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.
[97] Experiments and Observations, vol. II. pag. 50. Last Edition.
[98] That is, charcoal produced by the decomposition of spirits of
wine. Vol. II. pag. 39.
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.