Page:Popular Science Monthly Volume 15.djvu/478

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THE idea that matter is an aggregate of minute particles, each of which possesses all the essential properties of the mass, is as old as Democritus, but it was left for the present century to crystallize the conception of the atom in clear and accurate expression. The atomic theory, revived and vitalized by the illustrious Dalton, has not simply been able to survive the conflicts in which many an older theory has been wrecked: it has itself been a prime mover of revolutions. It is doubtful whether without it the recent advances in chemical and physical science could have been made.

But the atom in chemistry is not the atom in physics; they are of a different order. When the idea of a chemical atom came to be clearly conceived so that atom could be defined, as it is, to mean the smallest particle of an elementary substance which can enter into the composition of a compound, the most natural, if not, indeed, the inevitable corollary would be that the compound itself must be made up of parts, each of which, containing only the minimum number of its constituent atoms competent to give it character, must be the smallest particle of that substance which can possibly exist. To distinguish this minutest portion of a substance from the chemical atoms of which it is composed, the French called it the molecule—literally the little mass; and this word molecule, homeless in the English language less than one hundred years ago, expresses an idea which now lies at the foundation of modern physics.

It has been said that the science of astronomy is the demonstration of the law of gravitation. Indeed, what evidence have we of the truth of Newton's grand generalization, except that it explains the phenomena of the skies? So, in the outset, we may say that the science of physics is the demonstration of the molecular theory of the constitution of matter, since it explains phenomena, suggests research, directs experiment, classifies and unitizes wide ranges of apparently diverse results, to an extent unparalleled by any other.

This theory boldly affirms a limit to the divisibility of matter, and thus seems to defy the logic of the metaphysician, who, passing the limit set by the necessary imperfections of manipulation, carries the process of subdivision mentally downward through the scale of littleness, until, finding no place where his conception of subdivision must halt, declares that no limit exists. But the physicist does not deny the logic of the metaphysician; he simply remembers that mental conceptions need not of necessity represent the realities of nature's processes,

  1. Read before the Poughkeepsie Society of Natural Science, December, 1878.