158 Dr. J. Kerr's Electro-optic 



The initial extinction was very slow and impure ; but it was 

 perfected by careful regulation of the temperature, and by the 

 application of a stationary neutralizing plate of glass. Like 

 difficulties occurred in most of the experiments with the fusion- 

 cell, and they were generally overcome in the same way. 



Tried as a nonconductor, the plate of phosphorus gave no 

 optical effect. Tried then as a conductor, with moderate 

 charges of the jar, it gave a good restoration, — not so strong, I 

 thought, as that of bromine, but equally regular and pure, 

 strengthened by tension parallel to lines of force, and weak- 

 ened to extinction by compression. Phosphorus therefore, like 

 bromine, is purely positive. 



Judging from some facts that presented themselves in the 

 course of my experiments with fused phosphorus, lam almost 

 sure that, with better apparatus and proper arrangements, 

 this body would give a good optical effect as a nonconductor. 



Sulphur. — The fusion-cell was placed on its tripod between 

 the crossed Nicols, and all the pieces were put in final position 

 as for the optical experiment ; the cell was then filled with 

 sublimed sulphur, and the temperature of the air-bath was 

 raised gradually. As the sulphur melted, new quantities were 

 supplied till the cell was nearly full of liquid. The initial 

 extinction was now restored as quickly as possible by the use 

 of a thick neutralizing plate of glass placed in front of the 

 second NicoJ, and the sulphur was tried as a nonconductor. 

 When the machine was set in motion, the flame was restored 

 strongly from extinction; and the effect was found to be purely 

 positive, strengthened by tension parallel to lines of force, and 

 extinguished by compression. As the experiment went on for 

 a minute or more, the effect became weaker and weaker till 

 it was imperceptible. The insulation had evidently failed, the 

 temperature being too high for the apparatus. 



The Leyden jar being now introduced quickly, the sulphur 

 was tried as a conductor, and there was a strong restoration 

 obtained by discharge at every fifth turn of the plate. The 

 effect was still purely positive, strengthened by tension parallel 

 to lines of force, and extinguished by compression. In a few 

 minutes this effect also disappeared ; and it could not be reco- 

 vered by regulation of the temperature. In the meantime the 

 plate of sulphur had deteriorated very much from its first 

 purity. Perfectly clean at first, it was now filled with black 

 particles. 



After this first experiment, I made several attempts to obtain 

 a more constant effect, but without success. The conditions 

 of the experiment were too trying for the apparatus : the cell 

 was damaged in the first experiment, and was very soon de- 



