ν the weight by which the substance when weighed with brass weights will appear too light. The true weight, W, in vacuo will then be :
Now, with the true weight, W, in vacuo, the
specific gravity may be calculated according to the equation previously
given. To furnish a notion of the value of this correction, it may be
stated that a fragment of rock crystal weighing io grams will become
10'0031 grams, a grain of j parts in 10,000.
When
the specific gravity of a small gem is to be taken, an assay balance of
great accuracy may be advantageously employed."j" In this case the full
advantage of the delicacy of the instrument cannot be secured if water,
which has a high surface-tension, be the liquid in \vhich the stone is
weighed, the friction between it and the stone and immersed pan being
too great. Alcohol considerably diluted with water answers well. A fair
quantity is prepared and preserved in a well-stoppered and capped
bottle. Its specific gravity is best ascertained by means of Dr.
Sprengel's tube. In the following example of an experiment a dilute
alcohol of sp. gr. "8488 at 15° C, and containing about 80 per cent, by
weight of absolute alcohol, was used :
Specific gravity of brilliant-cut specimen of phenakite.
