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Hydrometer or

water, weight, liquid, liquids and equal

HYDROMETER OR AREOMETER.—An instrument for determining the specific gravities or relative densities of liquids. The principle upon which it works is that when a body floats on a liquid, and thus displaces a quantity of the liquid, the weight of the solid body is equal to the weight of the liquid which it displaces. The density of the liquid is determined either by the depth to which the hydrometer sinkg in the liquid, or by the weights required to cause it to sink to a certain depth. With both kinds, however, correction must be made for variations in temperature.

Baume's Hydrometer is largely employed on the Continent. It consists of two diffrenet instruments, one for liquids heavier than water, and one for those which are lighter. The first floats at the zero point, in distilled water, at a temperature of 58° F., and each degree marked downward indicates a density equal to one per cent. of common salt. The hydrometer for liquids lighter than water is poised so that the zero point of the scale is at the bottom of the stem when it is floating in a solution of i oz. of common salt in 9 oz. of water, and the depth to which it sinks in distilled water shows io°, the space between these two points being equally divided, and the same graduation continued to the top of the scale.* Fig. 222 is an illustration of a Baum hydrometer having the two different scales. For liquids lighter than water the figures on the left hand side are read, and for liquids heavier than water those on the ' right hand side.

As the whole weight of the hydrometer and the weights when adjusted in distilled water is to the number t,000, so is the whole weight when adjusted in any other fluid to the number expressing its specific gravity.

Nkliolson's Hydrometer or Areometer is constructed on a similar plan to Fahrenheit's. It has, however, in addition to the dish for the weights above, a small cup below for convenience in ascertaining the weight of a solid body in water. It is chiefly used for ascertaining the specific gravity of minerals (see fig. 63).

Fahrenheit's instrument consists of a little hollow ball, having a coun terpoise below, and a very slender stem above terminating in a small dish. The half length of the stem is distinguished by a fine line across it. The instrument is immersed in the liquid to this line by placing weights in the little dish above. Then, seeing that the part immersed is con stantly of the same magnitude, and the whole weight of the hydrometer is known, the last weight, added to the weights in the dish, will be equal to the weight of fluid displaced by the instrument, according to the laws of hydrostatics. The specific gravity will be found by the proportion.

Twaddle's this instrument o is equal to t,000, or the spec. gray, of dis tilled water, and each degree is equal to •oo3, so that by multiplying this number by the number of degrees marked on the scale and adding t the real spec. gray. is obtained.

A lcoholmeters, Argentometers, etc., are all simple hydrometers graduated so as to adapt them to the testing of spirits, silver solutions, etc.