THE CIRCULATION OF FOOD MATERIALS 299 



average, but the fact that the most important end-product of decomposition 

 is a gas (CO 2 ) is of great value in ensuring a rapid transference of carbon 

 from one locality to another, while at the same time the mobile nature 

 of water and its ready evaporation are factors of great importance in 

 maintaining the balance of nature. 



Plants are able to assimilate the ash constituents and compounds of 

 nitrogen only by means of the energy derived from the combustion of the 

 organic substance which is produced by means of solar radiation. Certain 

 organisms are even able to assimilate free nitrogen by means of chemical 

 energy, while on the other hand the decomposition of the synthesized 

 nitrogenous products may render available a larger or smaller supply of 

 kinetic energy ; and indeed a decomposition of nitrogenous organic com- 

 pounds which may or may not involve a liberation of free nitrogen is an 

 absolute necessity in the balance of nature. On the other hand, the 

 re-combination of free nitrogen (Sect. 68) can be brought about by certain 

 organisms and by electrical discharges. The chemical energy contained in 

 the organic or inorganic substances produced by the partial or complete 

 decomposition of organized bodies is in all cases derived from the work 

 done by the absorbed sunlight. Similarly, it is stored solar energy which 

 a nitrite bacterium derives from the oxidation of ammonia, and which 

 enables it to build up organic substance from carbon dioxide and water 1 . 

 This is the case again when ammonium nitrite formed in the air by electric 

 discharges is oxidized, for the energy of the lightning comes indirectly from 

 the sun. If a sufficient supply of oxidizable inorganic compounds was 

 deposited as the earth cooled, the oxidation of these might have afforded 

 the energy necessary for the existence of the first and most primitive 

 organisms, so that these need riot necessarily have been at the outset 

 furnished with the power of assimilating carbon dioxide (cf. Sect. 5). 



The perpetual changes on the surface of the earth are by no means 

 entirely due to the existence of life upon it. Indeed, a knowledge of 

 the changes to which inorganic nature is subject is of the utmost impor- 

 tance for a clear comprehension of vital phenomena ; but the former is 

 more the province of the chemist, the physicist, and the astronomer than 

 of the physiologist, though it must be remembered that these are convenient 

 limitations established by man, and not by nature, and that a comprehensive 

 view of the entire cosmic scheme is possible only by disregarding any 

 such artificial distinctions. 



Stationary chemical and physical equilibrium may be maintained 

 for longer or shorter periods both in animate and inanimate nature. Thus in 

 a living oak tree, a particle of carbon may be retained as a fixed constituent 



' I'feffer, Studien z. Energetik, 1892, p. 206. 



