Measurement and Cleavage of certain Primitive Crystals. 253 



Cyanite. 

 Fig. 7. 



On submitting some brilliant crystals to the reflecting goniometer, 

 it became evident that no reliance could be placed on their natural 

 planes. The form of the primitive crystal is an oblique prism. * The 

 incidence of M on T, given by Haiiy as 106° 6', varied very much ; 

 106° 6', 106° 10', 106° 20'. 



This substance is considerably hard and brittle ; but in the attempt 

 to cleave it in the direction of its natural joints, the same means "did 

 not succeed that usually does with other substances possessing those 

 characters. The pincers always bruised the lamina* in separating 

 them, which was fatal to precision. The most effectual means to 

 avoid this, I found to be that of placing the crystal on a table, and 

 supporting its under part in such a manner that the lamina* to be 

 separated should be perfectly at right angles with the table. A sharp 

 penknife then being placed in the desired direction, a smart blow 

 with a light hammer usually produced the effect. Several fragments 

 procured in this manner agreed in the incidence of M on T, 

 106° 15', and that of T on M on the adjacent plane over the edge 

 A 73° 45'; the former being 9' more than the measurement obtained 

 by Haiiy from the natural planes by means of the goniometer in 

 common use. 



A regular fracture in the direction of the terminal planes of the 

 primitive crystal is not so easily obtained as those parallel with the 

 lateral planes. I obtained one considerably brilliant, but not suffi- 

 ciently so for the use of the reflecting goniometer. 



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