DOUBLE REFRACTION IN ISOMORPHOUS SUBSTANCES. 26? 



of maximum elasticity. The examination of the rings led to 

 the same result. 



Double chlorides of copper and potassium — of copper and ammonium. 



BCl,CuCl+2H2 0. 



These chlorides appear as octahedrons with a square base, 

 111, the lateral angles being truncated by the faces of a square 

 prism, 001. The potassium double salt does not crystallize 

 distinctly ; that of ammonium presents, moreover, small octa- 

 hedral faces, 021, resting upon the edges of the base of the 

 prism; two contiguous faces of the octahedron. 111, form an 

 angle of 118° 18'. 



In these two salts the optical axis is that of maximum elasti- 

 city ; the doubly refractive power is nearly equal. 



The indices were measured only in the ammoniacal salt: — 



The small number of isomorphous substances which I have 

 as yet examined belonging to the very limited class of right 

 prisms with a square base, possess therefore analogous optical 

 properties associated with similar geometrical forms. 



Rhombohedral or Hexagonal System. 



Carbonate of Lime. 



CaO C02. 



Nitrate of Soda. 



NaON^O^. 



Sulphate of Potash. ? 



KO SO^. 



Calcite crystallizes in rhombodedrons of 105°, and nitrate of 

 soda in rhombodedrons of 106° 30', both possessing, as is known, 

 an optical axis, which is that of maximum elasticity, and a very 

 great power of double refraction, which does not differ much. 

 The sulphate of potash, the prismatic form of which approaches 

 very closely to that of arragonite*, is probably not very remote 



* J. F. L. Hausmann, PoggendorfTs Annalerij vol. Ixxxiii. p. 572. 



U 2 



