722 Application of Van der Waals' Equation to Magnetism. 



molecule, and I m and I mo being saturated values of the 

 intensity for a given temperature and for absolute zero 

 respectively. The ferromagnetic equation gives 



T -rK> ' 



when H is negligible, and at the critical temperature 



a'J 8 



and therefore 



T = 

 c &' 27' 



T_L27/ Ix 

 T c - I 8 V V' 



an expression which principally differs from the one above 

 by exhibiting a region of instability below I = iIo. 



Thirdly, according to the kinetic theory, the addition of 

 an external field to the molecular field leads to the formula 



A=*(Ti-T.), (<y> 



where k is the susceptibility and Ti is a temperature above 

 the critical temperature. 



The ferromagnetic equation can be made to yield the same 

 relation, by similar reasoning. 



Let the equation be written 



R'TI I 



a'I 2 = 



Io-I' 



when H is zero or is small enough to be neglected. If a 

 strong field is applied then T must be increased to T x to 

 preserve I at its former value, thus 



10 — 1 



By subtraction we get 



H = R'I or ^- T (T 1 -T). 



If T = T C , I will be small compared with I , and so we may 

 write 



j =a'(T t ^T.j, 



and hence 



~ =A:(1 1 I c ), 



