Since every known substance may be reduced in bulk by pressure, it follows that the particles of matter are not in actual contact, but are separated by interstices; and it is evident that the smaller the interstitial spaces the greater the density. These spaces appear to be filled with air in some cases, as may be inferred from certain semi-opaque minerals and other substances becoming transparent when plunged into water. Sometimes they may possibly contain some unknown and highly elastic fluid, such as Sir David Brewster has discovered in the minute cavities of various minerals, which occasionally causes them to explode under the hands of the lapidary; but as it is inconceivable that the particles of matter should act upon one another without some means of communication, it is presumed that the interstices of material substances contain a portion of the ethereal medium with which the regions of space are filled. The various hypotheses that have been formed as to the nature and action of the forces which unite the particles of matter, have been successively given up as science advanced, and now nothing decisive has been attained, although Professor Mossotti, of Pisa, by a very able analysis, has endeavoured to prove the identity of the cohesive force with gravitation. As the particles of material bodies are not in actual contact, he supposes that each is surrounded by an atmosphere of the ethereal medium, which he conceives to be electricity; moreover he assumes that the atoms of the medium repel one another, that the particles of matter also repel one another, but with less intensity, and that there is a mutual attraction between the particles of matter and the atoms of the medium, forces which are assumed to vary inversely as the square of the distance. Hence, when the material molecules of a body are inappreciably near to one another, they mutually repel each other with a force which diminishes rapidly as the infinitely small distance between the material molecules augments, and at last vanishes. When the molecules are still farther apart, the force becomes attractive. At that particular point where the change takes place the forces of repulsion and attraction balance each other, so that the molecules of a body are neither disposed to approach nor recede, but remain in equilibrio. If we try to press them nearer, the repulsive force resists the attempt; and if we endeavour to break the body so as to tear the particles asunder, the attractive force predominates and keeps them together. This is what constitutes the cohesive force, or force of aggregation, by which the molecules of all substances are united. The limits of the distance at which the negative action becomes positive vary according to the temperature and nature of the molecules, and determine whether the body which they form be solid, liquid, or aëriform.