viii PREFACE 



unlike the electromagnetic theory of Light, must be 

 regarded for the present as no better than a working 

 hypothesis. It is a hypothesis of stimulating char- 

 acter, and of great probability, but its truth is still 

 an open question that is probably not going to be 

 speedily closed. 



I am indebted to Professor Larmor for information 

 about some recent theoretical work, and for the 

 substance of Appendix M ; I have also to thank Mr. 

 Gwilym Owen, of the University of Liverpool, for 

 assistance in the revision of the proof. 



As "an introduction to an allied subject, the book 

 called Becquerel Rays, by the Hon. K. J. Strutt, 

 is to be recommended ; and the standard treatise 

 of Professor Eutherford on Radioactivity is well 

 known. I have avoided dealing at length with 

 the topics so conveniently to be found in these 

 writings. I have also barely touched on the large 

 subject of ' ionisation ' : it was difficult to do so 

 without overloading the principles with detail, a 

 knowledge of which is nevertheless necessary for 

 investigators. The treatise of Prof. J. J. Thomson, 

 The Discharge of Electricity through Gases, con- 

 tains a mass of information and original work 

 highly valued by physicists. 



The present book is intended throughout for 

 students of general physics, and in places for special- 

 ists, but most of it may be taken as an exposition of 

 a subject of inevitable interest to all educated men. 



OLIVER LODGE. 

 THE UNIVERSITY OF BIRMINGHAM, 

 July, 1906. 



