182 BRIDGMAN. 



arrangement of minimum potential energy. But this is not the ar- 

 rangement of smallest possible volume. As pressure is increased the 

 molecules may be forced to turn into the positions of Figure 35, 

 occupying the minimum volume. In this arrangement the attractive 

 centers have been torn apart, and the potential energy has been 

 increased. 



This point is one of wide importance. If we suppose that the 

 energy of temperature agitation is the same in two different phases at 

 the same temperature, the mere existence of transitions for which 

 there is no latent heat and all the work done by the external pressure is 

 stored up as an increase of internal energy, afl'ords conclusive evidence 

 that the forces with which the atoms act on each other cannot be 

 effectively situated at the centers of the atoms. The assumption of 



Fig. 34. Fig. 35. 



Figure 34. Hypothetical substance composed of approximately square 

 atoms. 



Figure .35. The atoms of Figure 34 in another arrangement. The ar- 

 rangement of largest volume is that of minimum potential energy. 



central forces is one that has been very widely used, from the early 

 WTitings of Poisson on theoretical elasticity to the recent specula- 

 tions of Griineisen. These considerations show that this is not an 

 adequate method of representing the inter-atomic forces, at least 

 when the atoms are close together as in a solid. The effective centers 

 of atomic attraction are not situated at the geometrical centers of the 

 atoms, but must be nearer the surface. It is furthermore probable 

 that the centers of attraction are very near the surface, because the 

 nearer they are to the surface, the easier it is to conceive, without doing 

 violence to our other conceptions of the atom, how it is that pushing 

 the geometrical centers closer together may pull the centers of attrac- 

 tion further apart. 



