149-152] Anisotropic Media 133 



composing it a number of charged electric particles, or electrons. It is to 

 the motion of these that the conduction of electricity is due. In a dielectric 

 there is no conduction, so that each electron must remain permanently 

 associated with the same molecule. There is, however, plenty of evidence 

 that the electrons are not rigidly fixed to the molecules but are free to move 

 within certain limits. The molecule must be regarded as consisting partially 

 or wholly of a cluster of electrons, normally at rest in positions of equilibrium 

 under the various attractions and repulsions present, but capable of vibrating 

 about these positions. Under the influence of an external field of force, 

 the electrons will move slightly from their equilibrium positions we may 

 imagine that a kind of tidal motion of electrons takes place in the molecule. 

 Obviously, by the time that equilibrium is attained, the outer surface of the 

 molecule must be an equipotential. This, however, is exactly what is required 

 for Mossotti's hypothesis. The conception of conducting spheres supplies 

 a convenient picture for the mind, but is only required by the hypothesis in 

 order to make the surface of the molecule an equipotential. We may now 

 replace the conception of conducting spheres by that of clustered electrons 

 by this step the power of Mossotti's hypothesis to explain dielectric phenomena 

 remains unimpaired; while the modified hypothesis is in agreement with 

 modern views as to the structure of matter. 



On this view, the quantity a tabulated in the sixth column of the table 

 on p. 132, will measure the radius of the outermost shell of electrons. Even 

 outside this outermost shell, however, there will be an appreciable field of 

 force, so that when two molecules of a gas collide there will in general be a 

 considerable distance between their outermost layers of electrons. Thus if 

 the collisions of molecules in a gas are to be regarded as the collisions of 

 elastic spheres, the radius of these spheres must be supposed to be con- 

 siderably greater than a. Now it is the radius of these imaginary elastic 

 spheres which we calculate in the Kinetic Theory of Gases : there is therefore 

 no difficulty in understanding the differences between the two sets of values 

 for a given in the table of p. 132. 



It is known that molecules are not in general spherical in shape, but, as 

 we shall now see, there is no difficulty in extending Mossotti's theory to cover 

 the case of non-spherical molecules. 



ANISOTROPIC MEDIA. 



152. There are some dielectrics, generally of crystalline structure, in 

 which Faraday's relation between polarisation and intensity is found not 

 to be true. The polarisation in such dielectrics is not, in general, in the 

 same direction as the intensity, and the angle between the polarisation and 

 intensity and also the ratio of these quantities are found to depend on the 



