Siliceous earth is not affected by the strongest heat, except by means of a burning lens, or dephlogisticated air.
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.
_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.
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.