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LXVIL On the Composition and Origin qf^ Porcelain Earth, 

 By Henry S. Boase, M.D.^ Secretary of the Royal Geolo^ 

 gical Society of Cornwall, <^c,^ 



"LT AVING lately seen in the Annates de Chimie et de Phy- 

 -*■-*■ sique (torn. Ixii. p. 225.) an interesting paper on Kaolin 

 by Berthier, the conclusions of which, however, do not seem 

 to me to be perfectly satisfactory, I am induced to offer a few 

 remarks, together with an account of some experiments on 

 the Cornish kaolin, in hopes of calling attention to this cu- 

 rious but complicated problem in " the chemistry of geo- 

 logy." 



Berthier commences by observing that in a former analysis 

 he found the kaolin of Limoges to be composed of 



Silica 46-8 



Alumina 87*3 



Potassa 2*5 



Water .* 13*0 



99-6 

 The presence of the alkali he attributed to a mixture of un- 

 decomposed felspar; and this being deducted, he considers 

 that the remaining pure plastic clay is a silicate of alumina, re- 

 presented by the formula AS + Aq, as also given by For- 

 chammer for the porcelain earth of Bornholm. But by a re- 

 cent set of experiments Berthier has found a considerable 

 portion of magnesia in kaolin from various localities, as shown 

 by the following analyses of the pure argillaceous parts, pre- 

 viously separated from the intermixed substances, by the suc- 

 cessive application of sulphuric acid and of a solution of po- 

 tassa. 



Limoges. Pamiers. Elbogen. Dept. de TAllier. 



Silica 43-05 45' 61-4 56' 



Alumina 40*00 38* 23*2 37* 



Magnesia ... * 2*89 1*2 0*5 much 



Water 14*06 11*7 13*8 12*3 



100* 95*9 98-9 



These specimens of kaolin, with the exception of the first, 

 do not appear to have been well prepared ; indeed, even the 

 clay of Limoges cannot be compared with that of Cornwall, 

 since it contains from 20 to 25 per cent, of an undecomposed 

 mineral ; whereas the largest quantity which I have obtained 

 from good Cornish samples is under 10 and as low as 8*5 per 

 cent. 



* Communicated by the Author. 



