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THE   MELTING   POINT   OF   CAST   IRON.               355
points increase as the combined carbon goes down, this being the case independent of the amount of graphite present. One could hardly expect anything else, for that matter, gray cast iron being really a steel with a lot of mechanically mixed graphite, and white iron a combination of carbon with iron. Alloys melt at a lower temperature than any of their constituents, and so also white iron  really an alloy of carbon or some carbides of iron with iron  should melt sooner than the purer iron in the gray variety.
'' The fact, however, that steel melts at a much higher temperature than the grayest of irons in the table, shows that there are other considerations not to be overlooked in studying the molecular physics of cast iron. The principal reason for this lowering of tern " perature is the supposed solution of the graphite in the iron before actual melting takes place. To what extent this occurs and under what circumstances is not known, but may account for the difference in the melting points of steel and gray iron.
4 < Again, in melting steel in the cupola commercially, an absorption of carbon from the fuel takes place, the melting point is doubtless lowered a little, and the results obtained are tangible, even though care must be taken to get the whole of the charge down before pouring. In the air furnace the steel absorbs carbon by contact with the pig iron charged and melts off, the wasting of wrought iron or steel poking bars used for rabbling giving evidence of this occurrence.
4' The writer is especially pleased to see the full corrob-oration of Mr. West's elaborate experiments with the melting of white and gray irons. The contrast is remarkably sharp, and on the whole it shows us that