Page:Elementary Text-book of Physics (Anthony, 1897).djvu/147

§ 117] between the last weight and the sum of the weights of the solid and of the water filling the bottle is the weight of the water displaced by the solid. The ratio of the weight of the solid to the weight thus obtained is the specific gravity of the solid.

The specific gravity of a liquid may also be obtained by means of hydrometers. These are of two kinds—the hydrometers of constant weight and those of constant volume. The first consists usually of a glass bulb surmounted by a cylindrical stem. The bulb is weighted, so as to sink in pure water to some definite point on the stem. This point is taken as the zero; and, by successive trials with different liquids of known specific gravity, points are found on the stem to which the hydrometer sinks in these liquids. With these as a basis, the divisions of the scale are determined and cut on the stem.

The hydrometer of constant volume consists of a bulb weighted so as to stand upright in the liquid, bearing on the top of a narrow stem a small pan, in which weights may be placed. The weight of the hydrometer being known, it is immersed in water; and, by the addition of weights in the pan, a fixed point on the stem is brought to coincide with the surface of the water. The instrument is then transferred to the liquid to be tested, and the weights in the pan changed until the fixed point again comes to the surface of the liquid. The sum of the weight of the hydrometer and the weights added in each case gives the weight of equal volumes of water and of the liquid, from which the specific gravity sought is easily obtained.

The specific gravity of gases is often referred to air or to hydrogen instead of water. It is best determined by filling a large glass flask, of known weight, with the gas, the specific gravity of which is to be obtained, and weighing it, noting the temperature and the pressure of the gas in the flask. The weight of the gas at the standard temperature and pressure is then calculated, and the ratio of this weight to the weight of the same volume of the standard gas is the specific gravity desired. The weight of the flask used in this experiment must be very exactly determmed. The presence