opposite method of treating bodies as if they were, so far at least as
our experiments are concerned, truly continuous. This method, in the
hands of Green, Stokes, and others, has led to results, the value of
which does not at all depend on what theory we adopt as to the
ultimate constitution of bodies.

One very important result of the investigation of the properties of
bodies on the hypothesis that they are truly continuous is that it
furnishes us with a test by which we can ascertain, by experiments on
a real body, to what degree of tenuity it must be reduced before it
begins to give evidence that its properties are no longer the same as
those of the body in mass. Investigations of this kind, combined with
a study of various phenomena of diffusion and of dissipation of
energy, have recently added greatly to the evidence in favour of the
hypothesis that bodies are systems of molecules in motion.

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