Brongniart on (he Formation of Porcelain Cla\j. 123 



calcareous silicates contain so much organic matter, that 

 when heated, even in small quantities, in a close tube, they 

 give out an empyreumatic smell, and sometimes alkalina water, 

 and that the residue becomes black, but is again burned white 

 in the open air. All this is nothing else but the consequence 

 of the superficial water impregnated with such matters pene- 

 trating into the rocks, and there depositing from what it ac- 

 quired from the external surface everything that could be 

 united with the component parts of these rocks by a sort of 

 relationship. 



2. — Brongniart on the Conversion of the Felspar of Primitive 

 Pocks into Porcelain Clay. 



The frequent conversion of the felspar of primitive rocks 

 into porcelain clay or kaolin^ is intimately connected with the 

 views just mentioned ; and is an operation which would be 

 quite impossible if water did not penetrate the solid mass of 

 the rock, and produce a decomposition by which the kaolin is 

 formed from the previously existing crystallized or crystalline 

 felspar. Professor Alexander Brongniart has lately published 

 an essay on the formation and composition of kaolin, and the 

 following interesting statement is taken from it. 

 . During the long period in which J3rongniart has been Direc- 

 tor of the Royal Porcelain Manufactory of Sevres, he has 

 caused analyses to be made by the chemists there, and more 

 especially by Malaguti, of the kaolin of various countries, and 

 has likewise, in the course of his travels, himself visited most 

 of the localities. The analyses shew that kaolin consists of 

 aliimina and silicic acid, with or without potash or soda; but 

 that the relative quantities of alumina and silicic acid are not 

 in constant reciprocal proportions. The following are the 

 analyses of some of these porcelain clays : — . 



