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240 FRAGMEN TS OF SCIENCE.

as the various movements of the earth’s surface are exe-
cuted independently of the orbital revolution of our planet.

The vapor molecules are kept asunder by forces which,
virtually or actually, are forces of repulsion. Between
these elastic forces and the atmospheric pressure under
which the vapor exists, equilibrium is established as soon
as the proper distances between the molecules have been
assumed. If, after this, the molecules be urged nearer to
each other by a momentary force, they recoil as soon as
the force is expended. If they be separated more widely
apart, when the separating force ceases to act they again
approach each other. The case is difi’erent as regards the
constituent atoms.

And here let it be remarked, that we are now upon the
very outmost verge of molecular physics; and that I am
attempting to familiarize your minds with conceptions
which have not yet obtained universal currency even among
chemists; which many chemists, moreover, might deem un-
tenable. But, tenable or untenable, it is of the highest sci-
entific importance to discuss them. Let us, then, look men-
tally at our atoms grouped together to form a molecule.
Every atom is held apart from its neighbors by a force of
repulsion; why, then, do not the mutually repellant mem-
bers of this group part company ? The molecules separate
from each other when the external pressure islessened or
removed, but the atoms do not. The reason of this stabil-
ity is that two forces, the one attractive and the other re-
pulsive, are in operation between every two atoms; and the
position of every atom—its distance from its fellows—is
determined by the equilibration of these two forces. If the
atoms come too near, repulsion predominates and drives
them apart; if too distant, attraction predominates and
draws them together. The point at which attraction and
repulsion are equal to each other is the atom’s position of
equilibrium. If not absolutely cold——and there is no such

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