1911 Encyclopædia Britannica/Pyrophorus

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PYROPHORUS (Gr. πῦρ, fire, φέρειν, to bear), a substance which spontaneously inflames on contact with the air. One of the earliest known is that of Homberg, prepared by heating a mixture of alum and finely divided carbon to redness in a closed tube. On opening the tube and emptying out the black residue (consisting of potassium sulphide, aluminium sulphate and carbon) it promptly catches fire. Many readily oxidizable substances, especially when very finely divided, have the same property. Metallic iron and cobalt, when prepared under certain conditions, are pyrophoric, as is also ferrous oxide. Spontaneously inflammable liquids are also known, e.g. certain alkyl metallic compounds, phosphorus dihydride, &c.