346 STRUCTURE OF THE SHEEP. 



gen in expired air has nearly disappeared and carbonic acid 

 gas is found in its stead ; it also contains much aqueous va- 

 por, which is condensed in a visible form, at a temperature 

 of 60°. Thus, although the carbonic acid gas is much 

 heavier than common air, yet, partly from the aqueous va- 

 por which the expired air contains being much lighter, but 

 principally from its own increased temperature, the expired 

 air, notwithstanding its carbonic acid, is yet specifically 

 lighter than the atmosphere ; and consequently rises up- 

 wards, and thus, in great measure, is prevented from being 

 respired a second time. It has been found by experiment 

 with a portion of atmospheric air, containing 80 parts of ni- 

 trogen, 18 of oxygen, and 2 of carbonic acid, that, on being 

 respired, the nitrogen continued the same, but the carbonic 

 acid was increased to 13 parts, and the oxygen reduced to 

 5 ; whence it appeared that 11 parts of carbonic acid were 

 substituted for 13 of oxygen, 2 parts having entirely disap- 

 peared. Thus the disappearance of the greater portion of 

 the oxygen was accounted for by its being converted into 

 carbonic acid ; but there remained a small portion, whose 

 absence could not be thus explained, more particularly as 

 Sir H. Davy calculated that about 32 ounces of oxygen 

 were necessary for 24 hours' expenditure in a man ; but 

 only 26^ ounces are requisite for the formation of even 37 

 ounces of carbonic acid gas, giving us an unexplained sur- 

 plus of 5j ounces of oxygen, during the above period. By 

 some it was supposed that this surplus oxygen united with 

 the hydrogen thrown off by the blood, and is thus converted 

 into watery vapor : by others it is held, that this oxygen is 

 absorbed by the blood, and enters the circulation. Carbonic 

 acid gas is exhaled from the lungs in different quantities du- 

 ring different periods of the day, being generated in the 

 greatest quantity about noon, decreasing in the afternoon 

 and night, and again increasing in the morning. It also in- 

 creases in man by taking animal food. 



Sir H. Davy contended that a small portion of nitrogen is 

 absorbed by the blood ; but this has been denied by others. 

 The chief use of nitrogen, however, is to dilute the oxygen ; 

 for if the latter is inspired pure a sense of warmth is felt in 

 the chest, the heat of the skin is raised, the pulse quickened, 

 and other symptoms of excitement are produced. A given 

 quantity of oxygen will, however, support life longer than 

 the same quantity of atmospheric air. It has been computed 



