THE NATURE OF ELECTRICITY. 37 



There is no reference, in this supposed case, to any 

 visible stratification of a crystal, as the experimental 

 ray is usually admitted at right angles to such stratifi- 

 cation ; the reference is to an invisible arrangement of 

 the molecules. 



The same course of reasoning is applicable to heat, 

 or to electricity, but the phase of the heat wave, or 

 the electric wave, may be different from that of the 

 wave of light, so that a substance opaque to light, as 

 copper, might allow the free passage of heat or electric- 

 ity ; or a substance transparent to light, as glass, 

 might obstruct their passage. 



And the medium in which the electric energy travels 

 may be the ether which is supposed to pervade the 

 different kinds of matter ; or the matter itself, in any 

 of its various forms, solid, liquid, or gaseous : as it 

 has been shown that undulations' may take place in 

 each of them. 



CONDUCTIVITY FOR HEAT AND ELECTRICITY COM- 

 PARED. It is very remarkable, and must be something 

 more than mere coincidence, that conductivity for heat 

 and electricity is nearly the same in the same sub- 

 stances. A good heat conductor is a good electric 

 conductor; a non-conductor of heat is a non-conductor 

 of electricity. So that if we know either the conduc- 

 tivity or resistance of any substance for heat, we 

 have, practically, its conductivity or its resistance for 

 electricity. This will appear from the table follow- 

 ing, showing the results obtained by Wiedmann and 

 Franz. 



Hence if heat and light are modes of motion, travers- 

 ing various substances by undulations, we are warranted 

 in assuming the same with reference to electricity. 



