Page:Elementary Text-book of Physics (Anthony, 1897).djvu/146

132 is defined as the ratio of its weight to the weight of an equal volume of pure water at a standard temperature.

The specific gravity of a solid that is not acted on by water may be determined by means of the hydrostatic balance. The body under examination, if it will sink in water, is suspended from one scale-pan of a balance by a fine thread, and is weighed. It is then immersed in water, and is weighed again. The difference between the weights in air and in water is the weight of the displaced water, and the ratio of the weight of the body to the weight of the displaced water is the specific gravity of the body.

If the body will not sink in water, a sinker of unknown weight and specific gravity is suspended from the balance, and counterpoised in water. Then the body, the specific gravity of which is sought, is attached to the sinker, and it is found that the equilibrium is destroyed. To restore it, weights must be added to the same side. These, being added to the weight of the body, represent the weight of the water displaced.

The specific gravity of a liquid is obtained by first balancing in air a mass of some solid, such as platinum or glass, that is not acted on chemically by the liquid, and then immersing the mass successively in the liquid to be tested and in water. The ratio of the weights which must be used to restore equilibrium in each case is the specific gravity of the liquid.

The specific gravity of a liquid may also be found by means of the specific gravity bottle. This is a bottle fitted with a ground-glass stopper. The weight of the water which completely fills it is determined once for all. When the specific gravity of any liquid is desired, the bottle is filled with the liquid, and the weight of the liquid determined. The ratio of this weight to the weight of an equal volume of water is the specific gravity of the liquid.

The same bottle may be used to determine the specific gravity of any solid which cannot be obtained in continuous masses, but is friable or granular. A weighed amount of the solid is introduced into the bottle, which is then filled with water, and the weight of the joint contents of the bottle determined. The difference