xxii PREFACE. 



and other acids and bases have been retained in this volume, 

 adding the chemical formula where, as in analytical statements, 

 a doubt as to their meaning might arise. Assuredly, the diffu- 

 sion of scientific knowledge should not be needlessly hindered 

 by the adoption of a pedantic mode of presentation. 



The great breadth of the subject of this volume has ren- 

 dered inadvisable any extended bibliography, such as it has of 

 late become customary to add to works of this kind. References 

 have therefore been restricted to publications specially discussed, 

 and to such as are not widely known on account of limited 

 circulation. 



The author's warmest acknowledgments are due to Professor 

 R. H. Loughridge, of the University of California, for effi- 

 cient and sympathetic assistance, both in the revision of the 

 manuscript, and active personal help in the preparation of the 

 illustrations. \Yithout his cooperation the preparation and 

 publication of the volume would have been much longer de- 

 layed. 



Acknowledgments are also due for helpful suggestions and 

 criticism to Professors L. H. Bailey, of Cornell University, 

 F. H. King, of Wisconsin, and Jacques Loeb of the University 

 of California. 



E. W. HILGARD. 



BERKELEY, CALIFORNIA, 

 November 15, 1905. 



