Page:Popular Science Monthly Volume 80.djvu/423

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but for the most part the old ones remain. The atomic theory of matter, for example, was not even touched when radio-activity and the divisibility of the atom were brought to light, for nobody who had gone beyond the high school stage in science ever thought of asserting an indivisible atom, and that simply because we had no basis for asserting anything about the insides of the atom. We knew that there was a smallest thing which took part in chemical reactions, and we named that thing the atom, leaving its insides to the future, and the future proved itself abundantly able to take care of the trust.

Coming now to the first of our two theories, it is probably carrying coals to Newcastle to explain to an intelligent audience to-day what are the essential elements of the kinetic theory of matter, but I will at least carry enough of these coals to make a logical stepping-stone from the familiar to the unfamiliar.

The kinetic theory, then, when divorced from all non-essentials, is merely the assertion that everything in this world of ours is in a state of restless, ceaseless, seething motion, that all matter is composed of minute parts called molecules which are eternally pounding and jostling against one another. In gases these molecules are so far apart that the forces of attraction which exist between them are quite negligible and they dart hither and thither like gnats in a swarm, only with the stupendous speed of a mile a second (in the case of hydrogen) and richochette unceasingly against one another and the walls of the containing vessel, producing by this bombardment all the familiar phenomena of pneumatic tires and gaseous bodies generally. If you could magnify the air in an ordinary room just a thousand million times, that is, enough to make a good-sized marrowfat pea swell to the size of the earth, yore would see objects about as big as a football—we will not say of what shape, because we do not know anything about it, but they would probably be of the same shape in a given gas—and if the motions would stop long enough to enable us to get a snap shot of the whole situation, you would see on the average one of these objects in a cubical space ten feet on a side. Then if you let them go again you would see each of these footballs shoot on the average through thirty such imaginary cubical rooms before it hit another. This distance we call the mean free path of a gas molecule.

In the liquid state the molecules are packed closely together by cohesive forces, yet they continually wriggle and squirm over and around one another, so that if you will be content this time with a 10 million-fold magnification, the liquid would look very much like a mass of wriggling squirming maggots—not a pretty picture perhaps, but a fairly accurate one I think.

In solids the molecules are for the most part locked up tightly in crystalline forms so that their motions are reduced to mere trembling.