﻿366 Prof. E. Rutherford on the Mass and Velocity of 



weight 3' 96. If a helium atom carries the same charge as 

 the hydrogen ion. the value o£ ejm for the helium atom should 

 consequently be about 25 x 10 3 . I£ we assume that the 

 a, particle carries the same charge as the hydrogen ion, the 

 mass of the a particle is twice that of the hydrogen atom. 

 We are here unfortunately confronted with several possi- 

 bilities between which it is difficult to make a definite 

 decision. 



The value of ejm for the a particle may be explained on 

 the assumptions that the a particle is (1) a molecule of hydro- 

 gen carrying the ionic charge of hydrogen, (2) a helium 

 atom carrying twice the ionic charge of hydrogen, or (3) one 

 half of the helium atom carrying a single ionic charge. 



The hypothesis that the a particle is a molecule of hydrogen 

 seems for many reasons improbable. If hydrogen is a con- 

 stituent of radioactive matter, it is to be expected that it 

 would be expelled in the atomic, and not in the molecular 

 state. In addition, it seems improbable that, even if the 

 hydrogen were initially projected in the molecular state, it 

 would escape decomposition into its component atoms in 

 passing through matter, for the a particle is projected at an 

 enormous velocity, and the shock of the collisions of the 

 a particle with the molecules of matter must be very intense, 

 and tend to disrupt the bonds that hold the hydrogen atoms 

 together. If the a particle is hydrogen, we should expect to 

 find a large quantity of hydrogen present in the old radio- 

 active minerals, which are sufficiently compact to prevent its 

 escape. This does not appear to be the case, but, on the 

 other hand, the comparatively large amount of helium present 

 supports the view that the a particle is a helium atom. A 

 strong argument in support of the view of a connexion 

 between helium and the a particle rests on the observed 

 facts* that helium is produced by actinium as well as by 

 radium. The only point of identity between these two sub- 

 stances lies in the expulsion of u particles of the same mass. 

 The production of helium by both substances is at once 

 obvious if the helium is derived from the accumulated a 

 particles, but is difficult to explain on any other hypothesis. 

 We are thus reduced to the view that either the a particle is 

 a helium atom carrying twice the ionic charge of hydrogen, 

 or is half of a helium atom carrying a single ionic charge. 



The latter assumption involves the conception that helium, 

 while consisting of a monovalent atom under ordinary chemi- 

 cal and physical conditions, may exist in a still more elemen- 

 tary state as a component of the atoms of radioactive matter, 

 * Debierne, C. JR. cxli. p. 383 (1905). 



