Inductive Capacity of Transparent Insulating Media. 243 



may be briefly summarized thus: — Some bodies (such, for 

 example, as hydrocarbon oils* and paraffin-wax) agree with 

 Maxwell's law so well that the coincidence cannot be attri- 

 buted to chance, but certainly points to an element of truth 

 in the theory: on the other hand, some bodies, such as glass t 

 of various kinds, fluor-spar^:, Iceland sparj, and the animal 

 and vegetable oils § , have specific inductive capacities much 

 greater than is indicated by their refractive indices. 



How do these latter results really bear on Maxwell's theory? 

 The facts are these. Takiug the case of one substance as 

 typical, the refractive indices of light flint-glass are very accu- 

 rately known, the period of disturbance ranging from -r— - — 5-^ 

 j 1 8 B 4'0x 10 14 



second to =^ — ^n second ; the specific inductive capacity is 



known to be about 6*7, the time of electrical disturbance being 

 from jy^oo second to a few seconds. If from the observed 

 refractive indices we deduce by a formula of extrapolation the 

 refractive index for very long waves, we find that its square 

 is about one third of 6' 7. There can be no cpiestion about the 

 accuracy of the observed refractive indices; and I have myself 

 no doubt about the specific inductive capacity ; but formulge 

 of extrapolation are always dangerous when used far from the 

 actual observations. If Maxwell's theory is true, light flint- 

 glass should be perfectly transparent to radiations having a 

 wave-period of, let us sayjjy^o^ second; because this glass is sen- 

 sibly a perfect electrical insulator, its refractive index for such 

 waves should be about 2 - 6. Are there any facts to induce us 

 to think such a thing possible ? It is well known that in some 

 cases strong selective absorption of light in the visible spec- 

 trum causes what is known as anomalous dispersion ; that is 

 to say, the body which presents such selective absorption of 

 certain rays has a refractive index abnormally low for waves 

 a little shorter than those absorbed, and an index abnormally 

 high for waves a little longer than those absorbed ||. 



Light flint-glass is very transparent through the whole 

 visible spectrum, but it is by no means transparent in the infra- 

 red. If the absorption in the infra-red causes in light flint- 

 glass anomalous dispersion, we should find a diminished refrac- 



* Silow, Pogg. Ann. 1875, p. 382 ; 1876, p. 306. Hopkinson, Phil. 

 Trans. 1881, part i. p. 371. 



t ' Cavendish Researches/ edited by Clark Maxwell ; Schiller, Pogg. 

 Ann, 1874, p. 535; TVullner, Site. 1: layer. Abaci. 1877, p. 1; Hopkinson, 

 Phil. Trans. 1878, part i., 1881, part ii. 



X Rornich and Ncnvak, Wiener Sits. Bd. lxs. part 2, p. 380. 



§ Hopkinson, Phil. Trans. 1881, part ii. 



|| 'Theory of Sound,' by Lord Rayleigh, vol. i. p» 125. 



