344 BELL SYSTEM TECHNICAL JOURNAL 



Gravity is long-range, because it falls away gently with increase of distance; 

 cohesion is short-range, because it falls away precipitately. We shall soon 

 be meeting with other examples of either character. 



One other fact to illustrate the short-range quality of the cohesive forces: 

 When a kettle of water is boiling away on the stove, the amount of heat 

 consumed in dispersing the first cubic inch that departs is the same as is 

 spent in dispersing the second, and the third, and each of the others down 

 to and including the last. This could not be so, if the particles were drawn 

 together by important long-range forces; for then each cubic inch would be 

 easier to drive off than that which last preceded it into the vaporous state, 

 since there would be less of the liquid remaining behind to attract it. 



The celestial bodies — useful as they have been in showing us the laws of 

 motion — have therefore served us badly by hinting that gravity is the sole 

 attractive force, a hint which is quite misleading. In another important 

 respect they fail to give us a lead: they show us no examples of collision. 

 Collision, more commonly known as impact, is one of the most important of 

 earthly phenomena, as it is one of the most uncomfortable. The apple 

 which fell in the orchard of Newton, and inspired him with the law of gravita- 

 tion, may have been a legendary apple; if it was real, we may be sure that it 

 ended its fall in a collision — ended its fall, not its existence. It did not pass 

 through the globe and pop out of the ground in the Antipodes; it did not 

 instantly merge with the grass or the soil of the orchard; it bounced and 

 rolled a little, perhaps, and then lay quietly pressing against the earth, 

 entire and whole. The earth was impenetrable to the apple, as the apple 

 to the earth. 



We do not even have to look to impact, to be taught this lesson about 

 the impenetrable. Not less impressive than the fact that the piece of iron 

 sticks together, is the fact that it does not shrink. For any particular 

 choice of temperature and pressure, it has a particular volume which is its 

 own. Work or heat must be expended to dilate it or tear it apart alto- 

 gether, but also work must be expended to make it denser. 



Having ascribed to attractive forces the fact that it takes heat — or let 

 me say henceforward, energy — to vaporize a piece of matter solid or liquid, 

 we now ascribe to repulsive forces the fact that it takes energy to squeeze 

 the piece. The forces must be short-range — still more short-range than are 

 the cohesive forces, inasmuch as these come into play to capture the atoms 

 and hold them together, before those get their opportunity of crying "hold, 

 enough!" They must be very potent, for the most terrific pressures which 

 have been achieved by man do not avail to squeeze the most compressible 

 solid into half of its original volume. Why talk of artificial pressures? 

 everywhere in the globe of the earth, except within a hundred miles of the 



