whereby copper is produced and sulphur is eliminated as SO_{2}. The liberated copper interacts with the excess of iron sulphide usually present in the furnace charge, and enters the matte as sulphide, whilst the iron which is thus set free is oxidised and carried into the slag as silicate, the ultimate reactions being indicated approximately by the equation— 2Cu + FeS + _x_FeS ➡ Cu_{2}S . _x_FeS (matte) + Fe (oxidised and enters slag). Copper silicates readily interact with iron sulphides in the charge, producing copper sulphides and iron silicates, thus— Cu_{2}O . _x_SiO_{2} + FeS ➡ Cu_{2}S (enters matte) + FeO . _x_SiO_{2} (enters slag). 6(CuO . _x_SiO_{2}) + 4FeS ➡ 3Cu_{2}S (enters matte) + 4(FeO . _x_SiO_{2}) + 2_x_SiO_{2}(enters slag) + SO_{2}. All the above reactions lead to an enrichment of the matte in copper contents, and at the same time, to the transference of iron from the matte to the slag, and although the conditions in the more reducing atmosphere of the coke-fed blast furnace are not so favourable to the fullest operation of these reactions as are the more neutral conditions of the reverberatory, the addition of oxidised materials constitutes a valuable means of increasing the concentration in this method of smelting. The blast furnace is thus also particularly suited for the recovering of the copper from the oxidised residues, such as converter slags and scrap, “calcine-barrings,” and the like, which accumulate in very considerable quantities at a smelter, and which by reason of their carrying much copper as oxide or silicate, not only add their quota of copper to the products, but materially assist the concentration and the furnace operation generally. B. ii. _The Pyritic Principle in Blast-Furnace Smelting._—This is the most important principle introduced into modern blast-furnace smelting practice. It has been evolved by the application of the results of experiments conducted from two different points of view—one series mainly on a laboratory scale, the other from actual industrial practice. Starting from theoretical considerations, John Holway demonstrated by experiment that the heat of oxidation of the iron and the sulphur of pyritic copper ores was so great as to make their smelting a self-supporting operation under suitable conditions. On the other hand, within comparatively recent years, smeltermen as a result of working practice, have found that an increase of sulphides on the furnace charge has led to less and less carbonaceous fuel being necessary for the smelting operations, providing that the conditions in the blast furnace be sufficiently oxidising. In utilising these results for general blast-furnace practice, the extended and successful application of this pyritic principle has led to marked advance in modern working.