Mr. Lev)) on a new Mineral. 



131 



1823.] 



tive form of datolite may be consi- 

 dered as a right rhombic prism, 

 rig. 1, of 103 6 25', in which one 

 side of the base is to the height, 

 in the ratio of sec. 51° 42' 30" to 

 tang. 32° 14', or, nearly, as 13 to 

 15. Ha'uy, in the new edition of 

 his Traite de Mincralogie, has 

 preserved the determination he had previously given of the pri- 

 mitive form of datolite. It is, according to him, a right rhombic 

 prism of 109° 28', in which the length of one side of the base is 



to the height in the ratio of 3 to V 10. This determination 

 differs from the preceding by 6° 3' in the incidence of the lateral 

 planes. It is not likely, therefore, he had measured the same 

 angle as I have. I suppose that the lateral planes of his 

 primitive form are those of some modification of the one I have 

 adopted, composed of two pairs of parallel planes, and must 

 consequently be the result of some decrement either on the 

 angles of the base, or on the lateral edges. This modification, 

 two planes of which should be inclined at an angle of 

 109° 28', I have never observed, nor does any simple law of 

 decrement give any thing very near it. 



Prof. Mohs in his Natural Historical System of Mineralogy, 

 has called the crystallization of this mineral hemi-prismatic, and 

 as this denomination is applied only to those substances, the 

 crystalline forms of which may be conceived to be derived from 

 an oblique rhombic prism, it might be inferred that he does 

 not consider the primitive form of datolite to be a right rhom- 

 bic prism. But it appears to me that this inference would be 

 in contradiction with the incidences he has given for the faces 

 of the octohedron he takes for the ground form of this sub- 

 stance, since they are calculated in the hypothesis the result of 

 a decrement by one row on the edges of the base of the primi- 

 tive form adopted by Ha'uy. However, there can be no doubt, 

 I presume, from what I have stated, that the forms of datolite 

 can be derived from a right rhombic prism. 



I shall now describe the crystals from the Tyrol which I have 

 already mentioned. The greater 

 number of them have the form 

 represented by fig. 4, which is 

 obviously a rhombic prism, the 

 edges and angles of the base 

 of which are not similarly mo- 

 dified. Consistently with the law 

 of symmetry, this form cannot 

 be derived, like those of dato- 

 lite, from a right rhombic prism, 

 but only from an oblique one ; 

 and, therefore, according to the 



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