Physics. — ‘Note on the paramagnetism of solids.” By Prof. P. 
EHRENFEST. 
(Communicated at the meeting of December 18, 1920). 
1. The object of this note is to show that the validity of the 
Curir-LANGEVIN law for the susceptibility of solid paramagnetic sub- 
stances may be arrived at on a different theoretical basis from that 
discussed by Welss') Srern*) and Lrnz*) (comp. section 3 of this 
paper), namely on the following assumption : the atoms or molecules 
of a solid paramagnetic substance contain electrons circulating in 
definite, practically fixed orbits of lowest quantum-number (which 
we shall call ‘‘rest-orbits’’), so that, in the absence of an external 
magnetic field, there is no perceptible difference in the energy of 
the two opposite (right and left) directions of circulation. It will be 
further assumed, that with a magnetic field at a given tempera- 
ture 7’ the statistical distribution between right and left which 
corresponds to MH and 7’‘) will automatically establish itself. On 
these assumptions the CUrIr-LANGEVIN law will be found to hold 
c.f. § 4), and, in the case of a crystal symmetry or of acrystalline 
powder of any crystalline structure, even with the correct numerical 
factor 3 in the denominator. 
2. LANGEVIN's theory explains the fact, that paramagnetic gases 
1) P. Weiss, C. R. 156 (1913) 1674. 
3) O. Stern. Z. f. Phys. 1 (1920) 147. 
3) W. Lenz. Phys. Zschr. 21 (1920) 613. 
4) If the motions of the electrons are not submitted to any limitations by means 
of quantum-conditions and if the law of thermal statistics is applied to all the 
degrees of freedom, the body is found to be wnmagnetic; comp. H. A. Lorentz. 
Vortr. kinet. Theorie der Mat. u. Elektr. p. 188. (Teubner 1914); H. J. van LEEUWEN, 
Vraagstukken uit de elektronentheorie van het magnetisme p. 54. [Dissertation 
Leiden 1919). Accordingly in his theory of paramagnetism LANGEVIN assumes, 
that the ampère currents, which represent the elementary magnets, are not sub- 
jected to thermal statistics in their cyclic co-ordinates. But when, as in our case, 
quantum conditions are introduced into the statistic scheme, the possibility of 
paramagnetism returns, even if not a single degree of freedom remains outside 
the scheme. To this circumstance my attention was drawn by Prof. N. Bork in 
a conversation (1919). 
