water. The weight of tin with which this instrument is loaded ought
to be such as will make it remain almost in equilibrium in distilled
water, and should not require more than half a dram, or a dram at most,
to make it sink to g.

We must first determine, with great precision, the exact weight of the
instrument, and the number of additional grains requisite for making it
sink, in distilled water of a determinate temperature, to the mark: We
then perform the same experiment upon all the fluids of which we wish to
ascertain the specific gravity, and, by means of calculation, reduce the
observed differences to a common standard of cubic feet, pints or
pounds, or of decimal fractions, comparing them with water. This method,
joined to experiments with certain reagents[56], is one of the best for
determining the quality of waters, and is even capable of pointing out
differences which escape the most accurate chemical analysis. I shall,
at some future period, give an account of a very extensive set of
experiments which I have made upon this subject.

These metallic hydrometers are only to be used for determining the
specific gravities of such waters as contain only neutral salts or
alkaline substances; and they may be constructed with different degrees
of ballast for alkohol and other spiritous liquors. When the specific
gravities of acid liquors are to be ascertained, we must use a glass
hydrometer, as represented Pl. vii. fig. 14[57]. This consists of a
hollow cylinder of glass, a b c f, hermetically sealed at its lower
end, and drawn out at the upper into a capillary tube a, ending in the
little cup or bason d. This instrument is ballasted with more or less
mercury, at the bottom of the cylinder introduced through the tube, in
proportion to the weight of the liquor intended to be examined: We may
introduce a small graduated slip of paper into the tube a d; and,
though these degrees do not exactly correspond to the fractions of
grains in the different liquors, they may be rendered very useful in
calculation.

What is said in this chapter may suffice, without farther enlargement,
for indicating the means of ascertaining the absolute and specific
gravities of solids and fluids, as the necessary instruments are
generally known, and may easily be procured: But, as the instruments I
have used for measuring the gasses are not any where described, I shall
give a more detailed account of these in the following chapter.

FOOTNO