ses a condensing power, and thrusts them again out of
their natural places. Thus want of uniformity, the condition of motion,
is produced. In all such disturbances of matter there is an alternative
for the weaker element: it may escape to its kindred, or take the form
of the stronger--becoming denser, if it be denser, or rarer if rarer.
This is true of fire, air, and water, which, being composed of similar
triangles, are interchangeable; earth, however, which has triangles
peculiar to itself, is capable of dissolution, but not of change. Of the
interchangeable elements, fire, the rarest, can only become a denser,
and water, the densest, only a rarer: but air may become a denser or
a rarer. No single particle of the elements is visible, but only the
aggregates of them are seen. The subordinate species depend, not upon
differences of form in the original triangles, but upon differences of
size. The obvious physical phenomena from which Plato has gathered his
views of the relations of the elements seem to be the effect of fire
upon air, water, and earth, and the effect of water upon earth.
The particles are supposed by him to be in a perpetual process of
circulation caused by inequality. This process of circulation does not
admit of a vacuum, as he tells us in his strange account of respiration.

Of the phenomena of light and heavy he speaks afterwards, when treating
of sensation, but they may be more conveniently considered by us in this
place. They are not, he says, to be explained by 'above' and 'below,'
which in the universal globe have no existence, but by the attraction of
similars towards the great masses of similar substances; fire to
fire, air to air, water to water, earth to earth. Plato's doctrine of
attraction implies not only (1) the attraction of similar elements
to one another, but also (2) of smaller bodies to larger ones. Had he
confined himself to the latter he would have arrived, though, perhaps,
without any further result or any sense of the greatness of the
discovery, at the modern doctrine of gravitation. He does not observe
that water has an equal tendency towards both water and earth. So easily
did the most obvious facts which were inconsistent with his theories
escape him.

The general physical doctrines of the Timaeus may be summed up as
follows: (1) Plato supposes the greater masses of the elements to have
been already settled in their places at the creation: (2) they are four
in number, and are