CHEMICAL EFFECTS OF RESPIRATION. 301 



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 accom- 

 panies its restoration to the qualities of arterial 

 blood. In what the chemical difference 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 returned 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 pro- 

 bable that a small portion of the nitrogen gas is 

 consumed during respiration. 



For our knowledge of the fact of the disappear- 

 ance of oxygen, we are indebted to the labours of 

 Dr. Priestley. It had, indeed, been long before 

 suspected by Mayow, that some portion of the air 

 inspired is absorbed by the blood ; but the merit of 

 the discovery that it is the oxygenous part of the 



larger projDortion of oxygen than the latter ; and that the latter 

 contains a larger proportion of carbon than the former: the pro- 

 portions of nitrogen and hydrogen being nearly the same in both. 

 The following are the exact numbers expressive of these proportions : 



Carbon. Oxijgen. 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. 



