EFFECTS OF RESPIRATION ON THE AIR. 387 



would remain altogether without renewal, and the expired air would not be 

 found to have undergone any change. 1 That a change is effected, however, 

 in the whole volume of the air contained in the lungs, with every inspiration, 

 is indicated by the difference between the inspired and expired air ; and its 

 amount has been ascertained by Grehant by means of his method, in which 

 hydrogen gas is inspired, to be that about two-thirds of the pure air inspired 

 remains in the lung, whilst the other third is expelled mingled with two- 

 thirds of vitiated air. 2 



309. The total amount of air which passes through the lungs in twenty- 

 four hours, will of course vary with the extent and frequency of the respira- 

 tory movements; and these are liable to be affected by many circumstances, 

 but particularly by the relative degrees of repose and of exertion. More- 

 over, as any such computation must be based upon the datum of the ordi- 

 nary volume of breathing or "tidal" air, it is obvious that the estimates of 

 different observers must vary with the amount they adopt. Dr. Ed. Smith 

 found that during the day (6 A.M. to 12 night) the average quantity of air 

 inspired by several persons at rest was 502 cub. in. per minute, or a total 

 of 542,160 cub. in (nearly 314 cub. feet); and as the average quantity 

 during the night was about 400 cub. in. per minute, the total daily amount 

 was 686,000 cub. in. (397 cub. feet), which is in close approximation to the 

 results of Valentin, who estimated the total daily amount at 688,348 cub. 

 in. The quantity is largely increased by exertion, and Dr. Smith computes 

 that the total amount actually respired by the unoccupied gentleman, the 

 ordinary tradesman, and the hard-working laborer, would be 804,780 cub. 

 in., 1,065,840 cub. in., and 1,568,390 cub. in. ; and with 12 hours of Alpine 

 walking, as over the "Wengern Alp, 1,764,000 cub. in. (1020 cub. feet). 

 When carrying the weights, as by the soldier in his heavy marching order, 

 Dr. Smith found an increase of 7 cub. in. per minute for every 1 Ib. weight 

 carried. 



310. The alterations in the Air 3 which are effected by Respiration, consist 

 in its saturation with aqueous vapor, the elevation of its temperature to 

 95.5 or 97.43 Fahr.,*and in the removal of a portion of its oxyrjcn, and the 

 substitution of a quantity of carbonic acid usually rather less in bulk than 

 the oxygen which has disappeared. Probably about 4.35 per cent, may be 

 taken as the average amount of Carbonic acid added to the air, at the ordi- 

 nary rate of respiration ; whilst according to Vierordt, the amount of oxygen 

 removed is 4.782 per cent. ; the absolute diminution in bulk of the expired 

 air, when deprived of the moisture which it has acquired in passing through 



1 See Mr. Jeffreys's Statics of the Human Chest, in which this important point first 

 received due consideration. 



2 See for an account of his method Budge, Compendium de Physiologic, 1874, p. 78. 



3 The composition of ordinary atmospheric air in England is thus given by Dr. 

 Miller (Elements of Chemistry, vol. ii, p. 455) : 



Oxygen, . . . 20.61 



Nitrogen, . . . 77.95 



Carbonic Acid, . . 0.04 



Aqueous Vapor, . 1.40 



Nitric Acid, Ammonia, and Car- \ traccg 

 buretted Hydrogen, . . . / 



And in towns Sulphuretted Hy- | traceg 

 drogen and Sulphurous Acid, / 



When fully saturated, air at 50 F. is stated by Pettenkofer to contain about 4 

 grains of water per cubic foot ; at 68 F. it contains about 7. The average hygromet- 

 ric condition of the atmosphere at 50 F. is such that it contains about 3 grains per 

 ru Me foot, or about 75 per cent, of its full saturation. At the ordinary temperature 

 of the month of July, the atmosphere is capable of containing nearly three times as 

 much aqueous vapor as in December; nevertheless the quantity of moisture actually 

 present is generally greater in winter, owing to the greater rainfall. 



4 Wcyrich, Observations on the Insensible Perspiration of the Lungs and Skin, 

 1865. 



