Heat may be applied to vessels em ployed in chemical operations, directly, as in the open fire and reverberatory fur nace : or through the medium of sand ; the sand-bath : of water ; the water-bath of steam ; the vapour-bath : of air ; as in the muffle.
Changes produced by chemical processes. These consist chiefly in a new mode of aggregation, combination, and decompo sition.
The form of aggregation may be alter ed by fusion, vaporization, condensation, congelation, and coagulation.
Fusion is the conversion of a . solid into a liquid by the sole agency of calo ric. Substances differ very much in the degrees of their fusibility; some, as wa ter and mercury, existing as fluids in the ordinary temperatures of the at mosphere ; while others, as the pure earths, cannot be melted by any heat we can produce.
Liquefaction is commonly employed to express the melting of substances, as tallow, wax, resin, &c. which pass through intermediate states of softness before they become fluid. Fusion is the melt ing of substances which pass immediate ly from the solid to the fluid state, as the salts and metals, except iron and pla tinum.
When, in consequence of fusion, the substances operated on acquire a greater or less degree of transparency, a dense uniform texture, and great brittleness, and exhibit a conchoidal fracture, with a specular surface, and the edges of the fragments very sharp, it is termed vitri faction.
In general, simple substances are less fusible than compounds; for example, the simple earths cannot be melted singly, but when mixed are easily fused. The ad ditions which are sometimes made to re fractory substances, to promote their fu sion, are termed fluxes : which fluxes are generally saline bodies.
Thus, the alkalies potash and soda pro mote powerfully the fusion of silicious stones ; but they are only used for accu rate experiments. The white flux is a mixture of a little potash with carbonate of potash, and is prepared by deflagrating together equal parts of nitrate of potash and super-tartrate of potash. When an ox ide is at the same time to be reduced, the black flux is preferred, which is produc ed by the deflagration of two parts of super-tartrate of potash, and one of ni trate of potash. It differs from the for mer only in containing a little charcoal. Soap promotes fusion by being converted by the fire into carbonate of soda and charcoal.
Aluminous stones have their fusion greatly promoted by the addition of sub borate of soda.
Muriate of soda, the mixed phosphate of soda and ammonia, and other salts, are also occasionally employed for the same purpose.
An open fire is sufficient to melt some substances, others require the heat of .a furnace.
The vessels in which fusion is perform ed, must resist the heat necessary for the operation. In some instances an iron or copper ladle or pot may be used; but most commonly crucibles are employed. These are of various sizes, The large crucibles are generally conical, with a small spout for the convenience of pour ing out ; the small ones are truncated tri angular pyramids, and are sold In nests. The Hessian crucibles are com posed of clay and sand, and when good, will support an intense heat for many hours, without softening or melting ; but they are disposed to crack when sudden ly heated or cooled. This inconvenience may be on many occasions avoided, by using a double crucible, and filling up the interstice with sand, or by covering the crucible with a lute of clay and sand, by which means the heat is transmitted more gradually and equally. Those which ring clearly when struck, and are of an uniform thickness, and have a red dish brown colour, without black spots, are reckoned the best. Wedgewood's crucibles arc made of clay mixed with baked clay finely pounded, and are in every respect superior to the Hessian, but they are very expensive. The black lead crucibles, formed of clay and plum bago, are very durable, resist sudden changes of temperature, and may be re peatedly used, but they are destroyed when saline substances are melted in them, and suffer combustion when ex posed red hot to a current of air.
When placed in a furnace, crucibles should never be set upon the bars of the grate, but always upon a support. Dr. Kennedy found the hottest part of a fur nace to be about an inch above the grate. They may be covered, to prevent the fuel or ashes from falling into them, with a lid of the same materials, or with ano ther crucible inverted over them. When the fusion is completed, the substance may be either permitted to cool in the crucible, or may be poured into a heated mould anointed with tallow, never with oil, or what is still better, covered with a thin coating of chalk, which is applied by laying it over with a mixture of chalk diffused in water, and then evaporating the water completely by heat. To pre vent the crucible from being broken by cooling too rapidly, it is to be either re placed in the furnace, to cool gradually with it, or covered with some vessel to prevent its being exposed immediately to the air.