THE VALENCY AND SPECIFIC HEAT OF TIIF. MKTALS 595 



and magnesium, the affinity of iodine for calcium is feebler than that of 

 chlorine (and oxygen), and therefore it is not astonishing that calcium 

 iodide may be subjected to that decomposition, into which the chloride 

 and oxide enter with difficulty. 37 Metallic calcium is of a yellow 

 colour, and has a considerable lustre, which it preserves in dry air. Its 

 specific gravity is 1 '58. Calcium is distinguished by its great ductility ; 

 it melts at a red heat and then burns in the air with a very brilliant 

 flame, which is easily understood from the fact that finely-divided 

 infusible calcium oxide is formed. Judging from the fact that in 

 burning calcium gives a very large flame, it is probable that this metal 

 is volatile. Calcium decomposes water at the ordinary temperature, 

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

 calcium 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, with 

 which it forms salts. Generally it is combined with silica, or occurs 

 as calcium carbonate or sulphate. The calcium 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 



37 Calcium iodide may be prepared by saturating lime with hydriodic acid. It is a very 

 soluble salt (at 20 one part of the salt requires 0'49 parts and at 48 0'35 parts of water 

 for solution), is deliquescent in the air, and resembles calcium chloride in many respects. 

 It changes but little when evaporated, and, like calcium chloride, fuses when heated, and 

 therefore all the water may be driven off by heat. If anhydrous calcium iodide be heated 

 with an equivalent quantity of sodium in a closely-covered iron crucible, sodium iodide 

 and metallic calcium are formed (Lies-Bodart). Dumas advises carrying on this re- 

 action in a closed space under pressure. 



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

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

 lighted, and the heat developer! by its combustion serves for decomposing the limestone. 

 When the process is completed the kiln is allowed to cool somewhat, the lime raked out, 

 and the same process repeated. In the continuously-acting furnaces, constructed like 

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

 for burning the fuel, whose flame passes through the limestone and serves for its decom- 

 position. 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, especially clay, dolomite, and sand. Such limestones when burnt 

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

 pure lime which is obtained from purer limestone, and which is called rich lime. Tlu- 

 latter quality is characterised by its disintegrating into a fine powder when treated witk 

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



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