Theory of Surf ace Forces, 287 



an ideal plane into hemispheres. Since the hemispheres are 

 at rest, their total action upon one another must be zero, that 

 is, no force is transmitted across the interface. If there be 

 attraction operative across the interface, it must be precisely 

 compensated by repulsion. This view of the matter was from 

 the first familiar to Young, and he afterwards gave calcula- 

 tions, which we shall presently notice, dependent upon the 

 hypothesis that there is a constant attractive force operative 

 over a limited range and balanced by a repulsive force of 

 suitable intensity operative over a different range. In La- 

 place's theory, upon the other hand, no mention is made of 

 repulsive forces, and it would appear at first as if the attractive 

 forces were left to perform the impossible feat of balancing 

 themselves. But in this theory there is introduced a pressure 

 which is really the representative of the repulsive forces. 



It may be objected that if the attraction and repulsion must 

 be supposed to balance one another across any ideal plane of 

 separation, there can be no sense, or advantage, in admitting 

 the existence of either. This would certainly be true if the 

 origin and law of action of the forces were similar, but such is 

 not supposed to be the case. The inconclusiveness of the 

 objection is readily illustrated. Consider the case of the 

 earth, conceived to be at rest. The two halves into which it 

 may be divided by an ideal plane do not upon the whole act 

 upon one another ; otherwise there could not be equilibrium. 

 Nevertheless no one hesitates to say that the two halves 

 attract one another under the law of gravitation. The force 

 of the objection is sometimes directed against the pressure, 

 denoted by K, which Laplace conceives to prevail in the 

 interior of liquids and solids. How, it is asked, can there be 

 a pressure, if the whole force vanishes ? The best answer to 

 this question may be found in asking another — Is there a 

 pressure in the interior of the earth? 



It must no doubt be admitted that in availing ourselves of 

 the conception of pressure we are stopping short of a com- 

 plete explanation. The mechanism of the pressure is one of 

 the things that we should like to understand. But Laplace's 

 theory, while ignoring the movements and even the existence 

 of molecules, cannot profess to be complete ; and there seems 

 to be no inconsistency in the conception of a continuous, 

 incompressible liquid, whose parts attract one another, but are 

 prevented from undergoing condensation by forces of infi- 

 nitely small range, into the nature of which we do not further 

 inquire. All that we need to take into account is then covered 

 by the ordinary idea of pressure. However imperfect a theory 

 developed on these lines may be, and indeed must be, it pre-* 



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