Mechanism of Radiation. 449' 



Substituting for V the value given by equation (33), we 

 find that a /r is the chemical force, a being proportional to 

 the valency of the atom. 



Hence between two atoms there will be a chemical attraction 

 or repulsion along the line joining their centres, and also a 

 force (proportional to 1/r 3 at most) arising from the coarse- 

 grainedness of the ionic structure. Between two molecules 

 the latter force exists alone. 



This latter force (force of cohesion) will be vanishingly 

 small in a gas, where the distance between adjacent molecules 

 is small compared with scale of coarse-grained ness of atoms, 

 but may become very large in a solid. 



The force consists of a couple and a radial force. From 

 the form of V these are equally likely, if the atoms are 

 placed at random, to be in either direction. But if the atoms 

 (or molecules) are permanently near one another, the couple 

 will turn them into a position in which this potential is a 

 minimum (and therefore, speaking roughly, negative), and 

 the radial force dV/dr is now attractive. 



Electrodynamical Forces. — Permanent and Induced 

 Magnetism. 



§ 38. In addition to the forces just considered, there will 

 be electrodynamical forces arising from the motions of the 

 atoms or molecules. 



Each molecule of a solid will, in addition to its internal and 

 translational vibrations, possess a rotational vibration in the 

 field of intermolecular force. As we pass to greater amplitudes 

 of this vibration, we shall come to a point at which a vibration 

 through a finite angle gives place to a complete rotation. The 

 rotation is not of uniform velocity, since irregularities are 

 introduced by the field of force. In a rotating atom or 

 molecule we have the rotation of a system of charges of 

 electricity. The net result is therefore equivalent to a circular 

 current of electricity in the direction of rotation. 



Hence, in the case of vibration, the result is the emission 

 of electromagnetic waves ; but the force on the atom or 

 molecule varies in direction at each half-phase. 



In the case of complete rotation the current and forces are 

 of constant direction : the atom or molecule may he regarded 

 as a permanent magnet of moment proportional (except for 

 irregularities) to the angular velocity, and of axis coinciding 

 with axis of rotation. 



In general, the axes of these molecular magnets will be 

 distributed at random, so that the solid will exhibit no mag- 

 netic properties. 



