385 



dT _ öj _ 7' dö 

 ^H^~~^~~^dï\j ••.... (3) 



Here c^ is the specific heat for constant field. This equation 



shows that the thermic effect in question is greatly dependent on 



do 

 , hence becomes abnormally high in the iieigbourhood of the 



OJ H 



Curie-point, and this is what we intended to prove. 



da 



— being <^ 0, the temperature increases during the magnetisa- 

 tion, and reversely. According to the above-mentioned experiments 

 c would suddenly assume a lower value at the passing of the 

 Curie-point in upward direction, but this does not affect the con- 

 clusion drawn. The relation found is independent of Weiss' hypo- 

 theses, and sets forth the inter-relation between heat-effect and 

 disappearance of ferro-magnetism more clearly than the equations 

 given by Weiss and Piccard. 



When, with Weiss and Piccard, o is taken as independent vari- 

 able, the following is found from (1): 



dT _dH^TdH 

 dOr, dr^cr c„ d7\ 



a 

 Above the Curie-point — (7" — <9) = C, where <9 and C are con- 



U 



dH a ^ 

 stants, so that — ;-;- = — . On substitution of this in (4) an equation 

 1^ o 



is obtained which also occurs in the cited paper, but which is 



strictly proved here without having recourse to Weiss' special theory. 



It will be vainly tried to estimate the said change of the specific 



heat at the Curie-point purely thermodynamically. Thermodynamics 



gives, indeed, rhe change of ch with the value of H (resp. of 



c<T with a), and the difference between c^ and c,, but not the 



dependence on the temperature in question. To find this a molecular 



theory like that of Weiss, is indispensable. From this an expression 



for the internal energy g will have to be derived, and also for the 



ds de da 



c's, because c^ = - — and c„ = = U -— . 



dT, " dTff dTff 



In connection with the preceding paper I may be allowed to add 

 a few lemarks. 



The late Professor Kuenen had the intention to make a commu- 

 nication on the subject mentioned in the title at the Meeting of the 



25* 



