treated according to Van der Wools' s Equation. 335 



T. 



1. TN a paper, published in this Magazine in November 

 _I_ 1915, on "The Application of Van der Waals's 

 Equation to Magnetism " *, Curie's views f on the analogy 

 between the intensity of a paramagnetic material as a 

 function of field-strength and temperature and the density 

 of a gas as a function of the pressure and temperature were 

 discussed and extended, and it was shown that the relation 

 of ferromagnetism to paramagnetism may be regarded as 

 analogous to that of a liquid to its vapour. In short, just 

 .as we consider the gas law 



P 



- = RT 

 P 



as a special case of the more general equation 



where P, /?, and T are the pressure, density, and absolute 

 temperature of the gas, and a, p , and R are constants, so 

 we may treat the paramagnetic law 



?=R'T 



:as a special case of a more general equation, 



(H+a'l»)(J-iy=B'T, . . . . (1) 



where H, I, and T are field-strength, intensity of magneti- 

 zation, and absolute temperature respectively, and a' } I , and 

 R' are constants corresponding to the constants a, p , and R 

 in the fluid equation. The general equation to fluids applies 

 to liquids when the densit} r is large, and to gases or vapours 

 when the density is small ; and in the same way the general 

 equation to magnetism applies to ferromagnetism when the 

 intensity of magnetization is large, and to paramagnetism 

 when it is very small. 



In the paper to which reference has been made, the 

 application of these views of the ferromagnetic and para- 

 magnetic states was considered in so far as intensity of 

 magnetization is dependent on temperature, and values for 

 the constants a', I , and R' were obtained for each of the 



* Ashworth, Phil. Mag. vol. xxx. p. 711 (1915). 

 t Curie, * (Euvresf p. 330. 



