PREFACE TO THE THIRD EDITION. 
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The interval between the preceding and the present editions of this 
book has been enriched with notable and significant contributions to 
Bacteriology and related subjects. Therefore thorough revision, much 
rewriting and the introduction of new material has been necessary. 
The outstanding event in the diagnostic field is the development of 
the Kahn test. This has met successfully the need for a simplified 
procedure to supplant the Wassermann test and its modifications, and 
the author is deeply gratified to have this important section from 
Dr. Reuben Kahn himself. The demonstration of the etiology of 
scarlet fever, the discovery of the Dick test and the preparation of 
antitoxic sera for this dread disease marks an important era in clinical 
bacteriology. The study of the intimate chemistry of bacterial activity 
has received increasing attention, and many facts of undoubted impor- 
tance have been added to this newly-developed field. 
The entire question of Filterable \'iruses has been subjected to the 
ordeal of careful scrutiny, both through the interest created by the 
work upon Bacteriophage, and more thorough work upon the technique 
of filtration experiments. No inconsiderable part of the earlier work 
has been questioned, thereby, and the entire subject is in an indeter- 
minate state at the present time. 
Correlation of information about the Natural History of microbic 
disease is unifying existing knowledge, and indicating gaps which must 
be closed in before a true science of Preventive Medicine can be estab- 
lished. Indeed, the present seems to be a transitional period in Bac- 
teriology between the remarkable achievements of the past and the 
momentous developments that are to come. 
The writer acknowledges with much pleasure and gratitude his 
obligation to many friends and critics who have pointed out mistakes 
and made important suggestions for this edition. The selection of 
type, the spacing and the arrangement, chosen for legibility, ease of 
perusal and con\enience, are but a few of the many manifestations of 
interest upon the part of the publishers. Those who glance at this 
volume will share the writer's appreciation of these highly important 
details. 
A. I. K. 
Chicago, III. 
