On the Radioactivity of Radium and Thorium. 445 



The case of uranium is especially interesting on account 

 of two points. The existence of non-separable activity con- 

 sisting entirely of a rays, as in the case of thorium. The 

 non-existence of a rays in the radiation of uranium X. 

 This is probably an example of a general law that each type 

 of radioactive matter when got by itself, free from the 

 matter which produced it on the one hand and the products 

 of its further change on the other, gives rise to homogeneous 

 rays, and that in all cases the a, ray is the first to be produced, 

 the (3 ray only resulting in the final stages of the disintegra- 

 tion. Uranium gives more definite evidence on this point 

 than thorium, because the period of the change is long enough 

 and the experimental analysis simple enough to enable the 

 chemical separation of the different types of matter involved 

 to be fairly complete. The suggestion was put forward in 

 the first paper on thorium (Phil. Mag. 1902, iv. p. 392) that 

 the non-separable activities of thorium and uranium re- 

 spectively might possibly be caused by the simultaneous 

 production of a second type of matter in the changes in which 

 thorium X and uranium X are produced. Later (ibid. p. 584) 

 it was shown that this explanation is not necessary when 

 radioactivity is considered as an accompaniment of the change 

 occurring. This explanation has now been adopted, but a 

 fuller discussion of the nature of radioactive change is reserved 

 until after the case of radium has been dealt with. 



McGill University, Montreal, Feb. 20, 1903. 



XLIY. A Comparative Study of the Radioactivity of Radium 

 and Thorium. By E. Rutherford, M.A., D.Sc, Macdonald 

 Professor of Physics, McGill University, and F. Soddy, 

 M.A. {Oxon.)*. 



§ 1. '"T^HE elements thorium and radium are very closely 

 Jl allied in radioactive properties, notwithstanding 

 the enormous difference that exists in their relative activity. 

 Both produce radioactive emanations, and both emanations 

 in turn excite activity on surrounding objects, which in an 

 electric field is mainly concentrated on the negative electrode. 

 In the details of their properties, however, they differ very 

 widely; so that the behaviour of a radium compound, com- 

 pared with that of thorium under similar circumstances, often 

 exhibits very striking peculiarities. The explanation already 

 advanced for the case of thorium elucidates satisfactorily 

 everything that has so far been observed for radium: and, 



* Communicated by the Authors. 



