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14 FRAGMENTS OF SCIENCE.

that chills the earth upon a clear night; it is the return
of its 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 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. 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 pro-
portions, of two gases, oxygen and hydrogen. We know
how to bring these constituents together, and to cause them
to form water: we also know how to analyze the water,
and recover from it its two constituents. So, likewise, as
regards the solid proportions of the earth. our chalk-hills,
for example, are formed by a combination of carbon, oxy-
gen, and calcium. These are 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 com-
pound of oxygen and. silicium, called silica; and our or-
dinary clay is, for the most part, formed by the union of
silicium, oxygen, and the well—known light metal, alumin-
ium. 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, h0wever
small, has for every other particle. With gravity there is
no selection; no particular atoms choose, by preference,
other particular. atoms as objects of attraction ; the attrac-
tion of gravitation is proportional to the quantity of the
attracting matter, regardless of its quality. But in the
molecular world which we have now entered matters are

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