CHAPTER VIII.
PHYSICAL PROPERTIES.
P—Specific Gravity.
The fixing
of the specific gravity of a stone also determines its group position
with regard to weight; its colour and other characteristics defining
the actual stone. This is a safe and very common method of proving a
stone, since its specific gravity does not vary more than a point or so
in different specimens of the same stone. There are several ways of
arriving at this, such as by weighing in balances in the usual manner,
by displacement, and by immersion in liquids the specific gravity of
which are known. Cork is of less specific gravity than water, therefore
it floats on the surface of that liquid, whereas iron, being heavier,
sinks. So that by changing the liquid to one lighter than cork, the
cork will sink in it as does iron in water ; in the second instance, if
we change the liquid to one heavier than iron, the iron will float on
it as does cork on water, and exactly as an ordinary flat-iron will
float on quicksilver, bobbing up and down like a cork in a tumbler of
water. If, therefore, solutions of known but varying densities are
compounded, it is possible to tell almost to exactitude the specific
gravity of any stone dropped into them, by the position they assume.
Thus, if we take a solution of
