Medicine. 229 



blood exposed to this air. The most noted changes 

 observed to take place in the air itself subjected to 

 respiration are the following: a part of the air 

 respired entirely disappears; the rest becomes 

 impregnated with carbonic acid, and is loaded 

 with water in the state of vapour. For the know- 

 ledge of these changes effected in the air respired, 

 and for the numerous and laborious experiments 

 from which these conclusions were inferred, the 

 world is chiefly indebted to Priestley, Cigna, 

 Lavoisier, Menzies, Seguin, and Davy. 



Changes no less remarkable are found to be 

 produced in the blood exposed to the air in the 

 lungs. The principal of these are as follow: the 

 blood absorbs air; it acquires a florid red colour, 

 and the chyle mixed with it undergoes such alte- 

 ration as to lose its colour and disappear; it emits 

 carbonic acid, and perhaps carbon itself; and it 

 emits w^ater, and perhaps hydrogen. The writers 

 who have principally signalized themselves in 

 tracing and making known these changes in the 

 blood, are Priestley, Cigna, Fourcroy, Has- 

 senfratz, Beddoes, Watt, and, very lately, 

 Mr. Davy. 



The theories of this function, as deduced from 

 facts successively discovered, have varied accord- 

 ing to the number of such facts, and the impres- 

 sions which they made on difl^erent minds. Dr. 

 Priestley, the first of the modern chemical phi- 

 losophers, as was before remarked, who attempted 

 to investigate the use of respiration, seems to have 

 considered it, from some of his earliest experiments, 

 chiefly as an excretory process. He believed that 

 the blood, in passing through the lungs, gives out 

 phlogiston to the air, which, when expired, he 

 supposed to be loaded with that substance, and, 

 consequently, that the main purpose of respira- 

 tion is to discharge phlogiston from the blood. 



