RESPIRATION. 613 



respiration the air drawn into the lungs is nearly balanced by the 

 air thrown out But when the animal is placed in untoward cir- 

 cumstances, and is obliged to make forced inspirations, the bulk 

 of the air is diminished, and this diminution is inversely as the 

 volume of the air which the animal is obliged to breathe. 



3. It is well-known that atmospherical air (abstracting a little 

 vapour of water and a trace of carbonic acid gas) is composed of 

 eighty volumes azotic and twenty volumes oxygen gas. But 

 when it is thrown out of the lungs by expirations, the volume of 

 oxygen gas is diminished, being replaced by exactly the same 

 bulk of carbonic acid gas. Various experiments have been made 

 to ascertain how much of this principle is lost by respiration in a 

 given time ; but they by no means correspond with one another. 

 Indeed, it is extremely probable, if not absolutely certain, that 

 the degree of effect which the same animal produces upon the air 

 respired differs materially at different times, and in consequence of 

 different circumstances. Nothing, therefore, beyond an approxi- 

 mation can be expected from our experiments on this function. 



Dr Menzies was the first who attempted to ascertain the quan- 

 tity of oxygen consumed by a man in a day. According to him, 

 36 inches are consumed in a minute, and of course 51,840 inches 

 in twenty-four hours.* This estimate exceeds that obtained by 

 Lavoisier and Davy from their experiments. Lavoisier and Se- 

 guin estimate the quantity of oxygen consumed by a man in 

 twenty-four hours at 46,037 cubic inches, and this nearly coin- 

 cides with the results which Lavoisier obtained from his last ex- 

 periments, on which he was occupied when he was dragged to the 

 place of execution. With this also the experiments of Davy co- 

 incide very well. He calculates that 3 1 *6 inches of oxygen are 

 consumed in a minute, which, in twenty-four hours, make 45,504 

 inches.f 



But these determinations can be considered only as approxi- 

 mations. Upon examining the air expired from my own lungs, 

 I found that the volume of carbonic acid gas which it contained 

 differed considerably from day to day. In the month of May 

 1832, I analyzed air from my own lungs on ten consecutive days, 

 between eleven and twelve o'clock each day. The following ta- 

 ble exhibits the result : J 



* Bostock on Respiration, p. 81. f Davy's Researches, p. 433. 



\ Records of General Science, i. p. 28. 



