'312 Dr Brewster on a Ncxv Cleavage in Calcareous Spar. 



plates or veins of calcareous spar, varying in thickness from 

 the lOOOdtli of an inch upwards, and having a position trans- 

 verse to that of the rhomb which contained them, and I put 

 this conclusion beyond a doubt by inserting veins of calca- 

 reous spar and sulphate of lime between prisms of calcareous 

 spar having the same position as those which compose the na- 

 tural rhomb. 



This view of the subject has been adopted by scientific mi- 

 neralogists, and the reflecting faces referred to have been ap- 

 propriately denominated by Professor Mohs faces of composi- 

 tion. The cohesion between these faces of composition is ex- 

 tremely powerful ; but when a separation is effected by a smart 

 and well directed blow, the disunited faces possess the most 

 perfect smoothness and the highest polish. 



Hence we see the mistake committed by Count Bournon 

 and some other mineralogists, who, in obtaining these planes 

 of false cleavage, regarded them as real, and ascribed a new 

 primitive form to calcareous spar. In order to prove that a 

 real cleavage of this description does not exist in calcareous 

 spar, we have only to extract from a pure mass of calcareous 

 spar two rhombs, one perfectly homogeneous and free of veins, 

 as can easily be ascertained, and the other intersected with 

 veins, and we shall find it impossible to obtain any cleavages 

 in the former, while the latter will readily exhibit them, but 

 never in any other place but at a face of composition. 



After I had determined, in this manner, that the cleavages 

 described by Bournon and others arose from the separation of 

 the planes of a compound crystal, I was led by an accidental 

 observation to discover a secondary cleavage parallel to these 

 faces of composition, and existing in all crystals of calcareous 

 spar. During the experiments on the action of crystallized 

 surfaces upon light which I have published in the Philoso- 

 phical Transactions for 1819, I had occasion to grind down 

 an artificial face upon one of the edges of the rhomb which 

 contains the obtuse angle. The rough grinding was done upon 

 a coarse file without water, and when the face was slightly 

 smoothed for the purpose of seeing if it was equally inclined 

 to the two adjacent rhomboidal faces, I caused it to reflect 

 the light of a candle. My surprise was considerable when I 



