CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 249 



unknown and may be continually changing, so that the only possible principal value' 

 is the one that omits the contribution of neighbouring molecules altogether." 



When now we come to the case of crystalline media, the molecules which were 

 removed from our small spherical cavity would affect considerably the value of the 

 forcive at the centre, for there is in this case no averaging out on account of random 

 motions and orientations of the molecules originally occupying the cavity. As we do 

 not know the relative dispositions of the molecules composing the space lattice, or the 

 law of force which is operative between the molecules, it is quite impossible to 

 calculate the value of the intrinsic forcive for a point inside a crystalline medium, and 

 as the method of averages, the application of which is quite satisfactory when the 

 medium is in a fluid state, breaks down in the case of a crystalline medium, it is clear 

 that our only way of progress lies in indirect deduction from experimental facts which 

 record the change of physical properties accompanying the transition from the liquid 

 to the crystalline state. This is the method which has been adopted in the previous 

 portions of this work and it has been shown that the internal forcive at a point of 

 a crystalline medium is extremely large and comparable, if interpreted magnetically, 

 with the molecular field in ferro-magnetic substances (10 7 gauss). 



In Part III. it was shown that the potential energy of the molecules forming 

 a crystalline structure was sufficiently large to account for the magnitude of the 

 thermal energy required for fusion. On p. 201 of his " ./Ether and Matter," LARMOR 

 states : " These various actions (referring to the disturbance of configuration of steady 

 orbits in molecules by action of an applied magnetic field) involve energy terms for 

 each individual molecule, and the sum for all the molecules, if it could be formed, 

 would represent the total energy of the disturbance of the medium. But such a 

 mere aggregate of terms would be of no use for applications to matter in bulk ; what 

 we are concerned with there is the mechanical part of the energy, which must be an. 

 analytical function of the specification of matter by volume, determined as to 

 mathematical form by the character of the molecular actions, but with coefficients 

 whose values are to be obtained only by direct experiment." 



Although for a fluid medium the total energy of the disturbance of the medium, due 

 to the application of a magnetic field, has little significance, yet, in the transition from 

 the liquid to the crystalline state, during which the molecular field becomes operative, 

 the sum total of the energy disturbance of the medium due to the action of this molecular 

 field is representative of the latent thermal energy which is absorbed when the 

 crystalline medium is fused, and has a definite value for each particular substance. 



From the point of view of fluids, the intrinsic forcives mutually compensate and the 

 mathematical functions may be treated as analytic, their principal values being taken. 

 In a crystalline structure, however, the functions cannot be treated as analytic. 

 Indirectly we have obtained a measure of the intrinsic forcive in this case with the 

 aid of experimental data. The most we can obtain by direct experiment, however, is 

 a measure of the mean ,'molecular field, which expresses mathematically how the 



2 M 2 



