Mr. Abernethy's inquiry was first directed to ascertain what the skin actually gave off from the body; and secondly, what changes took place in the air which we draw into the lungs (inspiration). We will endeavour to give some idea of these experiments. They were very simple—they involved no cruelty, like those of Edwards—and they were many of them such as the public might repeat without difficulty.
Very useful would it be, if persons who have leisure would sometimes engage in physiological inquiries. They would find them to be extremely interesting; and a series of facts would be easily collected, from which the physiologist might obtain the most valuable information, but which, engaged as most of us are in applying physiology to the correction of disordered functions, we can seldom collect for ourselves, except in a few hours stolen from those occupied in an arduous profession, and perhaps by the sacrifice of paramount duties.
Mr. Abernethy's experiments were very numerous, and commenced in the summer of 1791; but the winter's cold obliging him to desist, they were renewed in the spring of 1792. Having referred to the experiments of Ingenhous and Cruikshank, together with an allusion to a paper (not then made public) by Lavoisier, he proceeds to describe his own.
Having a trough containing a large quantity of quicksilver, he filled a glass jar (sufficiently capacious to contain his hand and wrist) with that metal. He inverted it into the trough in the usual way of proceeding in collecting gases. He fixed the glass jar in a sloping position, that he might introduce his hand the more readily beneath the quicksilver. In this way, whatever was given off from the skin of the hand, rising through the quicksilver to the top of the glass, and of course displacing a proportionate quantity of quicksilver, could be made the subject of analysis.
He describes his first experiment as follows: "I held my hand ten minutes in the jar beneath the surface of the quicksilver, and frequently moved it in that situation, in order to detach any atmospheric air that might accidentally adhere to it, and afterwards introduced it into the inverted jar. The quicksilver soon acquired a degree of warmth which rendered it not unpleasant. Minute air-bubbles ascended to the top of the quicksilver, more speedily in the beginning of the experiment, more tardily towards the conclusion. After an hour had elapsed, I withdrew my hand; the bubbles of air, which now appeared on the top of the quicksilver, were, I suppose, in bulk equal to one scruple of water.
"In sixteen hours, I collected a half-once measure of air, which makes fifteen grains the average product of an hour. No kind of moisture appeared on the surface of the quicksilver. Some sucking-paper was put up, which was withdrawn unmoistened. My hand was always damp when taken out of the quicksilver. Whatever aqueous perspiration was produced, adhered to its surface, whilst the aeriform ascended to the top of the jar. To the air I had thus collected, I threw up some lime-water[20], when about two thirds of it were rapidly absorbed; to the remainder, I added a bubble of nitrous gas[21]; but could not discover any red fumes, nor any diminution of the quantity. I repeated this experiment six times, with similar though not uniform results. I believe it will be found that the air perspired consists of carbonic acid gas, or fixed air, a little more than two thirds; of nitrogenous gas, a little less than one-third. In one experiment, the nitrogen made only one-fourth part of the air collected; in another, I thought it exceeded one-third."
He then made a series of experiments of the same kind, but substituting water for the quicksilver, sometimes heating himself previously by exercise. The results of these were not materially different from those in which he held his hand in quicksilver; but they are less clear, because the carbonic acid gas given off seemed absorbed by the water. In the next series of experiments, he held his hand and arm in atmospheric air. In this case, he found that, in addition to the giving off of carbonic acid, a portion of the oxygen of the air became absorbed. This is exactly what happens in the lungs. Now, as the carbonic acid, when given off, is in both cases coincident with the disappearance of oxygen, and as carbonic acid is composed of oxygen and carbon, it had been usually conceived that the oxygen taken in, contributed to form the carbonic acid given off; and the idea is still entertained very generally.
The experiments of Abernethy, however, presently to be adverted to, in regard to the skin; and those of Edwards, long after, in regard to the lungs; satisfactorily prove, we think, that the carbonic acid is not at all derived in the manner supposed[22].
To test this matter, Mr. Abernethy confined his hand and arm in various gases containing no oxygen—as hydrogen, and then in nitrogen; but he found the carbonic acid gas still given off as before. He then placed his hand in a gas (nitrous oxide) containing oxygen; and lastly, in oxygen itself, to see if it increased, or otherwise affected the elimination of carbonic acid; but in neither of those experiments was the carbonic acid thrown off, increased, or in any way affected by it.
In a subsequent part of the paper, he remarks on the idea that physiologists entertained of the carbonic acid given off by the lungs being made by the oxygen inspired; but he says, very justly, that the quantity of oxygen is too small for the formation of so much carbonic acid gas as we find given out by those bodies; and that his experiments on the skin clearly prove that the exhaling vessels of the skin emit carbonic acid in a state of complete formation; and then adds, what it is difficult to estimate the merits of, without recollecting that it was said half a century ago (and before the experiments of Edwards), "and, doubtless, those of the lungs perform a similar office."