to the latter class. The bases most often found in slags from smelting ores are:-lime, magnesia, oxide of iron, and alumina. Both the silica and bases are derived from the earthy matter of the ore, the ashes of the fuel, the flux, and the material of which the furnaces or vessels are constructed. In melting and mixing base metals to form alloys, some of these elements oxidise and pass into the slag, and often form the chief bases when united with silica, or other acid substances.

The silicates most frequently produced are those with an iron base, of which the most fusible is the protosilicate 2FeO. SiO2. The protosilicate of manganese appears to have about the same fusing point as the protosilicate of iron; it has an olive-green colour, is opaque, and but slightly crystalline. It does not absorb oxygen in contact with air like the silicates of iron, and cannot therefore be employed as an oxidising agent. A double or multiple silicate containing two or more bases is often more fusible than a single silicate; hence a compound silicate of lime, magnesia, alumina, etc., is more fusible than either of these bases combined alone with silica, and melts readily when strongly heated. Likewise, oxide of manganese augments the fusibility of earthy silicates.

Oxide of zinc forms single silicates, which are practically infusible, and, as a general rule, this oxide diminishes the fusibility of silicates of iron, lime, etc. Protoxide of tin augments the fusibility of multiple silicates. The proto- and bi-silicates of copper melt easily; they augment the fusibility of earthy silicates. Oxide of copper is easily separated, especially in the presence of sulphur or arsenic, with which it forms regulus or speise respectively. The oxides of lead and bismuth are, next to the alkalies, the most fluxing bases. Silicates of lead melt at a red heat. Oxide of lead increases the fusibility of multiple silicates, but it is easily reduced by the presence of metallic iron.

Slags are either vitreous or stony, and not infrequently they are more or less crystalline. Rapid cooling tends to

produce the glassy variety, and by slow cooling the crystalline structure is induced. The stony condition is prevalent in slags in which earthy bases predominate. The object in forming slags in melting metals is to form readily fusible compounds, which shall contain the impurities it is desirable to remove, and at the same time be as free as possible from the metal or metalliferous substance undergoing preparation. When the slag is free from useful metals, it is said to be clean; but when a slag contains useful ingredients it is often advisable to remelt it with another charge, so as to extract valuable components, and render it sufficiently clean to be thrown away. It frequently happens that the slags produced in dealing with a given kind of metalliferous matter are useful as a flux or other agent in extracting or melting different metals.

When the charge is improperly prepared, and either silica or lime predominate, or when the amount of alumina or magnesia is very large, or the temperature insufficient, the slags are imperfectly melted, and present a kind of granular fracture.

§ 16. Fluxes.—A flux is a substance added to metalliferous matter, in order to unite with the foreign ingredients, and form a fusible slag. The flux employed in any given case varies with the nature of the bodies to be removed. Thus, if the impurity be of an acid nature, such as silica, the flux should be basic or neutral. If, on the other hand, a basic substance has to be separated, an acid flux will be generally required. In many cases of simple melting of metals it is desirable to remove foreign metals, which are present as impurities; it is then advisable to add an oxidising flux, in order first to oxidise the impurities, in which state they readily combine with an acid flux to form a liquid slag, or volatilise, or simply rise to the surface in virtue of their lower specific gravities.

It frequently happens that a slag is formed at one stage of an operation, and decomposed again by the action of some

reducing agent present. With regard to silicates, the most difficult to reduce are those with earthy bases, such as lime, magnesia, alumina, etc. When a slag contains several oxides, the most feeble bases will be reduced first, and the others in the order of their basicity. Suppose a compound silicate to contain the oxides of manganese, iron, tin, and lead; the oxide of lead will be the first to be reduced, then the oxide of tin, then the oxide of iron, and lastly the oxide of manganese. The composition of silicates has a marked influence upon their consistence when melted. The bisilicates, or silicates with an excess of silica, pass gradually from the solid to the liquid state, or conversely from the liquid to the solid state; they preserve for a long time a "viscous" or "plastic" consistency. The protosilicates or basic silicates, on the contrary, pass more or less quickly from the liquid to the solid state, and on cooling they set rapidly. They are very liquid when melted, and crack at the moment of solidifying.

