Page:Popular Science Monthly Volume 13.djvu/561

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tinued heat in porcelain furnaces. In fact, as soon as the larger portion of the boric acid has evaporated, there are evolved from the fiery, liquid mass small rubies, sapphires, or emeralds. This was discovered some twenty years ago, but the crystals were too small to make the process a remunerative one.

Far more satisfactory were the results of Frémy's recent experiments. They are based upon a different principle, namely, that of separating the argillaceous earth slowly from its usual combination with silicic acid, as it is found in Nature everywhere, by bringing to bear upon it a substance of stronger affinity for the acid. In consequence, small crystals of argillaceous earth are formed in the fiery, liquid "mother-liquor," which, in the course of further separation, grow slowly. In the glass-factories of M. Feil, quantities of this "mother-liquor" of precious stones, weighing from twenty-five to fifty pounds, were easily' kept in a fiery, liquid state for two and three weeks, and in this way very favorable results were obtained. The most advantageous process turned out to be the separation of the argillaceous earth from the silicic acid by means of oxide of lead, for which purpose a mixture of equal parts of pure porcelain-clay and red-lead was placed in a large crucible of fire-proof clay and exposed for weeks to an intense red heat. Usually, the lead also extracts the silicic acid which the walls of the crucible contain, and eats holes through them. Hence, to avoid losses, the precious-stone crucible should be placed in another.

After several weeks of patient waiting, vividly recalling the expectant watching of the old alchemists at their crucibles in which the philosopher's stone was to be created, the crucible is taken out and cooled. After destroying the crucible, the contents are found to consist of two strata, above a glassy one, consisting principally of silicate of lead, and below a crystalline one, containing the most beautiful crystals of argillaceous earth in round clusters. If nothing but argillaceous earth and red-lead has been placed in the crucible, these crystals are as colorless as glass. They will cut glass and rock-crystal, nay, even the very hard topaz; in short, they are precious corundums or diamond-spar, so called because, next to the diamond and crystalline boron, it is the hardest of all stones.

Now rubies, sapphires, and Oriental emeralds, are nothing but colored corundums, and the former two can be easily obtained by the addition of the requisite quantities of the coloring metallic combinations. When there was added to the mixture of argillaceous earth and red-lead two or three per cent, of bichromate of potash, the crystals showed the beautiful rose-color of the ruby; when only a small quantity of that salt was used, and simultaneously a still smaller quantity of oxide of cobalt was added, sapphires were obtained. The precious stones thus produced, as a rule, are covered with a firm crust of silicate of lead, which is best removed chemically by melting it with oxide of lead or potash, or by means of hydrate of fluor-spar. Among a number of pounds of