ART. 61. _Heat and Matter._--If it be true that heat is due to the vibrations of the aetherial medium, the question now arises, as to how a body may become heated, and by so doing be transformed into the three stages in which matter is found. We have already seen (Art. 36), that matter may be found in three forms, viz. solid, liquid, and gaseous, and that all these different forms of matter are composed of minute parts called atoms. In the case of the solid, the atoms are held closely together by some strong attractive power, termed cohesion; in the case of the liquid, the atoms have a greater freedom; while in the gaseous form they have a greater freedom of movement than when in either the liquid or the solid state. According to Young's Fourth Hypothesis (Art. 45), we find that all matter, and therefore all atoms have an attraction for the Aether, by means of which it is accumulated within their substance, and for a small distance around them in a state of greater density, and therefore of greater elasticity. In other words, as Aether is gravitative, every atom possesses an atmosphere of Aether in the same way that the earth has its atmosphere of air; and further, the aetherial atmosphere of each atom is densest nearest to the atom, gradually getting rarer and rarer the further the atmosphere recedes from the nucleus or centre, the elasticity or pressure being always proportionate to the density. Professor Challis, in his Dynamical Theory of Light and Heat, states that all the forces in Nature are different modes of pressure under different circumstances of the universal Aether, and as heat is a Force, and therefore a mode of motion, that also must be due to some form of pressure due to the vibrations of the Aether. Professor Challis[8] on this point says: "According to this theory, the atoms of any substance are kept in position of equilibrium by attractions and repulsions resulting from the dynamical action of the vibrations of the Aether which have their origin at the atoms. Each atom is the centre of vibration propagated equally from it in all directions, and that part of the velocity of the vibration which is accompanied by change of density (of the Aether) gives rise to a repulsive action on the surrounding atoms. This action is the repulsion of heat, which keeps the individual atoms asunder." With all these facts before us, we are now in a position to account for the changes of matter which take place when heat is applied to either a solid or a liquid body. We have already seen (Art. 36) that it is by the application of heat that matter in its solid form is changed into a liquid, and from a liquid into a vaporous or gaseous form. It is now for us to endeavour to form a mental picture as to how this is done.