Origin of Porcelain Earth, 351 



Werner was tlie first to attribute it to the action of water 

 containing free carbonic acid, by which the alkaH of the felspar 

 is gradually abstracted. This opinion prevailed among chemists 

 until Berthier showed that silica is also carried off dissolved 

 in the alkaline solution, leaving a silicate of alumina, the con- 

 stituents of which are consequently not in the same proportion 

 as in felspar. 



The late Dr. Turner has illustrated this subject in a very 

 clear and pleasing manner, (Philosophical Magazine for July 

 1833,) explaining the change by the following formula: 

 Felspar. Porcelain-earth. 



(Po + 3Si) + (Al + 9Si); (Al.+ 3iSi). 



The potassa is first set at liberty by the action of water and 

 carbonic acid, and the composition of the felspar being thus 

 subverted, the silica whilst in a nascent state is dissolved by 

 the alkaline solution with which it is in immediate contacts 

 " The formula," says Dr. Turner, " showed that every two 

 equivalents of alumina present in porcelain clay along with 

 three and a half of silica, corresponded in the original felspar 

 from which it was derived, to twelve equivalents of silica and 

 one of potash. Hence the quantity of silica carried off was 

 enormous." 



The process of decomposition from which kaolin results has 

 also been well described by Fournet as quoted by Becquerel 

 {Traite de VElectricite etdu Magnetisme^ tom. i. p. 503), who 

 has collected much interesting information on this subject. 



" ' Le feldspath, quand il est desagrege et terreux,' dit M. 

 Fournet, ... 'absorbe done Tacide carbonique, qui reagit sur les 

 silicates et s'empare de leur bases les plus fortes. La silice est 

 mise en liberie a un etat gelatineux qui lui permet de se dis- 

 soudre en certaine quantite, a la verite, dans les eaux et dans 

 les carbonates alcalins ; elle est alors entrainee par elles, et 

 donne naissance, suivant les circonstances, a des cristaux de 

 quartz hyalin, des fiorites, des agates, des opales, des concre- 

 tions calcedoines, et des silicates de nouvelle formation, telles 

 que les mesotypes, les chabasies, &c.'" 



These explanations are consistent with our present know- 

 ledge of chemistry; but they rest too much on induction, and 

 not on actual experiment. For, in the first place, before the 

 precise nature of the decomposition can be determined, it is 

 necessary to ascertain the composition of the felspar ; and I am 

 not aware that this has ever been done. At all events, the 

 felspar in the foregoing formula (which indeed is the one 

 usually referred to on this subject) is the mineral which enters 

 into common granite and not into protogine. And since these 

 rocks occur passing gradually into each other in alternating 



