4 PREFACE. 



number of papers, and in a fairly large number of journals in America, 

 Germany, England, and Switzerland. A general discussion of the 

 results thus far obtained would render reference to the work more 

 convenient. The last chapter of this monograph gives in concise form 

 such a discussion and summary. A bibliography of the papers and 

 monographs already published, bearing upon this theory, will make 

 reference to the literature simpler. 



Finally, it gives me great pleasure to thank the Carnegie Institution 

 of Washington for the generous aid with which they have supported 

 these investigations, and without which it would have been impossible 

 to do much of this work. 



HARRY C. JONES. 



