Electricity and Light. 449 



the faintest restoration, unless the closing plates are unduly 

 strained by the screw press. 



The primary circuit is closed, and the spark-terminals, placed 

 at first in contact with each other, are drawn apart slowly till 

 sparks pass through the liquid in the cell. The spark-terminals 

 are then pushed a little nearer to each other, till the discharge 

 passes wholly through the air; the primary circuit is broken, 

 and the apparatus is ready for experiment. To obtain a good 

 observation, the liquid should be very clean, the room some- 

 what darkened, and the light due midway between the ter- 

 minals. 



Plane of polarization at 45° to the horizon or to the lines 

 of force : as soon as the primary circuit is closed, there is a 

 very distinct flickering restoration of the light in the polan- 

 scope, the flicker keeping time with the oscillations of the 

 rheotome. The cleaner the liquid plate, the more distinct is 

 the effect ; and when the liquid is sensibly without speck, the 

 restoration of the whole image of the flame from pure initial ex- 

 tinction is admirably clear and delicate. 



Plane of polarization at 0°, or at 90° to lines of force : gene- 

 rally a small effect in the polariscope, much fainter than in the 

 former case, and tending to evanescence as the angular ad- 

 justments are improved throughout the part examined of the 

 electric field. 



This was the first successful observation which I made upon 

 any liquid dielectric. All the following experiments are with 

 the electric machine. 



28. The plate machine employed is a small one, strong enough 

 to send sparks through the liquid in the cell when it is worked 

 vigorously. It will be remembered that the prime conductor is 

 connected with earth by a copper wire, which is interrupted in 

 the polariscope by J of an inch of carbon disulphide. 



The only difficulty in these experiments is to get the liquid 

 clean enough. At the beginning of a series of observations the 

 cell is cleaned thoroughly, washed with alcohol or other proper 

 solvent, and rinsed repeatedly with ether, no cloth or other 

 rubber being applied. The cell is then charged with fresh bi- 

 sulphide, many times in succession if necessary, the liquid being 

 introduced through a filtering-funnel which has had its tube 

 drawn to a fine end. In this way specks of solid matter which 

 are present in the liquid as obtained from the apothecaries, 

 specks also of organic fibre or of other dust which would other- 

 wise enter from the surrounding air, are kept out of the cell. 

 But even when all precautions are taken, I find it very difficult 

 to get the liquid specklessly clean. In many cases, as soon as 

 the electric force begins to act, specks of dust formerly invisible 



Phil. Mag. S. 4. Vol. 50. No. 333. Dec, 1875. 2 G 



