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XL On a new Variety of Alum. By James Apjohn, M.D,, M.R.I. A., 

 Professor of Chemistry in the Royal College of Surgeons, Ireland. 



Read 10th April, 1837. 



The mineral to which I am about to draw the attention of the Academy was 

 given me by Mr. Smith of College-green, but I have since received a larger speci- 

 men of it from W. G. Atherton, Esq., a young gentleman who recently arrived 

 in Dublin from the coast of Africa, for the prosecution of his medical studies, 

 and who informed me that it occurs about midway between Graham's Town and 

 Algoa Bay, in beds of considerable extent, and composed of a number of strata, 

 whose aggregate thickness in some places amounts to at least twenty feet. It closely 

 resembles satin spar, or the finer forms of amianthus. The threads or fibres of 

 which it is composed, and which are very easily separated from each other, are 

 about six inches in length, perfectly transparent, and possessed of a beautiful 

 silky lustre. Upon exposure to air, however, they gradually lose this lustre, 

 and become opake on the surface, owing to efflorescence. The taste is astrin- 

 gent and sweet, or almost identical with that of common alum. The specific 

 gravity = 1.727, and the aqueous solution reddens litmus, and gives white 

 precipitates, with nitrate of barytes, and caustic potash. These latter properties 

 belong also to common alum. The precipitate, however, afforded by the caustic 

 potash was but partially dissolved by an excess of the alkali, and the residue, 

 though white at first, gradually became brown. A slight examination was suffi- 

 cient to show that this matter was oxide of manganese, and that the alkali held 

 alumen in solution. 



Having thus established the presence in the mineral of sulphuric acid, alumen, 

 and protoxide of manganese, it became highly probable that it was an alum in 

 which the alkali was replaced by oxide of manganese. But before drawing such a 



