4-6 A SYSTEM OF 



happen to catch fire, the uppermoft ftratiifn, 

 which now confifts of a mixture of iron and dif- 

 ferent kinds of rocks called Grabcrg, in the ac- 

 counts given of them, they might, perhaps, be 

 changed, partly into flag, and partly into Terra 

 Puzzolana. 



SECT. XL. 



The Second Order. 



The Siliceous Kind, Silice<z. 

 This filiceous earth is, of all others, the moft 

 difficult to defcribe and to diftinguifh perfectly : 

 however, it may be known by the follow- 

 ing characters, which are common to all bo- 

 dies belonging to this order. 



i. In its indurated ftate it is hard, if not in 

 regard to the whole, yet at lead in regard to 

 each particle of it, in a degree fufficient to 

 ftrike fire with fteel, and to fcratch it, when 

 rubbed againft it, though the fteel be ever 

 fo well tempered. 



2. When pure, and free from heterogeneous 

 particles, it does not melt by itielf, neither 

 in a reverberatory, nor in a blaft furnace. 



3. After being burnt, it does not fall to a 

 powder, neither in the open air, nor in wa- 

 ter, as the calcareous order does, but be- 

 comes only a little loofer and more cracked 

 by the fire, unlefs it has been very flowly, ' 

 and by degrees, heated. 



4. It excites no effervefcence with acids. 



5. In the fire it melts eafieft of all to a glafs 

 with the fixt alcaline fait •, and hence it has 

 got the name of Vitrefcent, though this 

 name is, properly fpeaking, lefs applicable 



to 



