( 651 ) 
meeting l 2 ), H. du Bois and Honda have, it is true, shown that this 
law of the dependence of susceptibility upon temperature is not 
generally valid, and that paramagnetism also occurs which is inde¬ 
pendent of the temperature or increases with increasing temperature. 
But it is by no means the case that the foregoing hypotheses should 
be discarded on that account; what we learn from experiment in 
this case is only that these suppositions are not sufficient to explain 
magnetism as a whole. In particular it will be necessary to revise 
Langevin’s hypothesis that the magnetic moment of a molecule is 
constant, or at least quasi-constant, and also that concerning the 
nature of the mutual action of the molecules, which until now has 
been represented by the introduction of the molecular field. For an 
at low estimation of the value of both of these hypotheses, experi¬ 
ments temperatures are especially valuable. 
For it is only at the absolute zero that the magnetization gives 
the sum of the molecular magnetic moments, as it is only then that 
heat-motion can no longer prevent the magnetization from attaining 
its full value; and at low temperatures, too, is the strongest demon¬ 
stration of the mutual action of the molecules to be expected, since 
they are then at the smallest possible distance from each other. 
b. Ferromagnetic substances. We have, therefore, aimed at the 
continuation of the curves connecting magnetization and temperature 
in the three ferromagnetic substances and in magnetite down to the 
neighbourhood of the absolute zero. By utilising the methods and 
appliances s ) suitable for long-continued Accurate measurements at such 
constant temperatures as are obtainable with liquid hydrogen, we have 
been able in our measurements to reach a temperature of 20°,3 K. with 
hydrogen boiling under atmosphere pressure, and of 14°,0 K. with 
hydrogen near its melting point. The number of degrees on the absolute 
scale which separate these experimental temperatures from the absolute 
zero is but such a small fraction of the number between the absolute 
zero and the CuRiE-point (even in the case of nickel this number is 
still so much as 648 Kelvin degrees) that, considering the nature of 
the curves, we may regard the saturation-magnetization at the absolute 
zero as being determined by our experiments. All this, of course, 
with the proviso that the phenomenon in the region to w hich extra¬ 
polation is carried should give no occasion for adopting another point 
of view. Since the object of the measurements was a determination 
of the satu ration-magnetization, it seemed suitable to direct the expe- 
1 ) These Proceedings Jan. 1910. 
2 ) H. Kamerlmgh Onnes, these Proceedings Sept. ’06, Comm. Leyden N°. 94/. 
