MAGNETISM 
vu. 3] 
95 
Honda and Okubo * have developed a theory of magnetism 
taking into account the rotational motion of the molecules, and 
have obtained an expression somewhat similar to that of Langevin. 
In dealing with a paramagnetic solid it is necessary to take 
into account the mutual action between the solid molecules in 
forming an estimate of the resultant orientation. The effect 
of taking this action into account may be regarded as equivalent 
to increasing the kinetic energy by a certain amount. If we 
assume further that this amount is a constant independent of 
temperature, the resulting value for the susceptibility takes the 
form 
N 2 M 2 
1 ~ 3 R(T + A) 7 :13 
where A is a constant. 
This relation was given by Onnes and Perrier, and was found 
to hold for a number of solid substances at low temperatures. 
Langevin’s theory of a paramagnetic gas has been extended 
by Weiss f so as to include paramagnetic substances generally, 
and also ferromagnetic substances. For this purpose he 
postulates the existence of a “ molecular field ” proportional to 
the intensity of magnetization acquired. Weiss assumed that 
the Weber elements, whatever they may be, in a ferromagnetic 
solid are free to rotate, but that they are subject, as regards 
rotation, only to constraints arising from the molecular field. 
His hypothesis enables us to co-ordinate a very large number 
of facts, but the nature of the molecular field remains a mystery. 
Weiss himself came to the conclusion that it is improbable that 
it can have a magnetic origin. Thus it appears that the classical 
theory of magnetism is left with two unsolved problems, the 
nature of the molecular field and the nature of the Weber 
elements. 
3. The Quantum Theory of Paramagnetism 
Many attempts have been made to apply the principle of 
the quantum theory in the subject of paramagnetism. Ooster- 
huis X and Keesom,§ in discussing the quantum theory of 
magnetism, proposed to substitute in formula 7 : 11 for the 
classical product &T corresponding to the mean energy of a 
degree of freedom, the quantum expression for the mean rotational 
* Honda and Okubo, Sci. Rep., vol. 7, p. 141, 1914. 
t Weiss, Journ. de Phys., vol. 6, p. 661, 1907 ; Ann. de Phys., p. 134, 
1914. An interesting review of this work on magnetism is to be found 
in Le Magnétisme by Weiss and Foex (Armand Colin, 1926). 
I Oosterhuis, Phys. Zeits., vol. 14, p. 862, 1913. 
§ Keesom, Phys. Zeits., vol. 15, p. 8, 1914.