PREFACE TO SECOND EDITION. 



The preparation of this edition has been made a work 

 of pleasure on account of the many kind words which have 

 been said concerning our first effort to collect the scattered 

 facts pertaining to the chemical factors in the causation of 

 disease. We must be allowed to express our gratification 

 at the general acceptance accorded to the statements which 

 we first made three years ago, and which were then re- 

 garded by many as extremely radical. At that time 

 many of the leading bacteriologists held to the "mechan- 

 ical interference" theory, and regarded the chemical pro- 

 ducts of germs as of some interest, but in no direct way 

 concerned in the causation of disease. Now the fact that 

 a germ is pathogenic is considered to be sufficient evidence 

 that it elaborates poisonous products, and the study of these 

 products is regarded as of the greatest importance in the 

 investigation of the germ and the disease Avliich it causes. 

 The interest in this subject is not confined to a study of 

 the causation of disease, but efforts arc being made to 

 secure immunity from disease and even to effect cures by 

 the employment of the bacterial products. This line of 



