Mechanism of Radiation. 451 



through a position in which their motion is opposed by an 

 intermolecular couple greater than the gyrostatic couple 

 caused by their angular rotation. In the former and more 

 general case, we have residual magnetism; in the latter, 

 permanent magnetism. 



II. A further effect of the external field H is seen upon 

 considering that the rotation of the molecules gives rise, 

 virtually, to an electric current. If the current were real, 

 the addition of the external magnetic force would give rise 

 to an induced current : in the present case it gives rise to a 

 retardation of the angular velocity about the lines of magnetic 

 force, the velocity being regarded as a vector. It may there- 

 fore be regarded as giving rise to a system of magnets of which 

 the direction is opposite to that of the magnetic force. 



Hence diamagnetism may be accounted for in the manner 

 of Weber (Maxwell, Treatise, § 838). 



§ 39. The great range of values which is known to be 

 possible for K, the coefficient of magnetic induction, can 

 easily be accounted for. Suppose (for the sake of simplicity) 

 that the rotation of a molecule is a " normal " degree of 

 freedom. This will be specified by a subsidiary temperature; 

 and if a be the mean energy, the law of distribution may for 

 illustrative purposes be taken to be 



E* ds/V 



where E is the energy of the rotation (proportional to nF). 



Let b be the value of E at which the motion changes from 

 oscillatory vibrations to complete revolutions. 



In a field of force H a couple acts on this magnet which is 

 proportional (as regards H and w) to Hw. If is the angular 

 displacement produced, the couple is also proportional to Ow 2 . 

 Hence is proportional to H/w. This displacement may be 

 regarded as the creation of a new magnet, proportional to 

 (H/w>) x w, and therefore proportional to H, and independent 

 of w. The total induction is therefore proportional to 



H I E* «d*/E; 



and therefore to the area of the curve 



which is on the right of the ordinate 



