In his experiments on the iron and pyrophyrus, the nitrous gas was evidently decomposed. From the great quantity of nitrogene produced in those on the charcoal, it seems likely that both the common charcoal,[99] and the charcoal of copper employed contained atmospherical air, which being dispelled by the heat of the lens, was decomposed by the nitrous gas: indeed, till I made the following experiment, I suspected that the carbonic acid produced, when the charcoal of copper was employed, arose from a decomposition of the nitrous acid, formed in this way. [99] Dr. Priestley says, “having heated iron in nitrous air, I proceeded to heat in the same air, a piece of charcoal not long after it had been subjected to a strong heat covered with sand. The sun not shining immediately, after the charcoal was introduced into the vessel of air, through the mercury by which it was confined, part of the air was absorbed; but on heating the charcoal, the quantity was increased. Having continued the progress as long as I thought necessary, I examined the air and found it to be about as much as the original quantity of nitrous air; but it was all phlogisticated air extinguishing a candle and having no mixture of fixed air in it.”—Experiments and Observations, Vol. II, page 39. I introduced a piece of well-burnt charcoal, which could hardly have weighed the eighth of a grain, whilst red hot, under a cylinder filled with mercury, and admitted to it half a cubic inch of nitrous gas. A slight absorption took place. The sun being very bright, I kept the charcoal in the focus of a small lens for near a quarter of an hour. At the end of this time the gas occupied a space nearly as before the experiment, and a very minute portion of the charcoal had been consumed. On introducing into the cylinder a small quantity of solution of strontian, a white precipitation was perceived, and the gas slowly diminished to about three tenths of a cubic inch. To these three tenths a little common air was admitted, when very slight red fumes were perceived. This experiment convinced me, that the attraction of charcoal for the oxygene of nitrous gas, at high temperatures, was sufficiently strong to effect a slow decomposition of it. To be more accurately acquainted with this decomposition, and to learn the quantities of carbonic acid and nitrogene produced from a known quantity of nitrous gas, I proceeded in the following manner. II. _Analysis of Nitrous Gas by Charcoal._ A quantity of nitrous gas was procured in a water apparatus, from the decomposition of nitrous acid by mercury. A portion of it was transferred to the mercurial trough. After the mercury and the jar had been dried by bibulous paper, 40 measures of this portion were agitated in a solution of sulphate of iron. The gas remaining after the absorption was complete, filled about a measure and half; so that the nitrous gas contained nearly ¹/₂₆ nitrogene.