232 Medicine. 



by respiration. The blood in passing through the 

 lungs absorbs a portion of air, and carries it along 

 with it through the blood vessels. In the course 

 of the circulation this air is gradually decomposed 

 by the blood, the oxygen and part of the azote 

 entering into new combinations, while at the same 

 time a portion of azote, of carbonic acid and water, 

 is evolved. On returning to the lungs, the blood 

 receives a fresh quantity of air, and, at the same 

 time, discharges the azotic gas, carbonic acid gas, 

 and watery vapour which had been formed during 

 the circulation. This theory of respiration by Mr, 

 Davy is believed to be the latest of those deserving 

 especial notice which belong to the eighteenth 

 century.^ 



Besides the general theories of respiration which 

 have been just stated, it will be proper to mention 

 a few of the leading discoveries on this subject;, 

 and the authors to whom they respectively belong. 



It was not till Dr. Priestley had discovered 

 that venous blood acquires a scarlet colour when 

 brought into contact with oxygen gas, and arte- 

 rial blood a purplish red colour when put in con- 

 tact with hydrogen gas; or, in other words, that 

 oxygen gas instantly gives venous blood the colour 

 of arterial, and that hydrogen, on the contrary, 

 gives arterial blood the colour of venous; — it was 

 not till the accomplishment of this discovery that 

 philosophers began to attempt any explanation of 

 the phenomena of respiration. 



To Dr. Priestley likewise belongs the merit of 

 that instructive experiment of enclosing blood in a 

 bladder, and exhibiting the passage of oxygen 

 through its moistened coats, by the florid colour 

 thence imparted to the blood, in order to demon- 



* Researches Chemical and Fbilosopbicali by HuMPHRT Davy, 8vo. l8oCr» 

 p. 477, &c. 



