continue at all when the elements are settled in their places? He answers
that although the force of attraction is continually drawing similar
elements to the same spot, still the revolution of the universe exercises 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