FORCES AND ATOMS 343 



forms chemical operations, and reaches not far from the particles with any 

 sensible effect." 



Further along in the Oplicks we read : 



"Thus Nature will be very conformable to herself and very simple, per- 

 forming all the great motions of the heavenly bodies by the attraction of 

 gravity which intercedes those bodies, and almost all the small ones of their 

 particles by some other attractive and repelling powers which intercede the 

 particles." 



With the powerful aid of Newton we have now distinguished between the 

 attractive force of gravity and another attractive force, for which I retain 

 the old-fashioned name "cohesion". I give another basis of distinction, 

 one which could not have been found until in the mid-nineteenth century 

 the equivalence of heat with mechanical work was established. Consider a 

 piece of solid or liquid matter, and put the question: how much work must 

 be done to tear its atoms apart and dissipate them into the infinite reaches 

 of space, if the only force whereby they act on one another is the attraction 

 of gravity? The question is answerable, if it is known how massive the 

 atoms are and how far apart (on the average) they are. These things are 

 known. The result of the computation is to be compared with the amount 

 of work which is actually expended — in the form of heat — when the solid or 

 liquid is volatilized into vapor. It is found that only about the billionth 

 part of a millionth part of the heat so spent is devoted to "breaking down 

 the gravitational bond", to doing work against the attraction of gravity 

 which is overcome when the atoms are dispersed . All the rest is required 

 for overcoming that more intimate force of cohesion. 



Gravity now is pushed into the background, and sinks into the relative 

 insignificance which may be gauged from the fact that in the endless specula- 

 tions of physicists and chemists as to how matter is built up and joined 

 together, it is completely left out. The force which dominates the planets, 

 which makes a hill so hard to climb and a height so dangerous to fall from — 

 how amazing that it should be trivial, compared with others which the flame 

 of the gas-jet vanquishes as the water boils out of the kettle! Trivial of 

 course by comparison only, and at small distances, not at great; or to phrase 

 the situation better, it is the force of cohesion which is trivial at great 

 distances, gigantic at small. This is the contrast which is implied by the 

 technical terms of physics, "long-range forces" versus "short-range forces". 



^ The computation for mercury was made by my colleague Dr. L. A. MacColl, on the 

 basis most favorable to gravity : by assuming mercury to be a continuum, or in other words, 

 to be made up of infinitesimal atoms infinitely close together — an assumption giving 

 the greatest possible value to the work required for spreading the mercury through infinite 

 space, if gravity be the only restraint. The latent heat of vaporization of mercury is 

 found by experiment to be 1.88.10^^ times this value. Thus the contrast mentioned in 

 the text is not contingent upon knowledge of the mass and spacing of the atoms, though 

 the knowledge is available if wanted. 



