104 THEORIES OF RESPIRATION. 



become stimulated beyond measure, and death is the 

 consequence. 



What the chemical changes are, which respiration pro- 

 duces in the blood what it receives and loses, aiv points 

 which have occupied considerable attention from chem- 

 ists and physiologists, and given rise to experiiiK- -ts, 

 whose results are any' thing but harmonious. To ac- 

 count for the carbonic acid in the expired air, some say 

 that carbon is secreted from the venous blood, into the 

 air-cells of the lungs, and there unites with the oxygen, 

 forming carbonic acid. They say that the only use of 

 the oxygen, is to decarbonise the venous blood, and that 

 no more oxygen is consumed, than is necessary for this 

 purpose ; that no oxygen enters into the arterial blood, 

 but its florid hue is the result of its loss of carbon. 

 The promulgator of this doctrine was Mr. Ellis, of 

 Edinburgh, and he applied it to the respiration of plants. 

 His experiments are numerous, and seem quite satisfac- 

 tory. Others say, that oxygen does enter the blood, and 

 that it is the chief object of respiration, to supply the 

 blood with this vivifying substance. The carbonic acid 

 in the expired air, they say, is secreted from the venous 

 blood. This was the x theory of Lavoiser, the celebra- 

 ted French chemist. The experiments of Dr. Ed- 

 wards, of Paris, prove that carbonic acid is formed in 

 the blood vessels, because the same quantity of car- 

 bonic acid is given out when the animal breathes hy- 

 drogen gas, as when he breathes atmospheric air. It 

 seems probable too, that some of the carbonic acid is 

 formed in the way which Mr. Ellis supposes. Oxygen 

 is of course absorbed, and while circulating in the ves- 

 sels, it probably unites with the carbon, and forms the 

 carbonic acid, that is secreted, though this is not the on- 

 ly source oi this gas. 



Emily. But now I do not understand any better, how 

 the changes in the air produce the changes in the blood, 

 which you mentioned. I see no manner oi connection 

 between these two set*; of facts. 



