PASTEUR AND FERMENTATION 13 



could undergo different kinds of fermentation, that each 

 was associated with a different kind of microscopic or- 

 ganism, and that he could bring about whichever t3rpe of 

 fermentation he desired by introducing the proper kind of 

 "yeast," as he called all such microorganisms. 

 The housekeeper should remember this. Milk, she well 

 knows, ordinarily sours with a smooth, firm curd. Occa- 

 sionally, however, the curd produced is full of bubbles 

 and has a cheesy odor. On rare occasions milk may go 

 bad without curdhng at all ; while if kept in an air-tight 

 container it may decompose in such a way as to smell like 

 rancid butter. Each of these types of decomposition is 

 due to a different kind of microorganism; and it was 

 Pasteur who first realized this. Pasteur's demonstration 

 of the fact completely disposed of Liebig's theory of 

 fermentation. 



Pasteur regarded this work as a mere side line, and 

 planned shortly to get back to his chosen field. But the 

 Fates willed differently. For at about this time (1865) 

 France was quite suddenly threatened with the loss of 

 one of its chief industries, silk-raising, on account of the 

 ravages of a disease among the silkworms. This disease, 

 known as pebrine, had bafHed some of the best minds of 

 France, and its ravages were becoming so serious as to 

 cause widespread alarm. Then some one thought of Pasteur, 

 and he was asked to study the problem. He hesitated for 

 some time; for he foresaw that if he went so far afield 

 in his work he might never get back to chemistry. But he 

 finally agreed. And he solved the problem — not in a day, 

 or a month, or a year even, but soon enough to save the 

 silkworm industry. It took him five years, and dis- 



