PREFACE TO THIRD EDITION 
The death of the author of this work at the very time that the 
second edition became exhausted has made it necessary for some- 
one else to undertake the task of revising it. No one could revise 
it as he would have done, but in adding the new material made 
necessary by the growth of agricultural bacteriology, his point of 
view has been kept constantly in mind, and it is hoped that the 
present edition is not very different from what it would have been 
had he revised it. 
The growth of agricultural bacteriology during the last eight 
years has been fully as great as during the period between the 
first and second editions of this book. Perhaps, if the author had 
lived, he would have rewritten large sections of the text. Under 
the circumstances, however, it has seemed best merely to change 
those sections dealing with subjects in which the growth has been 
the greatest. 
Greatest changes have been made in the sections on soil 
bacteriology, on control of milk supplies, on plant diseases, and 
on laboratory technic. The section on soil bacteria had to be 
changed partly because of new knowledge, but also because of 
present-day scepticism concerning much that was once taken for 
granted. In regard to the control of milk supplies, even greater 
changes have been necessary, the teachings of to-day seeming to be 
in some respects exactly the opposite from those of five or six 
years ago. The new knowledge acquired in regard to plant dis- 
eases has made it necessary to rewrite the list of fungous diseases 
given in the second edition. The section on laboratory technic 
has had several changes made in it, primarily to bring it in accord 
with the methods recently recommended in official reports of 
committees appointed by different national societies. 
Vv 
