I have before observed, that when animals are made to respire nitrous oxide, a certain absorption of the gas always takes place; but the smaller animals, the only ones that can be experimented upon in the mercurial apparatus, die in nitrous oxide so speedily and occasion so slight a diminution of gas, that I judged it useless to attempt to analise the residuum of their respiration, which supports flame as well as pure nitrous oxide, and is chiefly absorbable by water. In the infancy of my researches, I often respired nitrous oxide in a large glass bell, furnished with a breathing tube and stop-cock, and poised in water saturated with the gas. In two or three experiments in which the nostrils being closed after the exhaustion of the lungs, the gas was inspired from the bell and respired into it, a considerable diminution was perceived, and by the test of lime water some carbonic acid appeared to have been formed; but on account of the absorption of this carbonic acid by the impregnated water, and the liberation of nitrous oxide from it, it was impossible to determine with the least accuracy, the quantities of products after respiration. About this time likewise, I often examined the residuum of nitrous oxide, after it had been respired in silk bags. In these experiments when the gas had been breathed for a long time, a considerable diminution of it was observed, and the remainder extinguished flame and gave a very slight diminution with nitrous gas. But the great quantity of this remainder as well as other phænomena, convinced me that though the oiled silk was apparently air-tight when dry, under slight pressure, yet during the action of respiration, the moist and warm gas expired, penetrated through it, whilst common air entered through the wetted surface. To ascertain accurately, the changes effected in nitrous oxide by respiration, I was obliged to make use of the large mercurial airholder mentioned in Research I. of the capacity of 200 cubic inches. The upper cylinder of it was accurately balanced so as to be constantly under the pressure of the atmosphere. To an aperture in it, a stop-cock having a very large orifice was adapted, curved and flattened at its upper extremity, so as to form an air-tight mouth-piece.