The men who have been good enough to cooperate with 

 me in the preparation of this volume are masters of their 

 respective subjects ; and what they say represents, in sum- 

 mary form, some of the latest achievements in science. 

 The book does not pretend to be an exhaustive treatise, 

 but it does claim to review some of the more recent and 

 more suggestive work. 



My thanks are due to the distinguished contributors not 

 only for permitting me to use their articles, but for afford- 

 ing me the pleasure and instruction these articles have 

 given me. 



I am also indebted to the editors of the Transactions of 

 the American Institute of Electrical Engineers, the Jour- 

 nal of Industrial and Engineering Chemistry, the Journal 

 of the Washington Academy of Sciences, the Scientific 

 Monthly, Science, and the Journal of the American Medi- 

 cal Association, for permission to reprint some of these 

 articles. 



THE EDITOR. 



