PEEFACE. 



The historical method of treatment is the death of clear 

 exposition. Consequently, I have given scant considera- 

 tion to the evolution of the idea in time compared with its 

 evolution from the standpoint of simplicity of apprehen- 

 sion. Furthermore, what a man " wants to know " is less 

 the doer of the work than the work done. I have, there- 

 fore, made the workers incidental to their work. Never- 

 theless, these men represent the very elite of the army of 

 science. It is doubtful whether in any other field of hu- 

 man effort there are to be found so many men of tran- 

 scendent genius, magnificent training and splendid balance 

 of judgment as are to be found in the science of radio- 

 activity. It often happens in science that when a new 

 Alaska is discovered there is a rush of " tenderfeet " to 

 the district and it becomes difficult to extract from the 

 mass of material presented the pure metal from the " fool's 

 gold." This was notably the case with Rontgen's orig- 

 inal discovery, but it has been equally not the case since 

 radio-activity became a distinct science. These men are 

 the pride of their universities; and their names and work 

 will long hold out " against the wreckful siege of battering 

 days." Such are: J. J. Thomson, Becquerel, M. et Mde. 

 Curie, Rutherford, Ramsay, Crookes, Lodge, C. T. R. 



(vii) 



