XXII I'KITM 1. 



In a subsequent part of the memoir he proceeds to establish from these premises 

 the gent-nil equations of the Field and obtains the usual formula; for the mechanical 

 force on currents, magnets and bodies possessing an electrostatic charge. 



He also returns to and elaborates more fully the electromagnetic Theory of Light. 

 His equations shew that dielectrics can transmit only transverse vibrations, the speed of 

 propagation of which in air as deduced from electrical data comes out practically identical 

 with the known velocity of light. For other dielectrics the index of refraction is equal 

 to the square root of the product of the specific inductive capacity by the coefficient of 

 magnetic induction, which last factor is for most bodies practically unity. Various comparisons 

 have been made with the view of testing this deduction. In the case of paraffin wax and 

 some of the hydrocarbons, theory and experiment agree, but this is not the case with 

 glass and some other substances. Maxwell has also applied his theory to media which 

 are not perfect insulators, and finds an expression for the loss of light in passing through 

 a stratum of given thickness. He remarks in confirmation of his result that most good 

 conductors are opaque while insulators are transparent, but he also adds that electrolytes 

 which transmit a current freely are often transparent, while a piece of gold leaf whose 

 resistance was determined by Mr Hockin allowed far too great an amount of light to 

 pass. He observes however that it is possible "there is less loss of energy when the 

 electromotive forces are reversed with the rapidity of light than when they act for sensible 

 times as in our experiments." A similar explanation may be given of the discordance 

 between the calculated and observed values of the specific inductive capacity. Prof. J. J. 

 Thomson in the Proceedings of the Royal Society, Vol. 46, has described an experiment by 

 which he has obtained the specific inductive capacities of various dielectrics when acted 

 on by alternating electric forces whose frequency is 25,000,000 per second. He finds that 

 under these conditions the specific inductive capacity of glass is very nearly the same as 

 the square of the refractive index, and very much less than the value for slow rates of 

 reversals. In illustration of these remarks may be quoted the observations of Prof. Hertz who 

 has shewn that vulcanite and pitch are transparent for waves, whose periods of vibration are 

 about three hundred millionths of a second. The investigations of Hertz have shewn that 

 electro-dynamic radiations are transmitted in waves with a velocity, which, if not equal to, is 

 comparable with that of light, and have thus given conclusive proof that a satisfactory 

 thi-ory of Electricity must take into account in some form or other the action of the 

 dielectric. But this does not prove that Maxwell's theory is to be accepted in every 

 particular. A peculiarity of his theory is, as he himself points out in his treatise, that 

 the variation of the electric displacement is to be treated as part of the current as well 

 as the current of conduction, and that it is the total amount due to the sum of these 

 which flows as if electricity were an incompressible fluid, and which determines external 

 electrodynamic actions. In this respect it differs from the theory of Helmholtz which 

 al-o takes into account the action of the dielectric. Professor J. J. Thomson in his 

 Review of Electric Theories has entered into a full discussion of the points at issue 



