PASTEUE. 31 



effectually disprove the molecular motion theory. He, there- 

 fore added a small quantity of yeast ; as pure as possible, to 

 some cultivation fluid which had been effectually sterilized by 

 heat and contained no gluten, and carefully sealed it. After 

 this had passed through the stage of fermentation, another 

 flask was prepared in the same way; and under the utmost 

 precautions that nothing else should be introduced from with- 

 out, a drop was taken from the first with a pipette, and added 

 to the second, and so on, to the fiftieth generation, and in many 

 of his experiments more than this number. These flasks 

 were, of course, kept at the proper temperature for the develop- 

 ment of fermentation. The result of this series of experiments 

 seemed to satisfy Liebig's basis for the recognition of living 

 beings; the yeast plant reproducing itself after its own form, 

 for generation after generation, continually, and after the 

 fiftieth or the one hundredth generation, again producing its 

 characteristic effects upon must or wort, namely, vinous fer- 

 mentation. They did more than this. It is well known that, 

 under certain conditions of temperature, vinous fermentation 

 passes into acetic fermentation, with the production of vinegar. 

 Now, in his investigations, Pasteur found that he was able to 

 separate these forms of fermentation by special management 

 of the cultures. By taking at the beginning a fermentation 

 contaminated by the acetic process and keeping the temperature 

 high, the acetic acid plant, the microderma, flourished, while 

 the vinous plant, the torula, gradually died out. He thus 

 rendered the conditions unfavorable to the one and favorable 

 to the other ; and by this operation and the reverse one he 

 succeeded, after passing the culture through many generations, 

 in obtaining the vinous yeast plant entirely pure, and the 

 acetic yeast plant entirely pure, each of which would al- 

 ways produce its characteristic fermentation and nothing else. 

 And what is more, he found the plants to be characterized by 



