248 The Outline of Science 



are in a state of violent movement, and they form no union with 

 each other. If we want to force them to enter into the loose sort 

 of association which molecules have in a liquid, we have to slow 

 down their individual movements by applying severe cold. That 

 is how a modern man of science liquefies gases. No power that 

 we have will liquefy air at its ordinary temperature. In very 

 severe cold, on the other hand, the air will spontaneously 

 become liquid. Some day, when the fires of the sun have sunk 

 very low, the temperature of the earth will be less than 200 C. : 

 that is to say, more than two hundred degrees Centigrade below 

 freezing-point. It will sink to the temperature of the moon. Our 

 atmosphere will then be an ocean of liquid air, 35 feet deep, lying 

 upon the solidly frozen masses of our water-oceans. 



In a solid the molecules cling firmly to each other. We need 

 a force equal to twenty-five tons to tear asunder the molecules in 

 a bar of iron an inch thick. Yet the structure is not "solid" in the 

 popular sense of the word. If you put a piece of solid gold in a 

 little pool of mercury, the gold will take in the mercury between 

 its molecules, as if it were porous like a sponge. The hardest 

 solid is more like a lattice-work than what we usually mean by 

 "solid" ; though the molecules are not fixed, like the bars of a lat- 

 tice-work, but are in violent motion; they vibrate about equi- 

 librium positions. If we could see right into the heart of a bit 

 of the hardest steel, we should see billions of separate molecules, 

 at some distance from each other, all moving rapidly to and fro. 



This molecular movement can, in a measure, be made visible. 

 It was noticed by a microscopist named Brown that, in a solution 

 containing very fine suspended particles, the particles were in 

 constant movement. Under a powerful microscope these particles 

 are seen to be violently agitated; they are each independently 

 darting hither and thither somewhat like a lot of billiard balls on 

 a billiard table, colliding and bounding about in all directions. 

 Thousands of times a second these encounters occur, and this 

 lively commotion is always going on, this incessant colliding of 



