Van der Waals* Equation of State to Magnetism. 715 



-equation to fluids, and is consistent with the analogy between 

 the form of the curves of I=/(T) and p—f(T) to which 

 Ourie drew attention. Suppose, then, leaving for future 

 consideration the manner in which the intrinsic field is set 

 up, we write H; = a'I 2 , the equation to magnetism takes the 

 definite form 



(H + a'P)( I 1 -J-) = R'T. 



This may be called briefly the ferromagnetic equation and 

 the constants a', I , R' the fundamental magnetic constants. 

 Since this equation is precisely analogous to Van der Waals' 

 equation it ought to yield similar deductions, and it will be 

 a test of the general truth of the equation if these deductions 

 are in the main confirmed by experiment. Nevertheless, as 

 Van der Waals' equation of state is only an approximate 

 representation of the properties of a fluid, it is not likely 

 that an analogous equation to a phenomenon so different as 

 magnetism will express the results of experiments with equal 

 success. 



In what follows the application of this equation will be 

 considered so far as it relates to I = /'(T), leaving I = </>(H) 

 for future treatment. 



4. It is at once obvious that this equation is a cubic in I, 

 and consequently that there may be critical constants as Curie 

 surmised. They can be written in terms of the magnetic 

 constants after the manner of a fluid, substituting a! for cr, 

 R' for R, and I for p . Thus wc have for the critical tem- 

 perature 



T — — a ^° 

 c ~27 R' ' 



This equation is useful in allowing a value to be assigned to 

 a' when R/ and I are known, and it is now possible to 

 determine the magnitudes of the three magnetic constants in 

 the ferromagnetic equation in the following way. 



As mentioned above, from the principle of the continuity 

 of states, R / may be taken as the reciprocal of Curie's 

 constant A. 



For iron A = 0281 * .\ R' = 3-56 

 For nickel A = 0048 f .'. R' = 20'8 

 For cobalt A = 0-lCGJ .-. R'= 6'0 



* Curie, GEuvres, p. 327. Weiss and Foex, Arch, des Sc. 4 ser. t. xxxi. 

 pp. 4, 69 (1911). 



t Weiss and Bloch, Arch, des Sc. t. xxxiii. p. 293 (1912). 

 X AVeiss, Arch, des Sc. 4 ser. t. xxxi. pp. 5-19 & 89-117 (1911). 



3 A 2 



