which are taking place in the molecule of 

 the emanation are essentially different in character from those 

 occurring in the other radioactive bodies. It is known that 

 uranium, thorium, and radium emit two types of radiation. 

 One type is not appreciably deviable by a magnetic or an 

 electric field, and is very easily absorbed in matter. These will 

 be called the a rays. The others are deviable and more 

 penetrating in character, and will be called the j3 rays. In 

 addition I have shown that thorium and radium emit some 

 rays nondeviable in character, but of very great penetrating- 

 power. All of the radioactive substances including polonium 

 as well as " excited " bodies and the emanations give out these 

 a. rays. Their power of ionizing the gas is very much greater 

 than for the other types of rays emitted ; and it is probable 

 that the greater proportion of the energy radiated into the 

 gas is in the form of a. rays. The a. rays from differ ent radio- 

 active substances, including the emanations of " excited 

 bodies, do not vary very much in penetrating power. The 

 ""excited" radiations for thorium and radium are the most 

 penetrating in character and that of uranium the least. 



It has been difficult to offer a satisfactory explanation of 

 the nature of these rays. I have previously shown as untenable 

 the view that they are secondary rays due to the emission of 

 ft rays. I have been recently led, by a mass of indirect evidence, 

 to the view that the a rays are in reality charged bodies pro- 

 jected with great velocity. The ionizing effect of the rays is 

 due to the collision of the projected body with the molecules 

 of the gas, in the same way that the cathode rays ionize the 

 gas in their path. Such a projected particle probably pro- 

 duces many thousand ions in its path before its velocity i-* 



Phil. Mag. S. 6. Vol. 5. No. 25. Jan. 1903. I 



) 3 



