142 LECTURES ON 



wood is burned. It is collected, too, in great qualities in the gas- 

 works of large cities, being formed from the nitrogen of the bitumin- 

 ous coal which is distilled and imperfectly burned in the gas retorts. 

 The common name, spirits of hartshorn, originated in the fact of the 

 preparation of this substance from the horn of deer, (hart.) 



When combustion goes on with full access of oxygen a good deal 

 of nitrogen escapes in the free state, the hydrogen it might unite 

 with to "form ammonia being chiefly appropriated by the more active 

 oxygen. In proportion as the burning proceeds with a limited sup- 

 ply of oxygen, and at a lower temperature, more ammonia is proba- 

 bly formed. In presence of a fixed alkali, as potash, soda, or lime, 

 all the nitrogen of organic bodies may be converted into ammonia; 

 and it is by an ingenious use of this fact that the chemist is enabled 

 to determine with the greatest ease and accuracy the proportions of 

 nitrogen which organic bodies contain. 



As in decay, so in combustion, it easily happens that numerous in- 

 termediate products occur, especially when the supply of oxygen is 

 deficient. The oil, tar, smoke, and soot of ordinary fires are ex- 

 amples. All these substances, however, by access of more oxygen 

 at the proper temperature, may be fully consumed into the same final 

 products as mentioned under decay. The mineral matters of organic 

 bodies remain in this process as ashes. 



The third means of restoring to the gaseous state the elements so- 

 lidified by vegetation is found in the results of the animal functions, 

 viz: in nutrition and respiration. 



The non-nitrogenous food of animals, consisting always of the vege- 

 table carbo-hydrates, oils, &c, or of certain products of their trans- 

 formation, are chiefly burned in the body by the oxygen that the 

 lungs inhale, and the carbonic acid and water thus formed are 

 thrown out of the system with the exhaled air. If one breathes 

 through a tube into a glass bottle, the deposition of moisture proves 

 the existence of water in the expired air; and by forcing the breath 

 through lime-water the formation of the white precipitate of carbo- 

 nate of lime reveals the presence of carbonic acid. 



The lungs and the skin, which also constantly throw off the same 

 substances by perspiration, are the agencies whereby the gaseous 

 products of the oxydation of the food are restored to the atmosphere. 

 The kidneys and lower intestines remove a portion of the waste; the 

 former in the liquid, the latter in the solid form. A small part of 

 the nitrogen, of which animals require constant supplies in their 

 nutriment, is exhaled as ammonia from the lungs and skin; but as 

 the caustic characters possessed by this body, even when in combi- 

 nation with carbonic acid, would not be compatible with its copious 

 separation in the gaseous form, we must look to the liquid or solid 

 dejections for the excretion of nitrogen. In animals we find that the 

 kidneys dispose of this element. In the blood of man and quadru- 

 peds there may be detected a substance which the kidneys collect 

 and discharge from the body in large quantities through the urine. 

 This substance, from its occurrence, is termed urea. When pure, it 

 is a colorless or white body that may be procured in beautiful crys- 



