who make frequent raids impartially into either territory, usually carrying off rich spoils.
It is natural to inquire as to the size of these molecules and atoms of which we are thus assured the world is made. The question of the relative size is accurately answered by chemical analysis. We know, for instance, that the atom of oxygen weighs 15.88 times as much as that of hydrogen, and so on. But this gives no answer to the question as to the absolute size. It may seem that it would be impossible—even presumptuous—to attempt to estimate the size of particles which must be far beyond the reach of the most powerful microscope; but this has been accomplished. Time would not permit me even to outline the methods of wonderful ingenuity by which this problem has been attacked. The study of the laws of expansion of gases, the phenomena of the soap bubble, the action of the electric current, the blue of the sky, the settling of fine drops of mist or of specks of dust, these and other classes of phenomena have all contributed to the solution; and the evidence from such varied sources has been strikingly concordant.
Let me give you the results. Small indeed are these atoms, but not immeasurably small. So small that when they are expressed in ordinary units the mind shrinks from the attempt to grasp them. But the scientist is not limited to a single unit of measure. The geographer uses a mile, the carpenter a foot. The astronomer's staff with which he gauges the motions of the planets in their courses stretches from the earth to the sun; while in estimating the distances of the fixed stars the unit is the far greater distance traversed by light in a year. And so in the world of the little a convenient standard of comparison is the wave of light, some fifty thousand of which are contained in an inch, of the order of the thickness of a brightly colored soap bubble or of the smallest things that we can see with our best microscopes. Measured in these units, we find the diameter of a hydrogen atom to be about one two thousandth part of a wave of light, or, in our ordinary measure, a hundred-millionth of an inch. We hear much of millions, especially in the daily press, though perhaps we have but a vague conception of them. For example, we heard not long since of a celebrated fine of $27,000,000. If that fine had been paid, and paid in dollar bills, and the bills laid end to end, they would have reached from Maine to California. (I do not suggest this as a desirable method of laying out money, though we often meet with suggestions of even less merit; but to help in expressing the magnitude of the quantities with which we are dealing.) Now if each bill were replaced by an atom, and the line closed up, it would extend a quarter of an inch. Or we may express the result in another way: The diameter of an atom bears the same relation to that of a tennis ball that the
