We meet with the same staggering figures in the science of the infinitely little that we do in the science of the infinitely vast.  Thus the physicist deals with a quantity of matter a million million times smaller than can be detected in the most delicate chemical balance.  Molecules inconceivably small rush about in molecular space inconceivably small.  Ramsay calculates how many collisions the molecules of gas make with other molecules every second, which is four and one half quintillions.  This staggers the mind like the tremendous revelations of astronomy.  Mathematics has no trouble to compute the figures, but our slow, clumsy minds feel helpless before them.  In every drop of water we drink, and in every mouthful of air we breathe, there is a movement and collision of particles so rapid in every second of time that it can only be expressed by four with eighteen naughts.  If the movement of these particles were attended by friction, or if the energy of their impact were translated into heat, what hot mouthfuls we should have!  But the heat, as well as the particles, is infinitesimal, and is not perceptible.

II

The molecules and atoms and electrons into which science resolves matter are hypothetical bodies which no human eye has ever seen, or ever can see, but they build up the solid frame of the universe.  The air and the rocks are not so far apart in their constituents as they might seem to our senses.  The invisible and indivisible molecules of oxygen which we breathe, and which keep our life-currents going, form about half the crust of the earth.  The soft breeze that fans and refreshes us, and the rocks that crush us, are at least half-brothers.  And herein we get a glimpse of the magic of chemical combinations.  That mysterious property in matter which we call chemical affinity, a property beside which human affinities and passions are tame and inconstant affairs, is the architect of the universe.  Certain elements attract certain other elements with a fierce and unalterable attraction, and when they unite, the resultant compound is a body totally unlike either of the constituents.  Both substances have disappeared, and a new one has taken their place.  This is the magic of chemical change.  A physical change, as of water into ice, or into steam, is a simple matter; it is merely a matter of more or less heat; but the change of oxygen and hydrogen into water, or of chlorine gas and the mineral sodium into common salt, is a chemical change.  In nature, chlorine and sodium are not found in a free or separate state; they hunted each other up long ago, and united to produce the enormous quantities of rock salt that the earth holds.  One can give his imagination free range in trying to picture what takes place when two or more elements unite chemically, but probably there is no physical image that can afford even a hint of it.  A snake trying to swallow himself, or two fishes swallowing each other, or two bullets