DR T. THOMSON'S Analysis of some Minerals. 461 



VI. Sulphate of Alumina. 



The specimen of this mineral in my possession comes from 

 Rio Soldana in South America. I am indebted for it to the 

 kindness of CHARLES MACINTOSH, Esq. of Crossbasket, who had 

 got it from the late Sir RALPH WOODFORD, Governor of Trinidad. 

 There is a notice respecting it in the Annales de Chimie et de 

 Physique *, by M. BOUSSINGAULT, from which we learn that it 

 occurs in the transition-slate of the Andes of Columbia, either in 

 a state of efflorescence or in crystallized masses. HUMBOLDT ob- 

 served it in the clay-slate of Araya near Cumana. It is found 

 also in the clay-slate of Socono, and in many other places of South 

 America. It occurs in commerce, in spherical masses, and in the 



The constituents of this mineral I found to be, . 



Silica, ......... 52.744 



Lime, 31.684 



Soda, 9.600 



Magnesia, 1.520 



Peroxide of iron, ' ". . . . . 1.200 



Alumina, 0.672 



Water, 2.000 



99.420 



If we suppose the magnesia to have replaced a little lime, this mineral is a com- 

 |X)und of 



4 atoms bisilicate of lime, 

 1 atom tersilicate of soda. 



Thence its symbol is 4CS 2 + NS 3 , and it differs from table-spar, by contain- 

 ing 1 atom of tersilicate of soda united to 4 atoms bisilicate of lime ; whereas table- 

 spar is pure bisilicate of lime. 



* Tom. xxx. p. 109. 



3 M 2 



