By Sir WILLIAM THOMSON.


The now well known kinetic theory of gases is a step so important in the
way of explaining seemingly static properties of matter by motion, that it
is scarcely possible to help anticipating in idea the arrival at a complete
theory of matter, in which all its properties will be seen to be merely
attributes of motion. If we are to look for the origin of this idea we must
go back to Democritus, Epicurus, and Lucretius. We may then, I believe,
without missing a single step, skip 1800 years. Early last century we find
in Malebranche's "Recherche de la Verite," the statement that "la durete de
corps" depends on "petits tourbillons." [1] These words, embedded in a
hopeless mass of unintelligible statements of the physical, metaphysical,
and theological philosophies of the day, and unsupported by any
explanation, elucidation, or illustration throughout the rest of the three
volumes, and only marred by any other single sentence or word to be found
in the great book, still do express a distinct conception which forms a
most remarkable step toward the kinetic theory of matter. A little later we
have Daniel Bernoulli's promulgation of what we now accept as a surest
article of scientific faith--the kinetic theory of gases. He, so far as I
know, thought only of Boyle's and Mariotte's law of the "spring of air," as
Boyle called it, without reference to change of temperature or the
augmentation of its pressure if not allowed to expand for elevation of
temperature, a phenomenon which perhaps he scarcely knew, still less the
elevation of temperature produced by compression, and the lowering of
temperature by dilatation, and the consequent necessity of waiting for a
fraction of a second or a few seconds of time (with apparatus of ordinary
experimental magnitude), to see a subsidence from a larger change of
pressure down to the amount of change that verifies Boyle's law. The