OF RESPIRATION. 97 



employed in the case of guinea-pigs, pure carbonic acid was 

 produced and a portion of the oxygen replaced by nitrogen, this 

 portion, however, decreasing as the experiment proceeded. The 

 use of the nitrogen that we respire is unknown. 



The universality of respiration or something analogous among 

 living beings,* and all the circumstances attending its perform- 

 ance, render it probable, as my friend Dr.Prout justly remarks, that 

 it does something more than discharge a little superfluous carbon. f 

 He considers galvanism as an instrument extensively used by the 

 vital principle, and since galvanism must be produced by the 

 combination of carbon with oxygen, as it is in the battery by 

 the union of the metal and oxygen, one great additional purpose 

 of respiration becomes highly probable. 



Dr. Prout and Dr. Fyfe have found the quantity of carbonic 

 acid gas experience uniform variations. It is diminished by 

 mercury, nitric acid, vegetable diet, tea, substances containing 

 alcohol, depressing passions, and fatigue, and undergoes an 

 increase from day-break till noon, and a decrease from noon 

 till sun-set, remaining at the minimum till day -break.]: In the 

 experiments of Allen and Pepys, the formation of carbonic acid 

 gas slackened when the guinea-pigs fell asleep. 



* Fish and Crustacea purify thoir blood by the air contained in the wator 

 that they draw over their gills, and the former not only discharge carbon but 

 absorb oxygen and azote [Mew. (Tstrcucil. li. 55.) : the syren lacertina and 

 proteits anguina have both gills and lungs : insects have no lungs but openings 

 on the surface of the body leading to air-vessels that arc distributed in the 

 Interior. All the experiments of naturalists made it appear that no animal 

 could live without air, but M. Biot has lately asserted that what his countrymen 

 call Maps and tenehrions remain in as good a vacuum as can be formed for 

 any length of time without apparent inconvenience. Animals found in the 

 scGcetions and blood vessels of others must live without atmospheric air. 

 Vegetables occasion the same changes in the air as animals. — Ellis, Furth r 

 inquiries fnto the changes induced in atmospheric air, SfC. 



t Thomson's Annals of Philosophy. 1814. 



X 1. c. Dissert, [nangur. tfc. Edinb. 1814. The smallest quantity yet ob- 

 served was in a diabetic patient of mine, taking very large doses of opium and 

 nux vomica. 



II 



