THE STRUCTURE OF THE WORLD-STUFF 273 



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 ques- 

 tion 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 tbe reach of the most powerful microscope; but 

 this has been accomplished. Time would not permit me even to out- 

 line 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 solu- 

 tion; 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 tbe 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 com- 

 parison 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 micro- 

 scopes. 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 



vol. lxxvi.— 19. 



