238 Electric Inertia and the Inertia of Electric Convection. 



found to be about 10~ 10 , the corresponding value of n being 

 about 1*6. Hence /// = 10 -11 approx. This value does not 

 differ very materially from 2 x 10 ~ 12 , which is the estimate of 

 /j, which has been made for solid conductors in § 6. As the 

 latter estimate depends on the cube of molecular distance 

 which was assumed to be 10~ 24 , the difference between the 

 two numbers falls within the possible errors of estimation. I 

 do not, however, attach much importance to the apparent 

 equality of the numbers and mention it only as a remarkable 

 coincidence, which probably is accidental. For the quantity 

 called fi diminishes with increasing distance between the 

 molecules, and for gases at atmospheric pressure would be 

 50,000 times smaller than for liquids. There would therefore 

 be a very wide discrepancy between fi and /// in the case of 

 gases. Should the coincidence between /j, and /// in solids 

 prove to be more than accidental it would prove that the 

 greater part of the kinetic energy in a luminous vibration 

 traversing a transparent solid is accounted for by the kinetic 

 energy of the electrons attached to the molecules and set in 

 motion by the vibration. This proposition is obviously not 

 true in the case of gases, but may hold for solids. 



It would be of some interest to discuss the effects of 

 metallic reflexion in connexion with the equations which are 

 given at the beginning of this paragraph. H. A. Lorentz* 

 has already introduced a term depending on inertia in the 

 equations of motion of light, and pointed out that without 

 such inertia the electromagnetic theory of light could not 

 explain the known experimental facts. But even the inertia 

 term introduced by Lorentz, was not sufficient to account for 

 all the discrepancies between theory and experiment. My 

 equations differ from those of Lorentz by the introduction of 

 two constants fi and /// which need not be identical, for there 

 is no a priori reason why the inertia of the conduction current 

 should be the same as that of the displacement current. The 

 numerical results of Lorentz's investigation are not easily 

 interpreted, as he used the Helmholtz form of the equations, 

 which involves a large and unknown coefficient. 



Attention may be drawn in conclusion to several papers bv 

 P. Drude f " On the Electron-theory of Metals." 



* Zeitsehrift fur Math. u. Physik, vol. xxiii. p. 197 (1878). 

 f Annalen der Physik, vol. i. p. 566, vol. iii. p. 369, and Physik. Zeit- 

 sehrift, ol. i. p 1 61 . 



