On the corundum stone from Asia / [Charles Francis Greville].

Charles Francis Greville

Date:: [1798]

Catalogue details

Licence: Public Domain Mark

Credit: On the corundum stone from Asia / [Charles Francis Greville]. Source: Wellcome Collection.

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u a rhomboidal parallelopiped, truncated at each of its obtuse “ angles, by an equilateral triangular plane. “ You will have a correct idea of the other crystal, if you “ suppose the crystal represented in Plate IV. fig. 87. truncated “ at each of the summits of its pyramids, by an equila eral tri- “ angular plane, as in the preceding modification, Jbut deeper, “ and in so great a degree, that the three rhombic planes of each “ pyramid disappear, with the exception of three isosceles tri- “angles; this modification differs from the first, only by the “ hexaedral prism, and the deeper truncature at the summits of “ the pyramids/' It is therefore clear, that if the primitive rhomboid of Corun¬ dum decreased only at the superior angles of its laminae, it would exhibit exactly the first of these varieties of Mr. Werner's ruby, as in the annexed fig. 19. As to the second variety of Mr. Werner's ruby, it is equally clear, if in fig. 87, referred to by Rome' de l'Isle, (represented by the annexed fig. 20.) no more of the pyramid was left than the three small triangles 6, a, c, there would be precisely one of the forms of Corundum before described, to which the an¬ nexed figure 5 belongs. It may perhaps be objected, that the laminae appear to be parallel to the terminal planes, in the sapphire, and inclined, in the Corundum. There are crystals of Corundum, in which, very frequently, the laminae appear parallel to the terminal plane; I was at first, and for some time, deceived by that appearance. In other Corundum crystals, the laminae appear to be parallel to the prismatic planes; and, to conclude the instances of analogy, the superposition of rhomboidal laminae is sometimes observable F 2

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