ss 



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 chemical 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 uniformly 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 91 in 100 parts. In the sapphire 

 it amounts to 92, and in the ruby to 90 hundredths. The other con- 

 stituent parts are silica 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 hardness, 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 affinity 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 sub- 

 stances 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 chalce- 

 dony. 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, 1 . Lamellatcd fragments, not un- 



