

RESPIRATION. 251 



air a foot in diameter, and extending to the top of the at- 

 mosphere, would be equal in weight to a column of water of 

 the same diameter thirty-two feet high, or to a column of 

 mercury twenty-eight inches high. The pressure of the at- 

 mosphere then, upon the body of a common sized man, is 

 equal to between thirty and forty thousand pounds) Atmos- 

 pheric air is composed of oxygen, azote and carbonic acid, 

 in the proportion of (20 parts of oxygen, 78 of azote, and 2 

 of carbonic acid.\ 



19. ^Oxygen is an invisible air, or gas, and enters into the 

 composition of air, water, and all animal and vegetable sub- 

 stancesJ It is the supporter of combustion, and no animal 

 can live without it. What are called acids and oxyds in 

 chemistry, are oxygen combined with other substances, as 

 sulphur, salt, nitre, &c. f Azote can neither support combus . 

 tion nor respiration, at least in man ; though it is an ele- 

 ment in all animal matter, and in some vegetables.) In 

 these cases it is obtained, both from food and the air. 



20. It is now fully ascertained, that while the chemical 

 composition of the blood is essentially changed, its weight 

 always remains the same ; as the carbon discharged is pre- 

 cisely equal to the united weight of the oxygen and azote 

 absorbed, and the same change is effected by the respiration 

 of all animals, whatever be their rank in the scale of organ- 

 ization. It is worthy of remark, that plants and animals 

 produce directly opposite changes in the chemical constitu- 

 tion of the air. The carbonic acid given off by animals is 

 composed of oxygen and carbon ; this is decomposed by 

 vegetables, which absorb the carbon and give off the oxygen 

 to the air, which in its turn is absorbed by animals, and car- 

 bonic acid given off: so these two great departments of 

 organized structure furnish food for each other^ renovating 

 the air, and preserving it in a state of constant purit$ The 

 immense quantity of oxyen given off by vegetables, may be 

 inferred from the following experiment. About fifty leaves 

 were enclosed in a jar of air ; the surface of the whole being 



