HYDROM'ETER rOpoptrinor, hydrome• trion, vessel for hydrostatic measurement, from hydOr, water iorpoe, me Iron, measure), also known as Ant.omETtit An instrument used in determining the specific gravity of liquids, and in some instances of solid Specific gravity, for determining which the hydrometer is used, is the ratio between the weight of a given quantity of a substance and that of an equal volume of water at its tempera ture of maximum density, 4° Centigrade, so it follows that the specific gravity of two sub stances must be directly proportional to their weights when the volumes arc the same, or in versely as their volumes when the weights are equal. The hydrometer. which is a. hollow in strument of glass or metal designed to float up right in a liquid, makes use of the principle of Archimedes that the weight of the volume of liquid displaced by a body is equal to the weight of the body itself. In its simplest form a hydrom eter might consist of a graduated scale floating vertically in a liquid, and on which the level of the surface of the liquid it displaces could be measured. For example, an ordinary ruler if allowed to float upright in water would sink un til a certain division on the scale is reached. This reading multiplied by the area of the cross section of the ruler will give the volume of a mass of water equal to that displaced by the ruler, and equivalent to the weight of the ruler itself. Immersing the ruler in a liquid of dif ferent specific gravity, it will sink until the sur face touches some other division on the scale, and as before the volume of a mass equivalent to the weight of the ruler will he obtained. A ratio made with these two quantities representing the volumes of the two liquids would give the spe cific gravity, hut as the eross-section of the ruler is a constant quantity, it is only necessary to compare the two readings on the scale. The ordinary hydrometer consists of a glass tube terminating in two bulbs, the lower of which is filled with mercury or shot in order to keep the instrument steady in an upright position when immersed in a liquid. Such a hydrometer is ex tensively used in scientific and commercial work, as it can he graduated to furnish direct readings of the specific gravity of a liquid. The mark to which the ment would sink when placed in water is usually marked 1.000, and the weight of water displaced is equal to the total weight of the hydrometer. if the instrument. he placed in a liquid that has a less specific gravity than water. alcohol for example. it will sink until the surface of the liquid touches some point higher up on the stem, as in this ease it takes a greater volume of the lighter liquid to be equivalent to the weight of the hydrometer, The scale on which the readings are made IS' on a piece of paper contained inside of the stern. and the
method of graduation forms the chief distinction between the different instruments. In the in strument of Gay-Lussac. which is known as a volumeter, the water-point was marked 100, and the division was carried along the stein both above and below this mark on the basis that the amount of the stein betw•een any two scale divi sions should equal of the volume of the part immersed in water. With an instrument graduated in this manner the specific gravity is obtained by dividing the reading into 100. In practice, it is more convenient to use a hydrom eter whose scale is graduated to read specific gravity direct, but in this event the scale is ir regular, and great care must be exercised in the construction and testing of the instrument. The accuracy of a hydrometer is greatly increased by making its stem as slender as possible, and con sequently increasing the space between the divi sions, so it is customary to construct a number of instruments, each having a limited range, and designed for liquids of different densities. The hydrometer when used for a special purpose or substance, frequently undergoes modifications. The aleoholmeter, for example, is so graduated as to give at once the percentage of pure alcohol in a mixture of alcohol and water. The urinom eter, lactometer, saeharinieter, and other instru ments are all hydrometers used for special pur poses. Hydrometers with arbitrary scales also play an important part in scientific work, though the present tendency is toward the exclusive use of specific gravity values. In the Baurni7- scale, which is encountered frequently in chemical proc esses, a. second fixed point on the hydrometer stem is determined in addition to that given by water. This is obtained by dissolving one part by weight of common salt in nine parts of water, and then the space between these two points is divided into ten equal parts, which are called degrees. The water-point is marked 10 on the scale, and the division is carried beyond for 40 degrees. For liquids heavier than water the sec ond fixed point is determined by immersion in a solution of 15 parts of salt in S5 parts of water, and the space between it and the water-point, which is marked 0, is divided into 15 equal divi sions. The scales of Cartier and Beek are also used, though less frequently than that of Baum'j.