Part 8 (1/2)

_Of Siliceous Earth_

Siliceous earth seems to be formed by nature from chalk, perhaps by the introduction of some unknown acid, which the vitriolic acid is not able to dislodge It abounds in h to strike fire with steel, as _flint_, _rock crystal_, and most _precious stones_ It is not acted upon by any acid except the fluor and phosphoric, but especially the for then dissolved in water, makes _liquor silicum_, from which the purest siliceous earth may be precipitated by acids For this purpose about four tiht of alkali hts of alkali and siliceous sand is ht and excluding the weather fro various useful utensils To lass perfectly colourless, and at the salass_, manufacturers use a certain proportion of calx of lead and est heat, except by isticated air

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 aluillaceous earth

This species of earth is ductile ater; it then hardens and contracts by heat, so as to be of the greatest use in for _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 therrees of extreme heat

The ductility of clay see 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 _enerally 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 ht of fixed alkali; which unites with the acid, and for 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 couishable by its great specific gravity, being four tih in this it resembles an _ore_, it has not been found to be nesia_

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 fro soluble in water

_Asbestus_, which containsdestructible by heat, though it is so 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 soht 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 foruish 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

LECTURE XXI

_Of Ores_