There are many other methods of separating the respirable from the
noxious part of the atmospheric air, which cannot be taken notice of in
this part, without anticipating information, which properly belongs to
the subsequent chapters. The experiments already adduced may suffice for
an elementary treatise; and, in matters of this nature, the choice of
our evidences is of far greater consequence than their number.

I shall close this article, by pointing out the property which
atmospheric air, and all the known gasses, possess of dissolving water,
which is of great consequence to be attended to in all experiments of
this nature. Mr Saussure found, by experiment, that a cubical foot of
atmospheric air is capable of holding 12 grains of water in solution:
Other gasses, as the carbonic acid, appear capable of dissolving a
greater quantity; but experiments are still wanting by which to
determine their several proportions. This water, held in solution by
gasses, gives rise to particular phenomena in many experiments, which
require great attention, and which has frequently proved the source of
great errors to chemists in determining the results of their
experiments.

FOOTNOTES:

[9] It will likewise be necessary to take care that the air contained in
the glass, both before and after the experiment, be reduced to a common
temperature and pressure, otherwise the results of the following
calculations will be fallacious.--E.

CHAP. IV.

_Nomenclature of the several Constituent Parts of Atmospheric Air._

Hitherto I have been obliged to make use of circumlocution, to express
the nature of the several substances which constitute our atmosphere,
having provisionally used the terms of _respirable_ and _noxious_, or
_non-respirable parts of the air_. But the investigations I mean to
undertake require a more direct mode of expression; and, having now
endeavoured to give simple and distinct ideas of the different
substances which enter into the composition of the atmosphere, I shall
henceforth express these ideas by words equally simple.

The temperature of our earth being very near to that at which water
becomes solid, and reciprocally changes from solid to fluid, and as this
phenomenon takes place frequently under our observation, it has very
naturally followed, that, in the languages of at least every climate
subjected to any degree of winter, a term has been used for signifying
water in the state of solidity, when dep