208 ORGANIC FOODSTUFFS. 



instance, more than fructose. According to E. PRIOR and 

 H. SCHULZE (I.), the permeability of the cell membrane varies in 

 the several species of yeast. 



The amount of carbohydrates consumed in the formation of 

 cells depends on the rate of reproduction, and therefore on 

 the nature and extent of the influences controlling same. 

 PASTEUR (VII.), for a series of experiments, calculated the 

 consumption for this purpose to be about i per cent, of the total 

 saccharose consumed ; and BALLING (I.) stated that 5.323 parts 

 of dry matter of yeast are formed for every 100 parts of wort 

 extract (not only carbohydrates) disappearing in primary fer- 

 mentation during the reproduction of bottom-fermentation yeast. 

 In an experiment by GILTAY and ABERSON (II.), one part of yeast 

 was obtained for every 3.8 parts of the total sugar consumed. 



262. Inorganic Sources of Nitrogen. 



When, in the course of his controversy with Liebig on the 

 character of alcoholic fermentation, and on the nature of yeast 

 as a living organism (see p. 121, vol. i.), PASTEUR (XXIII.), in 

 1858, made his victorious discovery that this ferment is also 

 active in a solution containing nitrogen solely in the form of 

 ammonium tartrate, the term "yeast " was still very vague, the 

 question whether wine yaast or beer yeast consisted of several 

 species of organisms probably differing considerably in their food- 

 stuff requirements had not come up for discussion, and there was 

 no reliable means available for separating such a mixture of 

 species into its components, and then examining the latter 

 separately. Hence no clear light could be thrown on the matter 

 by argument on the point of these observations. LIEBIG 's (II.) 

 statement in 1869 that he failed to obtain either fermentation or 

 reproduction of the sowing, in an accurate repetition of Pasteur's 

 experiment, was, in the opinion of the latter (XXIV.), sufficiently 

 disposed of by the offer to perform the experiment again in the 

 presence of any trustworthy person appointed by his opponent, 

 and produce as much yeast as the latter " could reasonably 

 desire." An objection urged by MILLON (II.) was controverted 

 in [864 by DUCLAUX (XVI.), and by degrees Pasteur's assump- 

 tion that yeast is able to satisfy its nitrogen requirements from 

 inorganic sources exclusively, assumed the position of an un- 

 assailable law, observations to the contrary being reported with 

 diffidence. A. MAYER (I.), whose researches on the nitrogen 

 requirement of yeast in 1869 led him to adopt substantially the 

 same opinion as Pasteur, observed as did also the latter, and 

 subsequently NAEGELI (IV.) as well that " the nutrition of 

 yeast at the expense of ammonium salts always proceeds with 

 somewhat greater difficulty than with nitrogenous yeast extract," 

 and added, " in the former case a large?' number of well organised 



