74 MICROBES AND TOXINS 



well employ, as part of its food, substances already oxidised 

 and incapable in any way of furnishing heat, the conditions 

 being that these are made to enter into a nutritive compound 

 in the manufacture of which heat-furnishing transformations 

 occur in sufficient amount. For example, the nitric bacteria, 

 as Winogradsky has shown, can take their carbon from 

 carbonic acid on condition that at the same time they trans- 

 form nitrous acid into nitric. Similarly, the ferments which 

 fix nitrogen can take up this gas from the air on condition 

 that they destroy at the same time by oxidation sugar or some 

 other hydrocarbon capable of furnishing heat during oxidation." 

 (E. Duclaux). 



Cellular protoplasm elaborates food by means of the 

 diastases which it contains, and has peculiar wants and 

 preferences according to the species. It must therefore be 

 difficult for a microbe to find the particular nourishment which 

 suits it best. The artificial cultures of microbes which are 

 so useful in scientific research and in medicine demand 

 practically for each microbe those food-stuffs which nourish it 

 in nature. 



Nutrition of the Mucedinese (Raulin's Experi- 

 ments). — Raulin succeeded in composing with perfectly pure 

 sugar and mineral salts an artificial medium more favourable 

 to the growth of Aspergillus niger than any occurring in 

 nature. It is thus a cultivation in the fullest sense of the 

 word. 



This medium was prepared by a series of trials and 

 demanded a most admirable patience, for he had to make 



