136 WHAT DOES MAN LIVE UPON? 



not always accompanied by the production of light, but in 

 which a quantity of heat, bearing a definite proportion to 

 the amount of oxygen consumed, is always liberated. 

 Nitrogen, on the other hand, has but small affinity for 

 the other substances ; it is scarcely combustible, but readily 

 unites with hydrogen to form ammonia. 



The four elements under consideration form numerous 

 compounds by their union one with another ; but only two 

 classes of these have a very deep importance in relation to 

 the organic world. One of the classes comprehends the 

 substances which are compounded of all four elements. 

 This includes albumen, fibrine, caseine, and gelatine. All 

 animal bodies are formed out of these substances, which 

 when separated from them as dead matter, all pass rapidly 

 by decomposition into water, ammonia, and carbonic acid, 

 which are diffused through the air. The second class, on 

 the other hand, includes the substances devoid of nitrogen, 

 namely gum, sugar, starch, the liquors prepared from 

 them, such as spirit, wine, beer, and, lastly, all the various 

 kinds of fat.* All these merely pass through the animal 

 body, since the carbon and hydrogen are burnt off by 

 means of the oxygen received in respiration, and are expired 

 as carbonic acid and water. By this slow but uninterrupted 

 process of combustion is maintained the heat indispensible 

 to life. But by the recent brilliant discoveries in chemistry 

 and physiology, we have become aware that the animal 

 body is incapable of composing from their elements, or of 

 forming from any other substance excepting caseine, the 

 substances, albumen, fibrine, &c., absolutely necessary to its 

 development and support ; that the animal must indeed 

 receive substances ready prepared, in order to apply them 

 to its nutrition, or to convert them into gelatine for the 



* See Mitchell's " Treatise on the Adulterations of Food, and the 

 Chemical means to detect them," 12mo. London, 1848. 



