OF NITRATE OF POTASH. 293 



4al,'of which all the parts shall have the same relation as they 

 had in nature to the axes and sides of the primitive crystalline 

 form. 



III. Nitrate of Potash. 



This salt possesses the most remarkable optical properties 

 of any crystal that is at present known, and its various actions 

 upon light are of the most anomalous and instructive charac- 

 ter. 



The crystals which I employed were all equiangular hexae- 

 dral prisms ; and the light was transmitted through two natu- 

 ral faces, separated by another face, so that they were inclined 

 to each other at an angle of about 60°. This inclination is by 

 no means convenient for measuring refractive and dispersive 

 powers ; but I attempted in vain to form artificial faces incli- 

 ned at a less angle, and those means which I had found suc- 

 cessful with other soft crystals, completely failed when applied 

 to this salt. 



When a candle was viewed through the nitrate of potash, I 

 observed a double refraction very much greater than that of 

 calcareous spar, — a phenomenon which gave me the more sur- 

 prise, as the Abbe Hauy, who examined many splendid cry- 

 stals of this salt, ascribes to it the property of simple refrac- 

 tion. 



The least refracted image was a circular mass of white ne- 

 bulous light, condensed at its centre, into a very faint image of 

 the candle, but without any strong prismatic tinge ; while the 

 light which had suffered the greatest refraction, formed a di- 

 stinct and highly coloured image. The great interval between 



Vol, VII. P, II. P p the 



