VII EXPERIMENTS IN NUTRITION 77 



The composition of this fluid is not a matter of guess- 

 work, but is the result of careful experiments, and is deter- 

 mined by the following considerations. 



It is obvious that if we are to study alcoholic fermentation 

 sugar must be present,^ since the essence of the process is 

 the formation of alcohol from sugar. 



Then nitrogen in some form as well as carbon, oxygen, 

 and hydrogen must be present, since these four elements 

 enter into the composition of protoplasm, and all but the 

 fiist-named (nitrogen) into that of cellulose, and they are 

 thus required in order that the yeast should live and 

 multiply. The form in which nitrogen can best be assimi- 

 lated was found out by experiment. We saw that in the 

 manufacture of beer the yeast cells obtain their nitrogen 

 largely in the form of soluble proteids : green plants obtain 

 theirs largely in the simple form of nitrates. It was found 

 that while proteids are, so to say, an unnecessarily complex 

 food for Saccharomyces, nitrates are not complex enough, 

 and an ammonia compound is necessary, ammonium tartrate 

 being the most suitable. Thus while Saccharomyces can 

 build up the molecule of protoplasm from less complex food- 

 stuffs than are required by Amoeba, it cannot make use of 

 such comparatively simple compounds as suffice for Haema- 

 tococcus : moreover it appears to be indifferent whether its 

 nitrogen is supplied to it in the form of ammonium tartrate 

 or in the higher form of proteids. 



Then as to the remaining ingredients of the fluid — 

 potassium and calcium phosphate and magnesium suli)hate. 

 If a quantity of yeast is burnt, precisely the same thing 

 happens as when one of the higher animals or plants is 

 subjected to the same process. It first chars by the libera- 



' It is a matter of iinlifference whether cane-sugar or grape-sugar 

 is used. 



