180 PHYSICS OF THE ELECTRON 



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 grad- 

 ually 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 con- 

 tain 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 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 eman- 

 ation 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 

 borne out 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, Ramsay and Soddy observed an 

 exactly opposite result. The volume diminished with time to a small 

 fraction of its original value. This diminution of volume was due to 

 the decomposition of the emanation into a non-gaseous type of matter 

 deposited on the walls of the tube, and followed the law of decrease to 

 be expected in such a case, namely, the volume decreased according 

 to an exponential law with the time, falling to half value in four days. 

 The helium produced by the emanation must have been absorbed by 

 the walls of the tube. Such a result is to be expected if the particle is 

 a helium atom, for the a particle is projected with a velocity sufficient 

 to bury itself in the glass to a depth of about ^ mm. This buried 

 helium would probably be in part released by the heating of the tube, 



