VI PREFACE. 



have given this work in the past and for the aid which they have tendered for 

 the future. 



I wish to express my thanks to Professor J. S. Ames for ample space in 

 the Physical Laboratory for carrying out the physical part of this work; to 

 Professor J. A. Anderson for a large number of valuable suggestions in con- 

 nection with the work; to Professor A. H. Pfund for advice in connection with 

 the construction of the radiomicrometer; and to Dr. J. Sam Guy, who will 

 continue this work, for aid especially in preparing the various salts and solu- 

 tions of the salts of dysprosium and samarium. 



Harry C. Jones. 

 June 1911. 



