ASSIMILATION IN THE DARK 291 



206. The Nitro-Bacteria. 



differ from the species already described, not only from a chemico-physiological, 

 but also from a morphological point of view, being smaller and more slender. 

 The cells are an elongated oval, mostly pear-shaped, 0.5 p in length and 0.15- 

 0.25 p. in breadth, and are therefore among the smallest of all known organisms. 

 In liquid cultures they develop and congregate to form a thin, mucinous 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 Nitrobacter 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 nutrient 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 riitro-bacterium, but also three other 

 species of (saprophytic) bacteria which thrive 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 bacteriologist. 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 containing 

 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 organisms, then as Winogradsky 

 ascertained by comparative investigations 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 

 pentavalent 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 nitrification 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. IMMEN- 

 DORFF (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 lixi viable nitrates, which may, 

 moreover, expose the materials to wasteful reduction processes, should be pre- 

 vented in the manure heap. On this account endeavours should be made to 

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



207. Assimilation in the Dark. 



The incapacity (recorded in 204) of the nitrifying bacteria to grow on 

 nutrient gelatin is, in the main, attributable to their general dibtac>te for organic 



