64 A. W. Stewart on 



The actual ))ositions of all these elements in the Periodic 

 System were established for the most part by the work of Dr. 

 Fleck, who carried out the most extensive investigation in this 

 difficult region of the subject. Some idea of the complexity of 

 the problem solved by him may be gained from the fact that 

 certain of the elements which he examined fall to half their 

 original quantity in an extremely short time. A gramme of 

 radium-A, for example, will leave only half a gramme unde- 

 composed after four minutes and twenty seconds. Thus Dr. 

 Fleck had not only lo cope with the noi'nial difficulties of 

 assigning to an element its proper position in the Table but in 

 addition he had to work with the greatest rapidity if any 

 measurable (piantity of his material was to be left intact at the 

 end of his experiments. 



On the basis of Dr. Fleck's results, which were obtained in 

 Soddy's laboratoiy, Prof. Soddy put forward the generalization 

 now known as >Soddy's Law ; but before the enunciation of this 

 is given it may be best to examine a simple case which is already 

 familiar to us. 



Radium belongs to Group 11 of the Periodic Table. In 

 disintegrating, it ejects an alpha-particle and is converted into 

 niton, which occupies a position in the Zero Gronp of the Table. 

 Further, since the ejection of the helium atom entails the loss of 

 four units of atomic weight, niton is four units lighter than 

 radium. Thus in the case of what is called an " alpha ray 

 change " the descendant element occupies a position in the 

 cohmin next-but-one to the left of the parent ; and the atom of 

 the descendant is four U7iits lighter than that of the parent 

 element. 



On the other hand, ivhen an element changes into another 

 by the ejection of an electron {Beta-ray change), the descendant 

 occupies a position one column to the right of the parent. 



The italicized text above gives the Sodciy Law, which governs 

 the character of the relations between the parent ele-ment and its 

 descendant as far as the Periodic Table is concerned. 



