Prof. Gr. Quincke on Electrical Expansion. 39 



foil, and contracted parallel to it. The glass behaves like a 

 negative crystal* (Iceland spar) with optic axis parallel to 

 the warmed plate. 



The line of greatest heating parallel to the tinfoil or the 

 optic axis of a negative crystal must correspond, in the case 

 of electric double refraction, to the shortest line of electric 

 force between the metallic electrodes, as, in fact, Dr. Kerr \ 

 has asserted, and as I have found confirmed. 



Moreover those portions of insulators in the neighbourhood 

 of the shortest line of electric force for those substances which 

 Dr. Kerr calls negative (glass, fatty oils, &c), behave like 

 optically negative crystals with optic axis parallel to the 

 shortest line of electric force ; " positive " substances (sulphide 

 of carbon, &c.) behave like optically positive crystals with 

 the optic axis parallel to the shortest line of electric force. 



More precisely, solid and fluid insulators between metallic 

 electrodes must be regarded as unequally expanded bodies, 

 which act optically like an aggregate of numerous small crys- 

 tals. The resin called by Dr. Kerr "clear amber resin" 

 behaves oppositely to glass when subjected to electric double 

 refraction, and will probably decrease in volume like the fatty 

 oils under the influence of electric force. 



39. The optical phenomena observed with substances which 

 conduct electricity badly, completely confirm the production 

 of expansion and contraction by electric force observed in 

 other ways. 



40. The cause of electric expansion and of changes of elas- 

 ticity produced by electric force is to be sought in a twisting 

 and displacement of the molecules of the insulator — which 

 place themselves with their greatest length in the direction of 

 the resultant electric force, so that their electric moment is a 

 maximum. That small particles of glass and other insulators, 

 suspended in badly-conducting fluids, do actually take up such 

 a position, has been shown by WeylJ. If the particles are 

 scattered not in a fluid, but in a mass not perfectly solid, 

 similar changes must take place, but more slowly. 



* Compare F. E. Neumann, Abh. Berl Ah. 1841, ii. p. 6. 



f Phil. Mag. (4) 1. p. 337, 1875. 



\ Reichert and Du Bois's Arch, 1876, p. 721. 



