712 Dr. J. It. Ashworth on the Application of 



The constant R' in the paramagnetic equation corresponds 

 to the constant R, in the gas equation. 



The second important conclusion to which Curie was led 

 was that ferromagnetic substances are transformed pro- 

 gressively when they are heated, and tend to assume the 

 properties of substances feebly magnetic as the temperature 

 passes and increases beyond the point of transformation *. 

 Curie pointed out that there is a close analogy between curves 

 of intensity of magnetization of a ferromagnetic as it changes 

 to the paramagnetic state in a constant field, and curves of 

 density of a liquid as it passes to the gaseous state at con- 

 stant pressures when the temperature is raised. The analogy 

 between the curves of I=/(T) and/>=/(T) prompted him 

 to ask the question : Are there critical constants for magnetism 

 as there are for fluids? 



The parallel which exists between fluids and magnetism 

 suggests that the change of state which takes place in each 

 at the critical temperature is due to causes of the same 

 kind. From the point of view of molecular theories the 

 passage of a fluid from the liquid to the gaseous state takes 

 place when the molecules of the fluid are released from 

 reaction one on the other and become independent, and in 

 like manner we may imagine that the passage from the 

 ferromagnetic to the paramagnetic state takes place when 

 the molecular magnets pass from reaction one upon the 

 other to freedom from mutual control. Further molecular 

 theories of fluids show that there must be a limit to the 

 density under the action of high pressures and low tempe- 

 ratures, and correspondingly molecular theories of mag- 

 netism show that there must be a limit to the intensity 

 of magnetization under the influence of strong fields and 

 low temperatures. 



Thus we may regard the ferromagnetic and paramagnetic 

 states in magnetism as analogous to the liquid and gaseous 

 states in fluids. 



2. The correspondence between the laws of gases and the 

 laws of paramagnetics, the continuity of the gaseous and 

 liquid states established by Andrews, and the like continuity 

 of the paramagnetic and ferromagnetic states established by 

 Curie, together with the fact that the density of a liquid 

 and the intensity of magnetization of a ferromagnetic both 

 tend to a limit, suggest that a general equation applicable to 

 fluids may be applicable to magnetism, and lead one to try 

 if the extension which Van der Waals made to the gas 



* Curie, (Euvres, p. 329. 



