A SYSTEM OF 
8S ■ 
e, La ferre noire^ at Montmartre. 
/. Violet, alfo from Montmartre 
SECT. LXXiX. 
B. Indurated, Indiirata. 
Is commonly untfluous to the touch, 
and more or lefs dilEcult to be cut or turned, 
in proportion to its different degrees of hard- 
nefs ^ is not diffufible in water, grows hard, 
and is very refradlory in the fire ; pounded 
and mixed with water, it will not eafily co- 
here in a pafte : ho wever, if it is managed with 
care, it may be baked in the fire to a mafs, 
which, being broke, fnews a dull and po« 
* Thefe contain a phlogifton, which is difcovered by ex- 
pofing them to a quick and ftrong fire, in vvhich they become 
quite black interiorly, affuming the appearance of the com- 
mon flints, not only in regard to colour, but alfo in regard 
to hardnefs : but if heated by degrees, they are firft white, 
and afterwards of a pearl colour. The fatter they feent to 
be, which may be judged both by their feeling fmooth and 
unduous, and by their fhining, when fcraped with the nail, 
they contain a larger quantity of the inflammable principle. 
It is difficult to determine, whether this flrongly adherent 
phlogifton is the caufe of the above-mentioned pearl colour, 
or prevents them from being burnt white in a ftrong fife ; 
yet no heterogeneous fubftance can be extraded from them, 
except fand, which may be feparated from fome, by means 
of water, but which fand does not make out any of the 
conftituent parts of the clays . If they be boiled in aqua 
regis, in order to extrad any iron, they are found td lofe 
their vifcofity. In the lefs unfluous clays, I have found pure 
quartz in greater and fmaller grains; but ftill I would not 
venture to ailert, that one is produced from the other, accord- 
ing to the rule I have laid down in Se£l. ix. i. I have like- 
Wife found this fort, upon certain occafions, attraft the phlo- 
gifton in. the fire. Thefe remarks may ferve as hints for the 
lefs experienced, who have a mind to examine thofe clays, 
which are of fo great confequence for their oeconomical 
ufes. 
rous 
