When heated in a powdered condition in excess of air, copper sulphide is oxidised, oxides of copper and sulphur being produced. There occur probably several intermediate reactions, and several intermediate products are formed, but the main effect is represented by the equations— ┌─ │ Cu_{2}S + 2O ➡ 2Cu + SO_{2} ─┤ │ 2Cu + O ➡ Cu_{2}O └─ which take place simultaneously, the copper represented in the first equation being oxidised spontaneously according to the second, and the resultant is the reaction Cu_{2}S + 3O ➡ Cu_{2}O + SO_{2}. In the furnace operations, some of the SO_{2} in the presence of air and oxidisable material, and in contact with the heated brickwork becomes oxidised to SO_{3}, which, interacting with the oxides and sulphides present, combines to form copper sulphate and cupric oxide. At a higher temperature the sulphate is again decomposed to CuO and SO_{3}, some of which passes off and is free to oxidise more sulphide; the rest is decomposed to SO_{2} and oxygen. These reactions occur during the roasting of charges containing copper sulphides. =Copper Mattes.=—On smelting a furnace charge which contains both copper and sulphur, the sulphur appears to have a stronger attraction for the copper than for any of the other metals usually present, and only when this affinity has been satisfied is the excess sulphur free to combine with other constituents of the charge. The fusible copper sulphide which is thus produced, has the power of mixing completely with any more sulphides which may be present, especially with sulphide of iron. The fused sulphides resulting from such furnace operations are termed _copper-mattes_. They may contain from a mere trace to upwards of 80 per cent. of copper, and in ordinary work, sulphide of iron is the other constituent present in the greatest proportion, but sulphides of nickel, silver, zinc, or lead, etc., may also be found, as well as arsenides and antimonides. These facts relating to the collection of the copper as a constituent of a fused sulphide product, form the basis of modern copper-smelting work. In view of the practical importance of the mixed sulphides, the diagram representing their equilibrium requires notice. A number of workers have studied the question with widely differing results. Röntgen made an exhaustive investigation of the system FeS—Cu_{2}S, and published a very complete diagram of the series, working with FeS and Cu_{2}S in the pure state.