438 



a. For some substances (crystallized ferrous-sulphate, crystallized 

 manganese sulphate etc.) the relation x^^ constant was found to hold 

 as far down as the temperature of liquid nitrogen, but at hydrogen 

 temperatures ^/^T was always found smaller than would follow from 

 Curie's law. 



b. With other substances (e, g. anhydrous manganese sulphate) the 

 product yi^T began to diminish at once when the temperature was 

 lowered below room-temperature; in that case the relation x( ^4- A) = 

 constant appeared I o hold for the range between ordinary tempera- 

 ture and nitrogen-temperatures ; in liquid hydrogen, however, further 

 deviations became apparent. 



c. With platinum the susceptibility was found to change very 

 little with temperature and in the range between liquid nitrogen 

 and liquid hydrogen to be almost independent of the temperature, 

 entirely' in accordance with the results of Honda and Owen who 

 found the product 'ji^T for platinum (and for many other elements) 

 to diminish continually with falling temperature. 



§ 2. Langevin ^) in his kinetic theory of paramagnetism has given 

 a theoretical deduction of Curie's law. He finds, that the suscepti- 

 bility is inversely proportional to the kinetic energy of rotation of 

 the molecules. When this energy is taken proportional to the absolute 



temperature, this gives x proportional to — or tT = constant, i.e. 



Curie's law. 



§ 3. It seems natural to try an explanation of the deviations from 

 Curie's law by dropping the assumption that the rotational energy 

 U of the molecules is proportional to the absolute temperature and 

 adopting a different temperature relation for U'^). 



In a different connection Einstein and Stern ^) were led to the 

 assumption that the rotational energy of a molecule with two degrees 

 of freedom is equal to 



hv 1 



+ ^hv (1) 



elcT_i 



1) P. Langevin, Ann. chim. phys. (8) 5, p. 70; 1905. 



2) Kamerlingh Onnes and Perrier ascnbed the deviations from the law to 

 the electrons freezing to the atoms, in consequence of Planck's vibrators coming 

 to rest. In that case, if the rotational velocities remain proportional to l^T, the 

 susceptibility approaches a constant value. In this paper a theory is developed in 

 which the elementary magnets themselves remain unchanged. 



=5) A. Einstein and 0. Stern, Ann. d. Phys. 40, p. 551; 1910. 



28 

 Proceedings iloyal Acad. AiiiSteruLim. Vol. XVI 



