INTRODUCTION xv 



broader and more fundamental than those known 

 as ' electric ' are indirectly involved ; and we are 

 now beginning to have some hope of obtaining 

 unexpected answers to riddles such as those con- 

 cerning the fundamental properties of matter which 

 have proposed themselves for solution throughout 

 the history of civilisation. Problems of this kind 

 have aroused interest and attention ever since men 

 began to escape from the struggle for bare exist- 

 ence that most immediately practical of all occupa- 

 tions and felt free to devote themselves, some to 

 art, some to literature, some to the accumulation 

 of superfluous wealth, and some to the gratuitous 

 pursuit of philosophical speculation,, exact experi- 

 ment, and pure theory. To this comparatively 

 leisured group I now address myself. 



