Divisibility, one of the general properties of matter, usually classed with impenetrability, extension, etc. The proposition that there is no limit to the mathematical subdivision of matter is universally admitted; but the question in its relation to physical science is a different one, and its treatment depends upon the adoption of one of two theories. Formerly it was not infrequently supposed that matter is continuous; at least, the idea that it consisted of ultimate atoms of a definite magnitude, and therefore that each body was composed of a certain limited number of them, was not entertained. According to the old doctrine of the homogeneity of matter, therefore, its divisibility physically could only be limited by the means employed, and in a theoretical point of view was logically considered as infinite. But the doctrine of Dalton in regard to definite and multiple proportions has been so well established by the investigations of chemistry, and researches into the molecular constitution of matter, particularly gaseous bodies, that physicists now generally hold that the subdivision of matter is limited physically, that at last the atoms become separated from one another, and these being the primordial particles, no conception can be formed of any subdivision of them.

As examples of the great extent to which subdivision may be carried without arriving at the ultimate atoms, or even at the molecules of which they are composed, may be taken the spinning of the spider's web, which contains 4,000,000 fibres, which of course are composed of organic molecules of a compound structure, spun together within the diameter of a human hair; the odor of musk, which may pervade a room for years by the exposure of only a single grain; and the extent to which sulphuret of lead may be diffused by adding together solutions of almost infinitesimal quantities of a salt of lead and sulphuretted hydrogen. Dr. Thomson in this way obtained sulphuret of lead in quantities sensibly appreciable, but which must, according to his computations, have been divided into at least 500,-000,000,000 parts; and yet it is uncertain whether this division extended to the binary molecules of the substance. The most striking examples, however, of the extent to which matter may be divided, and at the same time manifest its presence by the exhibition of a degree of energy, are furnished by the experiments of Tyndall with minute quantities of gases, vapors, and perfumes, in which he exhibits their influence upon the diathermancy of atmospheric air and the elementary gases.

He found that a quantity of watery vapor so small as to be inappreciable by any other test would increase the absorptive power of dry air to the obscure rays of heat to such an extent as to cause a marked difference in the deflection of the needle of a galvanometer. The equality which he has also established as existing between the diathermancy of the elementary gases and their mechanical mixtures, indicating that their elementary particles assume uniform and therefore definite wave vibrations, when in a free state they are subjected to the action of uniform heat waves, has thrown so much light upon the subject of molecular physics that the conclusion that elementary atoms or primordial particles limit the divisibility of matter cannot be resisted.