CHANGES IN THE AIR BY RESPIRATION. 283 



of bodily weight, while after hibernation was fully established it was 

 reduced to 26 cubic centimetres per kilogramme per hour. 



The absorption of oxygen, accordingly, in the process of respiration, 

 is directly associated, so far as regards its rapidity and amount, with the 

 physiological activity of the living organism. 



Increase of Carbonic Acid. The expired air usually contains, in 

 man, about 4 per cent, of its volume of carbonic acid, which it has ab- 

 sorbed in its passage through the lungs. Rather less than 13 cubic 

 centimetres of this gas are accordingly given off with each ordinary 

 expiration ; and as we have found that 10,000 litres of air are habitually 

 inhaled and discharged during twenty-four hours, this will give 400 

 litres of carbonic acid as the amount expired per day. This quantity 

 is, by weight, 186 grammes, or rather less than one pound and three- 

 quarters avoirdupois. 



The rate of exhalation of carbonic acid by respiration varies in the 

 same manner and according to the same conditions as the absorption 

 of oxygen. In a general way it may be said, as the result of many 

 trustworthy observations, both in animals and man, that the quantity 

 of carbonic acid exhaled during a given time, in proportion to the weight 

 of the body, is increased by muscular exertion or by any physiological 

 activity of the system, and is diminished by quietude, during sleep, and 

 in a state of inanition. 



These facts have been established more particularly for the human 

 subject, in a special series of investigations by Prof. Scharling, 1 who 

 found that the quantity of carbonic acid exhaled was greater during 

 digestion than in the fasting condition. It was greater in the waking 

 state than during sleep ; and in a state of activity than in one of repose. 

 It was diminished by fatigue, and ~by most conditions which interfere 

 with perfect health. 



It is also known that in man the habitual rate of exhalation varies 

 according to age, sex, constitution, and development. These variations 

 were very fully investigated by Andral and Gavarret, who found them 

 to be very marked in different individuals, notwithstanding that the 

 experiments were made at the same period of the day, and with the 

 subject as nearly as possible in the same condition. Thus they found 

 that the quantity of carbonic acid exhaled per hour in five different per- 

 sons was as follows : 



QUANTITY OP CARBONIC ACID PER HOUR. 

 In subject No. 1 ...... 19,770 cubic centimetres. 



" 2 15,888 " 



" 3 , . . . . . 20,475 " 



" 4 20,475 " 



11 5 26,060 " 



With regard to the difference produced by age, it was found that from 

 the period of eight years up to puberty the quantity of carbonic acid 



1 Annales de Chimie et de Physique. Paris, 1843, tome viii. p. 490. 



