EXPERIMENTS IN NUTRITION 77 



tion of carbon, then as the heat is continued the carbon 

 is completely consumed, going off by combination with the 

 oxygen of the air in the form of carbon dioxide ; at the 

 same time the nitrogen is given off mostly as nitrogen gas, 

 the hydrogen by union with atmospheric oxygen as water- 

 vapour, and the sulphur as sulphurous acid or sulphur 

 dioxide (SO 2 ). Finally, nothing is left but a small quantity 

 of white ash which is found by analysis to contain phos- 

 phoric acid, potash, lime, and magnesia ; i.e., precisely the 

 ingredients of the three mineral constituents of Pasteur's solu- 

 tion with the exception of sulphur, which, as already stated, 

 is given off during the process of burning as sulphur dioxide. 



Thus the principle of construction of an artificial nutrient 

 solution such as Pasteur's is that it should contain all the 

 elements existing in the organism it is designed to support ; 

 or in other words, the substances by the combination of 

 which the waste of the organism due to destructive meta- 

 bolism may be made good. 



That Pasteur's solution exactly fulfils these requirements 

 may be proved by omitting one or other of the constituents 

 from it, and finding out how the omission affects the well- 

 being of Saccharomyces. 



If the sugar is left out the yeast-cells grow and multiply, 

 but with great slowness. This shows that sugar is not 

 necessary to the life of the organism, but only to that active 

 condition which accompanies fermentation. A glance at 

 the composition of Pasteur's solution will show that all the 

 necessary elements are supplied without sugar. 



Omission of ammonium tartrate is fatal : without it the 

 cells neither grow nor multiply. This, of course, is just 

 what one would expect since, apart from ammonium tartrate, 

 the fluid contains no nitrogen without which the molecules 

 of protoplasm cannot be built up. 



