ure. These air-molecules, if closely packed (without any intervening space), would only fill about 3000 of the space taken up by the air as it is. They rush about in this void with the quickness of rifle-bullets. Every point of our skin is struck by at least five thousand millions of these little bullets every second. Their number is so great that every cubic inch of air contains no less than twenty-one trillions of them, and the same is true of all gases. They are so small that they are utterly beyond our powers of perception.
The smallest object which the best and most powerful combination of lenses, as now produced, would still enable us to recognize, requires a diameter of at least 4000000 of an inch, but of oxygen-molecules three hundred could be placed side by side before they would cover that minute distance. Still smaller are the molecules of hydrogen.
Now, in order to get a clear idea of this air which we inhale, of this hail-storm of little worlds which we perpetually encounter without apparent discomfort, let us resort to a little arithmetic and imagination—not the imagination of the poet and romancer, which delights in pictures of the fanciful and ideal, without taking much account of facts, but the healthy imagination of the scientist, which moves among the sternest of all realities, and which, if rightly exercised, becomes a potent factor in the elucidation of truth.
In this glass of water I observe a little air-bubble. It has a diameter of perhaps one thirtieth of an inch. Let us magnify this tiny bubble ten million times; let us imagine it ten million times larger than it is now, but first let us retire to a safe distance; for, the moment we touch it with our magic wand, it becomes a globe eight miles in diameter. In this globe fifty thousand billion little bullets, of the size of No. 6 shot, are flying about in all directions with the quickness of rifle-balls. Whenever one of these molecules, these shot-grains, comes in contact with another (and this happens about eighty million times every second), it is deflected from its course and takes another direction, but without the slightest loss of its original speed.
It may be asked. How can we manage to exist amid such a torrent of projectiles? we ought to be instantly annihilated. But we have forgotten to apply the same magnification-scale to our own persons. Let us do so, and we become giants seven thousand geographical miles in height. One of our feet would more than cover the distance from Chicago to New York, and with the other we could conveniently step across the Atlantic to Europe. Let the whole atmosphere be magnified in the same proportion, and it will be understood why the hail of little bullets perpetually bombarding our skin would not inconvenience us, for that skin would