249. Occurrence.--The above are the chief compounds of Ca. The element itself is not found uncombined, is very difficult to reduce (page 141), is a yellow metal, and has no use. Its most abundant compound is CaCO3. Shells of oysters, clams, snails, etc., are mainly CaCO3, and coral reefs, sometimes extending thousands of miles in the ocean, are the same. CaCO3 dissolves in water holding CO2, and thence these marine animals obtain it and therefrom secrete their bony framework. All mountains were first laid down on the sea bottom layer by layer, and afterwards lifted up by pressure. Rocks and mountains of CaCO3 were formed by marine animals, and all large masses of CaCO3 are thought to have been at one time the framework of animals. Marble is crystallized, transformed limestone. The process, called metamorphism, took place in the depths of the earth, where the heat is greater than at the surface. 250. Lime.--If CaCO3 be roasted with C, CO2 escapes and CaO is left. CaCO3 - CO2 = ? This is called burning lime, and is a large industry in limestone countries. CaO is unslaked lime, quicklime or calcium oxide. It may be slaked either by exposure to the air, air-slaking, when it gradually takes up H2O and CO2; or by mixing with H2O, water-slaking. Ca0 + H2O = Ca(OH)2. Great heat is generated in the latter case, though not so much as in the formation of KOH and NaOH. Like them, Ca(OH)2 dissolves in water, forming lime-water. Milk of lime, cream of lime, etc., consist of particles of Ca(OH)2 suspended in H2O. 251. Uses of Lime--CaO is infusible at the highest temperatures. If it be introduced into the oxy-hydrogen blow-pipe (page 28), a brilliant light, second only to the electric, is produced. Mortar is made by mixing CaO, H2O, and Si02. It hardens by evaporating the extra H2O, absorbing CO2 from the air, and uniting with Si02 to form calcium silicate. It often continues to absorb CO2 for hundreds or thousands of years before being saturated, as is found in the Egyptian pyramids. Hence the tenacity of old mortar. Hydraulic mortar contains silicates of Al and Ca, and is not affected by water. What are the uses of mortar? Being the important constituent of mortar and plaster, lime is the most useful of the bases.