466 DR T. THOMSON'S Analysis of some Minerals. 



VIII. Soda-Alum. 



Some years ago Dr HOOKER received several specimens of 

 native alum from Dr GILLIES, who resided at the time at Men- 

 doza, a city near the foot of the Andes, and about 800 miles west 

 from Buenos Ay res. Dr HOOKER was so good as to put some of 

 the specimens into my possession that they might be analyzed, 

 and their constitution determined. The specimens were ticket- 

 ed, " Native alum from the province of St Juan." They are in 

 irregular nodules, rather smaller than a hen's egg. From the 

 rocky fragments occasionally attached to them, they seem to 

 have been imbedded in a slate, having a blue colour, very soft, 

 and bearing some resemblance to the slate-clay usually accom- 

 panying the coal beds in this country. But these stony frag- 

 ments are too minute to enable us to determine with accuracy 

 the true position of the rock to which they belong. 



The alum is white, and composed of fibres adhering longitu- 

 dinally, and having some breadth, but very little thickness. It 

 bears some resemblance to fibrous gypsum, but is much harder, 

 not being scratched by the nail, though it is readily enough by 

 the knife. It is sectile, the outer fibres are white and opaque, 

 as if they had lost a portion of their water. But internally the 

 fibres are transparent, and have a glossy or rather silky aspect, 

 shewing that they retain a good deal of water of crystallization. 

 The specific gravity of the transparent portion is 1.88. It tastes 

 precisely like alum, but is much more soluble in water. For 100 

 parts of water, at the temperature of 62, dissolve 377.3 parts of 

 it, and boiling water takes up any quantity of it whatever. 

 When heated it behaves precisely like common alum. 100 parts 

 of it exposed to a red heat lose 46.55 parts of their weight. But 

 this is not pure water, but water holding some sulphuric acid in 

 solution. 



