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 of the relative weights of atoms. The numbers he obtained as representing the atomic weights were in many cases erroneous; but they were rectified by the work of Berzelius (1779-1848), and even now these constants of nature are subject to frequent revision. It was, however, to the genius of Dalton that the atomic weights of the elements were first comprehended.

Since Dalton's time, the sizes, intervals, and velocities of atoms have been ascertained. These problems have been solved by Clausius, Kelvin, Clerk Maxwell, and others from various sides: "from a comparison with the wave-lengths of light, with the tenuity of the thinnest films of soap-bubbles just before they burst, and from the kinetic theory of gases, involving the dimensions, paths, and velocities of elastic bodies, constantly colliding, and by their impacts producing the resulting pressure on the confining surface." For instance, one cubic centimetre of air contains twenty-one trillions of molecules; the average distance between each molecule equals ninety-five millionths of a millimetre; the average velocity of each molecule is four hundred and forty-seven metres per second; and the average number of impacts received by each molecule is four thousand seven hundred millions per second.

In 1865 Loschmidt of Vienna, twenty-one years after Dalton's death, calculated that the diameter of an atom of oxygen was the one-ten-millionth of a centimetre;