1915^ Radio-Activity and Pekiodic System 29 



Still another thought-inspiring fact which would seem to 

 have important bearing on the nature of the atom and hence 

 the meaning of the periodic system is the ease with which 

 certain elements by a change of valence change their chemical 

 character and form distinctive series of salts as if they 

 had been transformed into different elements. This causes 

 some confusion and what would ordinarily be called forcing 

 in the present tabulation of the system, and it will be recalled 

 that Mendeleeff, in his earlier tables, actually placed certain 

 of the metals, as copper and mercury, in two different groups, 

 assigning each two different places. Signs are seen in the 

 work of Barbieri and others of a tendency to place certain of 

 the elements in different groups according to valence. 



I believe that one should keep in mind the idea involved 

 in Patterson-Muir's definition of an element as a collection or 

 group of properties. Thus there are weight, electro-chemical 

 nature, affinity, valence and other properties by which we 

 recognize it and differentiate it from other elements and to 

 which our knowledge of it is necessarily limited. There is a 

 more or less definite gradation in these properties from ele- 

 ment to element, showing an inter-relationship, and yet 

 scarcely in itself justifying the conclusion that any one prop- 

 erty determines the others or that they are dependent upon it. 

 While it is true that it is hardly possible to dissociate these 

 properties 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 enter- 

 ing 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 radio-active substances are paving the way for an 

 understanding of the laws on which this system is based. 



Radio-activity was regarded by Mme. Curie as an atomic 

 property and this was the guiding thread which led to the 

 discovery of radium. Of course, this preceded by a number 



