on Radioactive Change. 579 



possible that any of the new types of radioactive matter — 

 e. (/., uranium X, thorium X, the two emanations, &c. — can 

 be identical with any of the known elements. For they 

 remain in existence only a short time, and the decay of their 

 radioactivity is the expression of their continuously dimin- 

 ishing quantity. On the other hand, since the ultimate 

 products of the changes cannot be radioactive, there must 

 always exist at least one stage in the process beyond the 

 range of the methods of experiment. For this reason the 

 ultimate products that result from the changes remain 

 unknown, the quantities involved being unrecognizable, 

 except by the methods of radioactivity. In the naturally 

 occurring minerals containing the radio-elements these 

 changes must have been proceeding steadily over very long 

 periods, and, unless they succeed in escaping, the ultimate 

 products should have accumulated in sufficient quantity to be 

 detected, and therefore should appear in nature as the in- 

 variable companions of the radio-elements. We have already 

 suggested on these and other grounds that possibly helium 

 may be such an ultimate product, although, of course, the 

 suggestion is at present a purely speculative one. But a 

 closer study of the radioactive minerals would in all 

 probability afford further evidence on this important question. 



§ 3. The Material Nature of the Radiations. 



The view that the ray or rays from any system are 

 produced at the moment the system changes has received 

 strong confirmation by the discovery of the electric and 

 magnetic deviability of the a ray. The deviation is in the 

 opposite sense to the /3 or cathode-ray, and the rays thus 

 consist of positively charged bodies projected with great 

 velocity (Rutherford, Phil. Mag., Feb. 1903). The latter 

 was shown to be of the order of 2*5 10 9 cms. per second. The 

 value of e/m, the ratio of the charge of the carrier to its mass, 

 is of the order 6 10 3 . Xow the value of e/m for the cathode- 

 ray is about 10 7 . Assuming that the value of the charge is 

 the same in each case, the apparent mass of the positive 

 projected particle is over 1000 times as great as for the 

 cathode-ray. Now e/m = 10 i for the hydrogen atom in the 

 electrolysis of water. The particle that constitutes the x ray 

 thus behaves as if its mass were of the same order as that o( 

 the hydrogen atom. The a rays from all the radio-elements, 

 and from the various radioactive bodies which they produce, 

 possess analogous properties, and differ only to a slight 

 extent in penetrating power. There are thus strong reasons 



