PREFACE TO THE FOURTH EDITION. 



In the present edition the scope and purpose of former editions 

 of the manual will be found unaltered, and, aside from verbal 

 changes where these seemed desirable, only such, modifications 

 as were required to bring the work up to date have been made. 

 The object aimed at in the revision has been to preserve the char- 

 acteristic features of the work, while introducing those modifica- 

 tions and additions which progress demanded. 



Recent advances made it necessary to enlarge the chapters on 

 bacterial poisons and on immunity, and the additional space 

 given to these subjects is justified by their importance not only to 

 the specialist in bacteriology, but to every well-qualified phy- 

 sician as well. The chapter on immunity in particular will be 

 found to be expanded in the matter of immunity proper, and 

 also in regard to the nearly allied processes of the formation of 

 specific agglutinins, precipitins and cytolysins. In the treat- 

 ment of these subjects the aim has been to present the well- 

 established results of investigations, and to state briefly the 

 conflicting views in regard to their theoretical explanation. 

 For, although Ehrlich's side-chain theory seems to have found 

 in America and some other fountries more general acceptance 

 than the other explanations that have been offered, those of 

 Bordet and of Arrhenius are deserving of consideration on ac- 

 count of the important experimental work which has been done 

 in their support, if for no other reason. 



Bacteriology is not a subject which can be learned from books 

 alone or without instructors, and the manual is intended to 

 supplement laboratory work and teaching, but not to supplant 



