Observations on various Liquids. 97 



conductor are very much shortened and attenuated when the 

 connecting-wires are placed ; and if the earth- wire be detached 

 from the cell, the earth- ball of the cell (with shaft projecting 

 a little way out of it) gives crackling discharge into the air 

 whenever the machine is worked vigorously. In one careful 

 trial of the electro-optic experiment, as for CS 2 , I obtained a 

 continuous restoration from pure extinction in the polariscope. 

 The effect was extremely faint, but perfectly regular, and was 

 neutralized by horizontal compression of glass. In several 

 following trials I could not recover this phenomenon regu- 

 larly. I have little doubt that the effect is real ; but it is one 

 of the faintest that I have ever observed. 



22. Carbon Dichloride (C 2 Cl 4 ). — Transparent, colourless, 

 and a very good insulator. Under electric action, as in the 

 preceding experiments (8, 9), carbon dichloride restores the 

 light from extinction in the polariscope. The effect is pure 

 and regular, and is neutralized perfectly by horizontal com- 

 pression of glass. The extinction-bands are well separated in 

 this liquid against a weight of three pounds on the first com- 

 pensator (11). Carbon dichloride stands, therefore, some- 

 what above benzol. 



As this liquid was expensive, and as there was only a small 

 quantity of it at hand, I had to be satisfied with charges that 

 were not quite clean. As the experiment proceeded, the im- 

 purities were apparently dissolved or absorbed in some way, 

 and the plate improved remarkably in its optical action. 

 Changes of the same kind were observed in some other liquids, 

 but in a less degree. 



23. At this point I may mention some apparently trivial 

 phenomena which I observed repeatedly in the course of the 

 preceding experiments. On one occasion, when the cell was 

 charged with CS 2 , the liquid had been allowed to evaporate 

 until its free surface was nearly as far down as the tops of 

 the balls. At the instant when the machine was set in mo- 

 tion, the surface of the liquid was deformed ; over the centre 

 of the field there was a hump raised which, by its form and 

 position, reminded me of the arched extinction-bands (11). As 

 long as the electric force was kept at a moderate and approxi- 

 mately constant intensity, I could not detect any sure appear- 

 ance of motion in the hump. Similar effects, though not so large, 

 were obtained in benzol, also in carbon dichloride and other 

 liquids. In cumol the hump was apparently as high as in CS 2 ; 

 but it was accompanied by vigorous movements in the liquid, 

 apparent currents from ball to ball along and through the hump. 

 In xylol the effects were very intense ; the movements were 

 more violent than in cumol ; and when only a large bubble of 



