5. Potassium nitrate (KNO), also called nitre and saltpetre, is largely used as an oxidising agent. It fuses below redness, and at a higher temperature is decomposed, yielding a large volume of oxygen, whereby the sulphur of metallic sulphides is converted into sulphurous acid, and the metals into oxides. Sodium nitrate acts in the same way.

6. Potassium bitartrate (THoKo), known also as cream of tartar or tartar. When pure this substance is white, but the variety chiefly used on the large, scale is coloured, and sold as red "argol"; this is cheaper, and contains other carbonaceous matters, which give it greater reducing power than pure cream of tartar. This reagent is very valuable in operations requiring much carbonaceous matter.

7. Potassium chlorate (KClO3).—This substance is sometimes used with nitre as an oxidising agent, especially in assaying.

8. Potassium cyanide (KCN).—This flux is valuable on account of the facility with which it fuses, and the readiness with which it reduces many metallic compounds when mixed with carbonate of soda. Common cyanide is preferable as a reducing agent, because it contains carbonate of potash.

9. Calcium oxide (CaO) or lime, is used in the caustic state, or combined with carbonic acid in the form of carbonate. It is a useful flux for silica and silicates; and is also used to remove sulphur and phosphorus from metals and their compounds.

10. Calcium fluoride (CaF) or fluor-spar. This substance acts as a flux in two ways-(1) by combining with silicates, forming fusible compounds; (2) by reacting with silicates and evolving the gas silicon fluoride SiF4. It forms fusible compounds with sulphates, such as plaster of Paris, and with phosphate of lime (bone-ash). It should be free from pyrites, blende, and galena, with which it is likely to be contaminated.

11. Lead oxide.-There are two oxides of lead of importance in treating metals, viz. litharge PbO, and red-lead PbO4. Both oxides are reduced by carbon or hydrogen,

producing metallic lead. Lead oxides, when melted, oxidise nearly all metals, except mercury, gold, silver, and platinum. With other oxides they form, easily fusible compounds. When heated with sulphur, lead oxides are reduced, and sulphurous acid is liberated. When oxide of lead in sufficient quantity is melted with an infusible silicate, a fusible double silicate is formed.

3

12. Manganese dioxide (MnO).-This substance is black in colour, opaque, and a good conductor of electricity. When heated alone it is infusible, but gives off oxygen, forming Mn,O, or Mn,O4, according to the degree of heat employed; heated with charcoal it is reduced to MnO. The facility with which it gives up oxygen makes it a valuable oxidising agent. With hydrochloric acid it is extensively used for generating chlorine. When strongly heated in a crucible lined with a paste of carbon it is reduced to the metallic state.

13. Silica (SiO2).—This body occurs in crystalline and amorphous forms; it is white, infusible, except at the very highest temperatures, non-volatile, insoluble in water and acids, except hydrofluoric; after ignition, it is decomposed by carbon in the presence of iron, copper, or silver at a white heat, forming silicides of those metals. The amorphous and gelatinous varieties are slightly soluble in alkaline carbonates, but readily soluble in caustic alkalies. It combines with all the bases forming silicates, and is therefore frequently employed to effect the fusion and separation of gangues in ores, the best forms to use being pure white sand and quartz.

14. China clay is essentially a hydrated silicate of alumina, and when pure may be represented by the formula (2A120, 3SiO2)+30H2; but clay is generally mixed with other silicates. It is white, and infusible in an ordinary furnace when heated alone, but readily unites with earthy and metallic gangues to form a fusible slag.

15. Glass is a mixture of silicates of sodium and potassium with some insoluble silicate, such as silicate of barium, mag

nesium, aluminium, iron, or lead. Being a compound silicate it fuses easily at a high temperature, and readily combines with lime and other bases containing little or no silica, so that it is often preferred to pure silica, and serves to economise borax. It is also employed as a covering in melting metals, so as to exclude the air. Plate or window glass, or green bottle glass is the most useful, but flint glass, which contains much oxide of lead, would be detrimental in many cases.

16. Ferrous sulphide (FeS) is chiefly used as a source of sulphuretted hydrogen. Roasted with easily decomposable sulphides, such as that of silver, it converts them into sulphates. Heated with oxides of copper, nickel, etc., it forms regulus. Heated in air it is oxidised to sulphate, and at a high temperature to oxide.

17. Iron pyrites (FeS2).-This body loses half its sulphur at a white heat, forming ferrous sulphide, and is used for similar purposes to that compound. It is chiefly employed in the metallurgy of copper, nickel, and cobalt.

