66 Mr Oxley, Magnetic Susceptibility with Temperature. 
Assuming the existence of chemically complex systems— 
chemically complex excludes the cases of the ferromagnetic 
elements in which the molecular field of Weiss is not negligibly 
small—it has been shown* that the work of du Bois and Honda 
on the temperature coefficients of paramagnetic and diamagnetic 
substances is not inconsistent with the assumptions at the base 
of the Curie-Langevin theory of magnetism, providing we sup- 
pose that these assumptions apply to substances composed of 
simple molecules only+—such for example as exist in gases at a 
temperature far from the point of liquefaction. Further, it does 
not follow that because the differential coefficient of the suscepti- 
bility with respect to the temperature is positive for some 
paramagnetic elements and that the rate of variation of dia- 
magnetic susceptibility with the temperature is not zero, that 
there is no distinction apart from mere positive and negative 
number between the nature of paramagnetism and that of 
diamagnetism f. 
It is important that the Curie-Langevin theory, which is the 
only quantitative theory of magnetism we possess, should not be 
discredited as failmg to account for individual cases. Each 
element has its characteristic molecular and atomic properties, 
and a general theory cannot be satisfactory unless it admits of 
modification to suit each peculiarity possessed by that element. 
Take the case of tin, which is one of the most complex elements 
from the magnetic point of view. According to du Bois and 
Honda the Curie-Langevin laws of paramagnetism and dia- - 
magnetism do notapply. They find, indeed, that the susceptibility 
of tin changes rapidly when the density changes and also during 
fusion. If we could watch the rotation of the particles of tin, 
under the application of the magnetic field, as the temperature is 
increased, we should probably witness violent disturbances at the 
points where the constitution changes. At one temperature the 
groups of particles are becoming simpler, at another they are 
becoming more complex. So long as the constitution of an 
element does not alter as the temperature is changed, the magnetic 
properties of that element will follow the Curie-Langevin laws, 
the Curie constant per particle having a value dependent upon the ~ 
constitution. For another stable molecular constitution which — 
does not alter over ranges of temperature, there will be a new — 
Curie constant per particle which will now determine the variation — 
of the susceptibility with the temperature. In the transition 
stage the Curie-Langevin laws cannot alone explain the phenomena, 
* A. H. Oxley, Proc. Camb. Phil. Soc., Vol. xvi. p. 486, 1912. 
+ Or to complex groups of molecules whose constitution does not vary over 
wide ranges of temperature. 
+ Loe. cit., p. 490. 
