weight which can be distinguished from the natural stones only by a close examination with the microscope. It is then observed that the artificial rubies contain cavities of a different outline and nature to those which occur in the rubies made in nature’s laboratory. These cavities are more or less spherical or pear-shaped in the artificial ruby and their walls are curved; in the natural stone the cavities are really negative crystals while their walls are angular. It was the French chemist Frémy who first made small rubies artificially, but subsequent workers, by employing larger quantities of material, modifying the ingredients taken, and allowing the fused product to cool more slowly, have achieved greater success. The exact way in which the so-called re-constituted rubies are made is not known, nor can absolute reliance be placed upon the descriptions which have been given of other methods of ruby-formation. The red spinel has also been made artificially, of good colour, and in large crystals. The spinel is a compound of alumina and magnesia, and by the aid of a substance such as boracic acid, which acts as a solvent for the constituents of spinel, but which volatilizes at very high temperatures, crystals of spinel having considerable dimensions, good colours, and the hardness of 8, have been obtained by several chemists. These stones, having been cut and polished, could not be distinguished by any test from the natural gems. Another method of operating, by which rock crystal and a considerable number of hard transparent and beautiful compounds of silica have been made, consists in causing two substances to act upon each other when both are in the state of vapour, sometimes with the aid of the vapour of water as a decomposing agent, and sometimes without. By the reaction of fluoride of aluminium and. boracic acid, fluoride of boron and alumina are produced, the latter crystallizing in colourless rhombohedra of white sapphire, or even, when chromium is added, taking the colours of ruby and blue sapphire. Similarly treated at a very high temperature in a lime crucible, the fluorides of aluminium and glucinum have been made
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ARTIFICIAL FORMATION
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