Though experiments on successive inspirations of pure nitrous oxide might go far to determine whether or no any nitrogene, carbonic acid and oxygene were products of respiration, yet I distinctly saw that it was impossible in this way to ascertain their quantities, supposing them produced, unless I could first determine the capacity of my lungs; and the different proportions of the gases remaining in the bronchial vessels after a compleat expiration, when atmospheric air had been respired. In some experiments (that I made on the respiration of hydrogene, with a view to determine whether carbonic acid was _produced_ by the combination of carbon loosely combined in the venous blood, with the oxygene respired, or whether it was simply _given out_ as excrementitious by this blood) I found, without however being able to solve the problem I had proposed to myself, that in the respiration of pure hydrogene, little or no alteration of volume took place; and that the residual gas was mingled with some nitrogene, and a little oxygene and carbonic acid. From the comparison of these facts with those noticed in the last section and in R. III. Div. I. there was every reason to suppose that hydrogene was not absorbed or altered when respired: but only mingled with the residual gases of the lungs. Hence, by making a full expiration of atmospheric air, and afterwards taking six or seven respirations of hydrogene in the mercurial airholder, and then making a compleat expiration, I conjectured that the residual gas and the hydrogene would be so mingled, as that nearly the same proportions should remain in the bronchial vessels, as in the airholder. By ascertaining these proportions and calculating from them, I hoped to be able to ascertain with tolerable exactness, the capacity of my fauces and bronchia, as well as the composition of the gas remaining in them, after a complete expiration of common air. IV. _Respiration of Hydrogene._ The hydrogene that I employed, was procured from the decomposition of water by means of clean iron filings and diluted sulphuric and muriatic acids. It was breathed in the same manner as nitrous oxide, in the large mercurial airholder. After a compleat voluntary exhaustion of my lungs in the usual posture, I found great difficulty in breathing hydrogene for so long as half a minute, so as to make a compleat expiration of it. It produced uneasy feelings in the chest, momentary loss of muscular power, and sometimes a transient giddiness. In some of the experiments that I made; on account of the giddiness, the results were rendered inconclusive, by my removing my mouth from the mouth-piece after expiration, before the assistant could turn the stop-cock.