[489] [There can be little doubt however that even Pliny was acquainted with our alum, but did not distinguish it from sulphate of iron, for he informs us that one kind of alum was white and was used for dyeing wool of bright colours.--Pereira’s Materia Medica, vol. i.] [490] [The alums, for at present several kinds are distinguished, are not merely combinations of sulphuric acid and the earth alumina, but double sulphates, the one constituent being sulphate of alumina, the other either sulphate of potash, sulphate of ammonia, sulphate of soda, &c. The alum of this country generally contains potash, that of France ammonia, or both potash and ammonia, hence the name potash-alum, ammonia-alum, &c.] [491] [Although native alum is not abundant, there is no question of its occasional occurrence.] [492] Plin. lib. xxxiv. c. 12. The same account is given by Isidor. Origin. lib. xvi. c. 2, and by Dioscorides, lib. v. c. 114. The latter, however, differs from Pliny in many circumstances. [493] Those who are desirous of seeing everything that the ancients have left us respecting their alum may consult Aldrovandi Museum Metallicum, Lugd. 1636, fol. p. 334. [494] Herodot. lib. ii. c. 180. [495] Diodor. Sic. lib. v. ed. Wesselingii, i. p. 338. [496] Tournefort, Voyage i. p. 63. Some information respecting the same subject may be seen in that expensive but useful work, Voyage Pittoresque de la Grèce, i. p. 12. [497] Diodor. Sic. lib. c. Strabo, lib. vi. edit. Almel. p. 423. [498] Copious information respecting the Spanish alum-works may be found in Introduccion à la Historia Natural de Espagna, par D. G. Bowles: and in Dillon’s Travels through Spain, 1780, 4to, p. 220. [499] The derivation of the Latin name _alumen_, which, if I mistake not, occurs first in Columella and Pliny, is unknown. Some deduce it from ἅλμη; others from ἄλειμμα; and Isidore gives a derivation still more improbable. May it not have come from Egypt with the best sort of alum? Had it originated from a Greek word, it would undoubtedly have been formed from στυπτηρία. This appellation is to be found in Herodotus; and nothing is clearer than that it has arisen from the astringent quality peculiar to both the salts, and also from στύφειν, as has been remarked by Dioscorides, Pliny, and Galen. [500] Etymol. p. 779. [501] Plin. lib. xxxiv. c. 12. [502] Noct. Att. lib. xv. c. 1. [503] 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.] [504] Wecker De Secretis, lib. ix. 18, p. 445. [505] One instance of its being used for this purpose is found in Ammianus Marcellinus, lib. xx. c. 12.