CHEMICAL EFFECTS OF RESPIRATION. 335 



The most obvious effect resulting from the ac- 

 tion of the air is a change of colour from the dark 

 purple hue, which the blood has when it is brought 

 to the lungs, to the bright vermillion colour, 

 which it is found to assume in those organs, and 

 which accompanies its restoration to the qualities 

 of arterial blood. In what the chemical differ- 

 ence between these two states consists may, in 

 some measure, be collected from the changes 

 which the air itself, by producing them, has 

 experienced. 



The air of the atmosphere, which is taken 

 into the lungs, is known to consist of about 

 twenty per cent, of oxygen gas, seventy-nine of 

 nitrogen gas, and one of carbonic acid gas. 

 When it has acted upon the blood, and is re- 

 turned from the lungs, it is found that a certain 

 proportion of the oxygen, which it had contained, 

 has disappeared, and that the place of this 

 oxygen is almost wholly supplied by an addition 

 of carbonic acid gas, together with a quantity 

 of watery vapour. It appears also probable that 

 a small portion of the nitrogen gas is consumed 

 during respiration. 



same in both. The following are the exact numbers expressive 

 of these proportions : 



Carbon, Oxygen. Nitrogen. Hydrogen. 



Arterial blood . . . 50.2 . . . 26.3 . . . 16.3 ... 6.6 

 Venous blood . . . 55.7 . . . 21.7 . . . 16.2 ... 6.4 

 Memoires de la Societe de Physique et d'Hist. Naturelle de 

 Geneve. T. v. p. 400. 



