i86 THE POPULAR SCIENCE MONTHLY. 



bodies. At length the vibrations become so energetic that the 

 fixed order in which the particles are arranged can no longer be 

 maintained. They begin to collide and interfere with each other ; 

 the whole artificial edifice of the crystal collapses. The molecules 

 can not regain their equilibrium — the snow has changed into 

 water. 



In liquids the molecules move about in all directions, yet none 

 of them can voluntarily separate itself from the main body. Just 

 as in a vessel, completely filled with live eels, each individual fish 

 may wriggle and move about among the others, yet it can not 

 detach itself or swim away from them. The liquid particles are 

 not yet sufficiently potent or energetic to overcome the pressure 

 exerted upon them by their surroundings. Above the water, 

 even in the open vessel, they have to encounter the pressure of 

 the atmosphere, the molecules of which keep uj) a constant and 

 vigorous bombardment against the liquid particles, forcing them 

 down. Still, this can not prevent that here and there a favorably 

 situated water-molecule pushes itself between the air-molecules ; 

 the water evaporates. Now, if the temperature is heightened — 

 that is, if the vibrations of the water-molecules increase to siich 

 an extent that they can hold their own against the pressure of 

 the air — then a condition of things is brought about familiar to us 

 under the name of boiling. The water-particles shoot about very 

 rapidly; they are no longer crowded together, they force their 

 way through the air-molecules and disperse — the water evaporates; 

 we have no longer liquid but gaseous water. 



In gases — as, for instance, the air, carbonic acid, etc. — the mole- 

 cules are in a state of vibration so violent that they fly about with 

 marvelous rapidity in all directions. 



Now, we are in possession of information — of pretty accurate 

 information — respecting these molecule - movements. The re- 

 searches of men like the late Prof. Kingdon Clifford, Prof. Helm- 

 holtz, and, above all, Sir William Thomson — one of the ablest 

 physicists and beyond comparison the greatest living mathema- 

 tician as well as one of the subtlest thinkers the world has ever 

 produced — the researches of men like these have thrown quite a 

 flood of light on this important and highly interesting subject. 

 Nor is our information merely confined to molecular movements 

 and vibrations ; the dimensions of the molecules themselves have 

 been approximately ascertained, because, from known facts of 

 pressure, friction, and heat-conducting capacity, very reliable 

 conclusions can and have been drawn. 



The air which surrounds us is a chaos of innumerable minute 

 solid bodies, flying rapidly about in all directions. Our skin is 

 perpetually bombarded by them, and it is this bombardment 

 which causes us to experience atmospheric resistance or press- 



