Sect. II.] Physiology, 293 



with it through the hlood vessels. In tlie course 

 of the circulation, this air is gradiinlly 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 ear])onie acid and water, 

 15 evolved. On returning to the lungs, the blood 

 receives a fresh quantity of air, and at the same 

 time discharges the azotic gas, carbon ie 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*. 



Beside the general theories of respiration wliich 

 have been just stated, it will be proper to mention 

 a few of the leading discoveries on this suljjeet, 

 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 arterial blood a 

 purplish red colour when put in contaet \\\i\\ hy- 

 drogen 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 3 — it was not till the 

 accomplishment of this discovery that philosophers 

 began to attempt any explanation of the pheno- 

 mena 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 



* Researches Chcmk'al and Thilosophiail by Miiirpliry Davy, 

 6vo^ 1800^ p. 477^ c\-c. 



