﻿the x particles expelled from Radium and Actinium. 367 



and that, after expulsion, the parts of the atom lose their 

 charge and recombine to form atoms of helium ; while such 

 a view cannot be dismissed as inherently improbable, there 

 is as yet no direct evidence in its favour. On the other 

 hand, the second hypothesis has the merit of greater simplicity 

 and probability. 



On this view, the a particle is in reality a helium atom 

 which is either expelled w T ith a double ionic charge or acquires 

 this charge in its passage through matter. Even if the a 

 particle were initially projected without charge, it would 

 certainly acquire one after the first few collisions with the 

 molecules in its path. We know that the a particle is a very 

 efficient ionizer, and there is every reason to suppose that it 

 w T ould itself be ionized by its collisions with the molecules in 

 its path, i. e. it would lose one or more electrons and retain 

 a positive charge. If the a particle can remain stable with 

 the loss of two electrons, these electrons would almost cer- 

 tainly be removed as a result of the intense disturbance set 

 up by the collision of the a particle with the molecules of 

 matter. The a particle would then have twice the normal 

 ionic charge, and the value of e/m, as found by measurement, 

 would be quite consistent with the view that the a particle is 

 an atom of helium. 



In a previous paper* I showed, from measurement of the 

 charge carried by the a rays, that 6'2 x 10 10 a. particles were 

 expelled per second from one gram of radium at its minimum 

 activity. This was based on the assumption that each a 

 particle carried a positive charge equal to the ionic charge of 

 hydrogen, viz. 3*4 x 10~ 10 electrostatic units. Assuming that 

 the a particle carries two ionic charges, the corresponding 

 number is reduced to one half of the above, viz. 3*1 X 10 10 . 

 This would make the calculated period of radium 2600 years 

 instead of 1300 years (see ' Radioactivity/ second edition, 

 1905, p. 457). In a similar way, the calculated volume of 

 the emanation released from one gram of radium would be 

 0*4 cubic mm. instead of 0*8 cubic mm. The calculated 

 volume of helium produced per year per gram of radium 

 would be 0"11 cubic cms. (' Radioactivity/ p. 481). 



On the hypothesis that the a particle is a helium atom, the 

 atomic weight of each product is diminished by four units, 

 in consequence of the expulsion of an a particle. On the 

 hypothesis that the a particle is half a helium atom carrying 

 a single ionic charge, the atomic weight is diminished by 

 two units instead of four. Taking the latter hypothesis, the 

 number of a particles expelled per second from one gram of 

 * Phil. Mag, August 1905. 



