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merly been inclofed, and prevented from evaporating ; 
of which kind are the fpars found in foflil fhells, 
wherein the bodies of the fhell fifh have perhaps 
lain and perifhed. 
5. We obferve, not only in the fmall cavi- 
ties of fames, but alfo in large caverns, fuch as 
thofe in the Peak in Derby (hire, Okeyhole in So- 
merfetlhire, and the famous grotto in the Greek 
ifland of Andparos, and in fhort wherever moifture 
defcends through the earth to a void fpace, and flops 
upon the inward furface, that it there forms cryftals, 
or fpars, or ftony concretions of fome fort or other ; 
of which fome are fo very imperfedt, as to have only 
the appearance of rude heaps of petrified matter, with- 
out any regular form, which chiefly happens where 
there is much moifture, and where it defcends, or 
foaks through pores fo large as to carry many earthy 
particles with it. 
6. To all which I muft add, that Sirlfaac Newton 
has made it appear, that the tranfparency of bodies is 
occafioned by the minutenefs of their pores, and the 
opacity of them by the largenefs of the pores, in 
which the rays of light being reflected from fide to 
fide are loft, and prevented from pafling through ; 
whence it is, that paper becomes tranfparent by being 
oiled, and the oculus mundi ftone by being foaked 
in water. 
Thefe are the principal obfervations on which I 
found my conjedtures ; and from hence I am induced 
to conclude, that all thefe above-mentioned fubftances, 
are formed by means of thofe cryftalline (perhaps 
faline) corpufcles with which the furrounding earth or 
porous ftones abound, and which probably are 
diffiifed 
