mineralogy. 
4 ? 
to this ordetj than to a great many otto 
earths 
SECT. XLL 
The mineral bodie's that are comprehended jii 
this order, are, indeed, fomewhat different from 
one another. This difference^ however, on firft 
fight may be difcerned ; but, in regard to their 
effefts in the fire, and other chemical experi- 
ments, it cannot be efceemed of any great coni'- 
fequence, at leaft while we are no farther ad- 
vanced in the art of decompounding thefe hard 
bodies, and as long as no one has thought it 
worth the trouble and expence to ufe thofe 
means which are already difeovered for this pur- 
pofe *, I mean the burning-glafs or concave mir-- 
ror j and to continue fuch experiments which Mr*. 
J It IS not yet known, if there is any loofe earth of thif 
kind to be found, or if the indurated one is produced of a 
clay, either pure, or mixed with the calcareous earth, which, 
afterwards has been difiblved, in order to produce this ; be' 
caufe I have not yet, at leall for my own part, found any 
loofe earth that I can fufpe^t to be a filiceous one, except 
that which remains after Hones of this kind are decayed, and 
which is found in form of a white cruft op the furface of 
thofe ftones that lye to the day, or on the earth. This being 
afterwards worn olF, and carried away by the water, is, 
haps, gathered together in form of ftrata. In the fame man- 
ner window-glafs iikewife moulders in length of time: buE 
it cannot, therefore, be fuppofed, that any fuch decayed par- 
ticles may, without being previoufty dilfolved in fome new 
meridruum, be reduced into their former fubftance. I am 
rather inclined to believe, that Tripoli is fuch a mouldered 
liiiccous earth, and that the method of Nature in producing 
inoft of the flinty kind, is fuch as we do not rightly knovir, 
nor have patience to follow, yet imagine that we in fotne 
meafure imitate it in making of glafs, iince both thefe have 
fome efteds common with one another. 
Pott 
