CONSTITUTION AND TEMPERATUEE ON MAGNETIC SUSCEPTIBILITY. 
251 
Oil furthor cooling, the nioleculGS coiitiiiiie to readjust thcmsGlves arid 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 liinding the pairs of molecules together, that a completel}^ new 
pairing of molecules takes place, resulting m spontaneous crystallization accompanied 
hy 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 tliis intrinsic 
field that the two latter media show rigidity. 
If H, 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 
4 
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 tlie 
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 witbin the molecule and we found that in the immediate neighbourhood of a 
molecule the a alue of this field, as determined from the properties of crystalline media, 
is of the order 10^ 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. 1). 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. 1a). 
In a gel, the whole collection of molecules is bound together into one homogeneous 
* See Part III., p. 86. 
t Lor. cit., p. 90. 
