T11K CONSTITUTION OF NA TURK. 5 
We on the earth's surface live night and day in the 
midst of ethereal commotion. The medium is never still. 
The cloud canopy above us may be thick enough to shut 
out the light of the stars; but this canopy is itself a warm 
body, which radiates its thermal motion through the ether. 
The earth also is warm, and sends its heat-pulses inces- 
santly forth. It is the waste of its molecular motion in 
space that chills the earth upon a clear night; it is the re- 
turn 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 ex- 
amine it in detail. When we look to our planet, we find 
it to be an aggregate of solids, liquids, and gases. Sub- 
jected 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 elemen- 
tary 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 analyze the water, and recover from it its 
two constituents. So, likewise, as regards the solid por- 
tions 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 def- 
inite 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 j J*/<? 
as an attraction which every particle of matter, however I 
small, exerts on every other particle. With gravity there / G** 
is no selection; no particular atoms choose, by preference, \ 
other particular atoms as objects of attraction; the attrac- \ ^ 
tiou of gravitation is proportional simply to the quantity J 
