The presence of lime silicate with the iron silicates has a marked influence on the fluidity of the slags, even when they are more highly siliceous, whilst on account of the lower atomic weight of calcium, lime will, weight for weight, flux off a greater quantity of silica than will ferrous oxide. In forming a slag of similar oxygen ratio, thus— Mono-silicate of lime, 2CaO . SiO_{2}, Lime : silica :: 112 to 60, or 1 part to 0·54 part. Mono-silicate of iron, 2FeO . SiO_{2}, Iron oxide : silica :: 144 to 60, or 1 part to 0·42 part; hence for the production of a slag of the same oxygen ratio, less weight of lime would be required to flux off the same weight of silica; in other words, the replacing values of the two oxides are as 112 to 144, or 7 to 9. Of the other bases which are occasionally present in slags, the proportions of the oxides of magnesium and zinc are sometimes considerable, the calculations being analogous to the previous cases. The case of alumina is anomalous, and its behaviour in slag production is not definitely understood. Many experienced workers hold the view that it tends to act either as acid or base, according to the proportions of silica. Thus, in a very siliceous slag, alumina in moderate quantity behaves as a basic oxide, forming aluminium silicates, and in very basic or low silica slags the alumina appears either to neutralise some of the excess base, acting as an acidic oxide, or to dissolve as such in the slag, whilst in intermediate cases it possibly behaves partly as an acid and partly as base. This view has recently been questioned, and it has been suggested by Shelby that alumina always acts as an acid in the formation of slags. The matter is thus one which requires further considerable investigation. Usually neither alumina nor zinc oxide behave very satisfactorily in the furnace when present in large quantities, tending to thicken the slags and to promote viscosity. =Anaconda Practice in Charge Calculations.=—An example of some of the practical considerations which enter into the calculation and making up of charges is well illustrated in certain particulars of the practice as conducted at Anaconda. Details of the materials charged over a period of one month are indicated in Table X. The important charge constituents available in large quantity include:— Cu. SiO_{2}. Fe(O). S. % % % % First-class smelting ore, 8·6 54·0 13·6 14·0 Concentrates, 10·9 26·0 32·0 32·0 Briquettes, 5·0 50·0 13·0 13·0 Lime-rock (flux), .. .. .. .. Old converter slags and residues, .. .. .. .. TABLE X.—BLAST-FURNACE CHARGE CALCULATIONS—TOTAL CHARGE, ALL FURNACES.