treated according to Van der Waals's Equation. 343 

 II. 



In this part the relation I = 0(H) will be treated from the 

 standpoint of the kinetic theory. 



Langevin's equation for a paramagnetic substance* is 



I/I = coth.i-- i (1) 



where #— j>t' » 



a being the magnetic moment of the molecule, H the field, 

 T the absolute temperature, and R the gas constant. 



Weiss supposes the mutual action of the magnetic mole- 

 cules in a ferromagnetic body to be equivalent to that of a 

 uniform field proportional to the intensity of magnetization f. 

 Thus the intrinsic or molecular field H; may be written 

 Hi=NI, where N is a constant, and if this field be added to 

 H the expression for x becomes 



-*^P> (2) 



which gives 1= ~uE lV ~~W ^ 



a linear equation in I and x for any given temperature. 



2. In fig. 3 the curve OCDFR is the graph of equation 

 (1), and the straight lines represent equation (3) for given 

 temperatures and fields, the tangent of the inclination of the 



line to the #-axis being given by — == T, and the intercept 



which the line makes with the vertical axis being =^.. The 



intensity of magnetization for any given temperature and 

 field is determined by the intersection of the curve with the 

 straight line since the intensity must satisfy simultaneously 

 both equations. 



Weiss treats H as negligible in comparison with NI (for 

 all but very low values of I), and the straight line therefore 

 passes through the origin. By varying the slope of the line 

 he traces the relation of intensity to temperature, but he 

 leaves the relation of intensity to field-strength out of con- 

 sideration. 



* Langevin, Ann. Chim. Phys. ser. 8, t. v. p. 70 (1905). 

 t Weiss, C. R. t. cxliii. no. 26, p. 1136, Dec. 1906. 



