With a view to discover where the power of nitrous air to diminish common air lay, I evaporated to dryness a quantity of the solution of copper in diluted spirit of nitre; and having procured from it a quantity of a _green precipitate_, I threw the focus of a burning-glass upon it, when it was put into a vessel of quicksilver, standing inverted in a bason of quicksilver. In this manner I procured air from it, which appeared to be, in all respects, nitrous air; so that part of the same principle which had escaped during the solution, in the form of _air_, had likewise been retained in it, and had not left it in the evaporation of the water. With great difficulty I also procured a small quantity of the same kind of air from a solution of _iron_ in spirit of nitre, by the same process. Having, for a different purpose, fired some paper, which had been dipped in a solution of copper in diluted spirit of nitre, in nitrous air, I found there was a considerable addition to the quantity of it; upon which I fired some of the same kind of paper in quicksilver and presently observed that air was produced from it in great plenty. This air, at the first, seemed to have some singular properties, but afterwards I found that it was nothing more than a mixture of nitrous air, from the precipitate of the solution, and of inflammable air, from the paper; but that the former was predominant. In the mixture of this kind of air with common air, in a trough of water which had been putrid, but which at that time seemed to have recovered its former sweetness (for it was not in the least degree offensive to the smell) a phenomenon sometimes occurred, which for a long time exceedingly delighted and puzzled me; but which was afterwards the means of letting me see much farther into the constitution of nitrous air than I had been able to see before. When the diminution of the air was nearly completed, the vessel in which the mixture was made began to be filled with the most beautiful _white fumes_, exactly resembling the precipitation of some white substance in a transparent menstruum, or the falling of very fine snow; except that it was much thicker below than above, as indeed is the case in all chemical precipitations. This appearance continued two or three minutes.