CHANGES IN THE AIR. 233 



j,3 to 4,6 per cent. ; Murray at from 6,2 to 6,5 per cent. ; 

 Fyfe at 8,5 per cent., and Irvine at 10 per cent. The mean 

 of tke whole of these observations is about 5,8 per cent. If 

 WP ^esume that errors had crept into some of these experi- 

 ments, it is still obvious that the quantity of carbonic acid 

 eliminated by different individuals, and at different times, is not 

 always the same. Front, whose skill in observation inclines 

 us to place the most implicit reliance on his results, found by 

 direct experiment that the time when the smallest quantity of 

 carbonic acid was produced, was shortly after midnight ; it 

 increased towards morning, and rose continually towards mid- 

 day, when it attained its maximum ; in the afternoon it 

 declined again, and sank continually through the course of 

 the evening, until it reached its minimum about midnight. 

 The formation of carbonic acid, therefore, experiences regular 

 fluctuations in accordance with the times of the day. Prout 

 observed, farther, that a larger quantity of carbonic acid was 

 produced in states of mental tranquillity, during gentle exer- 

 cise and with a low state of the barometer ; and that, on the 

 contrary, less was formed under the influence of active exer- 

 tion, depression of mind, and the use of spirituous liquors. 

 The estimates which we have of the absolute quantity of car- 

 bonic acid eliminated during a given time also vary greatly. 

 According to Lavoisier and Seguin, the quantity formed in 

 twenty-four hours amounts to 8,534 grains French ; according 

 to Davy, it is 17,811 grains English; according to Allen and 

 Pepys, it is 18,612 grains English. But these quantities 

 Berzelius has shown are far too great with reference to the 

 quantity of food consumed in the same interval of time.* 



* Berzelius observes (Thierchemie, 3tte Auf. S. 124), that upwards of 

 six pounds of solid aliment daily would be required to replace this loss of 

 carbonic acid, even were the whole of the carbon of the food to be elimi- 

 nated by the lungs in the shape of carbonic acid, and none to pass off with 

 the foeces, the bile, the urine, &c., which, however, is very far from being 

 the case. The above quantities must, therefore, be looked upon as exag- 

 gerated, though the observations themselves may be perfectly correct ; 

 the error, probably, lies in the reckoning ; during the short period that 

 such experiments last one or two minutes inspiration and expiration are 

 almost certainly forced or exaggerated ; the air is more rapidly changed, 

 and more carbonic acid is eliminated than during ordinary respiration. 

 The indications afforded by two minutes, under such circumstances, ap- 

 plied to the whole of the twenty-four hours, obviously raise the general 

 result far above the proper standard. 