18. Ferric oxide (Fe2O3).—This oxide is very stable, non-volatile, and of a red colour. At a white heat it gives up oxygen, forming Fe3O4. By heating with carbon, or carbonic oxide, it is reduced to the metallic state, but if much carbonic acid is present, ferrous oxide may be formed, which combines with any silica present, forming a fusible silicate. For this reason it is sometimes used as a flux. In refining iron it acts as an oxidising agent. In presence of sulphur it oxidises that element to sulphurous acid.

19. Zinc oxide (ZnO) is a powerful base; it forms combinations with alkaline earths and several bases, and has a strong affinity for alumina. It is reduced by carbon, carbonic oxide, and hydrogen. Zinc oxide and carbon in small quantity is added to molten copper for producing sound castings.

REFRACTORY MATERIALS

§ 17. For melting metals a furnace is required, built of, or at least lined inside with, a material capable of with

standing high temperatures without fusing, or softening, or decomposing by the heat to which it is subjected. As a rule the exterior is constructed of ordinary masonry, but the interior is lined with refractory material, the nature of which depends to a great extent upon the character of the operation to be performed in it. Refractory materials are either used in the natural state, such as silica, alumina, oxide of iron, magnesia, and fire-clay, or they undergo a preliminary preparation before use. In some cases the materials are moulded to the internal shape of the furnace. If they are not of a plastic nature like fire-clay, then clay, tar, or other binding material is intimately incorporated with them, in order to impart the necessary plasticity.

Quartz or Silica.-This substance neither softens nor melts at the highest furnace temperatures, and is therefore a valuable material for internal construction, either when mixed with refractory clay to form silica bricks, or when used alone as a lining for the beds of reverberatory furnaces. Sand is not composed of pure silica, but the small quantities of lime, oxide of iron, and clay usually present are not objectionable. Dinas clay is a highly refractory substance, occurring in the Vale of Neath, and contains about 97 per cent of silica, the remainder being lime, oxide of iron, alumina, alkali, and water. From this material, when mixed with 1 to 3 per cent of lime, bricks are made, which form valuable linings for the roofs of many reverberatory furnaces.

Alumina is quite as infusible as silica, and has the advantage of not generally combining with bases, and when it does so combine the aluminates formed are less fusible than silicates. But pure alumina is rarely found in nature, the nearest approach to it in large quantity is bauxite, which Berthier found to consist of 52 per cent alumina, 27·6 oxide of iron, and 20.4 per cent water; but the composition varies in different specimens. The ordinary mineral contains 3 to 5 per cent silica, 24 to 25 per cent oxide of iron, 50 to 60 per cent alumina, and 10 to 15 per cent water. It is generally dark red in colour like an iron ore, but some varieties exist

with but little iron, and are then white in colour and very refractory.

Lime and Magnesia. These are infusible bodies, strongly basic in character, but they form fusible compounds with silica and other acid bodies. This property is utilised in some steel furnaces, the interiors of which are lined with these oxides, which abstract phosphoric acid from the iron, forming stable phosphates. Lime and magnesia occur together in dolomite, from which material, after calcination, basic bricks are prepared.

Fire-Clay. The refractory bodies already referred to may combine together in certain proportions, without ceasing to be refractory. Fire-clay is a hydrated silicate of alumina with varying amounts of lime, magnesia, oxide of iron, alkali, etc., and some mechanically mixed silica. The plastic property which clays possess is due to the chemically combined water. In all cases the plasticity disappears when the clay has been baked, and it remains granular and powdery. The clays of the coal-measures, such as those of Stourbridge, are admirably adapted for making fire-bricks, although not pliant enough for pottery. In fact an excess of plasticity is a disadvantage for some metallurgical purposes, as bricks, for example, would crack at the time of baking.

§ 18. Crucibles, etc.-Earthen crucibles are made of fire-clay mixed with sand, burnt clay, or other infusible matter, so as to counteract the tendency which raw clay possesses of shrinking when heated. The bodies thus mixed with the clay expand, or do not contract on heating, having been already shrunk when burnt, and therefore act in an opposite manner to the clay. Such a composition must be able to resist a high temperature without softening, must not be friable when hot, and be capable of withstanding sudden changes of temperature without cracking, as, for instance, when a white hot crucible is brought out of a furnace into cold air.

Some crucibles are required to resist the corrosive action