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370                     METALLURGY OF CAST IRON.
like a fishmonger covered with  shining  fish   scales.
When metal is high in silicon, its surface may have a smooth, dead appearance devoid of life, and if the surface is disturbed with a rod or skimmer, it may act a great deal like cream upon milk. Were it not for its dull, silvery, quiet appearance and spark-less action, it might often be taken for hard iron. No. i iron, whether high in free carbon or silicon, when running from the cupola into a ladle or'from the furnace to the pig beds, throws off very few sparks, and those that do fly are chiefly caused by vibration of the metal from the running or spluttering of the stream, and fall as ordinary sparks, very different from those which come from harder or lower grades of melted iron.
Irons low in silicon and high in sulphur, from No. 7 to No. 10, which can be termed hard iron and also can be strong and weak, have peculiarities very pronounced to distinguish them from soft grades or No. i irons. In the ladle, such irons will, when "hot," show a smooth, bright appearance, with hardly a break on the surface, and as the mass becomes cool or "dulls down," it presents a dull, hazy, plastic appearance, which, if disturbed by a skimmer or rod, will act as if it were covered with an oxide or scum. While hot, it will often boil in the ladle .as if bubbles of gas were escaping from below. It also emits many sparks, which is the chief characteristic phenomenon of hard iron and cannot be better explained than in the language of Tomlinson, who says:
From all parts of the fluid surface is thrown off a vast number of metallic sparks, from the absence of carbon, which renders the metal sensitive to the oxidizing influence of the atmospheric air.ng them look