Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/682

Rh 044 ALUM the conclusion that sulphate of potash readily combines with sulphate of alumina. After Klaproth had discovered the existence of potash as an ingredient in leucite and lepidolite, it occurred to Vauquelin that it was probably an ingredient likewise in many other minerals. He recollected that alum crystals often make their appearance during the analysis of stony bodies ; and, considering that alum cannot be obtained in crystals without the addition of potash, he began to suspect that this alkali constituted an essential ingredient in the salt. A set of experiments, undertaken on purpose to elucidate this important point, soon satisfied him that his conjecture was well-founded. Accordingly, in the year 1797 he published a dissertation demonstrating that alum is a double salt, composed of sulphuric acid, alumina, and potash (Annales de Chimie, xxii. 258). Soon after, Chaptal published the analysis of four different kinds of alum, namely, Roman alum, Levant alum, British alum, and alum manufactured by himself. This analysis led to the same result as that of Vauquelin (Ann. de Chim. xxii. 280). Since that time alum has been admitted by chemists to be a triple salt, and various analyses of it have been made to determine its constituents. Vauquelin (Ann. de Chim. 1. 167), Thenard and Roard (ibid., torn. lix. 72), Curaudau (Journal de Physique, Ixvii. 1), and Berzelius (Ann. de Chim. Ixxxii. 258), successively published the results of their experiments. These analyses gradually led to an accurate knowledge of the composition of this salt. One of the most remarkable differences between the three species of alum is the solubility of each in water. At the temperature of 60, 100 parts of water dissolve 9 37 parts of ammoniacal alum. 14 79 parts of potash alum, 327 6 parts of soda alum. This great solubility of soda alum renders the manufacture of it very difficult. It does not easily crystallise; indeed, when the weather is hot, crystals of it can hardly be obtained. Its great solubility would render it more con venient and more economical for dyers and calico-printers, provided it could be furnished at the same rate with common alum. But the greater difficulty attending the making of it would probably prevent it from being sale able at a price sufficiently low to make it available as a mordant. Soda alum was first mentioned by Mr Winter in 1810, in his account of the Whitby alum processes (Nicholson s Jour. xxv. pp. 254, 255); but before that time it had been made by Mr Charles Macintosh of Crossbasket. Mr William Wilson, at Hurlet, near Glasgow, afterwards made it in considerable quantities. Specimens of it have been sent by Dr Gillies from the neighbourhood of Mendoza, in South America, where it occurs native in considerable quantity. These three different species of alum differ also some what from each other in their specific gravities, which are as follows: Ammoniacal alum 1 56 Potash alum 1*75 Soda alum T88 1 The word alumen, which we translate alum, occurs in Pliny s Natural History. In the 15th chapter of his 35th book he gives us a detailed description of it. By com paring this with the account of o-rvTmypta given by Diosco- rides in the 123d chapter of his 5th book, it is obvious that the two are identical. Pliny informs us that alumen 1 The soda alum whose specific gravity is here given was the native, from the province of St Juan, on the north of Mendoza. It contains less water, and therefore is probably heavier than common soda alum. was found naturally in the earth. He calls it salsugo- terrce. Different substances, he informs us, were dis tinguished by the name of alumen ; but they were all characterised by a certain degree of astringency, and were all employed in dyeing and medicine. The light-coloured alumen was useful in brilliant dyes, the dark-coloured only in dyeing black or very dark colours. One species was a liquid, which was apt to be adulterated; but when pure it had the property of striking a black with the juice of the pomegranate. This property seems to characterise a solution of sulphate of iron in water. It is quite obvious that a solution of our alum would possess no such property. Pliny says that there is another kind of alum which the Greeks call schistos. It forms in white threads upon the surface of certain stones. From the name schistos, and the mode of formation, there can be little doubt that this species was the salt which forms spontaneously on certain slaty minerals, as alum slate and bituminous shale, and which consists chiefly of sulphate of iron and sulphate of alumina. Possibly in certain places the sulphate of iron may have been nearly wanting, and then the salt would be white, and would answer, as Pliny says it did, for dye ing bright colours. Several other species of alumen are described by Pliny, but we are unable to make out to what minerals he alludes. The alumen of the ancients, then, was not the same with the alum of the moderns. It was most commonly a sulphate of iron, sometimes probably a sulphate of alumina, and usually a mixture of the two. But the ancients were unacquainted with our alum. They were acquainted with sulphate of iron in a crystallised state, and distinguished, it by the names of misy, sory, chalcanthum (Pliny, xxxiv. 12). As alum and green vitriol were applied to a variety of purposes in common, and as both are distinguished by a sweetish and astringent taste, writers, even after the discovery of alum, do not seem to have discriminated the two salts accurately from each other. In the writings of the alchemists we find the words misy, sory, chalcanthum, applied to alum as well as to sulphate of iron; and the name atramentum sutorium, which ought to belong, one would suppose, exclusively to green vitriol, applied in differently to both. When our alum was discovered is entirely unknown. Beckmann devoted a good deal of attention to the history of this salt, and published a curious dissertation on the sub ject; but his attempts to trace its origin were unsuccessful. The manufacture of it was discovered in the East, but at what time or place is totally unknown. It would appear that, about four or five hundred years ago, there was a manufactory of it at Edessa in Syria, at that time called Rocca, hence, it is siipposed, the origin of the term rock alum, commonly employed in Europe; though others allege that the term originated at Civita Vecchia, where alum is made from a yellow mineral which occurs in the state of a hard rock. Different alum works existed in the neighbourhood of Constantinople. About the time of the fall of the Grecian empire the art of making alum was transported into Italy, at that period the richest and most manufacturing country in Europe. Bartholomew Pernix, a Genoese merchant, discovered alum ore in the island of Ischia, about the year 1459. Nearly at the same time John di Castro, who was well acquainted with the alum works in the neighbourhood of Constantinople, suspected that a mine ral fit for yielding alum existed at Tolfa, because it was covered with the same trees that grew on the alum mine ral near Constantinople. His conjecture was verified by trials, and the celebrated manufactory at Tolfa established. Another was begun in the neighbourhood of Genoa ; and the manufacture flourished in different parts of Italy. To