550 Dr. Boase on the Composition and 



Breage. St. Stephens. 



Silica 40-15 39*55 



Alumina 36-20 38-05 



Magnesia 1*75 1*45 



Water 11-65 12-50 



Insoluble residue 1 ^^ ^r. o ►r/v 



(quartz and talc) |_^ ^ 



99-25 100-25 



The circumstance most to be noted in all the analyses of 

 kaolin hitherto made, is the great discrepancy in the results, 

 which is unavoidable, because this substance is necessarily he- 

 terogeneous, quartz and talc being mixed with the silicate 

 of alumina in variable proportions, even when washed with 

 the greatest care. Berthier's process will show pretty nearly 

 the quantity of silicate in the kaolin ; but it includes two sources 

 of error, for some alumina will escape between the scales of 

 the talc, and some quartz will be dissolved by the alkaline so- 

 lution. If, however, the earthy salt be completely separated, 

 this does not indicate the actual composition of kaolin as an 

 article of commerce, for the talc and quartz are not prejudicial 

 to it ; indeed protogine itself, under the name of china-stone^ 

 or petuntze, is ground and mixed with the porcelain-earth at 

 the potteries. 



I would also remark that the paper of Berthier, acceptable 

 as it is as an analytical essay on an interesting subject, affords 

 another instance of the great bias which the atomic system 

 gives to reduce experimental numbers according to this Pro- 

 crustean rule. Thus the kaolin is found to contain potassa ; 

 this is acutely referred to the presence of an unaltered mineral, 

 proved by analysis to be the source of the alkali. But then 

 its constituent parts, as obtained by experiment, are respec- 

 tively drawn out or cut off, in order to suit the calculated 

 composition of a potasso-magnesian felspar ; and the evil has 

 not stopt here, for the constituents of the kaolin are altered on 

 the same data ; and on these also it is attempted to explain 

 the nature of the process by which kaolin is produced from 

 felspar. 



1 now proceed to notice the perplexing problem concerning 

 the formation of porcelain-earth. 



Some have supposed this clay to be an original production, 

 or rather a friable deposit from which the subjacent crystalline 

 rock is now in progress of reconstruction. It is, however, at 

 the present day generally admitted that it owes its origin to a 

 chemical change effected in various kinds of felspathic rocks ; 

 but the precise nature of this change is yet unascertained. 



