Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/683

Rh ALUM 645 this country it was confined for the greater part of a century. Various manufactories of it were established in Germany by the year 1544. England possessed no alum works till the reign of Charles I. Thomas Chaloner, son of Dr Chaloner, who had been tutor to Charles, while hunting on a common in Yorkshire took notice of the soil and herbage, and tasted the water. He found them similar to what he had seen in Germany where alum works were established. In consequence of this he got a patent from Charles for an alum work. Since that time various alum works have been established in different parts of Great Britain, the most important now in operation being the Whitby works, originally established by Mr Chaloner; and the works at Pendleton, near Manchester, and Goole, York shire, and at Hurlet and Campsie, both in the neighbour hood of Glasgow. Several alum works exist in Sweden, particularly in West Gothland. There is one, for example, at Hsensseter, near the borders of the Wener Lake. But for a descrip tion of the Swedish works we refer to Bergmann s Ojnis- cula, i. 284, or English translation, i. 342. Various minerals are employed in the manufacture of alum, but by far the most important of them are the fol lowing three : alum-stone, alum-slate, bituminous shale. Alum-stone or Alunite was first observed at Tolfa, near Rome, in the 15th century, and afterwards in Hungary and several other places, chiefly in trachyte or other vol canic rocks. It appears to be produced by the action of sulphureous vapours on the felspars they contain, and gene rally occurs in compact, granular, or earthy masses, mixed with quartz or felspar. Small crystals are found in cavities, and are either rhombohedrons with angles of 89 10, and thus nearly cubes, or these with the polar angles replaced by the basal plane. The specific gravity ranges from 2-58 to 2-752, the compact varieties being the lighter. Its hardness is 3 5 to 4, or rather softer than fluor spar. It has a distinct cleavage perpendicular to the axis of the rhombohedron, and conchoidal fracture in other directions. The pure varieties are white and colourless, but it is often coloured greyish, yellowish, or reddish. The crystals de crepitate before the blowpipe, but are infusible, as well as the compact alunite. The alum is extracted from this mineral by repeated roasting and treating with water. The absence of iron accounts for the superior purity for which the Roman alum was long celebrated. Alum-slate is a far more abundant substance, occurring in beds in different formations. Thus it is common in the older Palaeozoic er Silurian strata of Scandinavia and Scotland. Generally it is distinctly slaty, but sometimes forms rounded balls or concretions. It contains much carbonaceous matter, and hence its colour is greyish or bluish-black. It has a dull lustre, is soft and sectile. It contains much disseminated iron pyrites, and on decom position in the air yields sulphate of iron, and alum as an efflorescence on the surface. Many of the shales or slate clays in the coal formation also contain much iron pyrites, and thus also produce alum when acted on by the atmosphere. Such are those used for manufacturing alum at Campsie and other places near Glasgow. Where they contain much bituminous matter they show a shining resinous streak and greyish- black colour, and are named bituminous shales. These burn when heated, with a pale flame and sulphureous odour. The alum slates at Whitby in Yorkshire belong to the Lias, and are used in the alum works in that neigh bourhood. In other places, as in many parts of Germany, similar beds are found in Tertiary formations, particularly in connection with the brown coal deposits. When fresh dug they often show no trace of alum, which only appears after exposure to the air, or when the decomposition of the iron pyrites is assisted by the action of heat. Several native varieties of sulphate of alumina and soda alum occur in South America, some of the most remarkable of which it may be proper to specify. 1. Sulphate of alumina, or Alunogene, was first found at Rio Saldanha, but is now obtained from several places in Europe and America. The colour is white, here and there tinged yellow, obviously from external impurities. It occurs in fine crystalline needles ; lustre silky; taste that of alum, but stronger; specific gravity, 1-6 to 1-7; soft; before the blowpipe behaves like alum. 2. Soda-alum. It occurs native in the province of St Juan, situated to the north of Mendoza, on the east side of the Chilian Andes, at about 30 S. lat. The alum is white, and composed of fibres adhering longitudinally, and having a certain breadth, but very thin. It bears some resemblance to fibrous gypsum, but it is harder, not being scratched by the nail, though the knife scratches it with great ease. It is sectile. The outer fibres are white and only slightly translucent, as if they had lost a portion of their water ; but the internal fibres are transparent, and have a silky aspect. It tastes precisely like alum, and is very soluble, water at the temperature of 62 dissolving 3-773 parts of it, and boiling water dissolving any quantity whatever. When exposed to heat, it behaves very nearly as common alum. 3. There is a mineral called aluminite, which was ob served in the environs of Halle many years ago, and which was afterwards detected by Mr Webster in clay resting on chalk at Newhaven in Sussex. This, if it were sufficiently abundant, would constitute an excellent material for the manufacture of alum. Its colour is snow-white. It occurs in reniforrn pieces of greater or smaller size ; fracture fine earthy ; dull ; streak glistening ; opaque ; adheres feebly to the tongue ; soils very slightly ; very soft ; feels fine, but meagre; specific gravity, 1/7054. It consists of alumina, sulphuric acid, and water. Four different processes are employed in the manufac ture of alum, according to the nature of the mineral from which the alum is to be extracted. The process employed at Tolfa is the simplest of all. If the Tolfa stone be kept constantly moistened with water for about two months, it falls to powder of itself, and yields alum by lixiviation. But this is not the pro cess employed by the manufacturers. The alum-stone is broken into small pieces, and piled on the top of a per forated dome, in which a wood fire is kindled. The smoke and flame of the wood penetrate through the pieces of alum-stone, and a sulphureous odour is disengaged, owing to the decomposition of a portion of the sulphuric acid in the stone. This roasting is twice performed ; the pieces of ore which the first time were at the edge of the dome, being the second time put in the middle. The process of roasting this stone requires considerable attention. If the heat be too great, the quality of jdelding alum is destroyed : if the heat be too small, the stone does not readily fall to powder. There can be little doubt that the unroasted stone would yield more alum than the roasted ; but probably the additional labour requisite in the latter case would more than swallow up the increase of product. The roasted stone, which has now acquired a reddish colour, is placed in rows between trenches filled with water. This liquid is so frequently sprinkled on it that the stone is always moist. In two or three days it falls to powder, like slacked quicklime ; but the daily watering is continued for a month. The success of this part of the operation is said to depend very much on the weather. When the weather is rainy, the alum is all washed