, 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.

For example, let us take an iron ball, and apply heat to it, either by
putting it in a furnace or suspending it in some way over an intense
heat. As the heat, which is vibratory motion of the Aether, begins to be
absorbed by the iron ball, it sets the atoms which compose the ball in
motion, urging them to separate, and thus cause the iron ball to expand
and increase in volume. As greater heat is absorbed, so greater motion
among the atoms is the result. So that the motion of heat is tending
all the time to expand the body, while they are held together by the
attraction of cohesion, whatever that may be. As the heat is further
increased, the iron ball begins to assume a liquid or molten form, its
atoms beginning to move about with greater freedom, tho