CLEAVAGE. 



69 



Crystals of certain species cleave with the greatest facility, as 

 those of Carbonate of Lime, Fluor, and Galena, which require 

 merely a slight sho<:k from a hammer to cause their separation into 

 fragments with even and parallel faces ; while those of other spe- 

 cies, as Arragonite and Apatite, require the aid of a knife or chisel, 

 and a delicate mallet, in order to arrive at the same result. It is 

 frequently necessary to learn beforehand the direction of the cleav- 

 age, which may be done by holding the crystal in a strong light, the 

 reflection of which renders the natural joints at once visible. 



A little practice is required in order to cleave minerals with neat- 

 ness. Crystals of Fluor, Calcareous Spar, Blende, and Sulphate of 

 Strontian, may be recommended to the student as good examples 

 for his early exercise. 



. 60. CLEAVAGE PLANES. 



The faces which result from cleaving a crystal may be 

 termed its cleavage planes. 



Cleavage planes differ with respect to the lustre they exhibit. 

 Those which are parallel, in the same crystal however, are always 

 similar in this respect, while those which are not parallel often pre- 

 sent considerable diversity. That it is perfectly easy from these 

 differences to ascertain the similarity or dissimilarity of cleavage 

 planes in a crystal. 



'. 61. DIRECTION OF CLEAVAGE CONSTANT. 



The direction in which the crystals of a species allow 

 themselves to be cleaved is constant. 



If two directions of cleavage exist at the same time, as in the 

 crystals of Sulphate of Strontian, wherever the faces corresponding 

 to them are obtained, they always intersect each other at the same 

 constant angles. This follows from the parallelism of all those fa- 

 ces of cleavage which lie in one and the same direction. 



. 62. FORM OF CLEAVAGE. 



A regular solid contained under cleavage planes is called 

 a form of cleavage, or a cleavage crystal. 



