About the beginning of this century, the properties of bodies were
investigated by several distinguished French mathematicians on the
hypothesis that they are systems of molecules in equilibrium. The
somewhat unsatisfactory nature of the results of these investigations
produced, especially in this country, a reaction in favour of the
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.
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