cid air_, which the
vapour of spirit of salt may properly enough be called, and had made
those experiments upon it, of which I have given an account in the
former part of this work, and others which I propose to recite in this
part; it occurred to me, that, by a process similar to that by which
this _acid_ air is expelled from the spirit of salt, an _alkaline_ air
might be expelled from substances containing volatile alkali.

Accordingly I procured some volatile spirit of sal ammoniac, and having
put it into a thin phial, and heated it with the flame of a candle, I
presently found that a great quantity of vapour was discharged from it;
and being received in a vessel of quicksilver, standing in a bason of
quicksilver, it continued in the form of a transparent and permanent
air, not at all condensed by cold; so that I had the same opportunity of
making experiments upon it, as I had before on the acid air, being in
the same favourable circumstances.

With the same ease I also procured this air from _spirit of hartshorn_,
and _sal volatile_ either in a fluid or solid form, i. e. from those
volatile alkaline salts which are produced by the distillation of sal
ammoniac with fixed alkalis. But in this case I soon found that the
alkaline air I procured was not pure; for the fixed air, which entered
into the composition of my materials, was expelled along with it. Also,
uniting again with the alkaline air, in the glass tube through which
they were conveyed, they stopped it up, and were often the means of
bursting my vessels.

While these experiments were new to me, I imagined that I was able to
procure this air with peculiar advantage and in the greatest abundance,
either from the salts in a dry state, when they were just covered with
water, or in a perfectly fluid state; for, upon applying a candle to the
phials in which they were contained, there was a most astonishing
production of air; but having examined it, I found it to be chiefly
fixed air, especially after the first or second produce from the same
materials; and removing my apparatus to a trough of water and using the
water instead of quicksilver, I found that it was not presently absorbed
by it.

This, however, appears to be an easy and elegant method of procuring
fixed air, from a small quantity of materials, though there must be a
mixture of alkaline air along with it; as it is by means of its
combination with this principle only, that it is possible, that so muc