252. Hard Water.--Review Experiment 76. The solubility of CaCO3 in water that contains CO2 leads to important results. Much dissolves in the waters of all limestone countries; and the water, though perfectly transparent, is hard; i.e. soap has little action on it. See page 187. Such water may be softened by boiling, a deposit of CaCO3 being formed as a crust on the kettle. Such water is called water of temporary hardness. MgCO3 produces a similar effect, and water containing it is softened in the same way. Permanently hard waters contain the sulphates of Ca and Mg, which cannot be removed by boiling, but may be by adding (NH4)2CO3. 253. The Formation of Caves in limestone rocks is due also to the solubility of CaCO3. Water collects on the mountains and trickles down through crevices, dissolving, if it contains CO2, some of the CaCO3, and thus making a wider opening, and forcing its way along fissures and lines of least resistance into the interior of the earth, or out at the base of the mountain. Its channel widens as it dissolves the rock, and the stream enlarges until in the course of ages an immense cavern may be formed, with labyrinths extending for miles, from the entrance of which a river often issues. In the long ages which elapsed during the slow formation of Mammoth Cave its denizens lost many of the characters of their ancestors, and eyeless fish and also eyeless insects now abound there. 254. Reverse Action.--Drops of water on the roofs of these caverns lose their CO2, and deposit CaCO3. Thus long, pendant masses of limestone, called stalactites, are slowly formed on the roofs like icicles. From these, water charged with CaCO3 drops to the bottom, loses CO2 and deposits CaCO3, which forms an upward- growing mass, called stalagmite. In time it may meet the stalactite and form a pillar. Notice that the same action which formed the cave is filling it up; i.e. the solubility of CaCO3 in water charged with CO2. 255. Famous Marbles.--The marble from Carrara, Italy, is most esteemed on account of a pinkish tint given by a trace of oxide of iron. The best of Grecian marble was from Paros, one of the Cyclades. The isles of the Mediterranean are of limestone, or of volcanic, origin, often of both. 256. Calcium Sulphate occurs in two forms, (1) with water of crystallization--gypsum, CaSO4 + 2 H2O, --(2) without it--anhydrite, CaSO4. The former, on being strongly heated, gives up its water, and is reduced to a powder-- plaster of Paris. This, on being mixed with water, again takes up 2 H2O, and hardens, or sets, without crystallizing. If once more heated to expel water, it will not again absorb it. When plaster of Paris sets, it expands slightly, and on this account is admirable for taking casts. 257. Uses.--Gypsum finds use as a fertilizer and as an adulterant in coloring-materials, etc. CaSO4 is employed in making casts, molds, statuettes, wall-plaster, crayons, etc.