1821.] primary Form of the Boumomte. 361 



an inch in breadth and thickness. From 16 of these crystals J. 

 have taken about 200 measurements, by means of the reflective 

 goniometer; and these, together with the cleavages I have 

 observed, enable me, as I trust, to put it beyond doubt, that the 

 form of the primary crystal is neither the square prism, nor the 

 cube. 



The annexed figure (Plate XI), fig. .15, represents one of 

 the most simple crystals : others exhibit several other planes, 

 which, however, are not important to the present question— ^-the 

 form of the primary crystal. 



This mineral may be cleaved with brilliant surfaces parallel to 

 the lateral planes P P", and c c' ; the latter are generally striated 

 in a direction perpendicular to the plane P' : the former meet at 

 an angle of 90° by the reflective goniometer, though sometimes 

 varying a few minutes ; the latter at about 93° 30' and 86° 30" ; 

 I say about 93° 30', because, owing to the extreme brittleness of 

 the substance, which readily yields even to the pressure of the 

 nail, it is difficult to obtain two perfect planes of cleavage on the 

 same specimen, and even when attained, they rarely agree to a 

 few minutes, owing either to the injury which the mineral sus- 

 tains internally by cleaving it, or, as is frequently the case in the 

 larger crystals, to their being composed of several smaller ones,* 

 bounded in common by brilliant surfaces. I have not been able 

 to discover a cleavage parallel to the terminal plane P'. 



It is known that the natural planes even of small crystals, 

 which, being the most perfect, are best adapted to the use of the 

 reflective goniometer, do not commonly give coinciding mea- 

 surements, but are subject to variations from 1 to about 40 

 minutes. Having submitted the annexed drawing, and the fol- 

 lowing measurements, to my friend H. J. Brooke, Esq. he 

 observes, that ' the inclination of the planes c on c being 

 93° 30', and there being cleavages parallel to the planes c c' and 

 P P", the primary form may be either a rectangular or a right 

 rhombic prism, the plane P' being assumed as the terminal 

 plane. As the planes c c' are mostly striated, the rectangular 

 ■prism P P" may be chosen to represent the primary form. 



'From the angle of 93° 30' which the planes c c' make with each 

 other, the terminal edges of the rectangular prism will be to 

 each other as 160 to 170 ; and assuming the angle P' on a to be 

 136° 16', and to be the result of a decrement by one row on the 



• It is to this circumstance of crystals being composed of several others, or of their 

 being in part constituted by after-deposition, that we may occasionally attribute some of 

 the planea attainable by fracture. \Ve seem to cleave the mineral, when, in reality, we 

 only separate two crystals, affording tolerably brilliant planes, but without that perfection 

 whicli belongs to those of actual cleavage ; these are, therefore, readily detected by the 

 experienced eye, and may be termed planes of composition. Such are the fractures 

 sometimes attainable parallel to the lesser diagonal of the primary prism of sulphate of 

 barytcs, and the planes that may sometimes be procured by taking off the edges of % 

 «ube of fluor. 



