Dr Brewster on a New Cleavage in Calcareous Spar. 313 



saw two images of the candle in place of one, one nebulous 

 and imperfect, and the other distinct though not very bright. 

 By increasing the angle of reflection to about 80° or more, I 

 soon determined that the imperfect image was reflected from 

 the new and slightly polished surface, while the distinct image 

 was reflected from a number of minute planes torn up by the 

 action of the file, and which had not been touched during the 

 process of smoothing and half polishing the artificial face. By 

 placing the rhomb upon a goniometer, I found that the mi- 

 nute planes were inclined ten or twelve degrees to the artificial 

 face ; and upon grinding down the other edges of the same 

 rhomb, and the corresponding edges of other rhombs, I found 

 that the minute planes were in every case equally inclined to 

 the adjacent faces of the rhomb, and that they had the same 

 inclination to the third face of the rhomb in the edge which had 

 been removed. 



I made the same experiments on the edges which contain 

 the acute angle of the rhomb, and on the obtuse and acute 

 summits of the rhomb, but I could not detect the slightest 

 trace of reflecting planes, and hence I conclude that there 

 is a secondary cleavage 'parallel to the edges of the obtuse angles 

 of the rhomb, and equally inclined to the adjacent rhomboidal 

 faces of which that edge forms the common section ; and that the 

 cleavage form of calcareous spar is a solid, bounded by these 

 six secondary cleavage planes, and the six primary cleavage 

 planes of the rhomb. 



The method above described of detecting secondary cleavage 

 planes has, so far as I know, not been previously used by mi- 

 neralogists, and is applicable to many other minerals, and par- 

 ticularly to salts, and crystals which are easily frangible. In 

 some cases a rasp or large toothed file is very useful ; and in 

 others gritty sandstones may be advantageously employed, 

 and sometimes used without water. 



While the determination of the cleavages of crystallized 

 bodies is important to the mineralogist as furnishing him with 

 useful discriminating characters, it is of still greater impor- 

 tance to the crystallographer and the natural philosopher. 

 If we consider it under the more general aspect of the deter- 

 mination of the force of cohesion by which the ultimate par- 



