Obfervations on the Flints of Chalk-beds. 305 
milky or opalefcent, then white, opake and rough 
fuperficially. We often find, that the furface of 
fmooth bodies grows rough, when it is under- 
going any chemical change, as in the familiar 
inftance of polifhed iron rufting. ‘The acquifition, 
_ or the lofs of fome conftituent principle, produces a 
difarrangement of the fuperficial particles. By 
filling a phial with fragments of flint and water; and 
then inverting it in water, the caufe of the change 
which the furface undergoes might be afcertained : 
Tis progrefs at leaft would, in all probability, be 
obferved. 
Ili. Many nodules are hollow. ‘Thefe contain 
either a white powder, or a cellular fpungy fub- 
ftance, which latter is more ufually the cafe. A 
few are fpherica, or nearly fo; moft are of an 
irregular roundifh or flatted fhape, with proceffes 
perforated by a hole, within which the contained 
porous matter appears, pointing outwards, and 
generally protruding as far as the orifice. A fpeci- 
men in my poffeffion might be thus exactly imitated. 
Take one of thofe oval phials, into which bent 
tubes are commonly inferted, for the purpofe of 
obtaining elaftic fluids by folution. Into this 
phial, put juft acid and chalk enough to raife a 
foam that fhall fill it; then conceive the foam to 
become concrete. In fome fpecimens, I have obferved 
the fpungy mafs to protrude beyond the orifice. 
And it feems to me obvious, from infpection, that 
the rarefied cellular fubftance, the powder, the 
Qq . perforated 
