Nearly every drink of water you take gives you some calcium salts, for "hard" water, such as comes from wells, springs, and rivers, gets this quality from com pounds of calcium and magnesium dissolved in it.
Calcium is a white or yellowish metal of no great importance in itself, but its compounds are very common and useful. Combined with phosphoric acid it makes calcium phosphate, which constitutes a large part of the bones of human and animal skele tons, and is very necessary to plant growth. Lime stone, marble, chalk, Iceland spar, coral, pearls, egg-shells, etc., are all made up mainly of calcium carbonate. Marble is a limestone which has under gone changes due to heat and pressure and has become crystalline (see Limestone; Marble). Chalk is com posed of calcium deposits which formerly were parts of minute organisms (see Chalk).
Iceland spar is in the form of transparent and color less crystals which have the power of producing double refraction of light and hence are used in the making of certain optical instruments. Calcium carbonate dissolves in ordinary waters, since they contain carbonic acid, and from such waters are formed the stalactites and stalagmites found in limestone caverns. The dripping water gradually evaporates and leaves a deposit of calcium carbonate in beautiful and fantastic forms.
Quicklime is the oxide of calcium; when exposed to air it takes up carbon dioxide and becomes calcium onate. It is then said to be "air-slaked" and seless as lime (see Lime). "Water-slaked" lime ticium hydroxide. When mixed with sand and it forms plaster; it absorbs carbon dioxide and . becomes calcium carbonate. Since there is little carbon diox ide in the air, there is plenty of time to put the plaster on the wall before it hardens.
Gypsum and plaster of Paris are calcium sulphate (see Gypsum).
Calcium carbide is made by heating pulverized lime stone or lime with coke in an electric furnace. With water, calcium carbide produces acetylene gas (see Acetylene). Cal cium carbide is also used in one of the new processes for the fixation of nitrogen. When strongly heated with nitrogen gas, it gives a mixture of carbon and cal cium cyanamide which is a widely used nitrogen fertilizer. When this calcium cyanamide is treated with hot water or super-heated steam, it produces calcium carbonate and ammonia (see Ammonia; Nitrogen).