(i.) The flux is preheated at little extra expense, there being usually plenty of heat to spare for this, and the roaster capacity is not unduly decreased. (ii.) Intimate mixing of the charge is assured, and this greatly facilitates the fusion and reaction. (iii.) More rapid and thorough roasting is effected, since the presence of the inert flux prevents clotting or undue sintering of the sulphides in the roaster. (iv.) The charge is found to be in a much better condition, both physically and chemically, for successful reverberatory smelting. Lime in the roaster charge appears to assist the thoroughness of the roast, whilst an incipient slag formation is commenced owing to the juxtaposition of basic oxides and silica, in the hotter parts of the roaster furnace. 2. _Rapidity of melting_ is an indispensable feature of modern work. The conditions necessary for rapid melting have been reviewed above. 3. _Use of Large Furnaces._—Reverberatory furnaces appear to have replaced the blast furnace in Great Britain somewhere about 1700, and by 1854 they were in general use in this country. At this period the usual dimensions were, for the hearth 13 feet by 9 feet, with a fire-box 4 feet by 4 feet, the furnace having a capacity of 12 tons per twenty-four hours. In Great Britain the size increased very slowly, and it was in the United States of America that the important increase in dimensions and in enormous outputs were developed. The work was commenced systematically in about 1878 by Richard Pearse (a Swansea-trained metallurgist) at the Argo Smelter in Colorado. Table V. indicates the gradual improvements in practice resulting from these developments (see also Fig. 23, p. 90). TABLE V.—DEVELOPMENT IN SIZE OF THE REVERBERATORY FURNACE. ─────┬─────────────┬────────────────┬─────┬───────────┬────────────── │ Fire─box │ Hearth │ │ │ Tons Ore per Year.│ Dimensions. │ Dimensions. │Stack│ Capacity. │ Ton Coal. ─────┼─────────────┼────────────────┼─────┼───────────┼────────────── 1878,│4' 6" × 5' │ 9' 8" × 15' │2' 9"│12 tons. │2·4 tons. 1882,│4' 6" × 5' │10' 4" × 17' 10"│2' 9"│17 " │2·43 " 1887,│4' 6" × 5' 6"│12' 8" × 21' 2"│3' 0"│24 " │2·67 " 1891,│4' 6" × 6' │14' 2" × 24' 4"│3' 0"│28 " │2·8 " 1893,│5' × 6' 6"│16' × 30' │3' 6"│35 " (43)‡│2·7 " (3·3) ‡ 1894,│5' × 6' 6"│16' × 35' │4' 0"│ (50)‡│ (3·7) ‡ 1903,│5' 6" × 10' │20' × 50' │5' 5"│ (70)‡│ (3·1) ‡ 1910,│8' × 16' │19' × 116' │ .. │ (275)‡│ (4·66)‡ ─────┴─────────────┴────────────────┴─────┴───────────┴────────────── ‡ The charges of calcines were fed whilst still red hot.