of Nitrate of Potash. 1 23 



whicli any theory can possess, that the spechnens of agate which 

 flepolaviz'e light must necessarily form two distinct images, — a 

 p]i£enomenon to which we have foimd a rapid approximation in 

 the carbonate of barytes. 



Th.e property which has now been explained, forms a simple 

 and infallible mineralogical character of the striated carbonate 

 of barytes ; and is particularly valuable to those who have been 

 perplexed by the numerous marks with which some writers have 

 laboured to' distinguish it from its kindred minerals. The as- 

 sistance, indeed, which optics afford in discriminating minerals, 

 is of the most extensive kind ; and it is much to be wished, that 

 mineralogists would exchange many of their vague distinctions 

 for those"^ unambiguous characters which bodies exhibit in the 

 modifications they impress upon light. 



The Abbe Hauy has, in some measure, begun this reforma- 

 tion^ and has set a brilliant example of what may be effected by 

 the aid of mathematical and physical acquirements. In his ad- 

 mirable work on Crystallogtiiphy, which has never been duly ap- 

 preciated in this country, he has created a new science, in whicJi 

 he has shown how to determine the integrant molecules of cry- 

 stallized bodies ; and how, from a few primitive forms, may be 

 derived that endless variety of secondary crystals which adorn 

 the mineral kingdom. The recent discoveries which have been 

 made in optics, enable us to give a new direction to these in- 

 teresting inquiries ; to determine the forms and even the angles 

 of crystals (iom their optical j)roperties ; and out of a mass of 

 shapeless fragments, to reconstruct an artificial crystal, 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 character. 



^The crystals which I employed were all equiangular hexaedral 

 prisms ; and the light was transmitted through two natural 

 faces, sej^arated by another face, so that they were inclined to 

 each other at an angle of about 6()\ This inclination is by no 

 means convenient for measuring refractive and dispersive powers; 

 but I attempted in vain to form artificial faces inclined at a less 

 angle, and those means which I had found successful with other 

 soft crystals, completefy 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 cal- 

 careous spar, — a phsenomenon which gave me the more surprise, 

 as the Abbe Ilaiiy, who examined many splendid crystals of this 

 talt, ascribes to it the property of simple refraction. 



The 



