eservoir of mercury, which it preserves
with more safety than any other vessel, being neither subject to
overturn, nor to any other accident. We operate with mercury in this
apparatus exactly as with water in the one before described; but the
bell-glasses must be of smaller diameter, and much stronger; or we may
use glass tubes, having their mouths widened, as in Fig. 7.; these are
called _eudiometers_ by the glass-men who sell them. One of the
bell-glasses is represented Fig. 5. A. standing in its place, and what
is called a jar is engraved Fig. 6.

The mercurial pneumato-chemical apparatus is necessary in all
experiments wherein the disengaged gasses are capable of being absorbed
by water, as is frequently the case, especially in all combinations,
excepting those of metals, in fermentation, &c.

SECT. II.

_Of the Gazometer._

I give the name of _gazometer_ to an instrument which I invented, and
caused construct, for the purpose of a kind of bellows, which might
furnish an uniform and continued stream of oxygen gas in experiments of
fusion. Mr Meusnier and I have since made very considerable corrections
and additions, having converted it into what may be called an _universal
instrument_, without which it is hardly possible to perform most of the
very exact experiments. The name we have given the instrument indicates
its intention for measuring the volume or quantity of gas submitted to
it for examination.

It consists of a strong iron beam, DE, Pl. VIII. Fig. 1. three feet
long, having at each end, D and E, a segment of a circle, likewise
strongly constructed of iron, and very firmly joined. Instead of being
poised as in ordinary balances, this beam rests, by means of a
cylindrical axis of polished steel, F, Fig. 9. upon two large moveable
brass friction-wheels, by which the resistance to its motion from
friction is considerably diminished, being converted into friction of
the second order. As an additional precaution, the parts of these wheels
which support the axis of the beam are covered with plates of polished
rock-cristal. The whole of this machinery is fixed to the top of the
solid column of wood BC, Fig. 1. To one extremity D of the beam, a scale
P for holding weights is suspended by a flat chain, which applies to the
curvature of the arc nDo, in a groove made for the purpose. To the
other extremity E of the beam is applied another flat chain, i k m, so
constructed, as to be incapable of lengthening or