_Of Argillaceous Earth._
Argillaceous earth is found in _clay_, _schistus_, or _slate_, and in
_mica_; but the purest is that which is precipitated from a solution of
alum by alkalies; for alum consists of the union of vitriolic acid and
argillaceous earth.
This species of earth is ductile with water; it then hardens and
contracts by heat, so as to be of the greatest use in forming _bricks_,
or stones of any required form or size. By means of the property of clay
to contract in the fire, Mr. Wedgwood has constructed an excellent
thermometer to measure the degrees of extreme heat.
The ductility of clay seems to depend upon some acid, probably the
vitriolic, adhering to it; for it loses that property when it is burned
into a brick, but recovers it when it has been again dissolved in an
acid.
_Of Terra Ponderosa._
_Terra ponderosa_, or _marmor metallicum_, is generally found in two
states, viz. united to vitriolic acid, when it is called _calk_, or to
fixed air, when it is called _terra ponderosa aerata_.
To obtain it pure from its union with the vitriolic acid, it must be
melted with about twice its weight of fixed alkali; which unites with
the acid, and forming a saline substance, may be washed out of it. In
this state it contains water, and therefore, when exposed to heat, will
yield fixed air; whereas the terra ponderosa aerata will not yield fixed
air by heat only, but when steam is made to pass over it when red hot.
This proves that water is essential to the composition of fixed air.
This stone is distinguishable by its great specific gravity, being four
times as heavy as water; but though in this it resembles an _ore_, it
has not been found to be metallizable.
_Of Magnesia._
This species of earth is found in _steatites_, or _soap rock_, _Spanish
chalk_, _asbestus_, and _Muscovy talck_; but the purest is got by
dissolving _Epsom salts_ (which consists of this earth united to the
vitriolic acid) and precipitating it by a mild alkali. In this state it
becomes united to fixed air, which may be expelled by heat. It is then
_calcined_, or _caustic_, but differs from quick-lime by not being
soluble in water.
_Asbestus_, which contains much of this kind of earth, is remarkable for
not being destructible by heat, though it is sometimes found in flexible
fibres, so as to be capable of being woven into cloth.
_Muscovy talck_ is remarkable for the thin and transparent flakes into
which it is divisible, and thereby capable of various uses.
There are some other distinct species of earth, particularly one brought
from Botany Bay, and another called _Stontiate_, from the place where it
was found in Scotland; but they have not as yet been much examined.
All stones formed by nature are compounded, and to distinguish them from
one another, and ascertain the parts of which they consist, is the
subject of _lithology_, a very extensive branch of knowledge.
All the simple earths are nearly, if not absolutely, _infusible_; but
when they are mixed they may all be fused.