gjig ANALYSES OF TALC AND MtCA« 



have a pvecise knowledge of the substance taken as the 

 standard of compahson." 



As. it is the same with mica, Mr. Klaproth undertook a 

 comparative analysis of these two minerals, with a view to 

 assist those naturalists, tvho do not confine themselves to ex* 

 ternal characters in their classification of substances, but 

 pay attention likewise to their chemical characters. 



Lamellar ulc It was proper for this analysis to choose a talc, that should 



»f St. Gothard answer very strictly to all the mineralogical characters of the 

 species, and accordingly that of St, Gothard appeared well 

 suited to the purpose. 



described. Its colour is a silvery white, in some parts verging to an 



apple-green : in mass it is very brilliant, with a pearly lus- 

 tre; its lamellar fracture is wavy : it is translucid, and the 

 thin laminae are transparent : it is tender, soft, flexible 

 without being elastic, greasy to the touch, and moderately 

 heavy. 



Action of heat By calcination this talc lost half a part per cent, but no 



**" '*' other remarkable change took place in it» 



Exposed to the heat of a porcelain furnace, in a charcoal 

 crucible, it was hardened, fell to pieces like a schist, ac- 

 quired a whitish gray colour, and was fused in some places. 

 Exposed to the same heat in a clay crucible, the result wa9 

 the same, except that the colour had become a yellowish 

 white. 



The results of its analysis were 



Component. Silex 62 



P^"'- Magnesia 30*5 



Oxide of iron 2*5 



Potash 2*75 



l^oss by calcination ...♦*. 0*5 



98-25. 



No chrome in Though the talcs that have a greenish colour afe said to 

 grwri talcs. contain chrome, Mr. Klaproth could not find any percepti- 

 ble trace of it* 



Mr.- 



