Fig. 3. 



1824.] Crystalline Forms of Artificial Salts. 21 



Fig. 3 is one of the common bi-pyramidal 

 crystals, whose relation to the preceding 

 figures may be perceived from the corres- 

 ponding letters on the planes. 



The union of these three crystals at an 

 angle of 120° is a fact which requires some 

 little explanation. 



It has been hitherto supposed, that when 

 crystals mutually intersect or penetrate each 

 other, the planes of intersection are parallel 

 to (he primary planes, or to secondary planes 

 resulting from some simple law of decre- 

 ment. And the theoretic order of arrange- 

 ment of the molecules of crystals seems to require this mode of 

 combination in intersecting crystals. But the union of the three 

 crystals which compose fig. 2 is probably the result of simple 

 accretion, in the same manner as crystals of different substances 

 are found adhering strongly together by the close apposition of 

 their surfaces ; with this distinction however between the two 

 cases, that the union of the crystals in fig. 2 is the result of some 

 law which does not govern the accidental union of heterogene- 

 ous crystals, nor always even of homogeneous ones. 



I have been led to this conclusion relative to the structure of 

 these by-pyramidal crystals, from having observed an apparently 

 intersected crystal of chrysoberil, separate easily into six seg- 

 ments, the planes of section being sufficiently bright for measure- 

 ment with the reflective goniometer, and inclining to each other 

 at an angle of b'0°. 



Sulphate of Soda. 



I do not find any distinct cleavage in the 

 crystals of this salt, whose primary form is 

 an oblique rhombic prism. 



PonM,or M' 101° 20' 



P one, ore' 133 18 



Pon/t 107 44 



Pone' 130 45 



MonM' 80 24 



M on/* 130 12 



Mon/ 162 38 



Mon/f 139 48 



Nitrate of Lead. — Nitrate of Barytes. 



The primary form of these salts is a regular octahedron, but 

 the crystals are commonly so much flattened in one direction 

 that their relation to the primary form is not very apparent. 



