It now becomes necessary to withdraw or _cast_ the metal. This is done by forcing a way through a channel left beneath the dam in the masonry of the hearth, and closed with clay; the inner portion of this is baked hard, and requires to be broken through with a steel point. As soon as the passage is opened, the metal runs out, and is received in a long trench formed in the sand floor of the moulding house, to which are adapted a number of less trenches, at right angles, each containing about one hundred weight of metal. The metal in the longer trench is also broken into pieces of the same size, and the ingots thus formed are called _pigs_, whence the term for this variety, _pig iron_. From one to three days will elapse from the time of the first charge until the furnace can be tapped, and pigs cast. From that time the casting succeeds with tolerable regularity, according to the working of the furnace, and at intervals depending upon the volume of the charge, and the capacity of the hearth. It appears probable that the fusion of the iron is effected always by a direct chemical union of that metal with carbon, in the proportion of two atoms of the former to one of the latter. This constitutes, as we have seen, the white variety of pig iron. But as it continues, generally speaking, in the furnace, long after its fusion takes place, it acquires a temperature higher than its proper melting point, and a tendency to separation takes place, the iron retaining in combination no more of the carbon than is necessary to maintain it in a fluid state at the increased temperature. Thus the grey variety of pig iron is formed; and on casting it, the carbon, in a form similar to that of plumbago, is disseminated throughout the mass, or forms on its surface the efflorescence that is called kish, and which is always a sign of a high quality in the iron it accompanies. In conformity with this theory, we find that a high temperature in the furnace always produces grey cast iron; and that a low temperature, from whatever cause it may arise, renders the iron more or less inclining to white. So also if the metal be not exposed to the heat for a sufficient length of time, it becomes white. Karsten classes these several causes of whiteness in the product, in the following order:-- "In conformity with the observations that have hitherto been made, white cast iron is obtained: "1. By the use of ores that are too easily fusible, or which is the same thing, by an excess of flux, by a want of density in the charcoal, and by too strong a blast, even when the working of the furnace is regular. "2. By a surcharge of ore, which deranges the action of the furnace, and produces impure cinder, containing uncombined iron. "3. By boshes of too rapid a slope, and a blast of too great a velocity; and this may occur even where the cinder is pure.