Hydrometer, Or Areometer, an instrument for determining the specific gravity of liquids. It generally consists of some buoyant body, as hollow glass or copper, weighted at the bottom and supporting a graduated stem, or one having a definite mark. There are two kinds, those of constant and those of variable immersion. Those of constant immersion are made to sink in the tested liquid, whether dense or light, to the same depth, by balancing with weights. Those of variable immersion have no movable weights, but rise or fall according to the density of the liquid. Nicholson's hydrometer, fig. 1, is of the first kind. As usually constructed, when this instrument is immersed in water it requires a weight of 1,000 grains to make it sink to a certain mark on the stem. According to the principle of Archimedes (see Hydromechanics), the weight of the instrument, together with the 1,000 grains which it sustains, is equal to the weight of the volume of water displaced. If the instrument is placed in a liquid lighter or heavier than water, and the weight changed until it sinks to the same depth, the specific gravity of the liquid will be indicated by the formula g = W+20 / W+1000, where W is the weight of the instrument, and w that of the weights placed upon the pan.

If w is less than 1,000 grains it will show that the liquid is lighter, and if it is more than 1,000 grains it will show that it is heavier than water. This instrument may also be used to find the specific gravity of solids, or as a delicate balance. For these purposes it has a small cup or wire cage suspended at the bottom to hold the body, which may be either heavier or lighter than water. To find the specific gravity of a solid, let it be first weighed in air, by placing upon the pan a piece of the substance which weighs less than 1,000 grains. Suppose, the substance to be sulphur, and that 440 grains are required to be added to make the instrument sink to the mark on the stem, the weight of the sulphur is, evidently, 1,000 - 440 = 560 grains. Now, what it loses if weighed in water will be the weight of an equal bulk of water, and this will be found by placing it in the cup or cage at the bottom, and adding sufficient weights to those in the pan at the top to bring the mark to the level of the water.

If it requires the addition of 275.2 grains, that amount will represent the weight of a volume of water equal to the sulphur; consequently the specific gravity of the sulphur will be 560/275.2 = 2.03. If the body is lighter than water, it will of course require the addition of more than its weight to the pan, and for immersion it will require to be placed in the wire cage. Fahrenheit's hydrometer differs from Nicholson's in being constructed of glass, and having a constant weight of mercury in a bulb at the lower end. Its use is therefore restricted to the weighing of fluids. - Of hydrometers of variable immersion, Baume's is the one most frequently used, and furnishes a good example of the class. Two instruments, of different forms, are represented in figs. 2 and 3. They are made of glass; their stems are hollow and lighter than the fluid in which they are immersed. Fig. 2 is called a salimeter, and is used for estimating the proportion of a salt or other substance in solution. It is graduated in the following manner: Being immersed in water at a temperature of 12° C., the point to which it sinks is marked 0°; it is then placed in a solution containing 15 parts of common, salt to 85 of water, the density of which is about 1.116, and the point to which it sinks is marked 15, and the interval divided into 15 equal parts; the graduation is then extended downward, generally terminating at 66°, which corresponds to the density of sulphuric acid.

When the instrument is to be used for liquids lighter than water, the zero is not placed at the point to which it sinks in pure water, but at a point to which it sinks in a solution containing 10 parts of common salt to 90 of water. The point to which it sinks in pure water was marked by Bau-me 10°, and the graduation was continued upward to the highest point to which the stem might be immersed in the lightest liquid. Fig. 3 represents the instrument for liquids lighter than water. The graduation of these hydrometers is arbitrary, and is an indication of the strength of the liquid only after trial. - Hare's hydrometer, a very valuable instrument, but one which has not been much employed, acts upon the principle of the barometer, and yields directly results of definite comparison; it is represented in fig. 4. A n-shaped tube has its legs, of equal length, placed in shallow vessels, one containing the liquid to be tested, and the other a liquid taken as a standard, as water. A partial vacuum is then produced in the tube by exhausting the air by means of an air pump, the mouth, or otherwise, making use of the stopcock to facilitate the operation.

It is evident that the height of the liquid column will be in the exact inverse proportion to the specific gravity of the liquids. - Hydrometers have various names, according to the purpose for which they are used: as lactometers, for estimating the amount of cream in milk, or the quantity of sugar of milk in the whey; vinometers, for estimating the percentage of alcohol in wine or cider; and there are acidometers and sac-charometers.

Fig. 1. - Nicholson's Hydrometer.

Fig. 2. Salimeter.

Fig. 8. Alcoholimeter. Baume's Hydrometers.