ndex upon the sector is now observed, and we calculate
what number of cubical inches correspond to each degree. We then fill a
second and third bottle, and so on, in the same manner, with the same
precautions, and even repeat the operation several times with bottles of
different sizes, till at last, by accurate attention, we ascertain the
exact gage or capacity of the jar A, in all its parts; but it is better
to have it formed at first accurately cylindrical, by which we avoid
these calculations and estimates.

The instrument I have been describing was constructed with great
accuracy and uncommon skill by Mr Meignie junior, engineer and physical
instrument-maker. It is a most valuable instrument, from the great
number of purposes to which it is applicable; and, indeed, there are
many experiments which are almost impossible to be performed without it.
It becomes expensive, because, in many experiments, such as the
formation of water and of nitric acid, it is absolutely necessary to
employ two of the same machines. In the present advanced state of
chemistry, very expensive and complicated instruments are become
indispensibly necessary for ascertaining the analysis and synthesis of
bodies with the requisite precision as to quantity and proportion; it is
certainly proper to endeavour to simplify these, and to render them less
costly; but this ought by no means to be attempted at the expence of
their conveniency of application, and much less of their accuracy.

SECT. III.

_Some other methods of measuring the volume of Gasses._

The gazometer described in the foregoing section is too costly and too
complicated for being generally used in laboratories for measuring the
gasses, and is not even applicable to every circumstance of this kind.
In numerous series of experiments, more simple and more readily
applicable methods must be employed. For this purpose I shall describe
the means I used before I was in possession of a gazometer, and which I
still use in preference to it in the ordinary course of my experiments.

Suppose that, after an experiment, there is a residuum of gas, neither
absorbable by alkali nor water, contained in the upper part of the jar
AEF, Pl. IV. Fig. 3. standing on the shelf of a pneumato-chemical
apparatus, of which we wish to ascertain the quantity, we must first
mark the height to which the mercury or water rises in the jar with
great exactness, by means of slips of paper pasted in several parts