300 PHILOSOPHICAL TRANSACTIONS. [ANNO 1798. 



India polish and form, en cabochon, which is often called tallow-drop, from the 

 French lapidaries' term goutte de suif, convex, oval, or circular. A very small 

 quantity of the corundum powder would be required, as the action of the powdered 

 corundum and gems, on the lump of corundum, would, as appears from the depth 

 of the grooves, wear away from it a supply of powder, for the operation of polish- 

 ing. It appears to be a part of a larger mass, is of a purplish colour, and of the 

 same laminated texture as the crystals of corundum ; it has this peculiarity, there 

 appear cracks, branching irregularly across the laminae of the lump, which are 

 filled with homogeneous matter, distinguished however by the superior purity which 

 might be expected to arise from the degree of filtration required for its deposition 

 in the fissures. Some of these cracks, whieh terminate on the surface, appear to 

 have the same crystallized arrangement which characterizes the laminae of corun- 

 dum. The cracks not being in any degree influenced in their direction by the 

 laminae of the crystallized mass, it is probable they had not been consolidated, when 

 they cracked : and, from this specimen, we may expect to find corundum cementing 

 masses of stone, by the same process of stalactitical cementation by which quartz 

 and calcedony connect great nodules and masses of siliceous stones. 



In this specimen, I consider the veins as pure corundum, that is, having the same 

 specific gravity, hardness, and texture as corundum crystals ; and I found the whole 

 lump possessed all the qualities of corundum, except its specific gravity, which 

 amounted only to 2.785 ; and in this property it corresponded nearly with the 

 matrix of the corundum crystals, or the vein in which corundum is before stated 

 to be found; the specific gravity of which is 2. 768. The texture of the matrix 

 appears sometimes like adularia, and confusedly crystallized ; often compact, like 

 cipoline or primitive marble ; sometimes sparry, sometimes granulated, and, on the 

 outside of the vein, and near fissures, decomposed, and becoming opaque. In all 

 its states, it scratches glass, but not rock crystal, possibly from want of adherence 

 of its particles ; and in this it differs from the substance of the above lump, which 

 cuts glass and rock crystal with great facility. This lump, and the matrix of co- 

 rundum, appeared to possess the same properties as corundum, when examined by 

 the blow-pipe, with the different fluxes. 



The matrix of corundum having sometimes an appearance like adularia and feld- 

 spar, I ascertained, by Mr. Hatchett's scales, the specific gravity of adularia to be 

 2.558, and of feldspar 2.555. The corundum, and the lighter corundum of the 

 lump, cut adularia and feldspar ; the latter effervesced, and combined with soda, 

 which the former did not. It is therefore evident, that the matrix of corundum, 

 or substance of the vein, is a distinct substance from adularia and feldspar, and 

 nearly connected with corundum. The matrix or vein contains also a black sub_ 

 stance, like shorl, which, on closer observation, appears to be hornblende. This 

 substance Mr. Garrow had remarked to have the appearance of charcoal, and on 

 that account he had attributed the formation of these strata to the agency of fire. 



