ody, which radiates its thermal motion through the
aether. The earth also is warm, and sends its heat-pulses incessantly
forth. It is the waste of its molecular motion in space that chills
the earth upon a clear night; it is the return of thermal motion from
the clouds which prevents the earth's temperature, on a cloudy night,
from falling so low. To the conception of space being filled, we must
therefore add the conception of its being in a state of incessant
tremor.

The sources of this vibration are the ponderable masses of the
universe. Let us take a sample of these and examine it in detail.
When we look to our planet, we find it to be an aggregate of solids,
liquids, and gases. Subjected to a sufficiently low temperature, the
two last, would also assume the solid form. When we look at any one
of these, we generally find it composed of still more elementary
parts. We learn, for example, that the water of our rivers is formed
by the union, in definite proportions, of two gases, oxygen and
hydrogen. We know how to bring these constituents together, so as to
form water: we also know how to analyse the water, and recover from it
its two constituents. So, likewise, as regards the solid portions of
the earth. Our chalk hills, for example, are formed by a combination
of carbon, oxygen, and calcium. These are the so-called elements the
union of which, in definite proportions, has resulted in the formation
of chalk. The flints within the chalk we know to be a compound of
oxygen and silicium, called silica; and our ordinary clay is, for the
most part, formed by the union of silicium, oxygen, and the well-known
light metal, aluminium. By far the greater portion of the earth's
crust is compounded of the elementary substances mentioned in these
few lines.

The principle of gravitation has been already described as an
attraction which every particle of matter, however small, exerts on
every other particle. With gravity there is no selection; no
particular atoms choose, by preference, other particular atoms as
objects of attraction; the attraction of gravitation is proportional
simply to the quantity of the attracting matter, regardless of its
quality. But in the molecular world which we have now entered matters
are otherwise arranged. Here we have atoms between which a strong
attraction is exercised, and also atoms between which a weak
attraction is exercised. One atom can jostle another out of its place
in vi