PYROPHORUS is the name of an artificial compound, which ignites by exposure to the air. It was first made by Thunberg, in 1680, from a mixture of human excre ment and alum, but it may be prepared from alum by calcination, with the addition of various inflammable substances.
Three parts of alum are mixed with from two to three parts of honey, flour, or sugar, and this mixture is dried on the fire, in a glazed bowl, stirring it all the while with an iron spatula. The mixture at first melts, but gradually swells up and runs into dry lumps, which are pounded and again roasted, till no moisture whatever remains in them, the mass now resembling a blackish powder of charcoal. The above operation may be saved by directly mixing two parts of charcoal powder with five of burnt alum. The powder is now poured into a phial, with a neck about six inches long, and the phial, when three quarters full, is put into a crucible, which is exposed to the red heat of a furnace, for about a quarter of an hour, till the black smoke, which at first issues from the mouth of the phial, is succeeded by a sulphurous vapour, which commonly takes fire. When the sulphurous flame ceases
the phial is closed with a clay stopper ; and, when the fire is out, the powder is transferred as fast as possible into a dry and strait glass, made warm, and secured with a glass stopper.
A good pyrophorus may be made by calcining a mix ture of three parts of alum, and one of wheat flour, in a common phial, till the blue flame disappears, and preserv ing it in the same phial with a good stopper. By the exposure of the pyrophorus to the air, the sulphuret attracts its moisture, and produces a degree of heat capa ble of igniting the carbonaceous matter which is mixed with it.