INTRODUCTION. 23 



a controversy which lasted more than thirty years. The 

 agency ascribed to yeasts was energetically denied by many, 

 prominent among them Liebig; while it was sustained with 

 vigor by others. The latter extended the original conception 

 to include other sorts of fermentation and the putrefaction 

 of albuminous material. Different kinds of fermentation, 

 with different products, such as acetic acid and butyric acid, 

 were ascribed to the growth of different kinds of microbes. 



These microbes were found to be fungi of various sorts, 

 and chiefly one or another variety of bacteria. The most 

 celebrated among the students of fermentation was Pasteur, 

 the simplicity and kindliness of whose character excite our 

 admiration equally with his devotion to his work. 1 



Before the nature of fermentation was understood the 

 possibility of spontaneous generation had been universally 

 admitted. When vermin of various sorts appeared in putre- 

 fying material the conclusion was drawn that they had their 

 origin directly from it. Although that had long since been 

 disproved in the case of large organisms like worms and 

 frogs, still, as late as the middle of the last century, it was 

 held by many to account for the swarming microscopic life 

 found in fermenting fluids. A flask of meat broth left ex- 

 posed to the air will after a few days contain countless tiny 

 living things, chiefly bacteria. Pasteur and his supporters 

 showed that these bacteria were the progeny of others 

 already in the flask or which had fallen in from the air. 



When the flask of broth was boiled, no development of 

 organisms took place, if the entrance of germs from the 

 atmosphere was prevented. The latter was accomplished 

 by such devices as heating the air, passing it through sul- 

 phuric acid, using a flask with a long twisted neck or by 

 plugging the flask with cotton (Schroder and Von Dusch). 



1 See Louis Pasteur, His Life and Labors by His Son-in-Law, trans- 

 lated by Lady Claude Hamilton. 



