NUTRITION 253 



converted by a special organism, Nitrosomonas, into nitrite and this nitrite again 

 into nitrate by another organism, Nitrobacter. These bacteria are always present 

 in normal soil and act with such rapidity that only traces of either ammonia or 

 nitrite can be detected in the soil. 



The fact can readily be observed by adding about half a gram of soil to 50 c.c. of a culture 



fluid of the following composition : 



Ammonium sulphate - 0'5 Magnesium sulphate - O'l 



Sodium chloride - - - 0'5 Ferrous sulphate - - O'l 



Potassium acid phosphate - 0"25 Water to - - - - 1,000 



and adding to 50 c.c. about half a gram of solid magnesium carbonate to preserve neutrality. 



After four weeks or so, the ammonia will be found to have disappeared and nitrate to have 



taken its place. The existence of the latter can be shown by the reaction with diphenylamiue 



sulphate in sulphuric acid. 



The chemical process occurring is unknown, but the bacteria concerned 

 have rather extraordinary properties. Carbon dioxide will serve as source of 

 carbon, and in fact it seems that, in cultures in vitro, other more complex 

 carbon compounds are injurious, especially glucose or peptone. But, in order- 

 to synthesise cell stuffs from carbon dioxide, a supply of energy from without is 

 necessary. Light is out of the question, since the organisms do not possess 

 chlorophyll and, in point of fact, light is actually fatal to them. It has been 

 suggested that the oxidation of ammonia and of nitrite might afford sufficient 

 energy. The injurious effect of organic matter only applies to artificial cultures ; 

 in the soil, glucose has a beneficial effect, although other sugars are inert and 

 nitrogen compounds injurious. The organisms are killed by a temperature of 

 45 C. or by the absence of oxygen. 



Preparations containing nitrifying bacteria and called "nitragin" have been sold for the 

 purpose of improving the soil, but their beneficial effect is doubtful (see Miss Dawson's 

 investigations, 1898 and 1900). Probably most soils already contain abundance of the 

 organisms, and infertility is due to other causes (see Russell's monograph, 1912, p. 98). 



The normal use of the nitrates is to form plant proteins, but, in the absence 

 of oxygen, they rapidly disappear if not made use of. They are converted back 

 to nitrates and ammonia on the one hand, as shown by the dotted lines on the 

 diagram, and to free nitrogen, on the other hand, as shown by the thin line. Many 

 various forms of bacteria are concerned in the process, producing also a number 

 of substances other than those named. 



On account of the importance of nitrates as food for plants, and indirectly for 

 animals, any means of obtaining them from the atmosphere by non-living agency 

 is of value. It has long been known that the oxygen and nitrogen of the air, in 

 presence of water vapour, can be caused to combine by the electric spark, forming 

 nitric acid. Of recent years, the process has been developed on a commercial 

 scale by the use of large electric arcs, spread out by magnetic action ; considerable 

 quantities of nitric acid are made yearly in situations where there is abundant 

 water power. In Birkeland's process, worked chiefly in Norway, the arc is 

 produced by an electromotive force of 5,000 volts and is spread out into a disc 

 of 2 m. in diameter, having a temperature of some 3,000. Again, Calcium 

 cyanamide is formed when nitrogen is passed over calcium carbide, heated to 1,000. 

 Calcium carbide itself is produced in the electric furnace from lime and carbon. 

 Calcium cyanamide gives ammonia when acted on by water and was, at one time, 

 advocated for the purpose of supplying nitrogen to plants. Another process, 

 apparently of great efficiency, has recently been discovered by Haber (1913). 

 By compressing a mixture of hydrogen and nitrogen and causing it to be acted 

 on by a certain catalyst at a high temperature, ammonia is formed by direct 

 combination, in large proportion. By passing this over another catalyst, it is 

 oxidised to nitric acid. 



THE THREE CLASSES OF ORGANIC FOOD-STUFFS 



For the food of the higher animals, it is usually of advantage to make up the 

 requirements by a combination of certain proportions of fat, carbohydrate, and 

 protein. Fat is of value on account of its high potential energy, being less 



