HYDROMETER, an instrument for de termining specific gravity, as of fluids, by flota tion. For determining the specific gravity of a liquid, the instrument is floated in the liquid itself. When a solid body floats in a liquid, and displaces a quantity of the liquid, it is sup ported by the same upward pressure that for liquids are inversely proportional to the den sities of the liquids. On each of these principles a form of hydrometer is founded. One is called the constant weight hydrometer, the other the constant volume hydrometer. The first, usually made of glass, is shown in Fig. 1. It has a large hollow bulb, and below that a smaller bulb, weighted with mercury, to make the in strument float upright and it is surmounted by a cylindrical glass stem which is graduated, the merly supported the liquid which it displaces. The weight of the solid body is thus equal to the weight of the liquid that it displaces. Hence the depth to which the same solid body is immersed in a liquid is greater as the density of the liquid is greater. This makes it possible to test the strength or purity of various solu tions. And. likewise, the weights required to inunerse a given body equally deep in various divisions being usually marked on or within the stem. The depth to which the hydrometer sinks in the liquid gives the density.
Of constant volume hydrometers, Nichol son's hydrometer, the best known, possibly. is adapted for determining the specific gravities not only of fluids, but of solids also. It is shown in Fig. 2. It consists of a hollow cylin der of metal, surmounted with a very fine metallic stem, to the top of which there is at tached a plate or pan for weights. From the bottom of the metallic cylinder hangs a cup or basket. The whole instrument is weighted so as to float upright. On the fine metallic stem there is a marked point: and by putting weights on the upper pan the hydrometer is always made to sink precisely to the point. If the weight of the instrument itself is known, and also the standard weight, or weight required to sink it to the marked point in distilled water, the cal culation of the specific gravity of any liquid from an observation with the instrument is very easy. To determine the specific gravity of
solids, the instrument is placed in distilled water and the solid body is put on the upper pan. Weights are then added till the hydrometer sinks to the marked point. But the standard weight of the 'instrument being known, it is plain that the difference between it and the weights that must be added on the upper pan to the weight of the body whose specific gravity is to be determined must be the weight in air of that body. The body is now transferred to the basket below the instrument, and the weight of the solid in water is similarly determined. From these data the specific gravity of the solid is calculated. Hydrometers are variously named from their use, as areometer, one for determin ing the density of fluids; lactometer or galacto meter, one for measuring the density of milk, and usually having a scale graduated from 1.000 to 1.029; incorrectly called creameter, as it does not correctly measure the cream; sacharometer, one for determining sugar con tents; salinometer, one for testing saline solu tion, as the brine in a marine boiler; urinometer, one for detecting the presence of sugar, etc., in urine; volumeter, one for determining the specific gravity of a solid by measuring its volume in a liquid. Alcoholometers are com monly graduated to show pure alcohol as 200 and proof spirit (50 per cent alcohol) as 100. The chemical hydrometer is simply a balance in which equal volumes of liquid are weighed, or in the case of testing a solid, it is weighed in air and in distilled water of tested temperature. A simpler device for the same purpose, com monly called a specific gravity bottle or pycno meter, is a measuring flask which permits easy comparison with the known weight of a similar measure of water. In all these devices tem perature is a modifying condition, because fluids increase in volume with heat, and therefore it is customary to have one or more thermometers permanently attached to the apparatus. See SPECIFIC GRAVITY.