Following are the specific gravities of vari ous substances, as compared with water at F.; the temperature of the substances being also supposed to be 39° F., except when other wise specified. It is to be understood that the specific gravity of a solid varies to some extent with the physical condition of the solid. The foregoing values, however, are sufficiently accu rate for general purposes; and more precise Information is given in this encyclopedia, in many cases, in the articles upon the several substances named.
The term specific gravity is not now used as commonly as it formerly was, the density of the substance being spoken of instead; it being understood in such cases that it is the density of the substance .relatively to water that is meant, and not its absolute density as expressed in units of mass per unit of volume. In the system of units employed in the United States and in England, this double use of the word density may sometimes lead to confusion. The metric system is now, however, very largely used in scientific work, and here the confusion cannot well occur; for in the metric system the unit of mass (the gramme) is the mass of a cubic centimeter of water at its temperature of maximum density; and hence, in this system, the 'absolute density" of a substance, and its °density relatively to water," are one and the same thing.
In the case of gases the expression specific gravity is now almost never used; density hav ing practically displaced it. The accurate de termination of the density of a gas is very difficult, and calls for great experimental skill and the finest of apparatus. In the usual method of performing such a determination a pair of glass globes, as nearly identical as possible, are suspended from the respective arms of a delicate balance and weights are added at one end or the other until perfect equilibrium is attained. One of the globes is
then thoroughly exhausted by means of an air-pump, while the other is filled with the gas under examination. The equilibrium of the balance will be destroyed by this process, and if we restore it by adding weights to the arm carrying the exhausted sphere we shall thereby ascertain the weight of the gas in the other sphere. By repeating the experiment with gases of different kinds the weights of equal volumes of these different gases will be ascer tained; and if the various weights so obtained are all divided by the weight as obtained for hydrogen (for example?, the quotients will be the densities of the various gases, with respect to hydrogen. The relative densities of some of the more familiar gases are given in the follow ing table, it being understood that the pressure in every case is one atmosphere, and the tem perature F. The relative densities of the gases here given do not change much for con siderable variations of temperature and pres sure; but this constancy is only approximate, not absolute; and hence, in the interest of accuracy, it is desirable to state (as follows) the precise conditions under which the table is exact.
According to the experiments of Regnault, a cubic centimeter of hydrogen, at the freez ing point of water and under a pressure of MA millimeters of mercury at Paris, weighs 0.00008956 grams; and from this fundamental constant it is easy to find the weight, under