lost weight, being, in fact, buoyed up by the water ; now add carefully
more weights, till an equipoise is effected ; note the weight by which
this is obtained, and divide the original weight of the stone by it,
and the quotient will give the specific gravity required.
Example .· In weighing a white stone whose specific gravity we required, we found the weight of the stone to becarats. The weights effecting the equi-
poise,carats. We have, therefore,
orwhich is the specific gravity required. This
would indicate the specific gravity of a Diamond.
spring-balance, an instrument made in Munich, is often now used by
mineralogists on the Continent, and enables the specific gravity of
small specimens to be determined with great rapidity. Another
instrument much used is the Westphal balance, named from its inventor,
and constructed on the principle of the familiar steel-yard.
are various other methods for determining this physical constant, but
the above will suffice for ordinary purposes. Care should always be
taken to have the stone perfectly clean, and damped before operation,
whilst all adherent air-bubbles must be most carefully removed. In the
preceding pages the specific gravity of each stone has been given at
the end of its chapter.
The Optical Properties of Gems.