"The theory of the vitrification of oxides, aided by trials on a small scale, points out the kind of earthy mixture which ought to be employed, but it cannot fix the exact proportion of the different earths that ought to be adopted; nor does it teach the means of replacing an earth by its chemical equivalent, as, for instance lime, by magnesia. The solution of the question will depend rather upon the properties of the silicates of lime and magnesia at high temperatures, than upon the action of these silicates upon iron. It is hardly probable that the iron obtained from all ores, could be equally good, even if the most proper fluxes could be added to these ores. Those who have maintained this opinion, have erroneously imagined that the reduction of the ore could always be effected under the same circumstances, which would not be the case, even if these fluxes were ascertained and made use of." Most of the ores of iron require, before they are subjected to the process of reduction, a preparatory operation called roasting. This consists in exposing them to a comparatively low heat. The more important use of this process is to render the mass more susceptible of mechanical division, but it also serves in many cases to separate the sulphur and arsenic that may exist in the ore. There are some ores, as, for instance, those of a number of mines in Morris and Sussex counties, New-Jersey, which are so free from impurities, and which yield so readily to the mechanical means employed for separating them, that this process is wholly unnecessary; but such ores are rare, and the process of roasting must, generally speaking, be performed. The mechanical division, which exposes a larger surface to the action of heat and of the chemical agents, is called stumping; this is usually performed by appropriate machinery, but was in the infancy of the art effected by hand.