﻿68 Dr. C. V. Burton on the 



for sensible times, as in our ordinary experiments.'' Now we 

 have seen that conduction is not a perfectly continuous 

 phenomenon, but is due to innumerable encounters among 

 perfectly conductive particles, and without entering upon any 

 calculations (which indeed would be a difficult matter) we 

 can see that there are, broadly speaking, two reasons why the 

 opacity of metals is so much smaller than is indicated by 

 Maxwell's analysis : these are, heterogeneity of structure and 

 intermittence of contact. 



To realize the influence of heterogeneity of structure with- 

 out the complication of intermittent contacts, take the case 

 of a metal at the absolute zero of temperature. We have 

 then virtually to deal with a network of perfect conductors, 

 constituting a body which as a whole has perfect conductivity, 

 and of which even an excessively thin film would be an effectual 

 barrier to electromagnetic waves, provided that the wave- 

 length were great enough to justify us in treating the metal 

 as homogeneous. But if we consider an extreme case, where 

 the wave-length of the disturbance is negligible in comparison 

 with the dimensions of a single conductive particle, a very 

 thin layer of the metal will be far from absolutely opaque. 

 For the conditions of the problem will then be the same as 

 if we had ordinary luminous radiations obstructed by an 

 agglomeration of perfectly reflecting bodies of appreciable 

 size. Of course these extreme conditions are not realized in 

 the case of the light transmitted by a metallic film ; but if we 

 may suppose that the diameter of a conductive particle is not 

 quite negligible in comparison with a wave-length of light, it 

 is clearly to be expected that very thin layers of the metal 

 will fall short of that absolute opacity which in this case 

 would follow from the assumption of homogeneity. 



When we pass to the consideration of metals at ordinary 

 temperatures, the conductivity for steady currents is finite ; 

 but for electromagnetic waves of short period we cannot even 

 treat the metal as an agglomeration of finitely conductive 

 particles continuously in contact with one another. It is 

 evident that the shorter we make the period of the electro- 

 magnetic disturbance in comparison with the average inter- 

 collisionary period of a (perfectly) conductive particle, the 

 more nearly do the particles act as if permanently insulated 

 from one another, and the less efficiently does the metal per- 

 form the functions of an electromagnetic screen. 



Further considerations might be added concerning the 

 average interchange of electrification between colliding par- 

 ticles when the electromotive intensity tending to produce 

 such interchange is very rapidly alternating ; but enough has 



