producing metallic lead. Lead oxides, wh nearly all metals, except mercury, gold, silv With other oxides they form easily fu When heated with sulphur, lead oxides sulphurous acid is liberated. When oxide of quantity is melted with an infusible silicate silicate is formed. 3 12. Manganese dioxide (MnO,).—Th black in colour, opaque, and a good conduc When heated alone it is infusible, but g forming Mn2O, or MnO4, according to th employed; heated with charcoal it is reduce facility with which it gives up oxygen mak oxidising agent. With hydrochloric acid used for generating chlorine. When stron crucible lined with a paste of carbon it is metallic state. 13. Silica (SiO2). This body occurs in amorphous forms; it is white, infusible, ex highest temperatures, non-volatile, insoluble acids, except hydrofluoric; after ignition, it is carbon in the presence of iron, copper, or s heat, forming silicides of those metals. The gelatinous varieties are slightly soluble in alka but readily soluble in caustic alkalies. It co the bases forming silicates, and is there employed to effect the fusion and separation ores, the best forms to use being pure w quartz. 14. China clay is essentially a hydrat alumina, and when pure may be represented (2A120, 3SiO2)+30H2; but clay is general other silicates. It is white, and infusible i furnace when heated alone, but readily unite and metallic gangues to form a fusible slag. 15. Glass is a mixture of silicates of sodium with some insoluble silicate, such as silicate of nesium, aluminium, iron, or lead. Being a compound sili fuses easily at a high temperature, and readily combines lime and other bases containing little or no silica, so tha often preferred to pure silica, and serves to economise It is also employed as a covering in melting metals, so exclude the air. Plate or window glass, or green bottle is the most useful, but flint glass, which contains much of lead, would be detrimental in many cases. 16. Ferrous sulphide (FeS) is chiefly used as a sou sulphuretted hydrogen. Roasted with easily decomp sulphides, such as that of silver, it converts them int phates. Heated with oxides of copper, nickel, etc., it regulus. Heated in air it is oxidised to sulphate, and high temperature to oxide. 17. Iron pyrites (FeS,).-This body loses ha sulphur at a white heat, forming ferrous sulphide, a used for similar purposes to that compound. It is c employed in the metallurgy of copper, nickel, and coba 18. Ferric oxide (Fe2O3).-This oxide is very s non-volatile, and of a red colour. At a white heat it up oxygen, forming Fe3O4. By heating with carb carbonic oxide, it is reduced to the metallic state, 1 much carbonic acid is present, ferrous oxide may be fo which combines with any silica present, forming a f silicate. For this reason it is sometimes used as a flux refining iron it acts as an oxidising agent. In prese sulphur it oxidises that element to sulphurous acid. 19. Zinc oxide (ZnO) is a powerful base; it forms binations with alkaline earths and several bases, and strong affinity for alumina. It is reduced by ca carbonic oxide, and hydrogen. Zinc oxide and carb small quantity is added to molten copper for producing castings. REFRACTORY MATERIALS § 17. For melting metals a furnace is required, bu or at least lined inside with, a material capable of standing high temperatures without fusing decomposing by the heat to which it is subje the exterior is constructed of ordinary maso terior is lined with refractory material, the depends to a great extent upon the character to be performed in it. Refractory materials in the natural state, such as silica, alumina magnesia, and fire-clay, or they undergo a paration before use. In some cases the mater to the internal shape of the furnace. If th plastic nature like fire-clay, then clay, tar, material is intimately incorporated with th impart the necessary plasticity. Quartz or Silica.-This substance neit melts at the highest furnace temperatures, an valuable material for internal construction, eit with refractory clay to form silica bricks, alone as a lining for the beds of reverber Sand is not composed of pure silica, but the s of lime, oxide of iron, and clay usually presen tionable. Dinas clay is a highly refract occurring in the Vale of Neath, and contain cent of silica, the remainder being lime, alumina, alkali, and water. From this materia with 1 to 3 per cent of lime, bricks are mad valuable linings for the roofs of many reverber Alumina is quite as infusible as silica, and tage of not generally combining with bases, and so combine the aluminates formed are less fusible But pure alumina is rarely found in nature approach to it in large quantity is bauxite, w found to consist of 52 per cent alumina, 27.6 and 20.4 per cent water; but the composit different specimens. The ordinary mineral conta cent silica, 24 to 25 per cent oxide of iron, 50 alumina, and 10 to 15 per cent water. It is g red in colour like an iron ore, but some v softening 1. As a r he operat - either use ide of in minary pre are moul! re not of a her binding n order t softens therefore hen mixed hen used furnaces! quantities not objec ubstance t 97 per ch form it does nearest erthier f iron, ies in 5 per r cent exist with but little iron, and are then white in colour and refractory. Lime and Magnesia. These are infusible bodies, str basic in character, but they form fusible compounds silica and other acid bodies. This property is utilised in steel furnaces, the interiors of which are lined with oxides, which abstract phosphoric acid from the iron, for stable phosphates. Lime and magnesia occur togeth dolomite, from which material, after calcination, basic are prepared. P Fire-Clay. The refractory bodies already referr may combine together in certain proportions, without ce to be refractory. Fire-clay is a hydrated silicate of al with varying amounts of lime, magnesia, oxide of alkali, etc., and some mechanically mixed silica. The p property which clays possess is due to the chem combined water. In all cases the plasticity disap when the clay has been baked, and it remains granula powdery. The clays of the coal-measures, such as the Stourbridge, are admirably adapted for making fire-b although not pliant enough for pottery. In fact an of plasticity is a disadvantage for some metallurgical pur as bricks, for example, would crack at the time of baki § 18. Crucibles, etc.-Earthen crucibles are ma fire-clay mixed with sand, burnt clay, or other inf matter, so as to counteract the tendency which raw possesses of shrinking when heated. The bodies thus with the clay expand, or do not contract on heating, h been already shrunk when burnt, and therefore act opposite manner to the clay. Such a composition mu able to resist a high temperature without softening, not be friable when hot, and be capable of withsta sudden changes of temperature without cracking, a instance, when a white hot crucible is brought out furnace into cold air. Some crucibles are required to resist the corrosive of metallic oxides in the material operated u ashes of the fuel, so that a crucible should b is best adapted to the special purpose to wh applied. The component parts of a crucible are fi fine powder, and passed through a sieve, whose meshes will vary with the desired grain in the pot (the plasticity being closely the fineness of the particles; at any rate for this closeness of grain appears to be indispens fine powder is mixed with water and kneade consistence for use. The best results are obt a mixture of different fire-clays, the most those containing the largest amount of s minimum of oxides of iron and lime. The pre and soda in small quantity sensibly increases but they act advantageously in soldering together. Iron pyrites, which is frequently through clays, especially those from the coa perhaps the most injurious constituent. from such clay will become indented with smal even holes, when exposed to a prolonged high It follows then that the most refractory crucib made from pure clays, the nearest approach presented by some French clays. The fitness of a clay for making crucibles termined by moulding a portion into the shape or any form containing sharp edges, carefully dr and exposing to a high temperature in a covered some time. If very refractory, the test will s of fusion. If the edges are rounded it is a proo fusion, and if melted, the clay is useless. Clay vessels of all kinds may be tested to as power of resisting corrosion by melting in them litharge, red oxide of copper, and borax, and time this mixture will take to permeate them. resist this destructive action the longest will of c |