Fermentation and Kindred Phenomena. 79 



mentation, as there are several points which deserve attention. 

 We have seen that the yeast cell has the power of transforming 

 sugar into spirit and carbonic acid. Are these the only changes 

 it produces ? Pasteur's elegant researches have shown the con- 

 trary. He has proved that other substances are always found 

 in fermented liquids. One of these is glycerine, another succinic 

 acid. Then we have a mixture of other bodies which are 

 separated during the distillation of spirit, and are in chemical 

 properties allied to that substance. The mixture is termed 

 fusel oil. 



The yeast cell is composed, as I have explained, of two parts 

 essentially, viz., a bag or lining membrane of cellulose and an 

 interior of protoplasm. During fermentation, yeast is constantly 

 multiplying, so that its weight at the close of the operation is 

 six or seven times greater than it was at the commencement. 

 It is obvious that it must derive its nourishment from the 

 fermenting liquid. 



Pasteur showed by the most careful and convincing experi- 

 ments that the cellulose envelope was derived directly from the 

 sugar. We know, in fact, that a very close relationship exists 

 between the two bodies, and that their mutual transformation 

 is constantly occurring in the vegetable kingdom. But a 

 difficulty arises as regards the protoplasm, for it contains 

 nitrogen, and that element is absent from sugar. 



Here again Pasteur has given the correct explanation, and 

 has shown that yeast will not thrive for any length of time in a 

 pure sugar solution, but requires for its nourishment certain 

 salts and nitrogenous substances. These it finds in the juices 

 of fruits or in malt infusion, but if fermentation is to be con- 

 ducted with a pure sugar solution they must be added, at least 

 if the fermentation is to continue. 



Pasteur after various experiments succeeded in producing an 

 artificial medium in which yeast grows luxuriantly. It contains 

 in addition to water and sugar, tartrate of ammonium and yeast 

 ash, or in place of the latter an artificial ash containing the same 

 salts. We may compare with perfect propriety the ammonium 

 tartrate and yeast ash to artificial manures, which are now used 

 so extensively in agriculture. 



