412 SECTIONAL COMMUNICATIONS. 



If we examine the properties of a substance which, besides having 

 the structure of Langevin's paramagnetic body, possesses the molecular 

 field, we find that the stable state, in the absence of an external field, 

 is not the non-magnetic state, but a condition of magnetisation of finite 

 extent which I have called spontaneous magnetisation. And it can be 

 shown that this spontaneous magnetisation has the same numerical value 

 as that of saturation at the temperatui'e under consideration. One 

 must not contuse this saturation with the absolute saturation dealt with 

 by Langevin. The latter corresponds to the complete alignment of the 

 magnetic molecules, whilst the former difiers from it by the fact of the 

 thermal agitation of rotation. In a, ferromagnetic substance, apparently 

 unmagnetised, the spontaneous magnetisation has different directions 

 at different points, and the non-magnetic state is the result of the mutual 

 compensation of these magnetisations. The action of a field upon a bar 

 of iron thus consists, not of producing magnetisation, but of co-ordinat- 

 ing the spontaneous magnetisations of the different parts by rendering 

 them parallel. 



The molecular field theory has given, for the first time, the law of 

 the variation of magnetic saturation as a function of the temperature, 

 and this law has been verified by new experiments with magnetite. 



Ferromagnetism disappears at a certain temperature which has been 

 called the Curie Point. Above this temperature the suljstance is para- 

 magnetic. The molecular field has enabled us to find the law of this 

 paramagnetism ; the reciprocal of the coefficient of magnetisation -^ is 

 proportional to the excess of the temperature T over that of the Curie 

 Point 0, 



^ = ^ (T - 6). 



This law was capable of immediate verification by means of Curie's 

 experiments on nickel, carried out previously. It has since received 

 numerous confirmations from accurate experiments on the ferromagnetic 

 metals and many of their alloys. 



If we assume that the coefficient N of the molecular field has three 

 different values in thiee directions at right angles, we can find an 

 explanation for the remarkable properties of the crystal of pyrrhotine, 

 with its magnetic plane, and in this plane the rectangular directions of 

 easy and difficult magnetisation. 



Lastly, twO' applications of the molecular field in energy considera- 

 tions. It has long been known that the specific heat of ferromagnetic 

 substances displays an anomaly at the Curie Point. Some would speak 

 of a heat of transformation. But the theory of the molecular field has 

 shown, on the contrary, that the phenomenon consists of a discontinuity 

 of the true specific heat, which, at the Curie Point, falls abruptly to a 

 smaller value. The magnitudes of the discontinuity, calculated from 

 magnetic data and measured calorimetrically, have been found to be 

 concordant. 



The recently discovered magneto-caloric phenomenon consists of a 

 reversible variation of temperature which accompanies magnetisation. 

 It is quite different from hysteresis, which is irreversible and always 



