PREFACE TO FOURTH EDITION. 



During the fourteen years which have elapsed since the appear- 

 ance of the first edition of this book, the subject matter of which it 

 treats has increased in importance, has modified our conceptions of 

 disease, and has furnished facts which are now utilized in treatment. 

 Quite naturally, during the growth and development of the chem- 

 istry of the infectious diseases, this science has from time to time 

 changed the relative importance of different phenomena. When the 

 first edition of this book was written it was believed by those most 

 competent to speak on the subject, that the basic products of bacte- 

 rial growth constituted the chief factors in the causation of the in- 

 fectious diseases, but it has been shown by subsequent discoveries 

 that this conception is erroneous, and we now look for the specific 

 bodies among the synthetic substances formed within the cells of the 

 microorganism. This advance in knowledge has rendered the chief 

 title selected for former editions inappropriate, and accounts for the 

 change which we have made therein. The text has for the most part 

 been rewritten with the intention of curtailing the space given to 

 subjects which advanced knowledge has shown to be less important, 

 and also for the purpose of introducing new matter. We regret ex- 

 ceedingly that want of space has compelled us to omit altogether the 

 bibliography found in previous editions. References to the litera- 

 ture employed in the older editions have been omitted, while those 

 bearing upon investigations which have been made since the appear- 

 ance of the last edition are given in footnotes. Those to whom the 

 third edition is accessible will be able to look up every piece of work 

 referred to in this volume and satisfy themselves concerning the in- 

 terpretation which we have placed upon the original contributions. 

 In order to abbreviate as much as possible, we have omitted many 

 details given in the previous editions. This curtailment has of 

 necessity not been uniform throughout the book ; certain chapters 

 having been cut down much more than others. In some instances 

 the desire to take up less space has possibly led us to omit state- 

 ments of considerable importance. Especially is this true of the 

 chapter devoted to poisonous foods. Several new chapters have 



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