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."