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 2 + 2H 2 = NH 4 N0 2 . 



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 

 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 Graminese, 

 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 

 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 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 



FIG. 13. Arrangement for 

 studying the nitrifica- 

 tion of sewage. (Miss 

 H. CHICK.) 



