On the Composition and Origin of Porcelain Earth. 349 



The substances which Berthier separated from kaolin are 

 granules of quartz and minute white or yellowish-white scales 

 having a pearly lustre. The composition of the scaly mineral 

 he ascertained to be as follows : 



Limoges. Pamiers. By calculation. 



Silica 65-9 59-2 67-7 



Alumina 20*8 25'2 19-1 



Potassa 7'5 ... 9*8 



Soda 8-9 



Magnesia 2-8 0*5 3*4? 



Lime 1*9 



Water 1-0 3*2 



98* 98'9 100- 



This mineral Berthier regards as the peculiar felspar from 

 which the kaolin has been derived by decomposition. He 

 has taken some pains to establish this point; and has at- 

 tempted by calculation, founded on the above analyses, to give 

 the true atomic composition of this supposed felspar. I am, 

 however, inclined to think that he is mistaken : for the unal- 

 tered mineral which I have extracted from the Cornish kaolin 

 is decidedly a variety of talc, in an extreme state of commi- 

 nution, and its appearance under the lens exactly agrees 

 with that described by Berthier. Now this mineral abounds 

 in the talcose granite, or protogine, which by disintegration 

 furnishes the beds of porcelain clay ; but it does not experi- 

 ence any change, remaining in the clay, from which it is se- 

 parated in considerable quantity during the process of prepa- 

 ration for the potteries. It seems to be nacrite or scaly talc of 

 mineralogists, of which Vauquelin has given an analysis, nearly 

 approximating to that of the scales from Pamiers as above 

 quoted. 



In denying that kaolin has been formed, from the scaly mi- 

 neral with which it is intermixed, I am willing to admit that 

 it has been derived from a potasso-magnesian felspar which 

 may be similar in composition to that indicated by Berthier's 

 calculation. Indeed several years ago I suggested, on minera- 

 logical considerations, that the felspar of protogine probably 

 contains magnesia. This conjecture is now confirmed by the 

 detection of this earth in kaolin ; and I have since ascertained 

 that it is also present in the china-clay of Cornwall. 



The specimens examined were of the best quality from 

 Breage and St. Stephens. The process which I adopted was 

 very similar to that employed by Berthier, and gave the fol- 

 lowing results. 



