THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 43 



factors which result in the production of combined nitrogen. The 

 passage of electric sparks or of the silent discharge through moist 

 air leads to the production of ammonium 

 nitrite. 



N 



2H 2 



NHN0 



Every thunderstorm, therefore, will result in 

 the production of small quantities of ammo- 

 nium nitrite, which will be washed down with 

 the rain and serve as a source of combined 



i 



nitrogen to the soil. Every decaying vege- 



table or animal tissue serves as a source of 



ammonia, so that from various causes the 



soil may contain nitrogen in the form of 



ammonia or of ammonium nitrite. These 



forms of combined nitrogen are not, however, 



suitable for all classes. of plants. Most moulds 



can assimilate ammonia as ammonium car- 



bonate or as amino-acids or amines, provided 



that they are supplied at the same time with 



sugar, the oxidation of which will serve them 



as a source of energy. Some moulds, many of 



the higher plants, and especially the Gramineae, 



which include the food-producing cereals, 



require their nitrogen in the condition of 



nitrates. It is necessary, therefore, that the 



ammonia or nitrites in the soil shall be con- 



verted into this highly oxidised form. This 



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 nitromonas) 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 FIG. 13. Arrangement for 



time that in its passage through the filter the frying ^nitr^ca- 



fluid has lost its ammonia and contains the H. CHICK.) 



whole of its nitrogen in the form of nitrate. 



If the cylinder be tapped (Fig. 13) half-way down, say at A, the fluid 



will be found to contain, not nitrates, but nitrites. In this conversion 



the two kinds of microbes mentioned above are concerned. At the top 



of the cylinder the nitrous bacterium is present, in the bottom of the 



