656 SCIENCE PROGRESS 



than is necessary for this kind of calculation, where it is the 

 magnitude of a small whole number which is in question. The 

 value of the ratio m/e for the a-particle and the value of its 

 charge are both twice that of the hydrogen ion. So the mole- 

 cular weight of the a-particle is 4, in terms of that of the 

 hydrogen ion as unity. That is, the a-particle has a molecule 

 identical with that of helium gas and not a fraction of it. In 

 this respect it is unlike the hydrogen ion, which, as is everywhere 

 accepted, has a molecular weight one-half of that of hydrogen 

 gas. Sir J. J. Thomson finds that the positive ray in helium is 

 the same as the a-particle, though he has indications of a more 

 complex molecule (He 3 ) + , which is a highly interesting and may 

 be very significant observation. 



From Avogadro's constant, known certainly to ± 20 per 

 cent, the number of molecules of radium chloride in a gram is 

 easily calculated, the molecular weight of radium chloride being 

 known. The number of a-particles expelled from it per second 

 in its first disintegration in which the emanation is produced has 

 been determined by Rutherford, as is well known, by direct 

 counting measurements. We shall get a different value for the 

 period of average life of radium, or its reciprocal, the fraction 

 disintegrating per unit of time, if we suppose that one molecule 

 of radium chloride gives rise to one, two, three and so on 

 rt-particles. The period of average life found by several quite 

 independent methods is about 2,500 years, whereas that found 

 by assuming that one a-particle results per molecule of radium 

 chloride disintegrating in the first change is 2,560 years. Hence 

 one molecule of radium chloride gives in its first change one 

 a-particle, identical in mass with one whole molecule of gaseous 

 helium, one also in each of its four later a-ray changes, and 

 never less than the single whole molecule of helium. Again, 

 the experimental value for the volume of helium produced per 

 gram of radium per year (Dewar) is 0*164 c.c, whereas the 

 calculated value on the assumption that each a-particle is a 

 whole molecule of helium is 0*158 c.c, calculated to the same 

 radium standard in each case. This, in itself, constitutes a 

 simple independent proof of the point being argued. 



Now consider the radium emanation also produced. Again 

 we find it is produced in molecules of the same mass as exist in the 

 state of gas, one whole molecule per molecule of radium chloride 

 disintegrating. The volume of emanation in equilibrium with 



