CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 251 



On further cooling, the molecules continue to readjust themselves and the rigidity 

 increases until a glass is formed. Still lower temperatures, accompanied by further 

 molecular readjustment due to reduction of amplitude, may result in such a distortion 

 of the lines of force binding the pairs of molecules together, that a completely new- 

 pairing of molecules takes place, resulting in spontaneous crystallization accompanied 

 by thermal evolution as the more stable crystalline state is formed. 



Whatever may be the nature of the forces which hold the molecules of a liquid 

 together, we have, in addition to these forces, the intrinsic field referred to above, 

 when the substance passes into a rigid gel or crystallizes, and it is due to this intrinsic 

 field that the two latter media show rigidity. 



If H c be this intrinsic field,* I the local intensity of magnetization,! the potential 

 energy per unit volume associated with the gel or crystalline medium will be 



*.H..I. (l) 



and this will be over and above any potential energy which the molecules of the 

 liquid possess. This is also a measure of the mechanical stress which binds the 

 molecules of the gel or crystalline medium together and determines their rigidity. 



In Part III. we have given reasons for locating the source of the local molecular 

 field within the molecule and we found that in the immediate neighbourhood of a 



Fig. 1. 



molecule the value of this field, as determined from the properties of crystalline media, 

 is of the order 10 7 gauss. However the molecule is orientated, provided that 

 orientation is not variable with time, the local forcive will be of this order of intensity 

 and in some direction determined by the orientations of the two molecules between 

 which it acts. In a gel, as we pass from molecule to molecule, the direction ,of this 

 stress will be continually changing (fig. l). Throughout a crystal, on the other hand, 

 its direction will be constant and will in fact be one of the determining factors of a 

 particular form of crystalline symmetry (fig. IA). 



In a gel, the whole collection of molecules is bound together into one homogeneous 



* See Part III., p. 86. 

 t Loc. tit., p. 90. 



