Density is the proportionate quantity of matter in bodies of a given magnitude; thus, if a body contains more matter than another, both being of the same bulk, the former is said to be more dense than the latter, and that in proportion to the relative quantities of matter they contain; or if the former body contains the same quantity of matter as the latter, but under a less bulk, its density is greater in proportion as its bulk is less than that of the other. Hence the density is directly proportional to the quantity of matter, and inversely proportional to the bulk under which it is contained. The relative quantities of matter in bodies are known by their gravity or weight; for, according to Sir Isaac Newton, the original particles of matter being equal, and consequently endowed with equal gravity, bodies, or assemblages of those particles, will have a gravity proportionate to the number of particles contained in them. Hence, when a body, mass, or quantity of matter is spoken of, its weight or gravity is always understood, that being the proper measure of the quantity of matter.

The density of bodies is found by weighing equal bulks of each; for this purpose solids must be previously reduced to the same shape and size, but each fluid should fill the same vessel, in which they must be weighed separately. The density of fluids may also be determined by the following methods. First, 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 liquors in them; that is, inversely as their bulk. Secondly, by immersing a solid in the fluids, their densities may be readily compared; for if the solid be lighter than the liquids, the part immersed by its own weight will be inversely as the density of the fluid; but if the body be heavier, so as to sink in the liquid, it must be weighed in them separately, and the weight lost by the body in each will be directly as the density of the fluid. Sir Isaac Newton, and most of the other philosophers, are of opinion that there is no such thing as a space absolutely full of matter, and that consequently there is no substance in nature, either solid or fluid, that is perfectly dense: the densest bodies, according to Newton, consist of much more porous space than solid matter.