Note 503 — ALUM.

The halotrichum of Scopoli. The first person who discovered this salt to be vitriolic was Henkel, who calls it Atlas-vitriol. [The mineral halotrichite is, in a chemical sense, a true alum in which the sulphate of potash is replaced by the sulphate of the protoxide of iron. It is composed of one atom of protosulphate of iron, one atom of sulphate of alumina, and contains, like all the true alums, twenty-four atoms of water.]