,1. .
[ 358 1
held againft a compafs, the needle vfo
lîbly moves. When tried in the cru^
cible, it yields from nine to ten
pounds o f iron in every hundred
weight : it does not diifolve in the leaft
with fal fodæ, with great difficulty
with borax, and fcarce viiibly by fufible
urinous fait. It feems to contain
a great deal o f clay earth in its
compoftion, which may be extrailed
by all acid folvents.
It is well known that this earth,
when it is entirely free from any other
mixture, may, by means o f heat and
drying, be brought to that degree o f
hardnefs, as to give fire with a fteel,
which proceeds from the parts being
brought clofer together, and con-
trailed in a fpace only half as large.
By being thus contrafted, it obtains
a folidity and hardnefs ; and befides,
the furface is fo much diminiftied, in
proportion to the whole mafs, that
the water cannot penetrate any farther
to foften it.x^
We have almoft daily opportunities
Jn^the ftudy o f chemiftry o f convinc-^
i i i g
1. f ' - j >di
[ 359 ]
ing ourfelves, that a fubftance with
a fmall furface cannot be changed in
any manner by liquid folvents ; but
may however be attacked by them, in
proportion to tlie diiTerent degrees o f
pulverization ; nay, even a fubftance
that cannot be reduced by the fineft
mechanical divifion, may frequently
be feparated, as much as is neceftary,
by a chemical one ; that is to fay, by
a preceding folution in another folvent.
The attrailion is here in proportion
to the extent o f the furface ;
and the larger this is, the ilronger will
be the attack : confequently I cannot
believe that any clay, petrified by heat
or flow drying, can have undergone
any eifential change, but only that its
parts have fo contrailed themfelves
as to give it the hardnefs o f a flint
and prevent it from imbibing any viii-
ble quantity o f water. But as foon
as it has been diflblved by any acid
whatever, and its parts have by
this means been brought out o f its
former contrailion, to the requifite
degree o f finenefs and expaniion, it
becomes as foft as before, without the
Z 4 acid
I».'