of oxygen and hydrogen gas. If, again, upon mixing a portion of the
residuum with oxygen gas, red fumes are produced, we conclude that it
contains nitrous gas.

These preliminary trials give some general knowledge of the properties
of the gas, and nature of the mixture, but are not sufficient to
determine the proportions and quantities of the several gasses of which
it is composed. For this purpose all the methods of analysis must be
employed; and, to direct these properly, it is of great use to have a
previous approximation by the above methods. Suppose, for instance, we
know that the residuum consists of oxygen and azotic gas mixed together,
put a determinate quantity, 100 parts, into a graduated tube of ten or
twelve lines diameter, introduce a solution of sulphuret of potash in
contact with the gas, and leave them together for some days; the
sulphuret absorbs the whole oxygen gas, and leaves the azotic gas pure.

If it is known to contain hydrogen gas, a determinate quantity is
introduced into Volta's eudiometer alongst with a known proportion of
hydrogen gas; these are deflagrated together by means of the electrical
spark; fresh portions of oxygen gas are successively added, till no
farther deflagration takes place, and till the greatest possible
diminution is produced. By this process water is formed, which is
immediately absorbed by the water of the apparatus; but, if the hydrogen
gas contain charcoal, carbonic acid is formed at the same time, which is
not absorbed so quickly; the quantity of this is readily ascertained by
assisting its absorption, by means of agitation. If the residuum
contains nitrous gas, by adding oxygen gas, with which it combines into
nitric acid, we can very nearly ascertain its quantity, from the
diminution produced by this mixture.

I confine myself to these general examples, which are sufficient to give
an idea of this kind of operations; a whole volume would not serve to
explain every possible case. It is necessary to become familiar with the
analysis of gasses by long experience; we must even acknowledge that
they mostly possess such powerful affinities to each other, that we are
not always certain of having separated them completely. In these cases,
we must vary our experiments in every possible point of view, add new
agents to the combination, and keep out others, and continue our trials,
till we are certain of the truth and exactitude of our conclusions.

SECT. V.

_Of the