86 
Dr. Brewster on the Form , &c. 
ascertained that every cleavage of carbonate of lime in the direction 
of the long diagonal of the rhomboidal faces, arises from the separa- 
tion of the surface of a vein from the mass which it traverses. 
Hence it follows, that the cleavages obtained by Count Bournon 
exist only in those specimens which are crossed by interrupting veins, 
and therefore that the triedral prism is not the form of the integrant 
molecule of carbonate of lime. 
In the course of my experiments on the polarising structure of 
different minerals, I have discovered similar veins in arragonite, 
quartz, and other crystals, and, in a very perfect manner, in some of 
the alkaline and metallic salts. The crystallographer should there- 
fore be careful, while he is determining the primitive form of these 
or other substances, that he distinguishes the real cleavages of the 
homogeneous crystal from those faces which may be presented to him 
in cases of irregular crystallisation. 
