1823.] 



Crystalline Forms of Artificial Salts. 



287 



Chromafe of Ammonia, 



The primary form is an oblique rhombic prism. This may be 

 cleaved in the direction of its two diagonals, but there is not any 

 distinct cleavage parallel to the primary planes. 



The planes P are frec[uently rounded, and 

 the crystals are very thin. 



PonM, orM' 114^ 00' 



Pong 110 10 



Pone' 101 58 



P on A 122 31 



MonM' 98 8 



MonA 139 4 



Mong 135 47 



Chromate of Soda. 



I have not perceived any distinct cleavage in these crystals, 

 which effloresce so rapidly that the surfaces cease to reflect 

 the images of objects almost before the planes can be measured. 



The primary form is an oblique rhombic 

 prism, the crystals being sometimes consi- 

 derably lengthened in the direction of the 

 horizontal diagonal. 



PonM, orM' 101° 



Pong 100 



Pone. 133 



PonA 107 



MonM' 80 



Monh 130 



16' 

 20 

 20 

 43 

 4 

 8 



Fig. 1. 



L 



Subcarbonate of Soda, 



The primary form of this salt was given by Rome De L'Isle as 

 an octahedron with a rhombic base, which form has been adopted 

 by Haiiy, and other writers, evidently without examining the 

 crystals ; for even with the common goniometer, the difference 

 of more than 3° between the inchnations of 

 M on M', and e on e', of the annexed fig. 1, 

 might have been readily detected. 



On examining some large and bright 

 crystals received from Mr. R. Howard, I 

 observed that two of the four edges of the 

 supposed base of the octahedron were 

 replaced by narrow planes ; and on looking 

 tnrough the crystals, I perceived indica- 

 tions of cleavages parallel to the edges that 

 were not replaced. lii this direction, they 

 may be very easily cleaved, but I do not 

 find that they yield to cleavage parallel 

 to the replaced edge, or to any of the other 



