ATOMIC WORLDS AND THEIR MOTIONS. 187 



lire. These air-molecules, if closely packed (without any in- 

 tervening space), would only fill about -j^oir 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 combi- 

 nation of lenses, as now produced, would still enable us to recog- 

 nize, requires a diameter of at least 4 „ (/o ,) q of an inch, but of oxy- 

 gen-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 tak- 

 ing 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 eluci- 

 dation of truth. 



In this glass of water I observe a little air-bubble. It has a di- 

 ameter 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 mole- 

 cules, 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 Avithout 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 bom- 

 barding our skin would not inconvenience us, for that skin would 



