70 FOKMATION OF NITRITES FROM NITRATES 



upon by light with inductance of endothermic chemical reactions 

 and formation of more complex compounds of organic character. 



Our knowledge of the first steps in the assimilation of both carbon 

 and nitrogen so as to take their part in the organic compounds is 

 still very incomplete, but that regarding nitrogen assimilation is 

 much the more fragmentary of the two. 



It is true that the living -cell can by linkage of reactions utilise 

 the energy stored up in the form of carbohydrates to induce endo- 

 thermic reactions and build up fats and proteins. It has already 

 been indicated, and the subject will again be taken up in a later 

 chapter, that a certain amount of carbohydrates can be oxidised, 

 and the energy so set free within the cell can be transferred to reduce 

 another portion of carbohydrate to fat, or to reduce nitrogenous 

 compounds and build in ammo-groups to form proteins. Other 

 examples of such linked reactions are seen in the action of certain 

 bacteria and other unicellular organisms, such as azotobacter, the 

 nitro-bacteria of the nodules of the leguminosae, the philothionic 

 organisms which derive stores of energy from the oxidation of 

 sulphur or reduced sulphur compounds, and the iron bacteria which 

 similarly utilise the energy obtained by oxidation of metallic 

 iron or of ferrous compounds to build up organic carbon compounds 

 from carbon dioxide and water. Such linked reactions require, 

 however, the presence of a living cell containing protoplasm, 

 possessing as its substratum organic compounds containing both 

 carbon and nitrogen in very complex combinations. Moreover, 

 the substrata of reduced compounds so utilised in linked reactions 

 have demanded at earlier epochs the existence of living organisms 

 for their reduction by the conversion of the energy of sunlight. 

 None of these substances could have existed in a planet cooling 

 down from a red-hot condition on account of their chemical insta- 

 bility at higher temperatures. So also all the bound nitrogen in 

 vegetable and animal organisms, and their decomposition products, 

 such as coal, guano, and nitrates, must at one time have existed as 

 atmospheric nitrogen, for no nitrates or nitro- compounds could have 

 withstood the earlier high temperatures. The enormous stores of 

 compounds containing the oxides of nitrogen now used in warfare, 

 agriculture, and industry must have been formed endothermically 

 from atmospheric nitrogen and oxygen with uptake of energy, and 

 whether this occurred through the electric discharge of the thunder- 

 storm or by the agency of living organisms, the first source of the 



