40 



PHYSIOLOGY 



conversion is effected by a group of micro-organisms. There are a number 

 of bacteria (bacterium nitrosomonas) which have the power of converting 

 ammonia into nitrites. Others (bacterium nitro- 

 monas) convert nitrites into nitrates. If sewage 

 matter rich in ammonia is allowed to percolate 

 through a cylinder packed with coke and the process 

 be continued for several weeks, it is found after a 

 time that in its passage through the filter the fluid 

 has lost its ammonia and contains the whole of its 

 nitrogen in the form of nitrate. If the cylinder be 

 tapped (Fig. 13) half-way down, say at K, the fluid 

 will be found to contain, not nitrates, but nitrites. 

 In this conversion the two kinds of microbes men- 

 tioned above are concerned. At the top of the 

 cylinder the nitrous bacterium is present, in the 

 bottom of the 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 preparing 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 quanti- 

 ties 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 combination 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 may be isolated by 

 growing it on gelatinous silica free from any trace 

 of combined nitrogen, so that the organism has 

 to procure its entire nitrogen from the atmo- 

 sphere. Under such conditions the numerous other 

 micro-organisms of the soil die of nitrogen starva- 

 tion, and only the microbe survives which is able to utilise free nitrogen. 

 This organism, which he called dostridium 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 



FIG. 13. Arrangement for 

 studying the nitrifica- 

 tion of sewage. (Miss 

 H. CHICK.) 



