The Lydian or Touch-stone (15a), by reason of its hardness and black colour, has been used from remote ages to test the purity of precious metals. Hornstone (15a) is a variety of Silica without evident crystallisation, and generally presenting a more or less splintery fracture; in one kind, Flint (15b), the surface of fracture is generally shell-shaped (conchoidal), sometimes conical, as is well shown by specimens in the case. Chalcedony (15b), which has a lustre nearly that of wax, is translucent. The specimens of Enhydros from Uruguay (15d) are of especial interest as containing imprisoned water. Heliotrope, or Bloodstone (16a), is a green stone with red, blood-like spots. Next follow the Plasma and Chrysoprase, which are green stones: and the Sard, generally brownish-red; as also the Sardonyx, its banded variety. All were much prized by the ancients because, though hard and tough enough to resist ordinary wear and tear, they are more suited to the display of the engraver’s skill than the still harder and more precious stones. Then come the Agates (16b), chiefly formed of thin layers of porous Chalcedony of different colours, though the material of many of the white layers is a compact Semi-opal. Most are now brought from Uruguay, in South America, and cut and polished at Oberstein, in Germany, where in former times Agates were collected in quantity from the mountains of the district. Sometimes the layers are parallel, and the stone is then an Onyx, useful as a material for cameos: or the bands of a section are arranged in parallel sets of zigzag lines, and the stone is then called a Fortification-agate; but in the ordinary agate the layers are variously curved. Moss-agates, or Mocha-stones (16e), are varieties of Chalcedony enclosing moss-like forms of oxides of Manganese and Iron, and green earthy Chlorite. Carnelian (16e) is a beautiful red stone much valued by the engraver: its fracture has a peculiar waxy lustre, and is distinct from that of the Sard, which is dull and horn-like. Opal, including the Precious or Noble Opal (16f), among the specimens of which is a fine suite from Queensland presented by the late Professor Story-Maskelyne, is hydrated Silica. [Sidenote: Carbonates.] Witherite, the carbonate of Barium (18a), is used in the manufacture of plate-glass. Strontianite (18b), the carbonate of Strontium, is one of two minerals from which Strontium nitrate is made for use in the manufacture of fireworks, owing to the fine crimson colour which the salt gives to the flame: the Strontium minerals are also employed in connection with sugar-refining. Cerussite (18b) is the corresponding carbonate of Lead, and when abundant is a valuable ore of that metal.