50 Clay. [Book VI. 



Argill, or day, is foluble in the vitriolic, nitrous, and 

 muriatic acids, ^nd forms alum with the firft of thefe. 

 If concrete volatile alkali*, or any other of the alkalis, 

 is added to a folution of pure alum, the alkali and 

 acid unite, while the clay falls to the bottom, united 

 with only a fmall quantity of fixed air. The fluid 

 muft be abftracted by decantation, and the precipitate 

 wafhed with diftilled water, and dried. Pure clay 

 does not become cauftic by burning, but is contracted 

 in fize, and becomes very hard. The fpecific gravity 

 of this earth is only 1,669. 



The principal natural fpecimens of argillaceous 

 earth are boles, clays, marks, flates, and mica. In 

 none of thefe, however, except the flag-ftone, does the 

 argill amount to half the weight of the whole fubftance. 

 Silica abounds very much in common clays. Baked 

 clays ccnftitute all the varieties of bricks, pottery, 

 and porcelain. If baked in a ftrong heat, they give 

 fire with Heel. 



Silica, or JKnt, is foluble in only one of the acids, 

 the fluoric, yet during its precipitation, it is capable 

 of combining with mod of the mineral acids. In its 

 indurated ftare, it is always fufficiently hard to fcratch 

 and {bike fire with Heel. After being burnt, it does 

 not fall to powder as the calcareous earth does. It 

 produces no eflfervefcence with acids. It may be dif- 

 folved by the fixed alkalies, both in the dry and wet 

 way. When alkali and flint are expofed to the heat 

 of a glafs-houle furnace, if the alkali is only half the 

 weight of the filica, it produces a diaphanous and hard 

 glafs, but when the alkali is in double or triple the pro- 

 portion, the glafs deliquefces of itfelf, by attracting 

 the humidity of the atmofphere, and forms what is- 



The volatile &lts in a folid {late. 



called 



