Page:Popular Science Monthly Volume 13.djvu/560

542 magnificent windows of Gothic cathedrals, with their gorgeous colors, produced by combinations of metals in the molten state. The false precious stones made in Paris with so much perfection from heavy strass-glass are colored with metallic oxides in as lasting a manner as the genuine stones.

The first precious stone reproduced, not only in its appearance, but its real nature, and in all its component parts, is the lapis-lazuli, the sapphire of the ancients, not to be confounded with the sapphire of our modern jewelers. This untransparent stone, of a magnificent azure-blue color, was most highly prized by the ancient Hindoos, Assyrians, Persians, Jews, Egyptians, Greeks, etc.; and this irrefragably refutes the erroneous theory of some archæologists that the ancients were unable to distinguish the blue color. When pulverized, this stone furnishes the surpassingly beautiful ultramarine color with which the artists of the middle ages delighted to paint the mantle or gown of the Virgin Mary, although they had to pay the most extravagant prices for the pigment, which they always charged in the bills of those who had ordered a sacred picture from them. Some fifty or sixty years ago, Gmelin, the German chemist, discovered that this most beautiful of blue colors could be artificially produced by heating argillaceous earth with soda, sulphur, and carbon; and now that Guimet, the French chemist, has practically introduced this process, Europe manufactures annually about 100,000,000 pounds of this pigment, most of which is produced in Germany.

At a very early period chemists devoted their attention to the artificial reproduction of rubies and sapphires, which, as we have said before, consist of nothing but crystallized argillaceous earth, colored by minute particles of metals. Several decades ago, the chemist Gaudin succeeded in obtaining small ruby pellets from pure argillaceous earth, precipitated from dissolved alum and moistened with chromate of potash. The color of these rubies, according to the quantity of chromate which they contained, was either that of a rose or bordering on purple. The pellets were so hard that they easily cut glass, garnets, and topazes; but they were not crystals, and their transparency was by no means perfect. Similar experiments were made by the chemists De Bray, Sainte-Claire Deville, Caron, Sénarmont, Ebelmann, and others. It was long acknowledged that a crystallization of argillaceous or beryl earth had to be obtained, and to that end it was necessary to reduce them with the requisite quantities of the coloring metallic combinations into a state of fiery liquefaction. Boric acid was selected for that purpose, because when heated it slowly evaporates. It appears as vapor in volcanic countries, and is especially obtained in Tuscany. The belief that this fiery means of reduction had played in Nature a part in the formation of precious stones was perfectly justifiable; and so boric acid was placed in comparatively large quantities with argillaceous or beryl earth in open platinum crucibles, which were subjected to a