unavailing protests against their hard imprisonment. If a biological analogy would make the picture more vivid you can imagine the cages of a menagerie arranged in squares or other regular figures, while the caged animals pace restlessly to and fro between their bars.
Increase in temperature means in all cases increase in the kinetic energy of agitation of the molecules, whatever state they may be in, and one of the fundamental assumptions of the kinetic theory, and one which we can now definitely prove, at least for gases, is that at a given temperature the average kinetic energy of agitation of a molecule is a universal constant, independent of whether the molecule is little or big. If the exact meaning of this statement is not clear, then imagine a lot of molecules of different weights from the big mercury unit down to the little hydrogen unit, one two-hundredth as heavy, led out in succession to a punching bag or to the striking machine at a county fair, and asked to register their strengths. They would, by virtue of a single impact, drive the index to just the same height, the lubberly mercury molecule being able to hit no harder, despite his size, than the tiny hydrogen molecule. This means of course that the hydrogen molecule must have a much larger speed, in fact a speed fourteen times as great in order to make up for his small avoirdupois. Such in brief is the kinetic hypothesis.
This hypothesis has had a long and checkered career in the course of which it has met with nearly all the vicissitudes which can befall a physical theory. Put forth in its most fundamental aspects by Leuoippus and Democritus in the early dawn of Greek thought (about 440 B.C.) it was violently combated by the idealistic philosophers of the ancient world, especially by Plato and Aristotle, and remained altogether fruitless for two thousand years and more, in part, no doubt, because of the adverse influence of these great names. At the beginning of the modern awakening of the intellectual life it was resurrected by Descartes about 1630, and elaborated in considerable detail by Daniel Bernouilli in 1738. Nevertheless, up to the middle of the nineteenth century it remained to the world at large the rather fanciful and naive speculation of a mere handful of philosophers. And although it is significant that this handful contains the names of the most prominent and productive of the makers of modern physics—Newton, Boyle, Eumford. Joule, Clausius, Maxwell, Kelvin, Boltzmann—nothing is more surprising to the student brought up in the atmosphere of the scientific thought of the present than the fact that the relatively complex and intricate phenomena of light and electricity had been built together into fairly consistent and satisfactory theories long before the much simpler phenomena of heat and molecular physics had begun to be correctly understood.
The first tremendous success of the kinetic hypothesis came about