512 RICE 



ART. L 



We may look at this matter from another standpoint, a 

 purely static one. We can assume a molecular configuration 

 practically unchanging in average conditions and imagine a 

 plane to be drawn in the interior of the liquid. Across this 

 plane there will be exerted repulsions between molecules in very 

 close proximity to one another and attractions between mole- 

 cules rather more separated. These ideas resemble somewhat 

 those of Laplace who regarded the liquid as a continuum whose 

 neighboring elements attract one another, this attraction tend- 

 ing to make the liquid contract; such contraction would be 

 opposed by an internal pressure. These concepts of cohesion 

 and intrinsic ^pressure are quite familiar. The molecular picture 

 defines them a little more closely. The force between two 

 molecules for distances greater than a certain critical amount 

 is an attraction falling off in value very rapidly as the distance 

 increases. At the critical distance, which must approximate in 

 value to the size of a molecular diameter, the force is zero and 

 changes to a repulsion when the distance apart is decreased; 

 this repulsion must increase with very great rapidity as the 

 distance apart is reduced below the critical separation. Van 

 der Waals formulated these forces of cohesion and intrinsic 

 pressure in his famous equation 



a Rt 



V + ~2 = 



1)2 V — b 



for a/v"^ is nothing more than the cohesion varying directly as 

 the square of the density, and Rt/{v — h) is the intrinsic pressure 

 varying inversely as the excess of the volume of the fluid above 

 its irreducible minimum volume 6. The symbol p represents 

 the ordinary pressure with which we are concerned in the con- 

 ditions of equilibrium. When p is small the cohesion and 

 intrinsic pressure are nearly equal, which means that we have 

 on the average a molecular configuration in which the repulsions 

 and attractions across an internal plane nearly balance one 

 another. The reader will recall in our discussion of the theory 

 of elasticity (Article K) the warning that the stress-constituents 

 Xx, Xy, etc. (which in the case of a fluid reduce to —p) are not 

 to be confused with molecular attractions and repulsions, which 



