PERIODIC ARRANGEMENT OF ELEMENTS 177 



that the relation is not an exact one, but only approximate ; 

 for the difference between the actual atomic weight, and 

 that which would follow if one element were a compound 

 of other elements of lower atomic weights, would be caused 

 by the addition of a certain number of electric atoms to 

 the molecule. 



It must be confessed, however, that the basis for specu- 

 lations like these is a slender one ; the sole ground is the 

 undoubted fact that radium produces an emanation 

 which spontaneously changes into helium ; and also that, 

 in doing so, the emanation parts with a large number 

 of corpuscles carrying negative charges. Nevertheless, 

 enough is known to prove that there is a wide field for 

 experiment, and that the harvest will be a rich one; 

 further, the reapers' task will be one of extraordinary 

 interest. 



