412 RESPIRATION. 



water. It is evident that this was the gas which was ob- 

 served so many years before by Yan Helmont. 



In 1775, Priestley discovered that the air is composed of 

 oxygen and nitrogen, though he did not make use of these 

 names; and a few years later, showed that air which has 

 been vitiated by the respiration of animals is consumed by 

 vegetables, which return the elements necessary to the life of 

 animals. In a paper published in the Philosophical Transac- 

 tions for 1776, he proved that the change in the color of the 

 blood in the lungs is due to the absorption of the newly 

 discovered oxygen ; and showed, furthermore, that the inter- 

 change of gases between the air and the blood can take place 

 through membranes, as readily as when the two fluids are 

 brought directly in contact with each other. 1 



The discoveries above enumerated, though all bearing on 

 the great question, were simply isolated facts, and failed to 

 develop any definite idea of the changes of the air and blood 

 in respiration. The application of these facts was made by 

 the great chemist Lavoisier ; who was the first to employ the 

 delicate balance in chemical investigation, and whose obser- 

 vations mark the beginning of an accurate knowledge of the 

 function of respiration. With the balance, Lavoisier showed 

 the nature of the oxides of the metals ; he discovered that 

 carbonic acid is formed by a union of carbon and oxygen ; 

 and, noting the consumption of oxygen and the production 

 of carbonic acid in respiration, advanced, for the first time, 

 the view that the one was employed in the production of the 



1 BERARD attributes the discovery of oxygen to Bayen (op. cit., tome iii., p. 

 328). It is true that Bayen in 1774 evolved oxygen by heating the red oxide of 

 mercury, but he simply saw a gas given off, the nature and properties of which he 

 did not describe. Priestley first published his discovery of oxygen, with a descrip- 

 tion of certain of its important properties, in the same year ; and because he thus 

 described properties which distinguish this from every other gas, to Priestley is 

 generally, and justly, ascribed the honor of its discovery. Scheele, in Sweden, 

 obtained and described oxygen (" the air of fire ") shortly after it had been ob- 

 tained by Priestley, without the knowledge that his discovery had been anticipated. 

 His work was published in 1777. 



