I. Preliminaries.
In my account of the composition of nitric acid, in [Division I]. I gave an estimation of the quantities of oxygene and nitrogene combined in nitrous gas: I shall now detail the experiments on which that estimation is founded.
At an early period of my researches relating to nitrous oxide, from the observation of the phænomena taking place during the production of this substance, I had concluded, that the common opinion with regard to the composition of nitrous gas, was very distant from the truth. I had indeed analysed nitrous gas, by converting it into nitrous oxide, before I attempted to ascertain its composition by immediately separating the constituent principles from each other: and my first hopes of the possibility of effecting this, were derived from Dr. Priestley’s experiments on the combustion of pyrophorus in nitrous gas, and on the changes effected in it, by heated iron and charcoal.
This great philosopher found, that pyrophorus placed in contact with nitrous gas, burnt with great vividness, whilst the gas was diminished in volume to about one half, which generally consisted of nitrogene and nitrous oxide. He likewise found, iron heated by a lens in nitrous gas, increased in weight, whilst the gas was diminished about ½, and converted into nitrogene.[97]
He heated common charcoal, and charcoal of copper,[98] in nitrous gas by a lens. When common charcoal was employed, the gas was neither increased or diminished in bulk, but wholly converted into nitrogene; when charcoal of copper was used, the volume was a little increased, and the gas remaining consisted of ⁵/₇ nitrogene, and ²/₇ carbonic acid.
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.
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.