SPECIFIC GRAVITY. The specific grav ity of a body is the ratio of the mass of that body to the mass of an equal volume of some standard substance which is arbitrarily selected for purposes of reference. In the cases of solids and liquids it is customary to select water as the standard substance; and water is always understood, in connection with solids and liquids, unless the contrary is explicitly stated. Gases are also sometimes referred to water, but it is much commoner to compare the density of a gas with that of air or hydro gen. The specific gravity of a solid which is not affected by contact with water may be de termined by weighing the solid twice — once while it is surrounded with air, and once while it is submerged in water. The difference be tween the two weights so obtained gives the weight of a mass of water whose volume is equal to that of the solid; and hence, in order to determine the specific gravity of the solid, it is only necessary to divide the weight as ob served in air by the loss of weight when it is submerged in water. If the solid is soluble in water, or if it is affected by water in any other manner, its specific gravity must be determined by some indirect method. Many such methods are known. For example, we may replace the water by some other liquid, such as benzene, in which the solid is not soluble, and then per form the experiment precisely as described above. The result so obtained is the specific gravity of the substance with reference to the benzene (or whatever liquid is employed in its place) ; and in order to express the specific gravity of the body with reference to water we have merely to multiply its specific gravity with respect to benzene by the specific gravity of the benzene as compared with water.
The specific gravity of a liquid may be de termined very accurately by means of the "specific gravity bottle.° This is a bottle made of thin glass (so as to be light), and provided with a ground-glass stopper. The weight of the bottle is determined (1) when it is empty, (2) when it is filled with water, and (3) when it is filled with the liquid whose specific gravity is desired. From these data we can easily determine the weight of the water in the bottle, and also that of the second liquid; and upon dividing the latter by the former we obtain the specific gravity sought. The specific gravity of a liquid may be obtained more ex peditiously (though less accurately) by means of the simple instrument known as the hydrom eter. This consists essentially of a glass
bulb (usually cylindrical in shape), provided at its upper end with a graduated stem and weighted at its lower end with shot or mer cury, so that it will float in an upright position when placed in a liquid sufficiently dense to permit it to float at all. It is evident that such an instrument will be buoyed up more by a dense liquid than by a lighter one; and when the stem at its upper end has once been graduated correctly we have only to place the hydrometer in the liquid whose specific gravity is desired, and observe the depth to which it sinks, as read from the graduated stem. In Fahrenheit's form of the instrument there is only one graduation mark on the stem; but a small pan is pro vided at the top of the hydrometer, and the observation consists in determining what weights must be placed upon this pan in order to cause the instrument to sink to the standard level indicated by the mark on the stem. Hy drometers are often graduated for special pur poses, so that instead of indicating the specific gravity of the liquid in which are placed, their readings give at once the percentage of alcohol (or of some other substance) that is present. Such instruments are called alcohol ometers, salinometers, lactometers, etc., accord ing to the special kind of liquid for whose investigation they are graduated.
As the density of water varies with the temperature, it is necessary (if we are to have a standard of density that is definite and accu rate enough for scientific purposes) to specify the temperature of the water with which the comparison is made. In England the standard temperature of the water was formerly taken at 62° F.; but it is now the almost universal practice to make the comparison with water at its temperature of maximum density, or at about F. In the vicinity of this tempera ture, the density of water varies very slowly with the temperature, and hence in comparing the density of a substance with that of water at or near 39° F., it is not essential to observe the temperature of the water with any high degree of precision. It was, indeed, this con venient and practical fact which led to the adoption of 39° F., as the standard temperature of the water.