254 Dr. J. Kerr's Electro-optic 



or quarter turn of the plate, it gave a strong and purely posi- 

 tive effect from extinction, -without sensible disturbance of any 

 kind. 



A thing noticed in most of the fused fats, and strikingly 

 apparent in the case of spermaceti, was the sluggishness of 

 rise and fall of the electro-optic action. When strong electric 

 force was applied, the optical effect rose gradually to its full 

 intensity, through 1 or nearly 2 seconds ; and again, when the 

 apparatus was discharged, the light fell to sensible extinction 

 still more slowly, through 20 or more seconds. This was evi- 

 dently a thing of the same kind as the gradual rise and fall 

 of effect which I observed long ago in glass ; but the electric 

 stress in that case was much more intense, and the duration 

 of the phenomenon much greater, extending even to 20 or 30 

 minutes. 



Beesv:a:i\ purified but unbleached, was tried in the same 

 way as the fats, and was found to be purely negative, and 

 almost as strong as spermaceti. 



White wax, a colourless and very clear plate in the fusion- 

 cell, gave a finer and stronger effect, still purely negative. 



Other Compounds of C, H, X. 



24. Aniline, C 6 H 7 N, is purely negative. Two samples 

 were examined. One of them, tried with plate machine and 

 strongest charges of the jar, gave a faint effect which was 

 purely negative, strengthened by compression, and extin- 

 guished by tension. The other sample, which was much 

 deeper in colour, gave no trace of effect with the jar ; but 

 with the KuhnikorfFs coil it gave intense restorations, which 

 were purely negative by both tests. 



Albjla.mine, C 3 H 5 XHo, is very feebly negative. With 

 the Ruhmkorff-discharges, the restorations from extinction 

 were perfectly clear and regular, but very difficult to charac- 

 terize. Upon the whole, the effect appeared to be certainly 

 negative : but it was very faint, and somewhat impure at the 

 best. 



Capronitrih, G t H u N, is feebly but clearly negative. Was 

 tried in the same way as allylamine. and acted rather better, 

 the restorations from extinction being very pure and regular, 

 but still rather difficult to characterize. On the whole, the 

 effect was clearly negative, strengthened by compression, and 

 not affected by tension. 



Bemonitrile, C 6 H 5 CX, is purely and strongly positive. 

 Tried as a conductor, without the jar, it gave an intense and 

 very fine effect, which was strengthened by tension parallel to 

 lines of force, and extinguished perfectly by compression. 



