ALTERATIONS WHICH THE NUTRIMENT UNDERGOES. 551 



It has, however, been shown, more especially in the 

 case of yeast, by the investigations of Pasteur, Schiit- 

 zenberger, Mayer, and others,* that the amount of 

 nitrogen gradually diminishes when yeast is cultivated 

 in a pure saccharine solution ; and this is true not only 

 as regards the percentage quantity of nitrogen, but also 

 as regards its absolute amount ; it therefore follows that 

 nitrogenous materials are separated as excreta, and 

 disappear in a gaseous form. Such a loss of nitrogen 

 often occurs, when there is a relatively quick and copious 

 formation of volatile nitrogenous substances which is not 

 compensated for by the assimilation of nitrogen by the 

 cells. Further, if those nutrient materials which can 

 furnish carbon to the cells are absent, all those nitro- 

 genous products of decomposition which do not at the" 

 same time contain utilisable carbon in their molecules 

 (for example, ammonium salts, urea, oxamide), must 

 remain as useless excreta; and in such a case, the 

 diminution of the nitrogenous material is very readily 

 noticeable, but in reality only because the carbon is not 

 used up in such a sparing manner, and because the 

 continuous loss of the carbonaceous gases leads to the 

 exhaustion of this element. Finally, the amount of 

 other nitrogenous substances in the nutrient material 

 exerts an influence; if numerous highly nutritious 

 nitrogenous bodies are present, nitrogenous molecules of 

 an excrementitious character will appear much sooner, 

 and, being less suitable as nutrient materials, will not be 

 further used up. In cases of necessity, however, part 

 of the nitrogenous decomposition products may probably 

 again form nutrient material, and in this way there is a 

 very parsimonious circulation of the material. 



These facts enable us to some extent to under- Peculiar cir- 

 stand Bolton's experiments, referred to above, in which nitrogenous 

 some forms of bacteria can live and multiply rapidly in matenal - 

 pure distilled water, i.e., where an extremely minute 

 quantity of nutrient material is present. These experi- 



* Pasteur, Ann.Chim.phys.,(3) 58, 507. Schiitzenberger, Compt.rend., 

 1874, vol. 78. Mayer, (Inters, uber die alkokoL Gahrung: Heidelberg, 



