96 Dr. J. Kerr's Electro-optic 



them to have been accidental. They seem rather to indicate 

 some specific differences between the several liquids, probably 

 with reference to the distribution of electric force. 



18. Cymol (C 10 H u ). — Colourless, transparent, and very 

 distinguishable from benzol by its agreeable odour. It is an 

 excellent insulator, and acts very similarly to benzol in electro- 

 optic experiments, giving an optical effect of exactly the same 

 kind, equally regular and pure, and of much the same inten- 

 sity and range. The light restored by electric force in cymol 

 is always extinguished perfectly by compression of glass in a 

 direction parallel to the lines of force. 



19. Terebene (C 10 H 16 ). — When the cell is charged with my 

 only specimen of this liquid, the arrangements being other- 

 wise as in the fundamental electro-optic experiment (8, 9), I 

 find, contrary to expectation, that the light is rotated and sen- 

 sibly dispersed in its passage through the cell ; still there is a 

 good approximate initial extinction got between the red and 

 blue. The liquid acts as a very good insulator, the sparks 

 from prime conductor having apparently the same density and 

 length when the connecting wires are in and out of place (10). 

 In terebene, electric force evidently strengthens the light from 

 approximate extinction in the polariscope ; and this effect is 

 clearly weakened by horizontal compression of glass, and 

 clearly strengthened by horizontal tension. The effect is not 

 nearly so strong as in the members of the benzol series ; but 

 it is certain, and certainly of the same kind. My former ob- 

 servations on oil-of-turpentine with the old plate cell were at 

 least as satisfactory as these on terebene, 



20. Amylene (C 5 H 10 ). — Colourless, transparent, and exces- 

 sively volatile. Tested in the usual way, it acts as an excel- 

 lent insulator ; long sparks from the prime conductor, or from 

 connected ball of the cell, do not sensibly diminish in length 

 or intensity when the earth-ball of the cell is touched by earth- 

 wire or knuckle. In the electro-optic experiment, as for CS 2 

 (8, 9), amylene acts very finely, and in the same way as each 

 of the preceding hydrocarbons, the effect of electric action 

 from extinction in the polariscope being always neutralized 

 perfectly by horizontal compression of glass. Against a weight 

 of one pound on the fixed compensator (11), electrization deve- 

 lops the extinction-bands faintly, and moves them into the outer 

 parts of the field ; but against a weight of two pounds, the 

 effect of the strongest electric action attainable falls far short 

 of extinction at the centre of the electric field. Amylene 

 stands, therefore, between benzol and terebene. 



21. Valeric Acid (C 5 H 10 O 2 ). — Tested in the usual way, 

 this liquid is not a good insulator. Sparks from the prime 



