Fusion is performed with the intention of weakening the attraction of aggrega tion; or of separating substances of dif ferent degrees of fusibility from each other.
Vaporization is the conversion of a solid to fluid into vapour by the agency of ca loric. Although vaporability be merely a relative term, substances are said to be permanently elastic, volatile or fixed. The permanently elastic fluids or gases are those which cannot be condensed into a fluid or solid form by any abstrac tion of caloric we are capable of produc ing. Fixed substances, on the contrary, are those which cannot be converted into vapour by great increase of tempera ture. The pressure of the atmosphere has very considerable effect in varying the degree at which substances are con verted into vapour. Some solids, unless subjected to very great pressure, are at once converted into vapour, although most of them pass through the interme diate state of fluidity.
Vaporization is employed to separate substances differing in volatility ; and to promote chemical action, by disaggregat ing them.
When employed with either of these views, no regard is paid to the substances volatilized, whether from solids, as in us tulation and charring ; or from fluids, as in evaporation ; or whether the substances vaporized are condensed in proper ves sels : for example, in a liquid form, as in distillation ; or in a solid form, as in sub limation. Or whether the substances va porized are permanently elastic, and are collected in their gaseous form, in a pneu matic apparatus.
Ustulation is almost entirely a metal lurgic operation, and is employed to ex pel the sulphur and arsenic contained in some metallic ores. It is performed on small quantities in tests placed within a muffle. Tests are shallow vessels made of bone ashes or baked clay. Muf fles are vessels of baked clay, of a semi cylindrical form, the flat side forming the floor, and the arched portion the roof and sides. The end and sides are per forated with holes for the free transmis sion of air, and the open extremity is plac ed at the door of the furnace, for the in spection and manipulation of the process. The reverberatory furnace is commonly employed for roasting, and the heat is at first very gentle, and slowly raised to red ness. It is accelerated by exposing as
large a outface of the substance to be roasted as possible, and by stirring it fre quently, so as to prevent any agglutina tion, and to bring every part in succession to the surface.
Charring may be performed on any of the compound oxides, by subjecting them to a degree of heat sufficient to expel all their hydrogen, nitrogen, and supera abundant oxygen, while the carbon, be ing a fixed principle, remains behind in the state of charcoal. The temperature duced either by the combustion of other substances, or by the partial combustion of the substance to be charred. In the former case, the operation may be per formed in• any vessel which excludes the access of air, while it permits the escape of the vapours formed. In the latter, the access of air must be regulated in such a manner, that it may be suppressed when ever the combustion has reached the re quisite degree ; for if continued to be ad mitted, the charcoal itself would be dissi pated in the form of carbonic acid gas, and nothing would remain but the alka line and earthy matter, which these sub stances always contain. When combus tion is carried this length, the process is termed incineration. The vapours which arise in the operation of charring, are sometimes condensed, as in the manufac ture of tar.
Evaporation is the conversion of a fluid into vapour, by its combination with ca loric. In this process, the atmosphere is not a necessary agent, but rather a hin drance, by its pressure. This forms a cri terion between chemical evaporation and spontaneous evaporation, which is mere ly the solution of a fluid in air. It is per formed in open, shallow, or hemispheri cal vessels of- silver, tinned copper or iron, earthenware or glass. The neces sary caloric may be furnished by means of an open fire, a lamp, or a furnace, either immediately, or with the interven tion of sand, water, or vapour. The de gree of heat must be regulated by the na ture of the substance operated on. In ge neral, it should not be greater than what is absolutely necessary.