CHARCOAL
200
tort and nitre begin to be ignited, the vital air begins to come forth.
At the first it is very pure, provided the nitre be quite free from dust
or admixture of vegetable or animal matter. If such matter be pre
sent in it, though in exceedingly small quantity, some carbonic acid
gas is formed at first, and comes out mixed with the first portions of
vital air and some azote. After this, or from the beginning, if the
nitre was quite pure, we obtain a great quantity of good vital air,
which, if not quite pure, contains a fmall admixture of azotic gas.
But, towards the end of the operation, when the heat must be in
creased to a higher degree, the vital air comes over mixed with a more
considerable portion of azotic gas. The reason of this difference in
the purity of the vital air is, that in the beginning, the nitric acid
gives out only a part of the oxygen which it contains, (nearly )
and in this
and it is changed into nitrous acid, retaining perhaps
ſtate continues adhering to the alkalis. In the nitrous acid, therefore,
there is a deficient proportion of oxygen, and a superfluous quantity
of azote. When the proceſs is continued, and the heat increased to
produce a more complete decomposition of the acid, the azote must
neceſſarily make its appearance in a proportion continually increafing.
All theſe facts, and many more, to be mentioned as we proceed,
and which are fo well explained by Mr. Cavendifh’s discovery, con
firm the solidity of that discovery, making it one of the most impor
tant which the modern chemistry has produced.
One of the moît remarkable properties of charcoal, when recently
taken from the fire, is an attraction for a certain quantity of humidity,
and for various odorous and colouring matters of different fluids, con
taining animal or vegetable substances, subject to fermentation or cor
ruption,—as also for the acetous acid.
We have proofs of its attraction for humidity in many curious ex
periments of Mr. Scheele and Dr. Priestley. Although indestructible
by heat in close vessels without addition, yet, if moistened, it will yield
carbonic acid and hydrogenous gas. This may be repeated by an¬