_Specific Gravity_ is the proportion of the weight of a body to that
of an equal volume of some other substance adopted as a standard of
reference. For solids and liquids the standard is pure water, at a
temperature of 60° F., the barometer being at 30 inches.
Aëriform bodies are referred to the air as their standard. A cubic
foot of water weighs 1,000 ounces; if the same bulk of another
substance, as for instance cast iron, is found to weigh 7,200 ounces,
its proportional weight or specific gravity is 7·2. It is convenient
to know the figures representing this proportion for every substance
in common use, that the weight of any given bulk may be readily
determined. For all substances the specific gravity is used in various
tests for the purpose of distinguishing bodies from each other, the
same substance being found, under the equal conditions of temperature,
&c., to retain its peculiar proportional weight or density.
Hence tables of specific gravities of bodies are prepared for
reference, and in every scientific description of substances the
specific gravity is mentioned. _In practical use, the weight of a cubic
foot is obtained from the figures representing the density by moving
the decimal point three figures to the right, which obviously from the
example above, gives the ounces, and these divided by 16 gives the
pounds avoirdupois, in the cubic foot._
Different methods may be employed to ascertain the specific gravity of
solids. That by measuring the bulk and weighing, is rarely practicable;
as a body immersed in water must displace its own bulk of the fluid,
the specific gravity may be ascertained by introducing a body, after
weighing it, into a suitable vessel exactly filled with water, and then
weighing the fluid which overflows. The proportional weight is thus
at once obtained. Wax will cause its own weight of water to overflow;
its specific gravity is then 1. Platinum, according to the condition
it is in, will cause only from 1/21 to 1/21·5 of its weight of water
to overflow, showing its specific gravity to be from 21 to 21·5. But
a more exact method than this is commonly employed. The difference of
weight of the same substance weighed in air and when immersed in water,
is exactly that of the water it displaces, and may consequently be
taken as the weight of its own bulk of water. See Fig. 93 and rule and
example on page 95.
[Illustration: FIG. 91.]
_The specific gravity is then obtained by weighing the body first in
air, and then, suspended by a fibre of silk or a hair, in water, and
dividing the weight in air by the difference._ It is hardly necessary
to say that the substance examined must be free from mixture of foreign
matters, and especially from cavities that may contain air.
NOTE.—_Hydrometers_ are instruments for determining the relative
density of fluids; _distilled water_ is usually referred to as the
standard of comparison. They consist usually of a bulb or float
weighted at bottom so as to float upright, and having an elongated
stem graduated to indicate the density of the liquid by the depth to
which they sink therein.