The experiments of Letellier § on warm blooded animals agree in their results with those of Vierordt. He found that the quan tity of carbonic acid gas evolved from the body at the freezing point, was double of that at an elevated temperature, in the two mice and guinea-pig, and a little more in the canary and pigeon. There can, therefore, be no doubt that more carbonic acid gas is evolved from the body in a cold, than in a warrn tem perature.
Efrct of the seasons.—Dr. W. F. Edwards* ascertained, by several well-devised experi ments, that birds placed under exactly the same circumstances, and with the surrounding air of the same temperature, consumed more oxygen in winter than in summer, and this appears to be connected with that change in the constitution of the warrn-blooded animals in the colder regions of the earth, by which they are enabled to generate more caloric in winter than in summer.
Baronzetric pressure. —Lenallois found that when warm-blooded animalsbreathed air in a vessel under an atmospheric pressure reduced to 30 centimetres (11-811 English inches), the quantity of oxygen gas consumed was diini nished.i. Prout, on the other hand, informs us, that, in every instance in his experiments, any remarkable increase in the percentage of carbonic acid in the expired air was accom panied by a sinking barometer. t Vierordt tested the effects of a range of the barometric scale between 330"' (29.309 English inches) and 340'" (30'197 English inches), and has thrown the results into a tabular form. The measure of the expired air was calculated under the ordinary pressure of 336'" (29.811 English inches). He found that a rise of 5"/.67, (the mean between the experiments at the lovver and those at the higher pressures,) produced the following effects :— It increased the pulsations in one minute 1.3 respirations 0.74 5, expired air (cubic in.) 35.746 93 As, however, the percentage of the carbonic acid in the expired air was greater at the lower than at the higher pressures, in the proportion of 4.450 to 4.141, the difference betvveen the absolute quantity of that gas in the expired air at the higher exceeds so little that at the lower pressures, that it rnay be reckoned as nil.* Age, sex, and constitution of body.— The quantity of carbonic acid evolved from the body is not only influenced by the ingesta and the varying conditioni of the surrounding media, but also by the age, sex, and constitu tion of the body. The only important re searches into the effects which these last con ditions of the body have upon the evolution of the carbonic acid, are those of Andral and Gavarret -I., and Scharling ; and though they
are far from having exhausted the subject, they possess the merit of having been care fully and accurately conducted, and of being carried on in the right direction. Andral and Gavarret availed themselves in their experi ments of the apparatus suggested by Dumas and Boussingault. Part of this apparatus consists of a mask, which can be fitted air tight to the face, and having a tube on each side, on a level with the commissures of the lips, provided with valves permitting the ex ternal air to pass in, but preventing its pas sage outwards. In front of the mouth there is a large aperture for conducting outwards the expired air; and to this a tube can be at tached for conducting it into the receivers and other parts of the apparatus prepared for ascertaining the quantity of carbonic acid gas. A person can breathe through this apparatus without constraint ; and the experiments were all performed between one and two o'clock P. m., each lasting from eight to thirteen mi nutes, and the individuals experimented upon were placed, as far as possible, under the same conditions with regard to food, muscular exertion, and state of the mind. They ex perimented upon sixty-two individuals of dif ferent ages, and of both sexes. They restricted their valuation of the quantity of carbonic acid evolved from the lungs to one hour, being perfectly aware of the fallacy of at tempting to estimate from experiments so limited as to time, the quantity given off in the twenty-four hours. Scharling conducted his experiments in a diff'erent manner. He en closed the individuals experimented on in a box, perfectly air-tight, and so large as to permit a person to work, read, or even sleep, during the experiment. Tubes were fixed in the box, to admit the external air freely, and to conduct the expired air into an apparatus fitted for determining the amount of the car bonic acid. The individuals experimented on remained in the box generally for an hour at a time, sometimes an hour and a half, but also often from thirty to forty minutes only ; and precautions were taken to keep up a free circulation of atmospheric air through the box during the whole of the experiment. His experiments were performed upon six persons, of different ages and of both sexes.