5 4 2 THE POPULAR SCIENCE MONTHLY. 



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, As- 

 syrians, Persians, Jews, Egyptians, Greeks, etc.; and this irrefragably 

 refutes the erroneous theory of some archaeologists 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 chem- 

 ist, has practically introduced this process, Europe manufactures annu- 

 ally about 100,000,000 pounds of this pigment, most of which is pro- 

 duced in Germany. 



At a very early period chemists devoted their attention to the arti- 

 ficial reproduction of rubies and sapphires, which, as we have said be- 

 fore, 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 pot- 

 ash. 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, Senarmont, 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 pur- 

 pose, 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 long-eon- 



