When the outer coat of this stone is washed away by heavy rains, it has sometimes the appearance of burnished silver. An Italian shoemaker, in the year 1630, deceived by this appearance, carried home several pieces, hoping, by means of fire, to extract silver from them. But at the same time that he was disappointed in this expectation, he was surprised by a very unlooked─for phenomenon. All the pieces which he had thus attempted to melt, when they were afterwards exposed to the light, became themselves luminous. It is the singular property of the Bologna phosphorus, after it has undergone calcination in a particular manner, to become capable of imbibing so much light on exposure, for a little while, to the light of the sun, or even to the flame of a candle, that it will afterwards shine in the dark for an interval of from eight to fifteen minutes, like a glowing coal, but without any sensible heat. The light which it emits is sufficient to read by, provided the letters be placed near it. If well prepared, the stone will retain this extraordinary property for five or six years. The preparation of it is thus conducted. Pieces of sulphat of barytes are made red hot, for a few minutes, in a covered crucible placed in the middle of a fire, and then left to cool. When cool, they are pounded in a stone mortar, and sifted. The powder thus formed is made into a paste with a little gum arabic, and divided into long cakes, or cylinders, each about a quarter of an inch thick. These pieces are dried in a moderate heat, and then, by degrees, are exposed to a more violent heat, among charcoal, in a wind furnace. As soon as the coals of the furnace are half consumed, it must be filled a second time, and the phosphorus must be left undisturbed. When the coals are quite consumed the ashes must be carefully blown off with a pair of bellows, and the phosphorus will be found at the bottom of the grate. ──────────────────────────────────────────────────────────────────────── CLASS II.—SALTS. ORDER I.—EARTHY SALTS. ─────── ALUMINE FAMILY. 197. _ALUM is a substance of yellowish or greyish white colour, usually opaque, but sometimes transparent. When purified, it consists of slender, irregular, hair─shaped fibres, and has a sweetish, astringent taste._ The alum of commerce is an artificial production from the different kinds of stones which contain it. That called _Roman alum_, from its being procured from the neighbourhood of Rome, is usually considered preferable to the other sorts; but good alum of our own manufacture is equal to it in quality. The Levant, or _Roche alum_, is said to have had its name from the village of Rocca, the present Edessa, in Syria.