546 JAMES CLERK MAXWELL. 



and part of the energy will be dissipated as heat instead of 

 being imparted to the external layer of cells. The disturb- 

 ance will therefore continually diminish as it is propagated, 

 until it very soon becomes insensible. The behaviour is the 

 same as that of a driver and follower connected by a differential 

 wheel, whose epicyclic motion is retarded by forces of the 

 nature of friction. Hence electromagnetic disturbances can- 

 not be propagated in conductors of electricity, and we there- 

 fore infer that all true conductors are opaque to light. 



The transparency of electrolytes, such as saline solutions 

 and the like, offers no difficulty in the face of this conclusion, 

 as the transference of electricity in them is by a process 

 entirely different from true conduction and more allied to 

 the convection of heat, but Maxwell pointed out that the 

 transparency of gold leaf is much greater than the theory 

 would indicate. Thus the resistance of a particular piece of 

 gold leaf was such that it ought to transmit only 10~ 50 of 

 the light incident upon it, which would be totally impercep- 

 tible, while the amount of green light actually transmitted 

 by it was easily perceived. This result Professor Maxwell 

 could reconcile with the theory only by supposing "that 

 there is less loss of energy when the electromotive forces are 

 reversed with the rapidity of the vibrations of light than 

 when they act for sensible times, as in our experiments." 



We have seen that the velocity of transmission of an 

 electromagnetic disturbance in any medium is expressed by 

 the quotient of the square root of the elasticity divided by 

 the square root of the density of the dielectric. We have 

 learned that the elasticity is inversely proportional to the 

 specific inductive capacity of the medium while the density 

 corresponds with the magnetic permeability. Hence we 

 infer that the velocity of transmission of an electromagnetic 

 disturbance varies inversely as the square root of the 

 specific inductive capacity, and also inversely as the square 

 root of the magnetic permeability of the dielectric, and this 

 must be true for the velocity of light if light be an electro- 

 magnetic disturbance. Now the magnetic permeability of 

 most transparent media, such as glass, quartz, sulphur, hydro- 



