ave made no
experiments to ascertain.

As soon as this mixture of iron filings with, brimstone and water,
begins to ferment, it also turns black, and begins to swell, and it
continues to do so, till it occupies twice as much space as it did at
first. The force with which it expands is great; but how great it is I
have not endeavoured to determine.

When this mixture is immersed in water, it generates no air, though it
becomes black, and swells.

SECTION VI.

_Of NITROUS AIR._

Ever since I first read Dr. Hales's most excellent _Statical Essays_, I
was particularly struck with that experiment of his, of which an account
is given, VOL. I, p. 224. and VOL. II, p. 280. in which common air, and
air generated from the Walton pyrites, by spirit of nitre, made a turbid
red mixture, and in which part of the common air was absorbed; but I
never expected to have the satisfaction of seeing this remarkable
appearance, supposing it to be peculiar to that particular mineral.
Happening to mention this subject to the Hon. Mr. Cavendish, when I was
in London, in the spring of the year 1772, he said that he did not
imagine but that other kinds of pyrites, or the metals might answer as
well, and that probably the red appearance of the mixture depended upon
the spirit of nitre only. This encouraged me to attend to the subject;
and having no pyrites, I began with the solution of the different metals
in spirit of nitre, and catching the air which was generated in the
solution, I presently found what I wanted, and a good deal more.

Beginning with the solution of brass, on the 4th of June 1772, I first
found this remarkable species of air, only one effect of which, was
casually observed by Dr. Hales; and he gave so little attention to it,
and it has been so much unnoticed since his time, that, as far as I
know, no name has been given to it. I therefore found myself, contrary
to my first resolution, under an absolute necessity of giving a name to
this kind of air myself. When I first began to speak and write of it to
my friends, I happened to distinguish it by the name of _nitrous air_,
because I had procured it by means of spirit of nitre only; and though I
cannot say that I altogether like the term, neither myself nor any of my
friends, to whom I have applied for the purpose, have been able to hit
upon a better; so that I am obliged, after all, to content myself with
it.

I have found that this kind of air is readily procured