All metals combine more or less with sulphur, and form sulphides when sulphur is brought into contact with the metal, in the absence of oxygen or chlorine. When oxides are treated with sulphur in sufficient quantities to absorb all the oxygen in forming sulphuric acid, the sulphur remaining combines with the metal. When sulphates are treated in the presence of carbon or hydrogen, the oxygen of the sulphuric acid is abstracted, and sulphides remain. The chemical relation of sulphur to metal is similar to that of oxygen - that is, the number and equivalents of the sulphides correspond with the number and equivalents of the oxides of the respective metals - causing them to be more fluid and brittle when cold, and impairing their ductility when hot. Large quantities of sulphur cause a low degree of fusibility which is shown in the sulphurets of antimony, lead, copper, and iron, the fusibility in each decreasing more rapidly than the evaporation of sulphur. Iron pyrites melts at a low red heat; but when reduced to half its original quantity, by evaporating the sulphur, it requires a strong white heat to melt the sulphides. The presence of free oxygen is required for the removal of sulphur; nor can it be removed entirely when carbon, hydrogen, or any other reducing agent is present, an oxidizing influence and thorough exposure of the metal to oxygen being necessary.

Nevertheless, the partial decomposition which certain metallic sulphides undergo, when heated without the access of atmospheric air, is to the metallurgist a consideration of importance. Galena treated in this way suffers partial decomposition; so, in like manner, does the monosulphuret, or monosulphide of copper, - a sufficient amount of sulphur being evolved from it to yield disulphide of copper as the permanent fixed result. The higher sulphur combinations of iron, or chemically speaking, the sulphur salts of that metal, generated by the combination of two sulphurets or sulphides, also give a portion of their sulphur when exposed to high heat in close vessels. Monosulphide of iron, however, does not yield up any of its oxygen by the mere process of heating in close vessels. The sulphide of zinc (zinc blende) is unchanged by the highest temperature; so, in like manner, is the sulphide of silver. The sulphides of gold and of platinum are decomposed when heated into sulphur and their respective metals. The sulphide of mercury can be distilled without change. Sulphide of antimony melts at a high red heat, afterwards distils over unchanged. The mono and the ter-sulphide of arsenic (orpiment and realgar) both fuse, and distil without undergoing any decomposition.

By far the more important and usual method, however, of effecting the reduction of metallic sulphides, consists in exposing them to the combined agency of heat and atmospheric air - constituting, in point of fact, the operation of roasting. Usually, the change which ensues during the operation of roasting,, is the conversion of sulphur of the sulphide into sulphurous acid gas, which escapes: the original sulphide, either losing a part of its sulphur, and being thus reduced to the lower stage of sulphurization, or else, losing the whole of its sulphur, oxygen is absorbed in place of the latter. Occasionally the sulphurous acid first generated absorbs the necessary amount of oxygen, to change it into sulphuric acid, which combining with the metallic oxide simultaneously generated, gives rise to the sulphate of an oxide. This latter is the case when galena (sulphide of lead) is roasted, the final result of the operation being oxide of lead, and sulphate of oxide of lead. This change is eminently favorable to subsequent metallurgic operations of which galena is the subject If the galena be argentiferous, the following reactions ensue: The mixture of oxide of lead and sul-phate of the same oxide being heated to whiteness in contact with silver (of the argentiferous galena), oxidizes the silver by decomposition of the sulphuric acid, of the sulphate and oxide of lead; hence there results a mixture of oxide of silver and of lead - a mixture easily dealt with, and deoxidized by a subsequent operation. The sulphide and disulphide of copper are changed by roasting, into dioxide of copper and sulphurous acid, and sulphate of the oxide of copper, which latter, when the temperature is raised to the highest pitch, evolves the whole of its sulphuric acid and oxygen; leaving metallic copper. Monosulphide of iron by roasting undergoes many progressive changes; beginning with the formation of protoxide of iron and sulphurous acid, and ending in the development of sesquioxide of iron. Sulphide of zinc (zinc blende) slowly changes under the influence of roasting, first into oxide of zinc, and sulphate of the oxide; then into subsulphate of the oxide; and, lastly, into oxide exclusively. Sublimate of bismuth changes, under the influence of roasting, into oxysulphuret: sulphide of silver is decomposed, and yields metallic silver. Ter-sulphide of antimony changes under roasting into antimonious and antimonic acid. The sulphide and the sesquisulphide of arsenic are changed into arsenious and arsenic acids.

By a modification of the same process, sulphide of nickel admits of decomposition into a mixture of oxides and sesquioxides of that metal. Sulphide of cobalt is also decomposed into a mixture of oxide of that metal and sulphate of the oxide. Finally, the sulphides of gold, platinum, and mercury are also reduced to the metallic state, sulphurous acid gas being evolved.

Another element equal in importance to oxygen, requires the attention of the metallurgist. Chlorine has a tendency to induce metals to crystallize, and causes consequently fluidity and brittle ness. Chlorine removes all other matter from metals when the latter are in a state of fusion. Carbon, sulphur, and phosphorus are drawn off by it, and, if the heat is continued, the chlorine itself escapes with a portion of the metals, but only when a minute proportion is present; it is thus a powerful element in the purification of metals. Lead smelted from chlorides is purer than from oxides and sulphurets, and its proper application to smelting and refining purposes has a most beneficial influence. Zinc does not readily combine with iron unless chlorine be present; it removes oxygen from the protoxides, thus purifying the surface and preparing it for closer union with an alloy. All metals smelted under the influence of chlorine, are inclined to oxidize, unless it is removed entirely. It is harmless to the metals, powerful as a means of fluxing slags and ores, and producing fluidity; its use, therefore, ought to be much more extended than it has been.