CALCIUM. 153 



group of light metals. Similar to the alkali-metals, they decompose 

 water at the ordinary temperature with liberation of hydrogen; their 

 separation in the elementary state is even more difficult than that of 

 the alkali-metals. 



They differ from the latter by forming insoluble carbonates and 

 phosphates (those of the alkalies are soluble), from the earths by 

 their soluble hydroxides (those of the earths are insoluble), and from 

 all heavy metals by the solubility of their sulphides (those of heavy 

 metals are insoluble). The sulphates are either insoluble (barium) 

 or sparingly soluble (strontium and calcium). The hydroxides and 

 carbonates are decomposed by heat, water or carbon dioxide being 

 expelled and the oxides formed. In case of calcium carbonate this 

 decomposition takes place easily, while the carbonates of barium 

 and strontium require a much higher temperature. They are bivalent 

 elements. 



Occurrence in nature. Calcium is one of the most abundantly 

 occurring elements. As carbonate (CaCO 3 ) it is found in the form 

 of ^ale-spar, limestone^ chalk, marble, shells of gggg_ an( ^ mollusca, 

 etc., or, as acid carbonate, dissolved in water. The sulphate is found 

 as gypsum or alabaster, CaSO 4 2H 2 O; the phosphate, Ca 3 (PO 4 ) 2 , in 

 the different phosphatic rocks (apatite, etc.) ; the fluoride, CaF 2 , as 

 fluor-spar; the chloride, CaCl 2 , in some waters, and the silicate in 

 many rocks. It enters the vegetable and animal system in various 

 forms of combination, chiefly, however, as phosphate and sulphate. 



Calcium^pxide, Lime, Calx, CaO = 56 ( Quick-lime, Burned 

 lime), is obtained on a large scale by the common process of lime- 

 burning, wfyoJa4g the Beating pf limestone or any other calcium car- 

 bonate to about 800 C. (1472 F.), in furnaces termed lime-kilns. 

 On a small scale decomposition may be accomplished in a suitable 

 crucible over a blowpipe flame : 



CaCO 3 = CaO + CO 2 . 



The pieces of oxide thus formed retain the shape and size of the 

 carbonate used for decomposition. 



Lime is a white, odorless, amorphous, infusible substance, of alka- 

 line taste and reaction; exposed to the air it gradually absorbs 

 moisture and carbon dioxide, the mixture thus formed being^known 

 as air- slaked lime. 



Lime occupies among bases a position similar to that of sulphuric 