The most fusible silicates are formed by union of silica with fusible bases, such as soda, potash, oxide of lead, oxide of bismuth, etc.; and the silicate formed is fusible in proportion to the amount of base present. With infusible bases, on the contrary, the fusibility is greatest when neither acid nor base is in excess. Baryta BaO is more fluxing than lime CaO, and magnesia MgO is still less fluxing than lime. Alumina AlO is, among the common bases, the least fluxing.

Fluor-spar CaF, has a wide and well-deserved reputation as a flux. At a high temperature it melts into a transparent liquid. It is partly decomposed by silica, with the liberation of the gas silicon fluoride, but the greater part is found in the slag, in which it favours fusion and fluidity. When the bodies to be fluxed off are sulphates of lime, baryta, strontia, and bone-ash (phosphate of lime), fluor-spar is specially useful. A tenth part of fluor-spar is sufficient to liquefy sulphate of lead at a bright red heat.

Sulphur is often present in metalliferous substances, in the form of sulphides, such as silver sulphide, for example.

In that case metallic lead may be employed, which decomposes the sulphide of silver, forming sulphide of lead, and isolating the silver, which then unites with the excess of lead. In the same way iron decomposes lead sulphide, forming sulphide of iron and metallic lead. The metals of the alkalies and alkaline-earths are very effectual in removing sulphur from metallic sulphides. The metals are obtained for this purpose by reducing their oxides by carbonaceous matter in admixture with the metallic sulphide. In this case an alkaline sulphide is generally formed, together with an oxide of the metal. In some instances the decomposition of the metallic sulphide is incomplete. Certain sulphides, such as zinc sulphide, are rendered fusible by the addition of sulphide of iron, or iron pyrites. Many sulphides are fusible alone; and in most cases it is easy to transform them into oxides by roasting in atmospheric air, or by fusion with an oxidising flux.

When litharge PbO is melted with sulphides and arsenides, sulphurous and arsenious acids are volatilised, and an alloy of the metals with lead is formed. Other metallic oxides act in a similar manner, but, as a general rule, the sulphide is more or less converted into oxide, depending upon the amount of oxide used, and the relatively chlorous and basylous characters of the metals, and of sulphur and oxygen.

Lead carbonate acts on sulphides in the same way as lead oxide. Lead silicate is also oxidising, but in a less degree, the slags produced containing both metals. Lead sulphate is more oxidising than litharge. When it is melted in the right proportion with sulphide of lead, the whole of the lead is reduced. Copper and iron sulphates are generally oxidising agents for sulphides. Nitrates, such as saltpetre KNO, are the most powerful oxidising agents in melting operations. Basic iron slags are very active agents in promoting oxidation. For example, when iron containing carbon, silicon, sulphur, and phosphorus is strongly heated with basic silicate of iron these impurities are largely oxidised, forming carbonic oxide, silica, sulphurous acid, and phosphoric acid.

Following is a list of the most common fluxes, with a brief account of their properties and uses :

1. Ammonium chloride (AmCl) called sal-ammoniac. This substance is decomposed by several metals forming metallic chlorides and liberating ammonia, which property is taken advantage of in purifying gold. A similar reaction occurs with several metallic salts.

2. Sodium chloride (NaCl) or common salt, is employed for preserving the substance beneath from the action of the atmosphere, and to moderate the action of bodies which cause violent ebullition. It melts and volatilises at a red heat in an open crucible, but requires a white heat to vaporise it in a closed vessel. When heated to redness with silica it forms a readily fusible silicate. It forms fusible compounds with antimony and arsenic, thus removing them from other metals during the process of refining. As the crystals decrepitate when heated, common salt should be powdered before using as a flux.

3. Borax (B40,Na).-In the crystalline form it may contain 5 or 10 molecules of water, which are given off on heating, causing an enormous increase in volume, so that the vitrified form is much more suitable for assaying. It forms fusible compounds with silica, and nearly all bases, being especially useful in uniting with metallic oxides, sulphides, and arsenides. The commercial salt is adulterated with common salt and alum.

4. Sodium carbonate (Na,CO) has the property of oxidising many metals, such as tin, iron, zinc, etc., by the action of its carbonic acid, and as a consequence of this action it acts as a desulphuriser. It forms fusible compounds with silica and many metallic oxides; it also melts at a low temperature, absorbing many infusible substances, such as lime, alumina, charcoal, etc. In some cases it acts as a reducing agent, as in the case of chloride of silver. When mixed with carbonate of potash a double salt is formed, which fuses at a lower temperature than either taken alone, a property very useful in the fusion of silicates, etc.