Page:Proceedings of the Royal Society of London Vol 1.djvu/105

Rh duced in rubies and sapphires when cut in a particular direction. As to the manner of cutting these asteries, or star-stones, as they are usually called, it appears to be rather the effect of chance than of any determined theory. Some hints, however, are here given for the purpose, which chieﬂy recommend a proper attention to the primi- tive rhomboidal form of the crystal.

In the 9th section the author treats of the characters of the corun- dum afforded by chemiwl analysis; and here he anticipates the re- sults of the investigation we are to be favoured with by Mr. Chene- vix. These conﬁrm What he has all along maintained in this paper .of the identity of the several kinds of stones which he has classed under the name of Corundum. .

The ingredients are unifome found to be the same in all the species, differing only, and but in a small degree, in their propor- tions. The principal of these ingredients is argill or alumina, which, in the imperfect corundum from the Carnatic, Malabar, China, and Ava, consist of between 86% and 9] in 100 parts. In the sapphire it amounts to 92, and in the ruby to 90 hundredths. The other con- stituent parts are silim and iron; the former in the greatest propor- tion. Of the identity of stones which bear very different appearances, another example is here given in the instance of the felspar, reasons being assigned Why some species of schorl, the adularia, and some other substances, ought to be classed with it.

The author proceeds next to consider the sort of corundum which does not exhibit the smallest rudiments of crystallization, and which mineralogists have agreed to distinguish by the name of Compact Corundum. It resembles, in many respects, a coarse jasper; but its much greater degree of lardness, and its much higher speciﬁc gra- vity, render its true nature easily distinguishable. It has a lamel- lated appearance. The red sort, in particular, gives pretty strong sparks When struck with steel. It is phosphorescent, like crystal- lized corundum. Its speciﬁc gravity is 3902.

Three sections treat next of the matrices of different sorts of co- rundum ; which lead the author to dwell largely on a variety of sub- stances which hitherto were thought to have no kind of aﬁinity with this stone.

The ﬁrst section treats of the matrix of imperfect corundum from the peninsula of India, chieﬂy from the Carnatic, and of the substances with which it is accompanied. This matrix, as far as our present knowledge extends, appears to be a stone of a particular nature; sometimes of a loose granulated texture, not unlike a coarse sandstone; and at other times of a closer grain, similar to the kind of marble known by the name of Coarse-grained Saline Marble: both kinds are of a pearly gray colour, sometimes slightly tinged with green, and have a degree of semi-transparency not unlike chalcedony. Their speciﬁc gravities are inferior to that of felspar. In this substance the crystals of corundum are imbedded, nearly in the same manner as those of felspar are dispersed in porphyry or certain granites. The accompanying substances are, l. Lamellated fragments, not un-