Professor Davy points this out (Art. 60) where he says of heat, "It may with propriety be called a repulsive motion," while Professor Challis (Art. 61) states that "Each atom is the centre of vibrations propagated from it equally in all directions, which give rise to a repulsive action on the surrounding atoms. This action (he adds) is the repulsion of heat which keeps the individual atoms asunder." There have been many experiments undertaken which go to prove that a repulsive action between atoms and molecules is produced by heat. It has been demonstrated that certain coloured rings, known as Newton's rings, change their shape and position when the glasses between which they appear are heated, thus indicating the presence of a repulsive power due to the increased heat. If we consider the change of state that heat induces in matter, as, for example, from solid to a liquid, or liquid to a gaseous form, we are compelled to admit that heat possesses an expanding and therefore a repulsive motion. It is almost an universal law that heat expands and cold contracts, and the greater the heat absorbed, the greater the expansion. In the case of a solid being converted into a liquid, a much greater heat or repulsive motion is required to separate the particles, on account of the power of cohesion being greater in the solid than in the liquid. As Professor Tyndall[11] states when dealing with the stability of matter from the molecular standpoint: "Every atom is held apart from its neighbour by a force of repulsion. Why then do not the mutually repellent members of the group part company? The reason of this stability is that _two_ forces, the one attractive and the other repulsive, are in operation between every two atoms, and the position of every atom is determined by the equilibration of these two forces. The points at which attraction and repulsion are equal to each other is the atom's position of equilibrium. When the atoms approach too near each other, repulsion predominates and drives them apart; when they recede to too great a distance, attraction predominates and draws them together." If, therefore, there are TWO forces at work in the atomic world, viz. attraction and repulsion, then the question arises, Can that repulsive power be increased in any way, and if so, by what means? Such repulsive motion, as experiment and experience teach us, can be increased, and such increase may be derived from the absorption of heat which gives rise to increased atomic motion, and so to increased aetherial motion away from the atom, by which the repulsive action of one atom upon another is increased. Thus an atom's repulsive power may be increased by heat; the greater the heat absorbed, the greater the repulsive power that any atom or body exerts upon a neighbouring atom or body. We can therefore understand how it is, that a body when changed from a solid to a liquid condition occupies a larger space in the latter condition than in the former; or why a body when