86 Lectures on Bacteria. [^ ix. 



nutrient solutions containing compounds of ammonium, if a 

 small quantity of soil is added at a suitable temperature, the 

 optimum being 37° C, and with constant access of air. 



Thus the formation of saltpetre is a result of the vegetation 

 of Bacteria ; it ceases when these are killed ; it also commences 

 when these Bacteria, artificially reared, are placed by themselves 

 without soil in the proper nutrient solution. From this we must 

 conclude that we have here an oxidation produced by the Bacteria 

 which are widely diffused in the superficial layers of a moist soil. 



The morphology of these Bacteria is not yet clearly ascertained. 

 According to the above-named writers, the individual organism 

 is a very small delicate Micrococcus, somewhat resembling 

 M. aceti, and van Tieghem, in his text-book, has named it 

 M. nitrificans. But the appearance assumed by this form is not 

 clear from the descriptions, and Duclaux speaks of a mixture of 

 different forms. The importance of the processes calls for a 

 more exact study of them, that is, of the question, whether nitrifi- 

 cation is the exclusive function of a distinct species, or of several 

 species and their combinations. 



3. Acetous fermentation (25, p. 504; 40, 41,42). If an acid 

 nutrient solution containing a small percentage of alcohol is 

 exposed to the air at a temperature of about 30-40° C, vinegar 

 is formed in it, that is, the alcohol is oxidised into acetic acid. 

 The fluid is at the same time more or less clouded, and its sur- 

 face covered with a thin colourless membrane. This membrane 

 consists in most pure cases of mother of vinegar. Micrococcus 

 aceti, Bacterium aceti (Arthrobacterium aceti ; Mycoderma aceti 

 in Pasteur's earlier nomenclature). Pasteur showed twenty- 

 five years ago that this Bacterium lives and grows on the organic 

 and mineral substances contained in the solution, and absorbing 

 oxygen from the air oxidises the alcohol into acetic acid. The 

 exact proof was obtained by adding 4 per cent, of alcohol and 

 1-2 per cent, of acetic acid to pure nutrient solutions of the 

 kind described on pages 6r and 67, and then introducing into 

 the liquid an infinitesimal quantity of membrane of mother of 



