86 A SYSTEM G? 



Moreover, there are fome of this order which 

 grow ibft in water, and, when only moiftened, 

 become duclile and tenacious : thefe are commonly 

 called clays. Some crack in the water, after hav- 

 ing imbibed a fufficient quantity of it, but do 

 not grow fofter in it, and are therefore in the firft 

 degree of induration : fome imbibe the water, 

 but do hot crack or fall to pieces ; thefe are yet 

 more indurated : and finally, fome there arc, in 

 which the water has no ingrefs at all. Thus, by 

 following the fiacceflive gradation of induration 

 of a ilibftance, which throughout all thefe cir- 

 cumftances is eafily difcovered to be the fame, 

 one may with great reafon conclude, that the 

 hardnefs of the jafper may perhaps be the lad 

 degree of hardnefs, and that this ftone confe- 

 quently confifts of an argillaceous fubftance, 

 (Seel, Ixv.) that already pofTefTes a quality which 

 the other clays cannot acquire but in the fire ; 

 having, befides, the fame effecl as the boles (Sect. 

 Ixxxvi.) when melted in the fire together with 

 calcareous or other earths. 



SECT, LXXVIII. 



'^f. Porcellain Clay, Terra Porcettanea, vulga 



Argil I a Apyra. 



Is very refractory in the fire, and cannot in 

 any common ftrong fire be brought into fufion 

 any farther than to acquire a tenacious foftnefs, 

 without lofing its form : it becomes then of a 

 - dim mining appearance and folid texture, when 

 it is broke ; frrikes fire with fleel ; and has con- 

 fluently the belt qualities required, as a fub- 

 ilance whereof veffels capable of refilling a melt- 

 ing 



