NO. II STRUCTURE OF THE ATOM PARSON 63 



Again, while the theory gives a good account of the magnetic 

 changes that may be expected to accompany chemical changes, it has 

 up to the present yielded very little in explanation of ferromagnet- 

 ism. This phenomenon, after all, is generally recognized to be due 

 to a fortuitous alignment, which can occur only in favorable circum- 

 stances, of the magnetic effects of separate atoms or molecules ; and 

 the problems connected with it, when regarded from the present 

 fundamental point of view, are of a higher order of difficulty 

 altogether than the problems of paramagnetism : the two stand in 

 much the same mutual relation as the problem of the structure of a 

 solid bears to that of a simple molecule. This has been emphasized 

 by Curie and Weiss. 



In the study of the magnetic properties of gross matter, the most 

 important step in recent years is recognized to have been the formu- 

 lation of Curie's laws : i . Ferromagnetic substances have a transition 

 point, at different temperatures for different substances, at which 

 they lose most of their moment and become merely paramagnetic 

 (the " Curie point "). This transition is compared to that between 

 a liquid and its vapor. The moment of a ferromagnetic substance is 

 not proportional to the field intensity, but reaches a maximum value, 

 called the saturation value. (These things had long been appreciated 

 in the case of Iron.) 2. The susceptibility (k) of a paramagnetic 

 substance is inversely proportional to the absolute temperature (T) 

 — so that kT is a constant (the " Curie constant ") : k is inde- 

 pendent of the field intensity. 3. The susceptibility of a diamag- 

 netic substance is independent of both the temperature and the field 

 intensity. 



These results led to a consistent theory of " external " magnetic 

 phenomena based on Langevin's electronic orbit as the unit upon 

 which the field acts. Diamagnetism, due to currents induced in these 

 orbits, would be expected to be independent of the temperature, 

 because for other reasons, such as the constancy of wave-lengths of 

 spectrum lines, the interior of an atom is supposed to be not much 

 affected by atomic collisions. Paramagnetism is obtained when the 

 atoms of a substance have a magnetic moment great enough to out- 

 weigh the ever present diamagnetic effect : molecular vibrations are 

 bound to interfere with the orientation of these atoms by the external 

 field, and in this way the temperature relation found by Curie has 

 been explained by Langevin (loc. cit., §2). Ferromagnetism, only 

 possible below a critical temperature, is the state in which the dis- 

 turbing effect of the vibrations is overcome and the atoms alisrn them- 



