Operations on bodies that are absolutely fixed, as metals, stones, sand, _&c._ require only such vessels as are capable of containing those bodies, and resisting the force of fire. These vessels are little hollow pots, of different dimensions, which are called Crucibles. Crucibles can hardly be made of any thing but earth; they ought to have a cover of the same material fitted to shut them close. The best earth we know is that whereof those pots are made in which butter is brought from Bretagne: these pots themselves are exceeding good crucibles; and they are almost the only ones that are capable of holding glass of lead in fusion, without being penetrated by it. For the roasting of ores, that is, freeing them, by the help of fire, from their sulphureous and arsenical parts, little cups of the same material with crucibles are used; but they are made flat, shallow, and wider, above than below, that these volatile matters may the more freely exhale. These vessels are called Tests, or Scorifiers: they are scarce ever used but in the Docimastic art, that is, in making small Assays of ores. CHAP. XIX. _The_ THEORY _of_ CONSTRUCTING _the_ FURNACES _most commonly used in_ CHYMISTRY. Skill in conducting and applying fire properly, and determining its different degrees, is of very great consequence to the success of Chymical operations. As it is exceeding difficult to govern and moderate the action of fire, when the vessels in which any operation is performed are immediately exposed to it, Chymists have contrived to convey heat to their vessels, in nice operations, through different mediums, which they place occasionally between those vessels and the fire. Those intermediate substances in which they plunge their vessels are called Baths. They are either fluid or solid: the fluid baths are water or its vapours. When the distilling vessel is set in water, the bath is called _Balneum Mariæ_, or the _Water Bath_; and the greatest degree of heat of which it is susceptible is that of boiling water. When the vessel is exposed only to the vapours which exhale from water, this forms the _Vapour Bath_; the heat of which is nearly the same with that of the _Balneum Mariæ_. These baths are useful for distilling essential oils, ardent spirits, sweet-scented waters; in a word, all such substances as cannot bear a greater heat, without prejudice either to their odour, or to some of their other qualities.