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186 was not more than twenty million years old, and would not probably sustain life more than ten million years longer; for in that time the sun would be cool, and therefore organic life on this planet would be impossible. In conjunction with the late Professor P. G. Tait, he, in 1867, produced a classical work on Natural Philosophy. He devoted a vast amount of time in studying the constitution of matter—the sizes of atoms and molecules, and the force which holds them together. Thomson came to the conclusion from examining the thickness of the wall of a soap bubble, the electrical action of small copper and zinc discs, the refraction of light, and the dynamical theory of gases, that the molecules of air are about large enough to put twenty-five millions of them in a row an inch long. "Imagine a globe of water six inches in diameter magnified to the size of the earth, and its molecules in the same proportion; then, when the drop had become a world, the individual particle would be about the size of small shot, certainly not larger than a football." These calculations were before the discovery of the corpuscular radiations of radium, polonium, actinum, etc., which the experimental researches of Professor J. J. Thomson, of the Cavendish Laboratory, Cambridge, proved to be at least a thousand times smaller than the chemical atom; although the calculations of Lord Kelvin concerning the atom still remain true. He did not care for the latter-day work