STUDIES IN FERMENTATION 81 



The great progress that has been made in our 

 knowledge of fermentation is based on the sound 

 foundations laid down by Pasteur. Brewers are as 

 careful of their special varieties of yeast as stock- 

 raisers are of their breeds of cattle. Poor yeasts 

 produce undesirable fermentations. Many of the 

 impurities of "home brew" and the various alco- 

 holic liquors that are now secretly made and ped- 

 dled are due to the influence of micro-organisms 

 which regular manufacturers had learned how to 

 exclude. The doctrine of the constancy of specific 

 types has proven to be a guide of great value in the 

 solution of problems of fermentation both theo- 

 retical and practical. We shall see further indica- 

 tions of its far-reaching import when we come to 

 consider its relation to infectious disease. 



It was when Pasteur was in the midst of his 

 studies of fermentation that he had an opportunity 



r 



to join the Faculty of the Ecole Normale of Paris. 

 He decided to accept the new position, and in 1857 

 we find him installed as a professor in the institu- 

 tion which it was once his dream to be able to at- 

 tend. Although his duties in the school were many > 

 he made a laboratory out of the attic which we 

 have previously mentioned and began work on the 

 fermentation of alcohol. During a part of the 



