The percentage of carbonic acid in the expired air differs at different periods of the same expiration. As the air expelled in the first part of an expiration consists chiefly of that contained in the trachea and upper part of the air passages, its amount of carbonic acid gas must necessarily be smaller than that expelled at a later period of the expiration. Allen and Pepys found the carbonic acid gas in the first and last portions of air in a deep expiration to differ as widely as 3-5 and 9-5 per cent. Dalton states that while the ave rage carbonic acid in an ordinary expiration is 4 per cent., the last portion of a forced ex piration contains 6 per cent. Vierordt divided the air of an ordinary expiration as far as possible into two equal parts, and in twenty one experiments ascertained that while the average quantity of carbonic acid in the whole expiration was 4-48, the first half contained 3.72 per cent, and the last half 5-44 per cent. We have already seen, that Vierordt concludes from his experiments that the air, after a sojourn of about 40 seconds in the respiratory apparatus, has the same percentage of car bonic acid gas in the different parts of the lungs and air passages.
From the above details, it must be obvious that nearly all the attempts made to estimate exactly the average quantity of carbon evolved in the form of carbonic acid gas from the body in the 2.4 hours are entitled to very little con fidence. The greater number of these are founded on a few experiments performed upon one or a very small number only of indi viduals in a state of rest, and upon the result of a few respirations in some cases performed under constraint. The estimate of the amount of loss of carbon in the 24 hours from the lungs and external surface of the body, based upon the direct method of experiment, in which the greatest number of the circumstances that influence the evolution of carbonic acid gas from the lungs were taken into account, is undoubtedly that of Scharling, though this even must be regarded as an approximation only to the truth. Suppose we take the average estimate of the two adult males be tween 28 and 35 years of age for the 24 hours, as given by Scharling *, the loss of carbon by the lungs and skin is 3543-13 Troy grains, or 7.382 oz. Troy. t Liebig I has endeavoured to ascertain the quantity of carbon lost at the lungs and skin in the 24. hours by the indirect method of research, which he maintains to be by far the most trust-worthy. He proceeded to ascertain the quantity of charcoal in the daily food and drink of a body of soldiers, and after deducting the comparatively small quan tity of this substance that passes off in the fwees and urine, the remainder was taken as the amount of carbon that unites with oxygen, and escapes in the form of carbonic acid gas by the lungs and skin. From the data thus obtained he calculates that an adult male, taking moderate exercis.e, loses 13.9 oz. of
carbon daily by the lungs and skin ; and that 37 oz. of oxygen gas must be daily absorbed from the atmospheric air for the purpose of converting this charcoal into carbonic acid gas. From similar experiments upon the in mates of the Bridewell at Marienschloss (a prison where labour is enforced), he calculates that each individual lost in this manner 10•5 oz. of carbon daily ; while in another prison, where the inmates were deprived of exercise, this loss amounted only to 8.5 oz. daily.* Allowing that this indirect method of research is more accurate than the direct, —a point which we are not at present prepared to de termine, — the accuracy of the data upon which Liebig's inferences rest regarding the quantity of carbonic acid exhaled from the lungs and skin in an adult using moderate exercise, has been called in question by Schar ling. t He endeavours to prove, by an ana lysis of the food and drink allowed to the sailors on board of his Danish Majesty's vessels of.war, that the whole carbon taken daily into the body of each of these individuals must be somewhat less than 101 oz.• yet these sailors are subjected to harder work'than ordinary seamen.
The quantity of carbonic acid gas evolved from the body in respiration varies greatly in the different divisions of the anintal kingdom. It is greater in birds, in proportion to their bulk, than in the cold-blooded vertebrata, and still smaller in the invertebrata, with the ex ception of insects. Si The ascertainment not only of the absolute quantity of carbon which escapes from the body in the form of carbonic acid gas in the different classes of animals, but also the relative proportion of this to the weight of the body, is a matter of considerable physiological interest, especially with refer ence to the source of animal caloric. From the experiments of Scharling, Andral, and Gavarret, it is evident that the young of the human species relative to their weight consume considerably more oxygen gas, and evolve more carbonic acid gas by respiration, than the middle-aged ; and that the latter again evolve more carbonic acid than those far advanced into old age. Valentin and Brunner have calculated, from experiments performed on Valentin, who at the time was 33 years of age, that for every gramme weight (15.433 Troy grains) of his body, there was evolved .0089 Troy grain of carbonic acid gas, containing .0024 Troy grain of carbon ; and this calcu lation approximates pretty closely to one based upon the results of Andral and Gavarret upon the evolution of carbon, combined with those of Quetelet upon the average weight of the body at this period of life.* The follow ing table, calculated from the experiments of different observers, to show the quantity of carbon consumed in the 24 hours for every 100 grammes weight (1543.3 Troy grains) of the body in the four divisions of the verte brata, is given by Vierordt — Troy Grains.