SPECIFIC GRAVITY, or, more properly, specific weight, is a term used to express the weight of any gas, liquid, or solid, under some given volume ; but the unit of weight and unit of volume are usually such, that for some one particular substance the weight of the unit of volume shall be expressed by the unit of weight, or by 10 times, 100 times, or 1000 times that unit. Thus the numbers expressing the specific gravities of different substances denote the actual weights of those substances under the unit of volume; and hence the ratio between the numbers expressing the specific gravities of any two substances ie the same as the ratio between the weights of those substances under any two equal volumes, the density of the atmo sphere and also the states of those substances with respect to tempera ture being the same.
Distilled water is the substance usually employed for the purpose of comparing together the weights of all substances except the gases ; and because the volume of any substance varies with its temperature, in determining from experiment the specific gravity of any substance, the weight under a given volume is reduced to that which it would become at one constant temperature. The constant temperature adopted in this country is, in general, that which is expressed by 60° of Fahrenheit's thermometer.
From the experiments of Sir George Shuckburgh Evelyn it was found that at the temperature expressed by 39° Fahr., the height of the column of mercury in the barometer being 29/9 inches, the weight of a cubic foot of distilled water was 999.8066 ounces avoir dupois ; and reducing this weight conformably to the table of the densities of water at different temperatures which had been given by Biot, it will be found that at a temperature expressed by 60° Fehr., and when the height of tho mercurial column is 30 inches, the weight of the cubic foot of water is 999'54 ounces. But in the Parliamentary regulations, which were made in 1825, a cubic inch of water is stated to weigh 252.156 troy grains, the temperature being 62° Fabr., and the height of the barometrical column, 30 inches ; and 7000 troy grains are made equivalent to one pound avoirdupois : hence it follows that a cubic foot of water should weigh 997.136 ounces. Either of these numbers is sufficiently near 1000 to make it very proper that tbis last should be adopted for the specific gravity of water, since a change in the value of the avoirdupois ounce, which would be scarcely appre ciable in the ordinary transactions of commerce, would render the ounce an accurate and convenient unit of weight, while the cubic foot constitutes the unit of volume.
In France, since the employment of the decimal scale of weights and measures has become general, the cubic centimetre ('06),028 cubic inches English) is the unit of volume, and the gramme grains) is the unit of weight. The gramme having been determined by the weight of • cubic centimetre of distilled water of the tempera ture at which its density Is a maximum (39'.2 Fehr.). Thus the weight of a cubic centimetre of any substance being expressed by any number se of grammes, se is the specific gravity of that substance.
The numbers expressing the specific, weights of substances are also taken to represent their densities. Density, properly speaking,
denotes the degree of closeness of the particles of a substance to one another; but this is evidently proportional to the number of particles within a given volume of that substance; and since the weight of a body is only the sum of the actions of gravity upon all its particles, it follows that the densities of two substances under equal volumes will be proportional to their specific gravities. It follows also that if two substances have equal densities or specific gravities, their weights will vary with their volumes ; and that the weights of bodies are to one another in a ratio compounded of their specific gravities and volumes. Previously to describing the methods of finding the specific gravities of substances, It will be proper to explain the construction of the hydrostatical balance, which is the instrument employed for the purpose. The beam of this balance rests, as usual, on the lower cir cumference of a circular perforation in both sides of the fork which holds it, by a pin which is fixed in it perpendicularly to its length and depth, at a small distance above the common centre of gravity of the beam, scales, and weight. The fork is suspended from the middle of a horizontal bar, and this last is suspended from a spring at the top or the pillar which supports tho machine. Care is taken that the two arms of the beam are symmetrical, and that the points from whenee the scales are suspended are at clue' distances from its centre of gravity. Now let the substance winch is to be weighed be put in one of the scale dishes, and the number of grains necessary to keep it in equilibrio be put in the other. If the weight of the substance should be an exact number of grains, that weight is determined, but if not, and it were required to ascertain the weight within one-hundredth part of a Frain (for example), the following contrivance may be adopted. Suspend in a vertical position from the lower part of the scale con taining the substance to bo weighed, a brass wire, whose volume and weight have been previously determined, and let part of the length of this wire enter into water which is contained in a vessel underneath the scale. The scales with this wire thus attached to one of them being previously put in equilibrio when the surface of the water is at a certain mark on the wire, let the substance to be weighed be intro duced into the scale above the wire, and let weights be placed in the opposite scale till one grain more would be found too great : then gently raising the whole balance till, by the increase of the weight on the side of the scale containing the substance, in conse quence of a greater portion of the wire being out of the water, an equilibrium takes place. The wire being graduated so that 100 divisions correspond to a weight equal to one grain, the number of graduations on it between the surface of the water and the fixed mart before mentioned will enable the experimenter to determine the number of hundredths of a grain by which the weight of the substance in the scale exceeds the number of gminsalready placed in the opposite sale.