SPECIFIC GRAVITY. 421 SPECIFIC GRAVITY. sure is equal to metric density divided by the density of air = .001293. On the English system density is variously ex- pressed in pounds per cubic foot, pounds per cubic inch, grains per cubic inch. To reduce specific gravity referred to water at 62° F. to density on these scales multiply by 62.354(5, .0360848, 252.593, respectively. Methods of Measurement. I. Density. Two courses may be pursued to determine the density of a substance : ( 1 ) Weigh a portion of it and measure its volume; (2) determine its specific gravity referred to a substance of known density. For approximate purposes the former is best, particularly in the case of solids of regular shape. To attain a high degree of accuracy, however, is diflScult, since the direct determina- tion of volume is subject to many errors. Hence the density of most substances is found by deter- mining, by one of the methods below, its specific gravity referred to water or hydrogen, the density of each of these having been carefully investi- gated. The density of water is found by observing the loss in weight of a solid of regular shape when immersed in water, some of the best results being obtained with a glass cube whose exact dimensions were found by a method based on the interference of light. II. Specific Gravity of Solids and Liquids. Most of the methods given below are based on Archimedes's principle, which states that a solid floating or immersed in a liquid loses weight equal to that of the liquid it displaces. Hence if it is totally immersed the loss of weight is the weight of a volume of liquid equal to the volume of the solid. Hence: Weight of solid = specific gravity of solid. Loss of weight = specific gravity of liquid. Thus if either is known the other may be found. For exact purposes the temperature of the liquid must be known, weighings in air must be cor- rected for the buoyancy of the latter, and the solid must be supported in the liquid so that only a single fine wire cuts the surface. Air bubbles are also a serious source of error, and for this reason water when used should be freely dis- tilled or boiled. (A) Hydrostatic Balance. A chemical bal- ance (see Balance) so arranged that a vessel of liquid may be placed under one end of the beam without resting on the pan. This is done by re- placing the ordinary pan by a shorter (Fig. a) or else by resting the vessel on a bridge (Fig. 6) over the pan. BRIDGE AND PAN FOB USE WITH BALANCE. The solid is weighed in air, then suspended from a hook on the pan or pan support by a wire of known weight, so that it hangs totally sub- merged in the vessel of liquid, and again weighed. (B) Jolly Balance. A large delicate spiral spring supported by a vertical rod of adjustable length and carrying at the bottom two pans, one below the other. For description see Jolly Bal- ance. (C) Specific Gravity Bottle. A light glass flask with a ground stopper generally pierced with a fine hole so that it may be completely filled. The Hask is weighed empty, then full of water. Knowing the density of wa- ter, the internal volume is calcu- lated. The bottle may now be filled with a liquid of unknown density and weighed and the density calcu- lated. Or a weighed amount of solid in small pieces may be put in the bot- tle, which is then filled up with wa- ter and weighed. The difference be- tween the weight of the solid plus that of the bottle entirely filled with water and the bpeoific gravity bottle or pyk- weight observed is nometer. the weight of wa- A, simple form ; B, with ther- ter displaced by the solid. This is the most accurate method for liquids and solids in small bits. (D) Mohr's Balance. A balance beam di- vided into tenths carries suspended from one end by a platinum wire a glass cylinder, F, almost solid. On the other end is a counterpoise, P, balanc- ing the cylinder in air. Riders, A, B, C, D, are provided of weight equal to the loss of weight of the cylinder in water at 15° 0. and of 1-10, 1-100, f^P^TEft*" SPECIFIC GBATIIY, MOBB 8 BALANCE. 1-1000 this weight. When the cylinder hangs in a jar, E, filled with a liquid whose density is desired, these riders are adjusted on the beam so that balance is restored, and the position of the riders then gives directly the specific gravity referred to water at 15°. Thus with the riders
