alums contain r o'iTo • ^ may be easily purified by solution, granulation, crystallization, andwashing, as has been already explained.
Alum is made extensively in France from an artificial sulphate of alumina. For thispurpose clays are chosen as free as possible from carbonate of lime and oxyde of iron.They are calcined in a reverberatory furnace, in order to expel the water, to peroxydizethe iron, and to render the alumina more easily acted on by the acid. The expulsion ofthe water renders the clay porous and capable of absorbing the sulphuric acid bycapillary attraction. The peroxydation of the iron renders it less soluble in thesulphuric acid; and the silica of the clay, by reacting on the alumina, impairs itsaggregation, and makes'it more readily attracted by the acid. The clay should, therefore,be moderately calcined; but not so as to indurate it like pottery ware, for it would thensuffer a species of silicious combination which would make it resist the action of acids.The clay is usually calcined in a reverberatory furnace, the flame of which servesthereafter to heat two evaporating pans and a basin for containing a mixture of thecalcined clay and sulphuric acid. As soon as the clay has become friable in the furnaceit is taken out, reduced to powder, and passed through a fine sieve. With 100 parts ofthe pulverized clay, 45 parts of sulphuric acid, of sp. gr. 1*45, are well mixed, in astone basin, arched over with brickwork. The flame and hot air of a reverberatoryfurnace are made to play along the mixture, in the same way as described for evaporatingthe schist liquors. See Soda. The mixture, being stirred from time to time, is, at theend of a few days, to be raked out, and to be set aside in a warm place, for the acid towork on the clay, during six or eight weeks. At the end of this time it must be washed,to extract the sulphate of alumina. With this view, it may be treated like the roastedalum ores above described. If potash alum is to be formed, this sulphate of alumina isevaporated to the specific gravity of 1-38; but if ammonia alum, to the specific gravityof only 1-24; because the sulphate of ammonia, being soluble in twice its weight ofwater, will cause a precipitation of pulverulent alum from a weaker solution of sulphateof alumina than the less soluble sulphate of potash could do.
The alum stone, from which the Roman alum is made, contains potash. Thefollowing analysis of alunite, by M. Cordier, places this fact in a clear light:—
Sulphate of potash - - - - 18-53
Sulphate of alumina .... 38-50Hydrate of alumina .... 42-97
1OO-O0
To transform this, compound into alum, it is merely necessary to abstract the hydrateof alumina. The ordinary alum stone, however, is rarely so pure as the above analysiswould seem to show; for it contains a mixture of other substances; and the above are indifferent proportions.
Alum is very extensively employed in the arts, most particularly in dyeing, lakemaking, dressing sheep-skins, pasting paper, in clarifying liquors, &c. Its purity forthe dyer may be tested by prussiate of potash, which will give solution of alum a bluetint in a few minutes if it contain even a very minute portion of iron. A bit of nut-gallts also a good test of iron.
AMADOU. The French name of the spongy combustible substance, called in Germanzu ndersehwamm, prepared from a species of agaric, the boletus igniarius, a kind ofmushroom, which grows on the trunks of old oaks, ashes, beeches, &c. It must bePlucked in the months of August and September. It is prepared by removing themtter bark with a knife, and separating carefully the spongy substance of a yellowbrown color, which lies within it, from the ligneous matter below. This substance is •bbt into thin slices, and beat with a mallet to soften it, till it can be easily pulledsunder between the fingers. In this state the boletus is a valuable substance for stop-it mn°° Z * ng hemorrhages, and some other surgical purposes. To convert it into tinder°f receive a finishing preparation, which consists in boiling it in a strong solutionSometim d r y mg it, beating it anew, and putting it a second time into the solution,the disu '’.“tdeed, to render it very inflammable, it is imbued with gunpowder, whenceAll the Ctl<? o- of black and brown amadou.
mentous it °f the lycopodium genus of plants, which have a fleshy or fila-
Hindoos em U i Ctllre! yield a tinder ( l uite ready for soaking in gunpowder water. Thethick spongy', a leguminous plant, which they call solu, for the same purpose. ItsAMALGAM"’ being reduced to charcoal, takes fire like amadou,amalgam of th i ^hen mercury is alloyed with any metal, the compound is called anAMALGAMA » as > for example, an amalgam of tin, bismuth, &c.gold from certain This is a process used extensively in extracting silver and
°* their ores, founded on the property which mercury has to dissolve these