114 MODERN THEORIES OF ELECTRICITY AND MATTER, 



In the theory of atomic transformation the emanation of radium 

 is the first product of disintegration and is transformed in its turn. 

 The induced radioactivity to which it gives rise is considered as due 

 to a solid radioactive material, which results from the transformation 

 of the radium emanation. Three ditferent radioactive materials are 

 distinguished in the induced radioactivity, w^hich constitute three 

 successive terms of the transformation. Each transformation is also 

 accompanied by the emission of rays, and the expelled particles are 

 also counted among the resulting products. 



Induced radioactivity does not disappear completely; but there 

 remains after the lapse of a day a very feeble residue which persists. 

 in part for years, and which is believed to be adding new terms to the 

 series of successive transformations. 



A new fact of great interest has come to the support of the theory 

 of the transmutation of radioactive substances, and has, indeed, made 

 it almost indispensable. It has been proved that radium, a per- 

 fectly definite chemical element, produces continually another per- 

 fectly definite chemical element, helium (Ramsay and.Soddy). It 

 is admitted that helium is one of the products of the disintegration 

 of the atom of radium, and it is noteworthy that helium occurs in 

 all the radium-bearing minerals. 



The theory of the radioactive transformation lias been extended 

 to all the radioactive bodies, and investigations have been made to 

 determine if the radioactive substances heretofore considered as ele- 

 ments are not to be derived from one another. The origin of radium 

 itself has been sought in uranium. It is well known that radium is 

 found in the uranium-bearing minerals, and it appears from recent 

 researches that the proportion between the quantities of radium and 

 uranium is the same in all these minerals. Uraniiun may, then, be 

 thought of as a mother substance, which disintegrates Avith extreme 

 slowness, giving place to the production of radium and the products 

 which succeed it. It appears also to be probable that the last term 

 of the radioactive series is polonium. It may be recalled that ura- 

 nium was the substance in which the property of radioactivity was 

 discovered by M. Becquerel, and polonium is the first new substance 

 which was discovered by the aid of the phenomenon of radioactivity. 



A series of analogous considerations has been established for an- 

 other radioactive substance — thorium. In this case thorium as a 

 primary substance generates radio-thorium, a substance recently dis- 

 covered, which gives rise to the gaseous radioactive emanation of 

 thorium and various products of radioactivity induced by this emana- 

 tion. Actinium also gives place to a series of transformations simi- 

 lar to those of thorium, and it, like radium, produces helium. 



All the radioactive substances which have been studied sufficiently 

 from the point of view of their disintegration follow a law of decreas- 



