DENSITY

, that property of bodies, by which they contain a certain quantity of matter, under a certain bulk or magnitude. Accordingly a body that contains more matter than another, under the same bulk, is said to be denser than the other, and that in proportion to the quantity of matter; or if the quantity of matter be the same, but under a less bulk, it is said to be denser, and so much the more so as the bulk is less. So that, in general, the density is directly proportional to the mass or quantity of matter, and reciprocally or inversely proportional to the bulk or magnitude under which it is contained.

The quantities of matter in bodies, or at least the proportions of them, are known by their gravity or weight; every equal particle of matter being endowed with an equal gravity, it is inferred that equal masses or quantities of matter have an equal weight or gravity; and unequal masses have proportionally unequal weights. So that, when body, or mass, or quantity of matter is spoken of, we are to understand their weight or gravity.

From the foregoing general proportion of the density of bodies, viz, that it is as the mass directly, and as the bulk inversely, may be inferred the proportion of the masses, or of the magnitudes; viz, that the mass or quantity of matter, is in the compound ratio of the bulk and density; and that the bulk or magnitude, is as the mass directly, and the density inversely. Hence, if B, b be two bodies, or masses, or weights; and D, d their respective densities; also M, m their magnitudes, or bulks: Then the theorems above are thus expressed, viz, D B/M, and B DM, and M B/D; or D : d. : B/M : b/m, and B : b :: DM : dm, &c; or .

No body is absolutely or perfectly dense; or no space is perfectly full of matter, so as to have no vacuity or interstices; on the contrary, it is the opinion of Newton, that even the densest bodies, as gold &c, contain but a small portion of matter, and a very great portion of vacuity; or that it contains a great deal more of pores or empty space, than of real substance.

It has been observed above, that the relative density of bodies may be known by their weight or gravity; and hence the most general way of knowing those densities, is by actually weighing an equal bulk or magnitude of the bodies, whether solid or fluid; if solid, by shaping them to the same figure and dimensions; if fluid, by filling the same vessel with them, and weighing it.

For fluids, there are also other methods of finding their density: as 1st, by making an equilibrium between them in tubes that communicate; for, the diameters of the tubes being equal, and the weights or quantities of matter also equal, the densities will be inversely as the altitudes of the liquids in them, that is inversely as the bulk.

2dly, The densities of fluids are also compared together by immerging a solid in them; for if the solid be lighter than the liquids, the part immerged by its own weight, will be inversely as the density of the fluid; or if it be heavier, and sink in the liquids, by weighing it in them; then the weights lost by the body will be directly proportional to the densities of the fluids.

Density of the Air, is a property that has much employed the later philosophers, since the discovery of the Torricellian experiment, and the air-pump. By means of the barometer it is demonstrated that the air is of the same density at all places at the same distance from the level of the sea; provided the temperature, or degree of heat, be the same. Also the density of the air always increases in proportion to the compression, or the compressing forces. And hence the lower parts of the atmosphere are always denser than the upper: yet the density of the lower air is not exactly proportional to the weight of the atmosphere, by reason of heat and cold, which make considerable alterations as to rarity and density; so that the barometer measures the elasticity of the air, rather than its density. If the height of the barometer be considered as the measure both of the density and elasticity of the air, when the thermometer is at 31°, and b be any other height of the barometer, when the thermometer is at t degrees; then in this case, b is the measure of the elasticity, and ((466 - t)/435)b is the measure of the density of the air.

Density of the Planets. In homogeneous, unequal, spherical bodies, the gravities on their surfaces, are as their diameters when the densities are equal, or the gravities are as the densities when the bulks are equal; therefore, in spheres of unequal magnitude and density, the gravity is in the compound ratio of the diameters and densities, or the densities are as the gravities divided by the diameters. Knowing therefore the diameters of the planets by observation and comparison, and the gravities at their surface by means of the revolution of the satellites, the relation of their densities becomes known. And as I have found the mean density of the earth to be about 4 1/2 times that of water, Philos. Trans. 1778; hence the densities of the planets, with respect to water, become known, and are as below:

As it is not likely that any of these bodies are homo- | geneal, the densities here determined are supposed to be the mean densities, or such as the bodies would have if they were homogeneal, and of the same mass of matter and magnitude.