Every body immersed in a liquid is submitted to the action of two
forces: gravity which tends to lower it, and the buoyancy of the liquid
which tends to raise it with a force equal to the weight of the liquid
displaced. The weight of the body is either totally or partially
overcome by its buoyancy, by which it is concluded that a body immersed
in a liquid loses a part of its weight equal to the weight of the
displaced liquid.
This principle, which is _the basis of the theory of immersed and
floating bodies_, is called the principle of Archimedes, after the
discoverer. It may be shown experimentally by means of _the hydrostatic
balance_ (Fig. 92). This is an ordinary balance, each pan of which is
provided with a hook; the beam being raised, a hollow brass cylinder is
suspended from one of the pans, and below this a solid cylinder whose
volume is exactly equal to the capacity of the first cylinder; lastly,
an equipoise is placed in the other pan. If now the hollow cylinder be
filled with water, the equilibrium is disturbed; but if at the same
time the beam is lowered so that the solid cylinder becomes immersed in
a vessel of water placed beneath it, the equilibrium will be restored.
By being immersed in water the solid cylinder loses a portion of its
weight equal to that of the water in the hollow cylinder. Now, as the
capacity of the hollow cylinder is exactly equal to the volume of the
solid cylinder the principle which has been before laid down is proved.
[Illustration: FIG. 92.]
Minerals, if suspected of containing spaces, should be coarsely
pulverized, and then the second method may be conveniently applied
to determine their density—thus prepared, a higher result will be
obtained, and even metals when pulverized were found to give a greater
specific gravity than when this is determined from samples in their
ordinary state. Very fine powders may also be examined by the method in
use for ascertaining the specific gravity of fluids, viz.: by comparing
the weight of a measured quantity with that of the same quantity of
water.
A glass vessel called a specific gravity bottle is commonly employed,
which is furnished with a slender neck, upon which is a mark indicating
the height reached by 1,000 grains of water. The substance to be
examined is introduced till it reaches the same mark, and, the weight
of the bottle being known, only one weighing is required to obtain the
result.
The specific gravity of fluids is also taken by the instrument
called a hydrometer or alcometer. Such instruments are much used for
ascertaining the specific gravity of spirituous and other liquors, as
an indication of their strength. _If the solid body to be tested is
lighter than water, it must be attached to some heavy substance to
cause it to sink._ Its specific gravity is then calculated by dividing
its weight in the air by the sum of the weights of the attached body
both in air and in water, first subtracting from this sum the weight of
the two bodies together in the water.
Bodies soluble in water may be weighed in some other fluid, as alcohol,
ether, olive oil, &c., and their proportional weight to that of this
fluid being thus ascertained, their density compared with that of water
is readily calculated or they may be enveloped in wax or other suitable
substance to protect them, and then treated by the method just given
for substances lighter than water. Gaseous bodies are weighed in a thin
glass flask or other vessel made for the purpose, and provided with a
stop-cock. The vessel is exhausted of air before the introduction of
the gas.