627 



of deviations of lliis tvpe. Yet it is not lo he expected a priori that 

 Langevin's tiieorj would l)e followed in this case, for this theory 

 has been deduced for a gas with perfect rotational freedom of the 

 molecules and starts from the assun)ption of the equipartitioii of 

 energy in al degrees of freedom. Now the case of powdered gado- 

 linium sulphate at low temperatures does not correspond to either 

 of these assumptions. It is true that Langevin's theory has been 

 extended by Wmss') to powdered crystals, but Weiss confines him- 

 self to small values of the parjimeter (( ; on the other hand Ehhen- 

 fest') has develo[)ed a theory in which the relatition (2) is obtained 

 for crystal powdeis on the assumption of the existence of quanta 

 but then the saturation magnetisation is only half the value corres- 

 ponding to perfect parallelism of all elementary magnets and in the 

 preliminary experiments a higher vahie seemed to be reached. 



Coiitirmation and extension of the preliminar'y results was thus 

 very desirable; the same method has been followed as in the 

 previous work: the specific maguetisaiiou, it, is calculated from the 

 force F (in grammes] exerted ou the mass in by an inhomogeneons 



r) H 

 magnetic field with aid of the formula /''</ = /»(t — . A detailed 



oz 



study of the apf)aratus, the corrections and the sources of error, a 



comprehensive account of which has been given in the pieceding 



communication '), has made it possible to ailaiii a much greater 



accuracy than in the previous work, at least as far as the magnetic 



measurements are concerned. The (ietermination of the temperature 



from the va[tour pressure of the iiatli is still a weak point, especially 



since the vapour [)ressure law is as yet not sufficiently well 



known ^). The research relates lo the same tubes, Gd J and Gdll, 



that have served for the research in liquid hydrogen and that have 



been mentioned in the preceding communication (§ Sc). 



^ 2. Observations. The direct results of the observations may be 

 given first: tables I and II (/ being the number of ampères in the 

 magnet coils; H„ the field strength, in gauss, in the centre; F the 

 force in grammes, on the total mass of substance). 



With Gdll between the points N°. Jo aud N°. 28 jioints have 

 been left out in which the observations have been taken al increas- 



1) P. Weiss, Paris C. R. 156 (1913) p. 1674. According to 0. Stern (Zs. f. 

 Phys. 1 (1920) p. 147) Weiss' deduction is not sound. 



') P. Ehrenfest, these Proceedings 23, p. 989; Leiden Coram. Suppl. N". 44b. 

 ') H. R. WoLTJER, these Proceedings p. 613; Leiden Comm. No. 167/;. 

 ^) I.e. § Se. 



41* 



