990 
and solutions of paramagnetic salts obey Curie's law, in these cases, 
taking account of the smallness of 
ull 
a= 1 
rT’ ( ) 
it gives at the limit the Curte-LANGEVIN relation 
Nw’ 
ar (2) 
3r 7 
where X = molair susceptibility; MN = AvoGapro’s number 7 = —= 
N 
BoLTzMANN’s constant, j= magnetic moment of one molecule. Ex- 
periment shows that this relation also holds for certain solid para- 
magnetic substances. In particular KAMERLINGH Onnes has found that 
gadolinium sulphate follows the law exactly down to the lowest 
(Helium) temperatures. This is not what one would expect in view 
of the assumption underlying the derivation of the formula, namely 
that the orientation of the elementary magnets depends exclusively 
on: (1) the thermal motions, (2) the directing influence of the external 
field H. In solid (crystalline) bodies there must be additional forces 
of a different nature which also play a part in the orientation of 
the elementary magnets. 
3. lt follows from the papers by Weiss, Stern, and Lenz, that in 
the case of forces of that kind which depend on a crystalline structure 
the same relation is obtained, if only it is assumed that: 
1. The potential energy ® of the forces which try to keep the 
axis of an elementary magnet parallel to a definite direction & is 
centrally symmetrical, i. e. it is equal for any two opposite orien- 
tations of the elementary magnets. 
2. When the temperature or the field changes, the statistical 
distribution of the orientations, which according to BoLTZMANN 
corresponds to the new values of 7’ and H, and to ®, actually 
establishes itself; this involves, that there is no retardation in the 
necessary reversal of the elementary magnets (false equilibrium). 
4. Let us now consider a solid body containing electrons whose 
“rest-orbits’” satisfy the conditions mentioned in § 1. Fora particular 
electron let the magnetic moment of its orbit be u and its projection 
on the direction of the field + g cos 9, according to whether it 
circulates to the right or left). At the temperatures considered we 
1) Strictly speaking u itself depends on the orientation of the orbit relatively 
to the field H, since the velocity of the electron is affected by it. In a magnetic 
field it is not simply the mechanical momentum of the electron that is to be 
