476 THE RESPIRATORY EXCHANGE 



The experiments of Stephen Hales led to the discovery by 

 Black about the year 1757 that a quantity of " fixed air " was 

 given of? from the lungs during respiration, and the presence of 

 this gas constituted the chief difference between expired and 

 inspired air. 



A still greater advance was made in 1772, when Priestley pub- 

 lished his " Observations on Different Kinds of Air." By experi- 

 ments he proved that " fixed air," which is now known as carbon 

 dioxide, is produced during putrefaction, and by plants during the 

 night-time ; green plants growing in the sunlight restore the pro- 

 perty of supporting animal life to air which has been vitiated by 

 the respiration of animals or by the burning of a candle ; oxygen 

 and nitrogen are constituents of the atmosphere, and venous blood 

 becomes arterial by exposure to oxygen, a change which occurs 

 even when the blood is separated from the gas by a moist membrane 

 or by the thin walls of the blood-vessels of the lungs. Such were 

 Priestley's contributions to the knowledge of respiration; his ex- 

 periments were sound, but his views on respiration were erroneous, 

 vitiated as they were by his belief in the old theory of " phlogiston " 

 introduced by Stahl in 1697. Phlogiston was the material and 

 principle of fire, not fire itself, and respiration, according to Priest- 

 ley, was a phlogistic process, whereby the phlogiston absorbed 

 by animals with their food was discharged. Venous blood was 

 phlogisticated, arterial blood dephlogisticated ; a clot of blood 

 placed in " fixed " or phlogisticated air became very dark, but 

 regained its red colour when it was transferred to oxygen or 

 dephlogisticated air. This old theory was overthrown a few 

 years later by Lavoisier, who extended and explained correctly 

 the discoveries of Mayow, Black, and Priestley ; he showed that 

 there were differences in the so-called phlogistic processes. The 

 calcination of metals he proved, as Mayow had observed a hundred 

 years before, to be a combination with oxygen, whereby the metals 

 gained in weight ; respiration, on the other hand, was not only 

 an absorption of oxygen, but a production and discharge of carbon 

 dioxide produced by the union of the oxygen with carbon. Further, 

 in a joint research with Laplace, published in the year 1780, he 

 showed that the heat of an animal's body might be accounted 

 for by this process of combustion. This view he modified a few 

 years later by including in the process of combustion the oxida- 

 tion of hydrogen, for quantitative experiments showed that not 



