Marchand**, in his carefully-conducted ex periments on frogs, detected a quantity of' ammonia in the tube of his apparatus, con taining the concentrated sulphuric acid, and concludes that nitrogen in this combination is exhaled from the lungs and skin.
From a review of all the experimonts upon the nitrogen of the respired air, we perceive that though the evidence preponderates in favour of' the exhalation of a small quantity of nitrogen from the lungs ft, yet that it is not sufficiently conclusive to justify us in stating that its operation is constant. It appears, however, from the evidence adduced, that the nitrogen in the expired air is at least frequently increased in quantity in ordi nary respiration, but not to the extent of affecting materially the percentage of this gas in the respired air.* Valentin and Brunner, in their carefully conducted experiments, could detect no traces of hydrogen, carbonic oxide, or carburetted hydrogen, in the expired air.
By far the most important chemical change the atmospheric air undergoes during its so journ in the lungs, is a diminution in the quantity of its oxygen and an increase of its carbonic acid gas ; and it may be safely af firmed that all the other changes in the respired air are of trivial importance in the function of respiration, when conipared with this. There can be no doubt that the con. elusion drawn by Allen and Pepys from their experiments, that the amount of oxygen which disappears from the inspired air is exactly equal to the quantity required to form the carbonic acid that appears in the expired air, is incorrect ; for all the latest and most accurate experiments have confirmed the general accuracy of the results obtained by Lavoisier and Davy on this point, and have satisfactorily determined that a larger quantity of oxygen disappears from the inspired air than what is sufficient to forvn the carbonic acid gas present in the expired air.
Percentage and absolute quantity of carbonic acid gas in the expired air.— The results of the earlier experimenters on this point are of so little value that we need not refer to them. The following results have been obtained by some of the later experimenters :— The results-obtained by Brunner and Valen tin, and by Vierordt, appear especially trust worthy ; and though the number of experiments is too small to enable us to deduce averages with any confidence, yet we may in the meantime consider that, in an adult male of middle age, the average quantity of carbonic acid in the expired air is about 4.35 per cent.!..)The quan
tity of carbonic acid gas in the expired air is not uniform in the same individual, but varies repeatedly, even in the course of the twenty four hours, and these variations are deter mined by certain conditions of the body and of the surrounding media.
Period of the day.— Dr. Prout believed that he had discovered that the quantity of car bonic acid formed during respiration is always greater at one and the same period of the day than at any other ; that this maximum occurs between 10 A. M. and 2 P.M., or generally be tween 11 A. xi. and 1 P. M. ; and that the minimum commences about 8" 30' P. m., and continues nearly uniform till about 3" 30' A. in. The beginning and end of the period of minimum evolution of carbonic acid he believed to be connected with the beginning and end of twilight, and he adduces some experiments in favour of this opinion. In these experinients Prout attended only to the percentage of the carbonic acid in the expired air, and took no means to ascertain the volume of-air passing through the lungs at the time,— an omission which seriously diminishes their value.t Prout's results do not accord with the previous experiments of Brande $, nor with the subsequent experiments of Coa, thupe* and Vierordt. -1- It would appear, therefore. that the variations in the quantity of carbonic acid in the course of the day do not occur at uniform periods, independent of other circumstances, as Prout supposed. It is, however, proved by the experiments of Scharling $ upon the human species, by Bous singault § upon the turtle dove, and by Mar chand upon frogs, that the absolute amount of carbonic acid exhaled is very considerably less during the night than during the day. Scharling gives in the following table the relative proportion of the carbon exhaled during the day and night in six individuals upon whom he experimented : — The average proportion is 1 during thc night to 1.237 during the day, or, in othei words, nearly a fourth part more carbonic acid gas is evolved during the day than during the night.1 How much of the diminished evolution of carbonic acid during the night is dependent upon the languor and drowbiness incident to that period, and how much upon the absence of the sun's rays and other causes, it is at present impossible to determine. It appears that this diminished evolution of car bonic acid during the night does not require the occurrence of sleep, though no doubt it is increased by sleep.