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Methods of determining Specific Gravity
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The
substance is first weighed in air and then in water. In water it weighs
less due to the buoyancy caused by the displacement of an equal volume
of water according to the principle of Archimedes. The difference
between the weight of the substance in air and its weight in water,
i.e. the loss of weight is the weight of a volume of water equal to
that of the substance.
Since
gem-stones are very small when cut, a delicate balance has to be used
at least to one milligram. The usual methods for determining the sp.
gr. involve the use of (1) the Chemical Balance, (2) the Pycnometer,
(3) Heavy Liquids and the Westphal's Balance, and (4) the Jolly's
Spring Balance.
(1) The Chemical Balance •
The
stone is suspended from the lower hook of the left stirrup of a balance
by a thin cotton thread and its weight determined in air. Let this be
weight
is determined with the stone immersed completely in water in a beaker,
placed on a hydrostatic bench placed on the left pan taking care that
the pan does not touch any part of the bench. Let this be
the stone is
This method could be used for fairly large and medium specimens but not for cut and small stones, and also takes much time.
(2) The Pycnometer
The
Pycnometer or the specific gravity bottle is merely a bottle with a
ground in stopper having a fine bore through it and it is so
constructed that it has a specified volume,
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