The word _alumen_, which we translate _alum_, occurs often in Pliny; and is the same substance which the Greeks distinguished by the name of στυπτηρια (_stypteria_). It is described pretty minutely by Dioscorides, and also by Pliny. It was obviously a natural production, dug out of the earth, and consequently quite different from our alum, with which the ancients were unacquainted. Dioscorides says that it was found abundantly in Egypt; that it was of various kinds, but that the slaty variety was the best. He mentions also many other localities. He says that, for medical purposes, the most valued of all the varieties of alumen were the _slaty_, the _round_, and the _liquid_. The slaty alumen is very white, has an exceedingly astringent taste, a strong smell, is free from stony concretions, and gradually cracks and emits long capillary crystals from these rifts; on which account it is sometimes called _trichites_. This description obviously applies to a kind of slate-clay, which probably contained pyrites mixed with it of the decomposing kind. The capillary crystals were probably similar to those crystals at present called _hair-salt_ by mineralogists, which exude pretty abundantly from the shale of the coal-beds, when it has been long exposed to the air. _Hair-salt_ differs very much in its nature. Klaproth ascertained by analysis, that the _hair-salt_ from the quicksilver-mines in Idria is sulphate of magnesia, mixed with a small quantity of sulphate of iron.[96] The _hair-salt_ from the abandoned coal-pits in the neighbourhood of Glasgow is a double salt, composed of sulphate of alumina, and sulphate of iron, in definite proportions; the composition being [96] Beitrage, iii. 104. 1 atom protosulphate of iron, 1½ atom sulphate of alumina, 15 atoms water. I suspect strongly that the capillary crystals from the schistose alumen of Dioscorides were nearly of the same nature. From Pliny’s account of the uses to which alumen was applied, it is quite obvious that it must have varied very much in its nature. _Alumen nigrum_ was used to strike a black colour, and must therefore have contained iron. It was doubtless an impure native sulphate of iron, similar to many native productions of the same nature still met with in various parts of the world, but not employed; their use having been superseded by various artificial salts, more definite in their nature, and consequently more certain in their application, and at the same time cheaper and more abundant than the native. The alumen employed as a mordant by the dyers, must have been a sulphate of alumina more or less pure; at least it must have been free from all sulphate of iron, which would have affected the colour of the cloth, and prevented the dyer from accomplishing his object.[97] [97] “Quoniam inficiendis claro colore lanis candidum liquidumque utilissimum est, contraque fuscis et obscuris nigrum.”--_Plinii_, xxxv. 15.