Some chemists, and even Sir H. Davy himself, have supposed, that a quantity of azote is absorbed in the lungs, but the general result of the experiments are against this supposition ; and it is now almost universally admitted, that this gas, which composes so large a proportion of the air, is not affected by respiration, although the contrary supposition would be a very convenient addition to our hypothesis, by furnishing us with an obvious source of azote for those animals who do not receive it through the medium of the stomach.
The exhalation of a quantity of aqueous vapour from the lungs, mixed with the expired air, was too obvious a phenomenon to be overlooked, even by the earliest phy siologists; but, except the general fact, little certainty has been obtained respecting it. Hales attempted to as certain the quantity of this vapour, by breathing through wood ashes, and finding how much they had increased in weight. His experiments led him to conclude, that the water exhaled in twenty-four hours would amount to about 20 oz. ; while Menzies, who attempted actually to collect the water, reduced it to 6 oz. Lavoisier endea voured to solve the problem less directly, by instituting a calculation founded upon the composition of water, com pared with that of the other substances which are received into, and discharged from, the lungs. We have already stated, that he supposed there was an over proportion of oxygen after the formation of the carbonic acid ; and this oxygen, he imagined, was united to a quantity of hydrogen in the lungs, and thus formed water, the amount of which might be known from that of the oxygen employed In genious, hots ever, as this method of Lavoisier's may ap pear, there are some obvious considerations which will prevent us from placing any confidence in it. Indeed, the foundation of all the reasoning, the disproportion be tween the oxygen and the carbonic acid, is itself a very doubtful point. Besides, the exhalation of hydrogen from the lungs, and its union with oxygen, under these circum stances, is not analogous to any other operation in the ani mal economy ; while, at the same time, there is a much more obvious source for the water in the fluid which is secreted by the mucous membrane which lines the respi ratory organs.
After having thus examined in succession all the dif ferent changes which are supposed to be produced in the air by respiration, it may be desirable briefly to recapitu late the result of our inquiries. 1. Air that has been respired loses a portion of its oxygen; the quantity varies according to circumstances; but it may be estimated, upon the average, that a man consumes between 45,000 and 46,000 cubic inches, or about 15,500 grains, in twenty four hours. 2. A quantity of carbonic acid is generated, the amount of which also varies according to circum stances, probably in proportion to the consumption of the oxygen ; but there is a considerable difference of opinion respecting the relation which these quantities bear to each other, whether the quantity of oxygen that disappears be precisely equal to that of the carbonic acid which is gene rated. If we admit that all the oxygen is employed in
the production of carbonic acid, the quantity will be as before, between 45,000 and 46,000 cubic inches, or above 21,000 grants. 3. A quantity of water is emitted from the lungs, mixed with, or diffused through, the air of ex piration ; but the amount of the water is still undetermined. The two next supposed changes, the diminution in the bulk of the air, and the abstraction of a portion of azote from it, are points concerning which there has been a considerable diversity of opinion ; but, upon the whole, it appears that they are not supported by sufficient evi dence. Hence we may conclude, that the only changes which are certainly known, as being produced upon the air by respiration, are the removal of a portion of its oxy gen, and the addition of carbon and water.
We now proceed to consider the change which is ef fected in the blood by respiration, an inquiry attended with much more difficulty than that respecting the air, in proportion to the greater difficulty there is in ascertaining the exact chemical composition of the blood. The large quantity of blood which the lungs always contain, induced even the earliest physiologists to suppose that some im portant effect was produced upon it by this organ ; and the conjecture was strongly confirmed by the discovery of Harvey, that every part of the blood passes through the lungs in each circulation. The opinions that were enter tained respecting the nature of the change were extremely various, but they may be all reduced to three heads. It was supposed by some, that the change was merely me chanical ; that it consisted in mixing the different parts of the blood intimately together ; that it was condensed by pressure, or rarefied by the addition of a portion of air. A second set of physiolygists, among whom we find Lower, Hooke, I\ layow, and many of the Italians, conceived that the air imparts something to the venous blood, by which it is converted into the arterial state ; while a third class, in which are included Harvey, Boyle, Hales, and Haller, supposed that the blood discharges some noxious matter, which is carried off by the air. Considered in its general outline, we shall probably find the third of these opinions to be the most correct ; but they were all of them advanced rather as hypotheses than as deductions from facts ; and when their respective advocates attempted to go into any minute detail, they soon degenerated into false, or even absurd speculations.