Pyrophorus (Gr. , fire, and to bear), a substance which takes fire on exposure to the air. This property is possessed by several substances and mixtures specially prepared. Finely divided metals, as iron when reduced from the oxide at the lowest possible temperature by a current of hydrogen, exhibit it in a remarkable degree. The effect appears to be produced in all cases by rapid combination of the oxidizable substance with the oxygen of the air. An excellent pyrophorus is produced by calcining in a close crucible 6 parts of lampblack mixed with 11 of sulphate of potash; the product is a mixture of carbon and sulphuret of potassium. Homberg's pyrophorus is made by stirring a mixture of equal parts of alum and brown sugar in an iron ladle over the fire till it becomes dry; then heating the same in a red-hot vessel nearly closed as long as a flame appears at the aperture. It is then removed from the fire, and carefully stopped until required for the experiment. Tartrate of lead heated to dull redness in a glass tube becomes a brown powder, which when shaken out into the air ignites.