xii PREFACE TO THE FOURTH EDITION 



species, with especial consideration of their common occurrence, the 

 methods of their detection and recognition, and correlation to the 

 diseases which they incite. 



In other respects the book has been thoroughly gone over. In the 

 section on Biology and 'Technique a few methods, which we ourselves 

 have found it unnecessary to use, have been omitted. A few other 

 methods have Been adjded and a few of the most useful ones ampli- 

 fied. Minor changes/have been made in the section on Immunity. 

 The chapter on Streptococcus has been revised and considerable addi- 

 tions and changes have been made in the chapters on the Paratyphoid 

 and Typhoid 'Bacilli. The more recent work on the Schick Test and 

 on the Determination of Virulence of the Diphtheria Bacillus has 

 been incorporated. Minor changes have been made in the sections 

 on Bacteria in Water and Milk. 



Altogether it has been attempted to bring the material up to date, 

 especially in those features in which we think its greatest usefulness 

 lies, namely, as a guide for the actual performance of bacteriological 

 wort. It is a pleasure again to acknowledge the valuable assistance 

 of Prof . J. G. Hopkins in correcting and rearranging the book. . 



HANS ZINSSER. 



College of Physicians and Surgeons, 

 Columbia University, 

 New York City. 



