Equation of State to Magnetism. 381 



The Curie temperature, however, is the temperature at 

 which the substance loses its ferromagnetic properties. 

 Above the Curie temperature the magnetization always is 

 zero at an exterior field zero. Thus remanent magnetism 

 cannot exist in this slate. If we put 7 = in the reduced 

 equation we get a quadratic equation in 77 with roots: 



The highest temperature at which remanent magnetism is 



.27 



possible therefore is 3= ^, and not 3=1. , 



So we find for the Curie temperature a. value different from 

 the critical one given by the equation, which temperature is 

 analogous to the critical temperature of a fluid. Besides, in 

 his paper, " Proprietes Magnetiques " *, Curie lays stress on 

 the fact that the analogy between the two phenomena cannot 

 be pushed too far, because the conformity between p=/(P) 

 and I=/'(H) at constant temperature is doubtful; whereas 

 this conformity must exist if for a ferromagnetic substance 

 equation (1) holds good. 



Yours faithfully, 



Leiden, November 1916. EvA BRUINS. 



Remarks on Miss Bruins' Communication. 



By J. R. ASHWORTH. May 1917. 



THE distinction between the temperature at which a 

 ferromagnetic substance loses its ferromagnetic pro- 

 perties when the field is zero, and when it has the critical 

 value to which Miss Bruins very properly draws attention, I 

 have considered in a paper which appeared in this Magazine 

 in April of this year on the treatment of: I = </>(H). For 

 residual magnetism the temperature of disappearance of 



ferromagnetism theoretically is 777-. T c or - D ,°, instead of 



S a'l 

 — -rj^'i further, the residual intensity of magnetization, 



according to the equation, should fall to - I as the tempe- 



21 * 



rature rises to ^ T c , and then become zero, while under the 



critical field the intensity should fall to 5 I as the tempe- 



rature becomes T c , and afterwards, at higher temperatures, 

 fall towards zero asymptotically. 



* Annates de Chimie et de Physique, S. 7, t. v. p. 404 (1895). 

 Phil. Mag. S. 6. Vol. 34. No. 202. Oct. 1917. 2 D 



