454 Dr. J. Kerr on a new Relation between 



disulphide. In some plates the chain holds together perma- 

 nently ; and in such a case, if the chain is not very coarse, a 

 good effect may generally be obtained in the parts of the field 

 above and below the chain, as formerly in the second experi- 

 ment upon carbon disulphide. In other plates the chain breaks 

 up violently at the instant of discharge of the prime conductor 

 upon the knuckle; and in such a case I have sometimes obtained 

 a good effect by drawing sparks in rapid succession from the 

 prime conductor. When the specks are so few and of such 

 forms as not to give a chain, they still interfere noxiously, pre- 

 senting themselves as a set of sparkling points, which dart hither 

 and thither through the central parts of the electric field. 



After numerous trials, many of them ineffective, I got at 

 length some speck lessly clean plates of benzol. The principal 

 effects then presented themselves with beautiful distinctness and 

 perfect regularity. The experiments were the same as those 

 upon carbon disulphide already described, and were often re- 

 peated, particularly the first and second (29,30); and the effects 

 obtained were exactly similar in kind and equally distinct, though 

 less intense. Without more details, I give here the final note 

 which was taken at the time of observation. 



Benzol. — Plane of polarization at 45° to the lines of force ; 

 strong electric action : the light is clearly restored from extinc- 

 tion, and is then extinguished perfectly by compression of the 

 compensator in a direction parallel to the lines of force. Plane 

 of polarization at 0° or at 90° to the lines of force ; strong elec- 

 tric action, with or without occasional spark-discharge through 

 the liquid : not a trace of effect in the polariscope, the extinc- 

 tion equally pure before and after the instant of strong spark- 

 discharge from the prime conductor. 



Paraffin and Kerosene Oils. 



37. Young's Paraffin Oil. — This liquid is a strong insulator, 

 and gives a very transparent plate. Its action is similar to that 

 of benzol, but much weaker, Benzol, as I have just mentioned, 

 gave me often a distinct principal effect above and below a fine 

 interterminal chain of solid particles, but this liquid never. The 

 action, indeed, is so faint that there, was only one of several 

 arrangements tried which gave a good regular effect. 



Experiment. — All the pieces are laid as in the first experiment 

 with carbon disulphide (29), except the dielectric, which is now 

 placed close to the second Nicol, a few inches from the eye, so 

 that no light is received from the lamp except through points 

 of the electric field very near the centre. No result could be 

 accepted as decisive till the liquid was perfectly clean ; and the 

 observations were carried out accordingly. 



