THE VALENCY AND SPECIFIC HEAT OF THE METALS 605 



metal is volatile. Calcium decomposes water at the ordinary tem- 

 perature, and is oxidised in moist air, but not so rapidly as sodium. 

 In burning, it gives its oxide or lime, CaO, a substance which is 

 familiar to every one, and of which we have already frequently had 

 occasion to speak. This oxide is not met with in nature in a free 

 state, because it is an energetic base which everywhere encounters acid 

 substances forming salts with them. It is generally combined with 

 silica, or occurs as calcium carbonate or sulphate. The carbonate 

 and nitrate are decomposed, at a red heat, with the formation of lime. 

 As a rule, the carbonate, which is so frequently met with in nature, 

 serves as the source of the calcium oxide, both commercial and pure. 

 When heated, calcium carbonate dissociates : CaC0 3 =CaO + CO 2 . 

 In practice the decomposition is conducted at a bright red heat, in the 

 presence of steam, or a current of a foreign gas, in heaps or in special 

 kilns. 38 



Calcium oxide that is, quicklime is. a substance (sp. gr. 3'15) 



33 Kilns which act either intermittently or continuously are built for this purpose. 

 Those of the first kind are filled with alternate layers of fuel and limestone; the 

 fuel is lighted, and the heat developed by its combustion serves for decomposing the 

 limestone. When the process is completed the kiln is allowed td cool somewhat, the lime 

 raked out, and the same process repeated. In the continuously acting furnacefe, con- 

 structed like that shown in fig. 78, the kiln itself only contains limestone, and there are 

 lateral hearths for burning the fuel, wliose flame passes through the limestone and 

 serves for its decomposition. Such furnaces are able to work continuously, because the 

 unburnt limestone may be charged from above and the burnt lime raked out from below. 

 It is not every limestone that is suitable for the preparation of lime, because many 

 contain impurities, principally clay, dolomite, and sand. Such limestones when burnt 

 either fuse partially or give an impure lime, called poor lime in distinction from that 

 obtained from purer limestone, which is called rich lime. The latter kind is charac- 

 terised by its disintegrating into a fine powder when treated with water, and is 

 suitable for the majority of uses to which lime is applied, and for which the poor lime 

 is sometimes quite unfit. However, certain kinds of poor lime (as we shall see in 

 Chapter XVIII., Note 25) are used in the preparation of hydraulic cements, which 

 eolidify into a hard mass under water, 



In order to obtain perfectly pure lime it is necessary to take the purest possible 

 materials. In the laboratory, marble or shells are used for this purpose as a pure form 

 of calcium carbonate. They are first burnt in a furnace, then put in a crucible and 

 moistened with a small quantity of water, and finally strongly ignited, by which 

 means a pure lime is obtained. Pure lime may be more rapidly prepared by taking 

 calcium nitrate, CaN a O 6 , which is easily obtained by dissolving limestone in nitric acid. 

 The solution obtained is boiled with a small quantity of lime in order to precipitate the 

 foreign oxides which are . insoluble in water. The oxides of iron, aluminium, &c., are 

 precipitated by this means. The salt is then crystallised and ignited : Calf 2 O<j 



In the decomposition of calcium carbonate the lime preserves the form of the lumps 

 subjected to ignition ; this is one of the signs distinguishing quicklime when it is freshly 

 burnt and unaltered by air. It attracts moisture from the air and then disintegrates 

 to a powder; if left long exposed in the air, it also attracts carbonic anhydride and 

 increases in volume ; it does not entirely pass into carbonate, but forms a compound of 

 lh latter with caustic lime. 



