Observations on various Liquids. 243 



observations (2) and (3), though, they are not now on a dark 

 ground. 



(5) Observation (4) is carried forward, with only one change : 

 the plate K is laid with the line of compression horizontal. 

 There is an evident change of optical" effect. The flashing- 

 bands are fainter : and when the eyepiece revolves as rapidly 

 as possible, the bands are not widened nearly so much as in (4). 

 The optical effect of discharge appears thus to be weakened by 

 horizontal compression. 



(6) The compression of the plate K is increased by degrees, 

 and the observations (4) and (5) are repeated at intervals. 

 The contrast between the two cases comes out more and more 

 strongly, till at last, when the compression has reached a 

 certain value (a value greater than could have been antici- 

 pated) , the contrariety is manifestly perfect. When the line 

 of compression is vertical, the flashing bands are sensibly as 

 distinct as ever ; but when the line of compression is hori- 

 zontal, they are evanescent and, under good conditions, quite 

 imperceptible. 



(7) When the right degree of compression has been reached 

 by trial as in (6), the observer takes the piece K in his hands, 

 and turns it rapidly round the ray, backwards and forwards 

 through a right angle, the eyepiece revolving and the currents 

 passing all the time. The effects are brought out thus in a very 

 striking form, and are seen to be perfectly regular and decisive. 



An intense electric current through nitrobenzol is therefore 

 optically equivalent to tension along the line of discharge, or 

 to compression in the perpendicular direction. 



Summary. 



57. The principal results will now be summed up briefly. 



(1) When an insulating liquid is traversed by electrostatic 

 force, it exerts a purely birefringent action upon transmitted 

 light. In relation to this action, liquids are divisible into 

 two classes, the positive and the negative. 



(2) Positive liquids act as glass extended in a direction 

 parallel to the lines of electric force, or as plates of quartz or 

 other positive uniaxals with axes parallel to the lines of force. 

 Carbon disulphide is the best example. 



(3) Negative liquids act as glass compressed in a direction 

 parallel to the lines of force, or as plates of Iceland spar or 

 other negative uniaxals with axes parallel to the lines of force. 

 Oil of colza is one of the best examples. 



(4) In the following Table the positive liquids are arranged 

 as nearly as possible in the descending order of electro-optic 

 power, the larger and clearer intervals being marked by sepa- 



