46 ESSAYS ON BACTERIOLOGY. 



largely, if not entirely, due to the action of new prod- 

 ucts formed in the growth and multiplication of the 

 bacteria, and that the peculiar phenomena belonging 

 to the specific diseases and constituting their specificity 

 are chiefiy, if not wholly, due to the peculiar proper- 

 ties of the ptomaines formed by their specific germs. 

 Of these phenomena we have now to consider those 

 two which are of greatest interest to us in our contest 

 with the infectious diseases, namely, self-limitation 

 and immtmity. 



From time immemorial it has been known that 

 there was a group of diseases which were distinctly 

 self -limited; they did not run an indefinite course; 

 they ended of themselves. As the old saying is: "If 

 the patient lived long enough he would recover front 

 that disease." To this class of maladies- belong 

 measles, scarlatina, diphtheria, erysipelas, and others. 

 Here was a great mystery. What was the explana- 

 tion of it? Theories have been advanced, only to be 

 disproved or discarded. Those which have not been 

 disproved have been discarded or held in reserve be- 

 cause there was no way of testing their tratb or falsity. 



With the rise of modern bacteriology th.e liope be- 

 gan to be entertained that we might now come nearer 

 to the solution of the problem. Here again the anal- 

 ogy between the fermentations and the infectious dis- 

 eases threw a light along the way, indicating the di- 

 rection for the investigation to proceed. It is known 

 of certain fermentations not only that they are accom- 



