388 OF RESPIRATION. 



the lungs, being from ^ ( - } ih to -g^th of its volume, though its weight owing to 

 the sp. gr. of carbonic acid being greater than that of oxygen is rather in- 

 creased. Supposing the entire amount of air respired" per diem to be 

 1,000,000 c. in., or 578 cubic feet, or 3600 gallons, as given by Dr. Smith for 

 an ordinary tradesman, the absolute quantity of oxygen withdrawn from 

 the air in the respiratory acts may be estimated at about 5000 cubic inches 

 or 30 c. ft. (== 5 per cent.) ; the weight of which would be about 18,000 

 grains, or rather more than 2^ Ibs. avoirdupois. A curious calculation has 

 been made by Welcker, to the effect that the surface presented for aeration, 

 by the entire mass of the red corpuscles of the blood, amounts to no less 

 than 2816 square metres, or estimating the amount propelled through the 

 lung in each second to be 176 cubic centimetres, the surface presented by the 

 corpuscles contained in that quantity of blood would amount to 87 square 

 metres, or a square of 13 military steps to the side. It is deserving of notice 

 in regard to the elimination of Carbonic acid, that if the air be already 

 charged to some extent with this gas, the quantity exhaled is much dimin- 

 ished (Allen and Pepys). 



311. The absolute quantity of Carbonic acid exhaled from the Lungs is 

 liable to variation from so many sources, that no fixed standard can be as- 

 signed for it. The mean of a great number of observations, however, made 

 in different modes, and under different circumstances, would give about 160 

 grains of Carbon per hour as the amount set free by a well-grown adult 

 man, under ordinary circumstances. Taking this as the average of the 

 twenty-four hours, the total quantity of Carbon thus daily expired from the 

 Lungs would be 3840 grains, or 8 oz. troy, which is equal to 14,080 grains 

 of Carbonic acid. 1 According to Dr. Edward Smith, who has devised an 

 apparatus by which all the Carbonic acid exhaled may be collected for a 

 whole day without intermission except for food, the total amount evolved in 

 24 hours at rest (deduced from experiments on different persons) was 7.144 

 oz. of Carbon ; and he estimates that it should be increased to 8.68 oz. and 

 11.7 oz. for the non-laboring and laborious classes respectively, at their ordi- 

 nary rate of exertion. The chief causes of variation are, the Temperature 

 and Hygrometric state of the surrounding Medium, Age, Sex, Development 

 of the Body, Nature and Quantity of Food and state of the Digestive pro- 

 cess, Muscular Exertion or Repose, Sleep or Watchfulness, Period of the 

 Day, Season, and state of Health or Disease. These will now be considered 

 in detail : 



i. Temperature of Surrounding Medium. The amount of Carbonic acid 

 exhaled by warm-blooded animals, is greatly increased by external Cold, and 

 diminished by Heat. From Letellier's 3 experiments it appears that the quan- 

 tity of Carbonic acid exhaled by Mammals between 86 and 106, is less 

 than /itt/fthat set free near the freezing-point; whilst that which is exhaled 

 between 59 and 68 is but little more than two-thirds of the same amount. 

 The diminution occasioned by heat is still more remarkable in Birds ; which 

 exhale at the highest temperature scarcely more than one-third of that set 

 fiv<> at the lowest. The observations of Vierordt 3 upon himself show that 

 the same is true of the Human subject ; and a difference of 10 Fahr., ac- 

 cording to him, producing a variation of rather more than two cubic inches 

 in the amount of Carbonic acid hourly expired. Dr. Ed. Smith found that 



1 Schilling estimated the hourly elimination of Carbonic Acid to be on the aver- 

 age 30 grammes, or ">5f> grains, and by Panum it is given as 34 grammes or 525 

 grains. 



2 Annales de Chimio et <le Physique, 1845; and M. Boussingault's Memoires de 

 Chimic Agricole et dc Physiologic, 18-">4. 



3 Physiologic des Athmens, pp. 73-82. 



