RESPIRATION, AND ITS EFFECTS. 59 



which forms these cells; and by this means the important change 

 is eifected. 



There is, we well know, a considerable difference between the 

 expired and the inspired air ; the former is hot, the latter cold ; 

 this is healthy, that injurious ; one will support combustion and 

 life, the other is unfit for breathing and will extinguish a flame. 

 Tliere is but little difference in quantity between the air in its 

 different states ; but the oxygen in expired air has nearly dis- 

 appeared, and carbonic acid gas is found in its stead ; it also 

 contains much aqueous vapour, 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 vapour Avhich the expired air contains being much 

 lighter, but principally from its increased temperature, the ex- 

 pired air, notwithstanding its carbonic acid, is yet specifically 

 lighter than the atmosphere, and consequently rises upwards, 

 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 nitrogen, 18 of oxygen, and 2 of cai-- 

 bonic 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 1 1 parts of car- 

 bonic acid were substituted for 13 of oxygen, 2 parts having 

 entirely disappeared. 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 ])articularly as Sir H. 

 Davy calculated, that about 32 ounces of oxygen were necessary 

 for twenty-four 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 surplus of 5^ ounces of oxygen 

 during the above period. By some it was supposed, that this 

 surplus oxygen unites with the hydrogen thrown off by the 

 blood, and is thus converted into watery vapour : 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 quan- 

 tities during 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 increases 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 



