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. LECTURE XXI. _Of Ores_. Metallizable earths, commonly called _ores_, when united to phlogiston, make the metals, distinguishable for their specific gravity, their opacity, shining appearance, and fusibility. All the proper metals are _malleable_, and those which are not so are called _semi-metals_. The metals again are subdivided into the _perfect_ and _imperfect_. The former, which are _gold_, _silver_, and _platina_, suffer no change by fusion, or the longest continued heat: whereas heat calcines or dissipates the phlogiston of the imperfect metals, which are _mercury_, _lead_, _copper_, _iron_, and _tin_, so that they return to the state of earth; and this earth is always heavier than the metal, though of less specific gravity, having received an addition of weight from water or air: but these earths, or ores, being exposed to heat in contact with substances containing phlogiston, again become metals, and are then said to be _revived_.