In smelting-houses Iron ore is fused amidst charcoal, the phlogiston of which combines with the martial earth, and gives it the metalline form. The Iron thus melted runs down to the bottom of the furnace, from whence it is let out into large moulds, in which it takes the shape of oblong blocks, called _Pigs_ of Iron. This Iron is still very impure, and quite unmalleable. Its want of ductility after the first melting arises partly from hence, that, notwithstanding the previous roasting which the ore underwent, there still remains, after this first fusion, a considerable quantity of Sulphur or Arsenic combined with the metal. A certain quantity of quick-lime, or of stones that will burn to lime, is frequently mixed with Iron ore on putting it into the smelting furnace. The lime being an absorbent earth, very apt to unite with Sulphur and Arsenic, is of use to separate those minerals from the Iron. It is also of use to mix some such matters with the ore, when the stones or earths which naturally accompany it are very fusible; for, as the Iron is of difficult fusion, it may happen that the earthy matters mixed with the Iron shall melt as easily as the metal, or perhaps more easily. In such a case there is no separation of the earthy from the metalline part, both of which melt and precipitate together promiscuously; now quick-lime, being extremely refractory, serves on this occasion to check the melting of those matters which are too fusible. Yet quick-lime, notwithstanding its refractory quality, may sometimes be of use as a flux for Iron. This is the case when the ore happens to be combined with substances which, being united with lime, render it fusible: such are all arsenical matters, and even some earthy matters, which being combined with quick-lime make a fusible compound. When the ore of an Iron mine is found difficult to reduce, it is usually neglected even though it be rich: because Iron being very common, people chuse to work those mines only whose ores are smelted with the most ease, and require the least consumption of wood. Yet refractory ores are not to be altogether rejected, when another Iron ore of a different quality is found near them. For it often happens, that two several Iron ores, which being worked separately are very difficult to manage, and yield at last but bad metal, become very tractable, and yield excellent Iron, when smelted together: and accordingly such mixtures are often made at Iron-works.