Vlii PREFACE. 



Wilson, H. A. Wilson, Elster and Geitel, Soddy, Debierne, 

 McLennan, Allan, and many others. 



Their work consists of facts experimentally obtained and 

 theories based upon these facts. Their statements of fact 

 may, emphatically, be believed, and I trust that I have not 

 trifled with them. Their theories are as they would have 

 them acceptable so long as they explain the phenomena 

 observed. As these theories are presented in this book, 

 they cannot be said to rest upon any definite concensus of 

 scientific opinion. The science is too new. Each theory 

 is, rather, the work of some one man who is entitled to 

 speak as the authority paramount on that particular sub- 

 ject. I have taken them as so many defensible theses and 

 have presented them as clearly and forcibly as I might. 



If any portion of this book seems a little too enthusiastic 

 it is due to a vast admiration for the work of these mas- 

 ters of science. 



B. KR 



PREFACE TO THE FIFTH EDITION. 



The rapid sequence of editions through which this book has 

 passed has given me scant opportunity of altering it. I take the 

 opportunity, however, in this edition of correcting certain typo- 

 graphical inadvertencies that have caused me to write many 

 grievous letters of explanation to readers who thought that in 

 them they discovered inconsistencies of statement. I wish to 

 thank most cordially my readers both at home and abroad for 

 their letters of thanks and good-will. In a book that appeals 

 solely to the beauty of human reason and not at all to the emotions, 

 such letters are as unlocked for as they are welcome. I wish to 

 thank my critics ; it is deeply gratifying to me that they have 

 indicated nothing that I ought to retract. Finally, I wish to 

 thank the workers in radio-activity for their kind expressions of 

 praise and esteem. 



R. K. D. 



