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62                          METALLURGY   OF   CAST   IRON.
less expensive. All the above fluxes are used just as they are mined, being in no way burned or roasted  a treatment necessary to some grades of limestone  and will benefit, it is claimed, almost any flux of a rock character. When this is done with limestone it gives us quicklime, a form that requires less weight when charged than limestone. The action of burning or roasting causes the limestone to become friable, so as to largely eliminate its carbonic acid and other volatile matter and generally make a limestone more ready to unite with the impurities. While such treatment of limestone would naturally be expected to be economical, it has not proven so in all cases. When the fuel required to roast it is taken into consideration with that which may be saved in converting it into slag in the smelting of iron, there is considerable difference of opinion in regard to the question of economy for furnace practice.ething of a checked marble cast. It is obtained from the Benson Mines, New York, and instead of being called limestone as are the first two shown, it is defined as calcite by the shippers. It will be noticed that Nos. 2 and 3 have no sulphur. For many classes of work this is preferable to No. i As sulphur in limestone is similar in its effect to sulphur in fuel, it largely passes into the iron and raises its sulphur contents. For cupola work preference, as far as labor is concerned, would be given to Nos. 2 and 3 owing to these being more friable than No. i, but the furnace limestone No, i is will also show that a flux which, might work well in a furnace can often be well utilized in cupola practice:gases, heated stock, and liquid metal should enable can counteract the absorbing power ofee reactiony they are often composed of about equal parts of silica and alumina. Bricks should contain silica or alumina in proportion to the amount of heat or friction they are                                                 if j