402 Dr. J. R. Ashworth on the Theory of 



pressure has to be introduced in the theory of fluids ; and 

 this intrinsic field may be evaluated either from the known 

 facts of intensity of magnetization (I) as a function of the 

 temperature (T) or as a function of the field strength (H), 

 just as the intrinsic pressure in a fluid may be derived from 

 the relation of the density to the temperature or the density 

 to the pressure. In the theory of fluids the constant of the 

 intrinsic pressure (van der Waals\s a) has consistent values 

 by whichever method the calculation is carried out, but in 

 the theory of magnetism the constant of the intrinsic field 

 has two values which differ enormously according to the way 

 in which they are derived. For example, if the constant of 

 the intrinsic field is derived from I=/(T) it gives to this 

 field at the highest intensity a value about 10 7 in iron, and 

 similar values in nickel and cobalt, but if it is derived from 

 I = <£(H) it gives a value of the order of a few units. 



We have then to choose between two magnitudes for the 

 intrinsic field of entirely different orders, and the first step 

 will be to find grounds for excluding one or the other. 



The value of the intrinsic field derived from the equation 

 to the critical temperature in spite of its enormous magnitude 

 has been in general accepted, probably because such a large 

 intrinsic field explains successfully the behaviour of intensity 

 of magnetization under changes of temperature, and also 

 because it may be made to account for the rise in specific 

 h^at which the ferro-magnetic metals exhibit when the tem- 

 perature is raised up to the critical temperature. 



But while the effects of temperature on ferro-magnetism 

 are satisfied by the introduction into the equation of an 

 immense intrinsic field, the demonstration that this field is 

 entirely a magnetic field is wanting, and it is allowable to 

 suppose that it is in part, perhaps in greater part, a field of 

 force of some other kind than magnetic. 



At the critical temperature the sudden loss of magnetism 

 in ferro-magnetic substances although the most prominent is 

 not the only effect, and the modification which takes place 

 in other properties, such as electrical resistivity, thermo- 

 electric power, and specific heat, is evidence of a change in 

 ferro-magnetic substances which is probably akin to a change 

 of state. If we treat the intrinsic field as the independent 

 variable and the other properties as dependent upon it, then, 

 assuming this field is entirely magnetic and of enormous 

 magnitude, we ought to find some large effects in the de- 

 pendent properties when this magnetic intrinsic field is 

 varied or suppressed. 



There is an interesting change in the magnetic critical 



