﻿the a particles expelled from Radium and Actinium. 365 



of the a. particle from radium itself is in agreement with that 

 to be deduced from its range in air — a result which is only 

 to be expected if the a particle from radium has the same 

 mass as that from radium C. 



There then remains only one a ray product of radium, yiz. 

 the emanation, whose radiation has not been closely examined. 

 There is, however, no reason to suppose that the a particles 

 from the emanation differ in mass from those of the other 

 products. An examination of the complex pencil of rays 

 from a layer of radium in equilibrium shows no evidence of 

 the presence of a. rays which surfer an abnormal amount of 

 deflexion. I think there can be no doubt that the a particles 

 emitted from the various products of radium have an identical 

 mass, but differ only in the initial velocities of projection. 

 Although the mass of the a particles has been determined for 

 only a single product of thorium and of actinium, the analogy 

 with radium would lead us to expect that the a particle has 

 the same mass for all the products of these substances. 



We may thus reasonably conclude that the a particles 

 expelled from the different radio-elements have the same mass 

 in all cases. This is an important conclusion; for it shows 

 that uranium, thorium, radium, and actinium, which behave 

 chemically as distinct elements, have a common product of 

 transformation. The a particle constitutes one of the funda- 

 mental units of matter of which the atoms of these elements 

 are built up. When it is remembered that in the process of 

 their transformation radium and thorium each expel five a par- 

 ticles, actinium four, and uranium one, and that radium is in 

 all probability a transformation product of uranium, it is seen 

 that the a. particle is an important fundamental constituent 

 of the atoms of the radio-elements proper. I have often pointed 

 out what an important part the a particles play in radioactive 

 transformations. In comparison, the ft and 7 rays play quite 

 a secondary role. 



It is now necessary to consider what deductions can be 

 drawn from the observed value of e/m found for the a. particle. 

 The value of e/m for the hydrogen ion in the electrolysis of 

 water is known to be very nearly 10*. The hydrogen ion is 

 supposed to be the hydrogen atom with a positive charge, so 

 that the value of e/m for the hydrogen atom is 10^. The 

 observed value of e/m for the a particle is 5*1 x 10 3 , or, in 

 round numbers, one half of that of the hydrogen atom. The 

 density of helium has" been found to be 1*98 times that of 

 hydrogen, and from observations of the velocity of sound in 

 helium, it has been deduced that helium is a monatomic gas. 

 From this it is concluded that the helium atom has an atomic 



