. Annals of Philosophy. 1823.] tive form of datolite may be consi- dered as a right rhombic prism, rig. 1, of 1036 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'
. Annals of Philosophy. 1823.] tive form of datolite may be consi- dered as a right rhombic prism, rig. 1, of 1036 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 rho
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