506 



BIOLOGY OF THE MICRO-ORGANISMS. 



tion made by Pasteur in 1858, that racemic acid, 

 which is a combination of two forms of tartaric acid, 

 the one rotating to the right, and the other to the left, 

 was able to nourish the mould fungi, in the form of 

 racimate of ammonia, but that it was only the form 

 which rotated to the right which is taken up by the 

 fungi, while the other is left in the nutrient fluid. 



It is very difficult to compare the nutrient value of 

 the various sources of carbon, because they evidently 

 vary in their action in accordance with variations in the 

 sources of nitrogen ; and again, if we take care that the 

 nitrogenous materials are the same it becomes a ques- 

 tion whether they may not be assimilated in another 

 way when the carbon sources are different. Hence we 

 would apparently obtain a more exact mode of comparison 

 if we combined the sources of carbon and nitrogen, and 

 then subjected some of these combinations to compara- 

 tive experiments. In this way Nageli has constructed 

 the following scale, proceeding from the best to the 

 worst nutritive materials: 1. Albumen (peptone) and 

 sugar. 2. Leucine and sugar. 3. Tartrate of am- 

 monia, or sal ammoniac and sugar. 4. Albumen (pep- 

 tone). 5. Leucine. 6. Tartrate of ammonia; succinate 

 of ammonia ; asparagine. 7. Acetate of ammonia. 



Thus albuminous materials, and those belonging to 

 the group of carbo-hydrates, seem to be the normal nutri- 

 tive sources of the mould fungi, and it is chiefly these 

 materials on which they feed under normal circum- 

 stances. But, on the other hand, it is noteworthy how 

 widely the nutrient materials may vary, the fungi sub- 

 sisting with ease on very different chemical substances, 

 and also how the mould fungi seem to be able to main- 

 tain their existence as the result of their ability to 

 obtain nutriment from the most different sorts of 

 chemical substances. 



The supply of hydrogen and of combined oxygen is 

 partly provided for by the above-mentioned carbon and 

 nitrogen compounds, and partly by water and free 

 oxygen. Sulphur also takes part in the composition of 

 the organic constituents of the mould fungi, and is 



