COMPOSITION OF ATMOSPHERIC AIR. 



the lungs The venous blood, which is to be subjected to the, 

 salutary influence of this air, arrives, at the same time, in the lit- 

 tle vessels, which ramify in every direction over the sides of the 

 cells; consequently, it is through the very sides of these capil- 

 lary vessels that the air acts upon this fluid. 



21. The blood coming to the lungs is of a blackish red color, 

 and is not fit to support life in the organs ; but so soon as it comes 

 in contact with the air it changes its nature ; its color becomes o. 

 a bright red, regains its vivifying properties and acqufres all the 

 characteristics of arterial blood 



22. The atmospheric air which thus enters into the lungs, and 

 there produces so remarkable a phenomenon, is chiefly composed 

 of two substances which differ very much from each other ; 

 namely oxt/ye , and azote or nitrcyfu. 



23. Though the oxygen which enters into the composition of 

 the air forms but about one fifth (21 parts in the 100,) it is its 

 most important part, ft is to the oxygen that the air owes its 

 property of supporting life, and of sustaining the burning of com- 

 bustible bodies when inflamed. 



24. Azote, or Mtroyen, which enters into the composition ol 

 the air in the proportion of 79 parts in a 100, is unfit for respira- 

 tion, and incapable of supporting combustion. It seems to serv * 

 only to dilute the oxygen, and thus mitigate the otherwise too 

 irritating action of this gas 



25. By being breathed the air changes its nature; its oxygen 

 disappears little by little, and is replaced by another fluid called 

 cuibottic ctcid q(is. 



26. This ctirlmnw acid r/cw is composed of oxygen combined 

 with v.arln)n t derived from the blood ; instead of being fit to sup- 

 port life, it acts as a poison on animals that breathe it for a short 

 time, and causes death. 



27. On this account, by the respiration of animals, the air is 

 gradually vitiated, and, if it were not renewed, would soon occa- 

 sion asphyxia. 



21. What is th<- color of the bl oH when it enters the lungs? What 

 change does contact with the air produce upon it? 

 2'2. Of what is atmospheric air composed ? 



23. V\ hat is the p oportion of oxygen in the air ? What is the great use 

 of the oxygen ? 



24. \\ hut is the pro- ortion of nitrogen in the air? Is it capable of jap. 

 porting animal life ? What seems to be its use ? 



2-"). What effect is produced upon the air by breathing it 7 

 26. Of what is carbonic acid gas composed? Whence is the carbon 

 derived? How does it act on those animals that breathe it? 



27 What would happen if the air breathed by animals were not renewed. 



