Quicklime. The making of lime from limestone or chalk is called lime burning. The operation is carried out in a structure called a lime kiln, which is usually a barrel-shaped vertical shaft surrounded by substantial brickwork. There are two main methods of procedure, the one continuous and the other intermittent. In the continuous process, the kiln is filled up with limestone and fuel (generally coke) in alternate layers. Combustion is started at the bottom and maintained by a regulated draught. As the charge works down, the addition of limestone and fuel is continued from the top, while the lime is removed from the bottom of the kiln. The lime produced by this method has the ashes of the fuel mixed with it. To avoid this, the more modern type of kiln has four lateral fire grates outside the actual kiln. For the intermittent method, a kiln is required which has a fireplace at the bottom. Over this a rough arch is built of large pieces of limestone, laid dry, and then the kiln is filled up with pieces of limestone which decrease in size from below upwards. The fire is kindled beneath the arch and urged by a regulated draught. The heating is maintained for three days and nights, after which time the charge is allowed to cool down. Carbonic Acid Gas in Nature. Although the solvent action of carbonic acid is very small compared with that of strong acids, it is nevertheless great in comparison with that of water. This is shown especially in its action on limestone, an action from which several important consequences arise. Rain, as it falls through the air, dissolves a little carbon dioxide and, although this is only an exceedingly dilute solution of a very weak acid, its cumulative effect, especially in limestone districts, is very great; it hollows out enormous caves and causes the formation of those fantastic creations in stone known as stalactites and stalagmites. When a drop of water charged with carbonic acid gas falls on limestone, it dissolves a little of that substance, forming calcium bicarbonate, which may be regarded as a compound of calcium carbonate, carbon dioxide, and water. Little by little, the solid rock is hollowed out and a cave, or perhaps an underground watercourse, is formed. Again, the drop of water charged with calcium bicarbonate may find its way to the roof of a cave. As it hangs from the roof while it gathers strength to fall, a little of the carbon dioxide escapes, and a minute quantity of calcium carbonate is deposited. In this way, a stalactite looking like an icicle in stone gradually grows downwards. When the drop reaches the floor of the cave, a little time elapses before it sinks into the ground; again a little carbon dioxide escapes, and a small quantity of calcium carbonate is formed. Little is added to little, and in the course of ages the stalagmite grows upward from the floor and ultimately meets the stalactite to form a continuous column of glistening crystallized calcium carbonate.