420 THE POPULAR SCIENCE MONTHLY 



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 Leu- 

 oippus and Democritus in the early dawn of Greek thought (about 

 440 B.C.) it was violently com.bate(i>by the idealistic philosophers of the 

 ancient world, especially by Plato and Aristotle, and remained alto- 

 gether fruitless for two thousand years and more, in part, no doubt, be- 

 cause 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, Eum- 

 ford. Joule, Clausius, Maxwell, Kelvin, Boltzmann— nothing is more 

 surprising to the student brought up in the atmosphere of the scien- 

 tific 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 cor- 

 rectly understood. 



The first tremendous success of the kinetic hypothesis came about 



