SPECIFIC URAVITY.—The relative weight or gravity of any body or substance, consid ered with regard to an equal bulk of some other body, which is assumed as a standard of com parison. The standard for the specific gravities of solids and liquids is pure distilled water, at a temperature of 62 deg. Fahr., which is reckoned unity. By comparing the weights of equal bulks of other bodies with this standard, we obtain their specific gravities.

Thus the specific gravity of cast-iron is 7.21, that is, any particular mass of cast-iron will weigh 7.21 times as much as an equal bulk of water. The practical rule is, weigh the body in air, then in pure distilled water, and the weight in air, divided by the loss of weight in water, will give the specific gravity of the body. In designating the gravities of gases, the standard of unity is atmospheric air.* In order to calculate the amount of water to be added to a liquid to reduce it to a lower specific gravity, subtract the specific gravity of water from the required specific gravity, and the result will be the amount of liquid to be taken. The difference between the specific gravity of

the liquid and the required specific gravity represents the amount of water. For example, suppose we have a sample of sulphuric acid having a specific gravity of 1.845, and it is required to use an acid with a specific gravity of 1.3, how much water must be added to the strong acid to obtain the weak ? 1.845-1=.845=the amount of strong acid; 1.845-1.3=•545=the amount of water.

Thus, if we add 545 parts of water to 845 parts of strong sulphuric acid, we shall obtain an acid having a specific gravity of 1.3, or if we add i ounce 65 minims (fluid measure) of water to I ounce 6 drachms and 5 minims of strong sulphuric acid, we shall obtain about 23 drachms of sulphuric acid having a specific gravity of 1.3.