4 S ASYSTEMOF 
happen to catch fire, the uppermoft ftratunij 
which now confifts of a mixture of iron and dif- 
ferent kinds of rocks called Graherg^ in the ac- 
counts given of them, they might, perhaps, be 
changed, partly into flag, and partly into ^erra 
Puzzolana, 
SECT. XL. 
The Second Order. 
The Siliceous Kind, Silken, 
This filiceous earth is, of all others, the mofl: 
difficult to defcribe and to diftinguifh perfectly : 
however, it may be known by the follow- 
ing charaders, which are common to all bo- 
dies belonging to this order. 
1. In its indurated ftate it is hard, if not in 
regard to the whole, yet at leaft 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 itfelf, 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 ail 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 
