ON THE SrECIFIC REFRACTION OF SOLIDS. 



157 



gravity afc the same temperature was found to be 1'074'i ; heuce its 

 specific i-efraction for this ray is -5307. An alcoholic solution, containing 

 50-6 per cent, of phenyl ether, gave '5265 as the specific refraction for the 

 same line A. 



II. EucJc salt in solid state and in aqueous solution. 



Rock salt was chosen for these experiments for the following reasons :— 

 (1) the fact that it can be obtained in large transparent masses, (2) its 

 refraction is not influenced by its crystalline form, (3) its easy solubility in 

 water. For the determination of its refractive index a prism was cut from 

 a piece of a colourless and clear specimen ; and the prism then ground and 

 polished. The angle of the prism was determined by aid of the image of 

 the illuminated slit reflected from the two faces, and the refractive indices 

 for a, ft, and y determined in the usual manner. The specific gravity of 

 the prism was determined by weighing it first in air, and then in pure 

 benzene. In the preparation of the aqueous solutions, jjortions of the 

 rock salt from which the prism had been cut were used. The following 

 table contains the numbers obtained, and the specific refractions of salt for 

 A:— 



The specific refraction deduced from the aqueous solution, taking the 

 mean of (I.), (n.)> ^^^ (III.), is greater by -0098 than the value obtained 

 for the solid du-ectly. Further, the results from (I.), (II.), and (III.) do 

 not exhibit such agreement amongst themselves as was found in the case 

 of phenol, the extreme difference being -0172. 



III. Fused horax in the solid state and. in aqueous solution. 



Fused borax is one of the few soluble substances which can be easily 

 obtained in large transparent masses. After many futile attempts, it was at 

 last found possible to obtain it in the form of prisms. This end was attained 

 by casting liqiiid borax, which had been maintained in a state of fusion 

 for a considerable time (in order to remove the air-bubbles), in a mould 

 made of silver plates. The most successful experiments were made with 

 borax which, before pouring into the mould, had been allowed to cool 

 down, so as to render it comparatively viscous. The prisms so obtained 

 were, after annealing, ground and polished. The refractive indices of 

 three such prisms have been determined ; their specific gravities were 

 determined as in the case of rock salt. The borax used in the preparations 

 of the aqueous solutions was a portion of the same used to make the 

 prisms, The following table contains the numbers obtained : — 



