April 23, 1915] 



SCIENCE 



591 



nite gradation in these properties from ele- 

 ment to element, showing an inter-rela- 

 tionship, and yet scarcely in itself justi- 

 fying the conclusion that any one property 

 determines the other or that they are de- 

 pendent upon it. While it is true that it is 

 hardly possible to dissociate these prop- 

 erties from some conception of matter, such 

 conception has not yet reached its ultimate 

 analysis and until it has we are dealing 

 with the recognized properties alone. 



In the same year in which the periodic 

 system was forced to adjust itself to a zero 

 group another discovery was entering upon 

 its marvellous development which was to 

 open up new views as to the nature of 

 matter and radically affect the system. 

 The remarkable and illuminating results 

 obtained in the study of radioactive sub- 

 stances are paving the way for an under- 

 standing of the laws on which this system 

 is based. 



Eadioaetivity was regarded by Mme. 

 Curie as an atomic property and this was 

 the guiding thread which led to the dis- 

 covery of radium. Of course, this preceded 

 by a number of years Rutherford's an- 

 nouncement of his theory of successive 

 transformation or the disintegration of the 

 atom. It is a question whether the fact 

 that an atom is undergoing disintegration 

 is to be regarded as a property in the same 

 sense as the mass, valence, etc., but so long 

 as this change can not be induced, changed 

 or stopped and is known to take place only 

 in the case of a fraction of the elements it 

 is certainly distinctive and may be called 

 a property for lack of a better name. There 

 is, however, undoubtedly a cause for this 

 disintegration and this instability may be 

 due to some inherent property of the atom. 



At present there are some thirty-seven 

 radioactive bodies known, with the pos- 

 sibility of still others being identified. 

 Each has distinctive radioactive properties. 



For a number of these the chemical and 

 physical properties are known. Each is an 

 atom hitherto unknown and miist be con- 

 sidered a new element. Of course, the pres- 

 ent accepted arrangement of the periodic 

 system does not provide for so many addi- 

 tional elements and indeed is rather hope- 

 less for even the sixteen rare earth elements. 

 What is to be done with this embarrass- 

 ment of riches ? 



Soddy's study of the grouping in well- 

 known families of a number of the better 

 known radioactive elements according to 

 their chemical properties, combined with a 

 consideration of the kind of disintegration 

 by which it was produced led him to a gen- 

 eralization which would enable one to place 

 correctly any radioactive element whose 

 source was known, and at the same time 

 give an approximation as to its atomic 

 weight. 



Pajans arrived at the same generaliza- 

 tion independently from an examination 

 of the electro-chemical evidence, finding 

 that the product of an a ray change was 

 more electro-positive, while that of a (i ray 

 change was more electro-negative. Similar 

 conclusions from various evidence were 

 reached by Fleck and Russell. 



The generalization is as follows: 



When an a particle is expelled it carries with it 

 two atomic charges of positive electricity and the 

 expulsion of these two positive charges from the 

 atom affects the valency of the product, as Fajana 

 has pointed out, just as in ordinary electro-chem- 

 ical changes of valency. If the atom were 

 initially in G-roup IV., for example, its ion is 

 tetravalent and carries four atomic charges of 

 positive electricity. Two such charges having 

 been expelled with the a particle, the product is in 

 the di-valent Group II., non-separable from 

 radium. The mass in this case is four units less. 

 So with the /3 ray change. The |3 particle is a 

 negative electron and the loss of this single atomic 

 charge of negative electricity increases the posi- 

 tive valency of the product by one. Radium B, 

 for example (in Group IV.), expels a /3 particle 

 and becomes radium G (in Group V.). When- 



