Lecture Xx.

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.