I hope to be able to lay before you in the course of the term some of
the evidence for the existence of molecules, considered as individual
bodies having definite properties. The molecule, as it is presented to
the scientific imagination, is a very different body from any of those
with which experience has hitherto made us acquainted.
In the first place its mass, and the other constants which define its
properties, are absolutely invariable; the individual molecule can
neither grow nor decay, but remains unchanged amid all the changes of
the bodies of which it may form a constituent.
In the second place it is not the only molecule of its kind, for there
are innumerable other molecules, whose constants are not
approximately, but absolutely identical with those of the first
molecule, and this whether they are found on the earth, in the sun, or
in the fixed stars.
By what process of evolution the philosophers of the future will
attempt to account for this identity in the properties of such a
multitude of bodies, each of them unchangeable in magnitude, and some
of them separated from others by distances which Astronomy attempts in
vain to measure, I cannot conjecture.
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