PHYSICAL PROPERTIES OF MINERALS. 83
specific gravity of water 1.00: χ dividing 5 into
17.65, make it equal to 3.53, which is the exact specific gravity of
the mineral, and which is that of essonite. Instead of a hydrostatic
balance, we may as 'well use Nicholson's hydrometer, a simple and very
convenient instrument, con» sisting of a hollow glass cylinder (A), and
two dishes (B and C) filled with lead, in order to keep the instrument
upright. The hydrometer is put-in a glass vessel (E), filled with
water, and used as follows :
1st. The weight is determined which is required to sink the instrument to the mark D in" water.
2d.
The mineral is put in the dish Λ over the weight noted, that is
required, in addition to the mineral, to sink the hydrometer to D.
3d.
The same experiment is repeated by putting the mineral, after being
moistened and washed with water, in the dish C ; and now is A— B the
weight of the mineral in the air, and B—b the weight of a quantity of
water equal in volume to that of the mineral.
gravity.
For determining the specific gravity of substances or minerals lighter
than water, or which float in water, it is necessary to adhere to the
same method by the hydrometer. A heavier body, such as lead, after
determining the
