THE NITRO-BACTERIA. 379 



skin adhering firmly to the walls of the vessel. Compared with 

 their powerful oxidising action, the vegetative development of 

 these organisms is astonishingly slight. Spore formation has not 

 been found either in these or in the nitroso-bacteria ; and up to 

 the present no subdivision of the genus Nitrobader into species 

 has been made. 



BURRI and STUTZER (III.) in 1895 obtained from Hanoverian 

 soil a nitro-bacterium which they assert will thrive both on nutrient 

 gelatin and in bouillon, but (so it is said) exhibits no nitrifying 

 action in nutrient media of this kind, as a rule, and, indeed, loses 

 this power entirely, so that when re-transferred into mineral nut- 

 rient solutions it does not attack the nitrites placed at its disposal. 

 A careful examination of such a culture, obtained direct from the 

 above-named chemists, was made in 1896 by S. WINOGRADSKY (X.), 

 who showed that the alleged pure culture contained, not only the 

 nitro-bacterium, but also three other species of (saprophytic) bac- 

 teria which tli rive well in bouillon, a medium in which the nitro- 

 bacterium will not grow. Winogradsky's treatise is recommended 

 to the reader, more particularly because it mentions numerous 

 contingencies likely to arise in working, and render of no avail 

 the trouble bestowed on the nitrifying bacteria by the bacteri- 

 ologist. Furthermore, he gives a new recipe for a medium for the 

 pure cultivation of nitro-bacteria, more convenient in use than 

 gelatinous silica, viz., nitrite agar-agar, i.e. a mineral solution con- 

 taining nitrites and qualified by 1.5 per cent, of agar-agar. 



If the amount of nitrogen oxidised per unit of time be taken 

 as the standard for measuring the chemical energy of these organ- 

 isms, then as Winogradsky ascertained by comparative investiga- 

 tions the nitroso-bacteria will be found the more active of the two. 

 From this fact it is permissible to draw the further deduction that 

 the conversion of the trivalent nitrogen of nitrous acid into penta- 

 valent nitric nitrogen requires the expenditure of a greater amount 

 of internal force than is needed for the converse operation in the 

 oxidation of ammonia to nitrous acid. 



Both nitroso- and nitro-bacteria are always present in the soil, 

 the second type of organism immediately oxidising the nitrous 

 acid generated (from ammonia salts) by the first. Whether nitri- 

 fication begins already in the dung-heap, or has its first inception 

 in the field, is dependent on various circumstances. It will proceed 

 whenever a sufficient quantity of ammonia salts has been produced 

 by the fermentation of urea, provided air has ready access. Thus, 

 H. IMMENDORFF (III.) showed that in the outer layers of manure 

 heaps (especially horse-dung), the production of nitrous acid will set 

 in briskly in a few days. There are ample reasons why the formation 

 of the easily lixiviable nitrates, which may, moreover, expose the 

 materials to wasteful reduction processes, should be prevented in the 

 manure heap. On this account endeavours should be made to mini- 

 mise the aeration of the manure by battening the heaps well down. 



