Law of Molecular Force. 127 



involving b might be taken as representing the virial of the 

 repulsive forces that are supposed to act during the collisions 

 of molecules. This opens up the whole of the important 

 question of the repulsive forces or of the elastic properties 

 usually assigned to molecules ; and this is the part of the 

 kinetic theory in its actual state least satisfactory to the earnest 

 natural philosopher. Sir William Thomson only gave expres- 

 sion to the aspirations which his development of the gyrostatic 

 principle and its attendant speculations on ' ' Elasticity a mode 

 of Motion " have awakened in the minds of physicists for some 

 means of escape from those highly artificial conceptions of the 

 classical elasticians of laws of force changing from attraction 

 to repulsion, which still survive in the idea of an elastic mole- 

 cule, when, in his address to the Mathematical and Physical 

 section at the Montreal meeting of the British Association 

 (Nature, vol. xxx. 1884), he sketched the possibility of apparent 

 repulsion through attraction. If, therefore, we can prove that 

 molic force is always attractive, an important step will be 

 gained towards homogeneity in our conceptions of force in 

 nature, and there will be only the one general specific pro- 

 perty of attraction to explain by the ultimate theory of mole- 

 cular structure, which will account for the action of molecule 

 on molecule. At first sight it might appear as if the very form 

 of the equation of the virial, as applied in the kinetic theory 

 of gases, militated against the elimination of repulsive forces; 

 for the term § pa, which represents the virial of the applied 

 pressure, must, on a theory of pure attraction, really be con- 

 sidered as representing the virial of the attractions of the 

 molecules of the bounding walls on the molecules inside. Now 

 the pressure, whatever its origin, is equivalent to a repulsive 

 action from the walls on the fluid inside ; so that we have to 

 face the apparent contradiction of the virial of attractive forces 

 between two sets of molecules being replaceable by the virial 

 of an imaginary repulsion between them. But to avoid this 

 difficulty, we have only to make the conception, which in 

 itself will be found an important one in Physics and Che- 

 mistry, that the surface-molecules of two bodies in contact 

 always penetrate amongst one another. Let us see the bearing 

 of this idea on the determination of the virial of the attractions 

 of the molecules of a body B on the molecules of a fluid A 

 bounded by B. If a molecule of A gets to the far side of a 

 molecule of B, the attraction on it will be "in the same sense 

 as that in which we usually consider the pressure to act ; when 

 the molecule of A is on the near side of a molecule of B, the 

 attraction on it is in the opposite sense. The value of the virial 

 of the action of the molecules of B on those of A depends, 



