44 PHYSIOLOGY 



cylinder the nitrate bacterium is present. The conversion of ammonia 

 into nitrates by the agency of bacteria has been made the basis of a 

 method of treatment of sewage which is now very largely employed. 

 These different bacteria play an important part in all soils in pre- 

 paring them for the cultivation of crops. 



Is the total capital of combined nitrogen which is worked over 

 by these bacteria and utilised by the whole living world confined to 

 the small quantities produced by atmospheric discharges ? Of late 

 years definite evidence has been brought forward that such is not the 

 case and that organisms exist which can utilise and bring into com- 

 bination the free atmospheric nitrogen itself. Thus certain soils have 

 been found to undergo a gradual enriching in nitrogen although no 

 nitrogenous manure has been applied to them. Winogradsky has 

 shown that this fixation of nitrogen by soils is effected by a distinct 

 micro-organism, which he isolated by growing on gelatinous silica free 

 from any trace of combined nitrogen, so that the organism had to 

 procure its entire nitrogen from the atmosphere. Under such con- 

 ditions the numerous other micro-organisms of the soil died of nitrogen 

 starvation, and only the microbe survived which was able to utilise 

 free nitrogen. This organism, which he called clostridium pasteurianum, 

 grows well on sugar solution if free from ammonia and enriches the 

 solution with combined nitrogen. It is anaerobic, i.e. only grows 

 in the absence of oxygen. In the soil, where oxygen is constantly 

 present, it occurs associated in a sort of symbiosis with two species 

 of bacteria which are aerobic and protect it from the surrounding 

 oxygen. The mechanism by which this organism is able to fix free 

 nitrogen, and the nature of the first product of the assimilation are not 

 yet ascertained. Such an assimilation will serve to the organism as a 

 source of energy, since the application of heat is necessary for the dis- 

 sociation either of ammonium nitrite or of nitrous acid into nitrogen 

 and water, as is seen from the following equation : 



HN0 2 Aq. + 308 Cal. = H + N + 2 + Aq. 

 NH 4 N0 2 Aq. + 602 Cal. = 2N + 4H + 20 + Aq. 



In addition to this spontaneous fixation of nitrogen by humus, a 

 method has long been known to farmers by which the fertility of a 

 soil can be increased without the application of nitrogenous manures. 

 If a plot of land is to be left fallow it is a very usual custom to sow it 

 with some leguminous crop such as sainfoin. Careful experiments by 

 Boussingault, Lawes and Gilbert, and others have shown that the 

 growth of almost any leguminous crop in a soil poor in nitrogen may 

 result not only in the production of a crop containing much combined 

 nitrogen, but also in an actual increase of nitrogen in the soil from 

 which the crop is taken. It was then shown by the last two observers, 



