1128 
liquid mixtures of oxygen and nitrogen (KAMERLINGH Onnes and 
Perrine |. ¢.) on the other are not in favour of these explanations. 
3. a change of the field of force, in which the separate oxygen 
molecules are to be found, by an oppositely acting negative mole- 
cular field (KAMERLINGH ONNES and Perrier, Leiden Comm. N°. 139d) *). 
4. a change in the heat motion. We might for instance suppose 
that this motion no longer follows the laws of equipartition, but 
that, in agreement with the quantum-theory with assumption of a 
zeropoint-energy, at low temperatures it neutralizes the magnetisation 
to a higher degree than would be the case if the equipartition laws 
were still valid. (Krusom, Leiden Comm. Suppl. N°. 36c) ’). 
Without further discussing the arguments for or against the two 
latter ways of explanation, we shall in this paper only treat a 
question put by Prof. KAMrRLINGH ONNEs on occasion of discussions 
held at Leiden on magnetic problems. This question may be formu- 
lated in the following way. Might it not be possible that the occur- 
rence of A in (2) was caused by what is left, when we take the 
statistic mean, from a directing action of the couples of force, 
exerted by the oxygen molecules on each other when, ina magnetic 
field, the molecules come very near to each other? In our conside- 
rations we shall assume that with sufficient approximation the forces 
exerted by the oxygen molecules on each other may be treated as 
forces exerted by electric quadrupoles, as has been proved to be 
1) As to R. Gans, Ann. d. Phys. (4) 50, p. 163, 1916, comp. p. 1134, note 1. 
*) This hypothesis has first been tried by OosrterHuis, Leiden Comm. 
Suppl. No. 31, These Proceedings Vol. XVI (1913), p. 432, for paramag- 
netic salts, afterwards by KEEsomM, Leiden Comm. Suppl. No. 32, These Pro- 
ceedings October 1913, p. 454 and 468, for ferromagnetic substances. More 
recently v. WEIJSSENHOFF (Ann. d. Phys. (4) 49, p. 149, 1916) and especially 
REICHE (Ann. d. Phys. (4) 54, p. 401, 1917) have worked out the hypothesis 
by the methods of the quantum theory which had then been developed much 
further. They found a good agreement with the observations on the suscep- 
tibility of paramagnetic salts, comp. also A. SMEKAL, Ann. d. Phys. (4), 57, 
p. 376, 1918. LANGEVIN. too (Procés-Verbaux et Résumé des Communications 
de la Soc. franc. de physique 1919, p. 18) adheres to this way of explaining 
the deviations from the law of Curie. Especially from the results of the 
investigations on crystal structure with the aid of Röntgen rays we can hardly 
think any longer that in solids molecules rotate, in the heat motion, as a 
whole like elementary magnets in the sense of the “magnetic molecule’, a 
definition of which, in accordance with the ideas of Weiss, has been given 
in Leiden Suppl. No. 32a, p. 11, note 3. Thus we shall have to consider 
parts of the molecule, eventually atoms or parts of atoms, comp. O. STERN, 
Zs. f. Phys. 1, p. 147, 1920. Comp. also W. LENz, Physik. Zs. p. 613, 1920, 
P. EHRENFEST, Leiden Comm. Suppl. No. 446, These Proceedings Vol. XXIII 
(1921), p. 989. 
