six shillings for the present, and wait five or six
years longer (if I should live so long) for the opportunity of buying
the same thing, probably much enlarged, and at the same time a complete
account of all that has been done by others relating to this subject.

Though for the plain, and I hope satisfactory reason above mentioned, I
shall probably write no other _histories_ of this kind, I shall, as
opportunity serves, endeavour to provide _materials_ for such histories,
by continuing my experiments, keeping my eyes open to such new
appearances as may present themselves, investigating them as far as I
shall be able, and never failing to communicate to the public, by some
channel or other, the result of my observations.

In the publication of this work I have thought that it would be
agreeable to my readers to preserve, in some measure, the order of
history, and therefore I have not thrown together all that I have
observed with respect to each kind of air, but have divided the work
into _two parts_; the former containing what was published before, in
the Philosophical Transactions, with such observations and corrections
as subsequent experience has suggested to me; and I have reserved for
the latter part of the work an account of the experiments which I have
made since that publication, and after a pretty long interruption in my
philosophical pursuits, in the course of the last summer. Besides I am
sensible that in the latter part of this work a different arrangement of
the subjects will be more convenient, for their mutual illustration.

Some persons object to the term _air_, as applied to _acid_, _alkaline_,
and even _nitrous air_; but it is certainly very convenient to have a
common term by which to denote things which have so many common
properties, and those so very striking; all of them agreeing with the
air in which we breathe, and with _fixed air_, in _elasticity_, and
_transparency_, and in being alike affected by heat or cold; so that to
the eye they appear to have no difference at all. With much more reason,
as it appears to me, might a person object to the common term _metal_,
as applied to things so very different from one another as gold,
quicksilver, and lead.

Besides, _acid_ and _alkaline_ air do not differ from _common air_ (in
any respect that can countenance an objection to their having a common
appellation) except in such properties as are common to it with _fixed
air_, though in a different de