28 POPULAR SCIENCE MONTHLY. 



to the other inactive products produced during the radioactive changes. 

 The a particles expelled from the radioactive product are themselves 

 non-radioactive. The measurement of the ratio e/m shows that they 

 have an apparent mass intermediate between that of the hydrogen and 

 helium atoms. If the a particles consist of any known kind of matter 

 they must be atoms either of hydrogen or of helium. The actual 

 value of e/m has not yet been determined with an accuracy sufficient 

 to give a definite answer to the question. On account of the very 

 slight curvature of the path of the a particles in a strong magnetic 

 or electric field, accurate determination of e/m is beset with great 

 difficulties. The experimental problem is still further complicated 

 by the fact that the a particles escaping from a mass of radium have not 

 all the same velocity and in consequence it is difficult to draw a 

 definite conclusion from the observed deviation of the complex pencil 

 of rays. 



The results so far obtained are not inconsistent with the view that 

 the a particles are helium atoms, and indeed it is difficult to escape 

 from such a conclusion. On such a view, the helium, which is 

 gradually produced in the emanation tube, is due to the collection of 

 a particles expelled during the disintegration of the emanation and its 

 further products. This conclusion is supported by evidence of another 

 character. It is known that thorium minerals like monazite sand 

 contain a large quantity of helium. In this respect they do not differ 

 from uranium minerals which are rich in radium. The only common 

 product of the different radioactive substances is the a particle and 

 the occurrence of helium in all radioactive minerals is most simply 

 explained on the supposition that the a particle is a projected helium 

 atom. This conclusion could be indirectly tested by examining 

 whether helium is produced in other substances besides radium, for 

 example, in actinium and polonium. 



The experimental determination of the origin of helium is beset 

 with great difficulty on all sides. If the a particle is a helium atom, 

 the total volume of helium produced in an emanation tube should 

 be three times the initial volume of the emanation present, since the 

 emanation in its rapid changes gives rise to three products each of 

 which emits a particles. This is based on the assumption, which 

 seems to be fulfilled by the experiments, that each atom of each 

 product in breaking up expels one a particle. This at first sight offers 

 a simple experimental means of settling the question, but a difficulty 

 arises in accurately determining the volume of helium produced by 

 a known quantity of the radium emanation. It would be expected 

 that, if the emanation were isolated in a tube and left to stand, the 

 volume of gas in the tube should increase with time in consequence 

 of the liberation of helium. In one case, however, Eamsay and Soddy 



