380 , tROCKSSFOR 



and even affifb the fufion of clay. The Petitrl- 

 tfc of the Chinefe, ufed in making their porce- 

 lain, is a mixture of this latter ibrt ; and it is by 

 this means that they reduce their clay to fu- 

 iion. 



J v. Common Clay. 



CLAY of various degrees of purity is found iii 

 many places on the furface of the earth, but 

 fcarce any where in perfect purity. For fuch 

 of the common clays as have been examined, 

 have been found to contain a large proportion, 

 fometimcs no lei* than fevcnty in an hundred 

 parts of iilicious fand. Warning, indeed, de- 

 taches the fand, but a very fubtile iiliceous dull 

 dill remains, and cannot be feparated unlds by 

 the folution of the clay. Hence it appears, 

 why the mixture of clay and lime commonly 

 known by the name of marl, is fufible in the 

 fire. It is thought to be the lime that oc- 

 cafions the fulion, whereas it is the . Iiliceous 

 earth. 



The Swedifli clays are fufible without lime ; 

 but the rcafon of this cannot be precifcly ex- 

 plained. Some have thought that the iron ia 

 the Swedilh clay promotes its fulion. But 

 Rinmunn difcovcrcd by a feries of expcriments r 

 that clays contaminated with a large propor- 

 wonof ircn, are more refractory than thofe in 



\vhich 



