PREFACE TO THE TENTH EDITION. 



As in previous editions, we have kept before us the adapta- 

 tion of this book to the needs of the beginner. There have 

 been incorporated all advances that we regard as fundamental 

 to an understanding of the accepted principles and practices 

 of modern bacteriology. 



Opinion is now well stabilized upon the matter of variations 

 in bacterial species and the subject has been treated in appro- 

 priate places in both a general and particular manner. 



Since the last edition of this book there have been devel- 

 oped methods for the estimation of the exact acid or alkaline 

 strengths of solutions. These methods are based upon the 

 theory of dissociation of electrolytes under different condi- 

 tions of solution. They are now so simplified as to make 

 them adaptable to the routine estimation and correction of 

 reactions of culture media. In this connection an effort has 

 been made to explain the phenomenon of dissociation in such 

 manner as to make it comprehensible to those not versed in 

 the physics of solutions, and to clarify the steps, and reasons 

 therefor, taken in efforts to determine the reactions of 

 solutions, especially of culture media, by the estimation of 

 their hydrogen-ion content. The significance of pH values 

 as symbols of reaction and the use of indicators for different 

 pH values is explained. 



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