140 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



converted into resinous bodies by losing water; while, again, ammonia 

 through its combination with aldehydes may give rise to nitrogenous 

 bodies, such as alkaloids. We therefore see that in the appropriation of 

 the carbon and the carbon dioxide of the atmosphere the carbon becomes 

 fixed to form these various bodies synthetically in vegetable protoplasm, 

 while the oxygen is liberated. 



As regards the assimilation of hydrogen in the synthetical processes 

 occurring in vegetable cells, this evidently occurs from the decomposition 

 of water, as is proved by the circumstance that the oxygen liberated, 

 while sometimes less, is often in excess of that which is contained in the 

 C0 3 absorbed. Thus, in the vegetable cell the carbo-hydrates, such as 

 cellulose, starch, gum, and sugar, are made by the simultaneous reduction 

 of carbon dioxide and water under the influence of solar radiation. 



For nitrogen, the atmosphere is the sole source, though it may 

 possibly to a certain extent be derived from the nitrates in the soil. 

 When obtained from the atmosphere it is held in solution in the form of 

 salts, possibly in rain-water. All decomposing organic matters set free 

 ammonia, and therefore nitrates, particularly of potassium, are powerful 

 fertilizers, and increase vegetation b}^ supplying the nitrogen which is 

 essential in the development of albuminous bodies and crystallized 

 nitrogenous vegetable constituents. 



Of the minerals which are essential to vegetable life, such as 

 phosphates, silica, salts of lime and magnesium, and alkaline salts, they 

 are obtained partly from the atmosphere and partly from the soil. They 

 are contained in large amounts in all parts of vegetable matter, and will 

 deserve special consideration under the subject of the vegetable diet of 

 the herbivora. The mineral constituents of the soil and atmosphere 

 therefore play an important part in the phenomena of the development 

 of vegetable life. This we have seen to be essentially one of reduction. 

 In separating the oxygen from carbon and hydrogen a portion of their 

 affinity for ox}^gen is restored to these latter elements. For in CO 2 and 

 H 2 this affinity is completely satisfied ; that is, the energy which resides 

 in the atoms of carbon and hydrogen has not been destroyed by combi- 

 nation and transformation, and when these atoms unite with oxygen this 

 energy is dissipated as heat. To reduce these combinations, therefore, 

 the energy thus latent in the form of heat must be restored to the atoms 

 of carbon and hydrogen. Thus vegetables, in decomposing water and 

 carbon dioxide, making use of the heat of the sun, not only convert atoms 

 of carbon, hydrogen, and nitrogen into organic substances, but have at 

 the same time accumulated chemical energy. For all organic compounds 

 are capable of uniting with oxygen ; in other words, are combustible. 

 The energy restored under the name of affinity to the atoms is hence 

 derived from a portion of the solar radiation which is absorbed by plants 



