Mechanism of Radiation. 453 



For a gas this gives 



A=1 + 90/4tt, 



where is the fraction of the total volume which is occupied 

 by atoms. 



It is clear that this is of the right order of magnitude. For 

 oxygen, hydrogen, nitrogen at the density of the air the 

 refractive index is roughly 1*00025 *, giving 



£=1-0005. 



Substituting this value in the formula just found for k (and 

 neglecting the difference between atoms and molecules, since 

 this will not affect the order of magnitude of the quantities 

 concerned), we get = *OOO7. 



Now the density of liquid oxygen, nitrogen, &c, is roughly 

 1000 times that of air, so that the value of just found will 

 be at any rate comparable with the true value. 



Dielectrics, Conductors, and Electrolytes. 



§ 41. It will be seen that the foregoing theory of the 

 specific inductive capacity of a dielectric is virtually that of 

 Mossotti f. The atoms or molecules must not be regarded as 

 perfect conductors, but they arrange themselves so that their 

 external surfaces are equipotentials, and therefore behave like 

 conductors to all external points. 



In a solid body each ion will be in equilibrium under the 

 forces arising from all the ions in the solid. In the case of 

 some substances, the solid may be regarded as a collection of 

 atoms, or molecules, each individual atom retaining its identity; 

 whereas for other substances the body must simply be regarded 

 as a confused mixture of ions which have placed themselves 

 in equilibrium. 



The difference is that between a dielectric and a conductor. 

 If the former body is placed in an electric field, the molecules 

 will arrange themselves so that their surfaces are equipotentials, 

 and we have a dielectric of which the theory of Mossotti gives 

 a good account. 



When a body of the second sort is placed in an electric field, 

 the ions (possibly all ions, or possibly only the ions of the 

 outermost layer) will be free to move from one molecule to 

 another, and will therefore arrange themselves so that the 

 whole surface of the body is an equipotential. In one half of 

 the body there would be an excess of negative ions, in the 

 other half an excess of positive ions : the former is the so- 

 called " induced charge " of negative electricity, the latter of 

 * Preston, < Theory of Light,' p. 137. 

 f Maxwell, Elect, and Mag. § 62. 

 Phil. Mag. S. 6. Vol. 2. No. 11. Nov. 1901. 2 H 



