The specific gravity of fluids is also taken by the instrument called a hydrometer or alcometer. Such instruments are much used for ascertaining the specific gravity of spirituous and other liquors, as an indication of their strength. _If the solid body to be tested is lighter than water, it must be attached to some heavy substance to cause it to sink._ Its specific gravity is then calculated by dividing its weight in the air by the sum of the weights of the attached body both in air and in water, first subtracting from this sum the weight of the two bodies together in the water. Bodies soluble in water may be weighed in some other fluid, as alcohol, ether, olive oil, &c., and their proportional weight to that of this fluid being thus ascertained, their density compared with that of water is readily calculated or they may be enveloped in wax or other suitable substance to protect them, and then treated by the method just given for substances lighter than water. Gaseous bodies are weighed in a thin glass flask or other vessel made for the purpose, and provided with a stop-cock. The vessel is exhausted of air before the introduction of the gas. RULE FOR FINDING THE SPECIFIC GRAVITY OF A SOLID BODY. _Weigh the solid in air and then in pure water._ The difference is the weight of water displaced, whose specific gravity is 1.000. Then, as the difference of weight is to 1·000, so is the weight in air to the specific gravity; or divide the weight of the body in air by the difference between the weights in air and in water. EXAMPLE. A lump of glass is found to weigh in air 577 grains; it is then suspended by a horse hair from the bottom of the scale pan, and immersed in a vessel of pure water, when it is found to weigh 399.4 grains. What is its specific gravity? 577.0 Then, as 177.6 : 1 :: 577.0 : sp. gravity. 399.4 1 ----- ----- 177·6 the difference 177·6)577·0(3·248, Ans. 532·8 ----- 4420 3552 ---- 8680 7104 ----- 15760 14208 ----- 1552 [Illustration: FIG. 93.]