DR T. THOMSON on Asbestus, Chlorite, and Talc. 383 



It is obvious at first sight, that this mineral has no connexion 

 either with asbestus, or talc, or chlorite, as it is almost destitute 

 of magnesia, which constitutes a"n essential constituent of all 

 these minerals. The atomic constituents are, 



19 atoms silica, 

 15 atoms alumina, 



1 atom lime, 



0.6 atom magnesia, 



0.4 atom protoxide of iron, 



3^ atoms water. 



The atoms of silica amounting to 1 9, while those of the bases 

 are only 17, it is obvious that the lime, magnesia, and protoxide 

 of iron must be in the state of bisilicates. The constitution of 

 the mineral may be represented thus : . , 



15 A/S + CS 2 + (M +f/) S 2 + 3Aq. 



Thus it appears that the Cornish talc is nothing else than a 

 hydrous silicate of alumina, mixed or combined with a little bisi- 

 licate of lime, bisilicate of magnesia, and bisilicate of iron. I 

 propose to distinguish this mineral by the name of Gilbertite, in 

 honour of Mr DAVIES GILBERT, late President of the Royal 

 Society. 



The mineral called Bucholzite, from the Tyrol, imperfectly 

 described and analyzed by Dr BRANDES, is an anhydrous silicate 

 of alumina. I am indebted to the kindness of Professor NUTALL, 

 for very pure specimens of it from Chester on the Delaware. 



Its colour is greyish-white. It is composed of fibres often 

 curved, and which, when viewed through a microscope, assume 

 the appearance of flat and rather irregular four-sided prisms. 

 Lustre silky ; about the hardness of quartz. Specific gravity 

 3.193. Infusible before the blowpipe. 



