K TO SKCOND EDITION. 



TIIK preparation of this edition has Urn made a work 

 of pleasure on account of the many kind words which have 

 leen said concerning our first effort to collect the smttered 

 facts pertaining to the chemical factors in the causation of 

 di-'ase. \\Y mii-t !< allowed to express our gratification 

 at the "Jeii'Tal acceptance accorde<l to the statements which 

 we first made three yen's ago, and wliieh were then re- 

 isinled ly many as extremely radi<td. At that time 

 many of the leading bacteriologistB held to the " mechan- 

 ical interference" theory, and regarded the chemical pro- 

 duets of IMTIII- a- of some interest, hut in no direct way 

 concerned in the causation of disease. Now the fact that 

 a -crin is pathogenic is considered to l>e sufficient evidence 

 that it elaliorates poisonmi- products, and the study of tin .- 

 pro<luets is regarded a- of the greatest importance in the 

 investigation of the ^crm and the disease which it cau- 

 The interest in this sulijcct is not eonfine<l to a study of 

 the causation (' di-:a-r, hut efforts are In-ill^ made to 

 ire immunity from diseasi- and even to elli-et cures l>y 

 the employment of the li;i<-terial produets. This line of 



