CHANGES IN THE AIR IN KESPERATION. 411 



probable existence of some unknown vital substance in the 

 atmosphere. A few years later it was demonstrated by Ber- 

 noulli, that the existence of aquatic animals depends upon 

 air held in solution in the water. About this time Robert 

 Hooke performed his celebrated experiment of exposing the 

 lungs of a living animal, and maintaining the vital processes 

 by artificial respiration. He demonstrated that asphyxia 

 occurred when he ceased to change the air in the lungs, 

 though these organs were allowed to remain distended. 



Fracassati also showed that the red color of the upper 

 surface of a clot of blood was due to its exposure to the air ; 

 and Lower, examining the blood before and after its passage 

 through the lungs, in artificial respiration, showed that the 

 red color of arterial blood depends on the renewal of the 

 atmosphere. 



In 1674, Mayow published his work on Respiration, in 

 which he advanced the view that the air contained a princi- 

 ple, capable of supporting combustion, which is absorbed in 

 respiration, changes venous into arterial blood, and is the 

 cause of the heat which is developed in animal bodies. 1 The 

 importance of this discovery was not appreciated by the phys- 

 iologists of that day ; and it was more than a century before 

 it received its appropriate place in science. 



In 1757, Joseph Black, of Glasgow, isolated and studied 

 carbonic acid, which he called fixed air. He recognized this 

 gas in the expired air, by passing the breath through lime- 



1 We find the following passage in an analysis of the work of MAYOW on Res- 

 piration, published in the Philosophical Transactions, 1668, p. 833 : 



" The author * * * delivers his thoughts on the use of Respiration, waving 

 those opinions, that would have respiration either to cool the heart, or make the 

 Bloud pass through the Lungs out of the right ventricle of the heart to the left, 

 or to reduce the thicker venal blood into thinner and finer parts ; and affirming, 

 That there is something in the Air, absolutely necessary to life, which is conveyed 

 into the Bloud ; which, whatever it be, being exhausted, the rest of the air is 

 made useless, and no more fit for Respiration. Where yet he doth not exclude 

 this use, That with the expelled Air, the vapors also, steaming out of the Bloud, 

 are thrown out together." 



