ON EARTHS AND STONES. 305

fT. Tnere are. Argillaceous earths diffur greatly
from each other in colour, purity, and other qualities.
Some are perfectly white, as that of which tobacco-
pipes are made. Others are blue, brown, yellow, and,
in short, of all hues ; which they owe to mixtures of
other earths or metals. Those which burn red contain
# portion of iron. No clays are found perfectly pure ;
but they are mixed with more or less of other earths.
The common brick-clays contain a large proportion of
sand, which often makes them crumbly and perishable.
In general, the finest earthenware is made of the
purest and whitest clays; but other matters are mixed,
in order to harden and strengthen them. Thus porce-
lain, or china, is made with a clayey earth, mixed with
a stone of a vitrifiable nature, that is, which may be
melted into glass ; and the fine pottery, called gueen’s
ware, 18 a mixture of tobacco-pipe clay, and flints
burned and powdered. Common stoneware is a coarse
mixture of this sort. Some species of pottery are
made with mixtures of burned and unburned clay ;
the former, as I told you before, being incapable of
becoming soft again with water like a natural clay.

ff. Are clays of no other use than to make pot-
tery of P

7. Yes; the richest soils are those which have a
proportion of clay; and marl, which I have already
mentioned as a manure, generally contains a good deal
of it. Then clay has the property of absorbing oil or
grease ; whence some kinds of it are used, like soap, for
cleaning cloths. The substance called Fuller’s earth
is a mixed earth of the argillaceous kind; and its use
in taking out the oil which naturally adheres to wool
is so great, that it has been one cause of the superiority
of our woollen cloths.

ff. Then, I suppose, it is found in England ?

T. Yes. There are pits of the best kind of it near
Woburn, in Bedfordshire ; also at Reigate, in Surrey.
A clayey stone, called soap-rock, has exactly the fee,
and lock of soap, and will even lather with water.

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