372 Mr. 0. J. Lodge on a Mechanical Illustration 



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indicated of the close relation known to exist between electric 

 conductivity and conductivity for heat. They appear to de- 

 pond on two distinct things — the first on the "friction" 

 between the button and the cord, the second on the elasticity 

 of the rods or strings connecting the buttons together. Never- 

 tneless it is an experimental fact that the two apparently di- 

 stinct tilings are inverse functions of one another ; not impro- 

 bably they are, in substances of similar chemical properties, 

 simply inversely proportional to one another. 



§ 19. But now what happens when light foils on an elec- 

 trolyte ? If, indeed, the whole substance were composed, as 

 our model would suggest, of atoms in a nearly disjointed state, 

 it can hardly be doubted that much energy would be lost as 

 light in effecting decomposition. But in reality these free 

 " Clausian " atoms are very few compared with the atoms in 

 firm chemical combination at any one instant ; and all the 

 compound molecules would be highly elastic, and would pro- 

 pagate the light-oscillations perfectly well, the force being by 

 no means sufficient to pull the constituent atoms asunder in 

 most liquids. Hence electrolytes, though conductors, maybe 

 transparent. It may possibly happen that some of the waves 

 are to some extent destroyed by the occasional free atoms, 

 and that the absorption of light in deep transparent liquids is 

 partly due to this. Again, it may happen in some electrolytes 

 that the atoms in the compound molecules themselves are so 

 weakly associated as to be shaken asunder even by light-oscil- 

 lations, especially if their periods are synchronous. This dis- 

 sociation would be especially likely to manifest itself in case 

 one or both of the constituents tended, as soon as free, to 

 come out of the fluid in the solid state ; and accordingly it 

 is observed in a solution of sulphuretted hydrogen and in Pro- 

 fessor Tynclall's "actinic clouds." 



§ 20. If through a dielectric medium such as we have ima- 

 gined above (§ 18) we cause an electromotive force to act in 

 the direction of x, we must picture it to ourselves as a pulling 

 of half the cords which lie parallel to x in the positive direc- 

 tion, and the other half in the negative direction (see § 8, 

 foot-note) ; and the elastics being stretched by this action, an 

 x oscillation will take place about the new position of equili- 

 brium in a shorter period than before. Let fall in the direc- 

 tion of z a beam of plane-polarized light. If it were polar- 

 ized so that its vibrations occurred in the plane xz, it would 

 now travel quicker through the medium than when the electro- 

 motive force was not acting ; on the other hand, if its vibra- 

 tions occurred in the plane yz, it would travel apparently at 

 ?ts ordinary rate. (In reality its velocity would in this case 



