PREFACE TO NINTH EDITION. 



In this, the ninth edition of the Principles of Bacteriology, 

 a number of changes have been made, though they do not 

 constitute a departure from the original objects of the 

 work. It still remains a book for the beginner. 



The order in which some of the matter was presented has 

 been changed to what seems a more logical sequence. Several 

 chapters have been fully revised and rewritten, and the 

 section upon the physiological functions of bacteria has been 

 greatly extended. New matter bearing upon hemolysis, 

 complement-fixation, and the Ehrlich conception of the 

 reactions of immunity has been introduced. 



Such new principles and their applications as have been 

 found worthy of serious consideration have been incorporated, 

 and a large amount of old matter, important historically but 

 not otherwise necessary, has been eliminated. 



Nothing has been left undone to improve the book and 

 bring it abreast of the very active subject. It is hoped 

 that in its new form the work will continue to receive the 

 flattering reception accorded to its preceding editions. 



A. C. A. 



University of Pennsylvania, 1915. 



