THE VALENCY AND SPECIFIC HEAT OF THE METALS 603 



solidifies, then a compound of the calcium sulphate with two molecules 

 of water is produced ; and this may be regarded as derived from S(OH) 6 

 by the substitution of two atoms of hydrogen by one atom of bivalent 

 calcium. Natural gypsum sometimes appears as perfectly colourless, 

 or variegated marble-like, masses, and sometimes in perfectly colourless 

 crystals, selenite, of sp. gr. 2 '33. The semi-transparent gypsum, or 

 alabaster, is often carved into small statues. Besides which an anhy- 

 drous calcium sulphate, CaSO 4 , called anhydrite (sp. gr. 2'97), occurs 

 in nature. It sometimes occurs along with gypsum. It is no longer 

 capable of combining directly with water, and differs in this respect 

 from the anhydrous salt obtained by gently igniting gypsum. If 

 gypsum be very strongly heated it shrinks and loses its power of 

 combining with water. Gypsum also passes into anhydrite if it be 

 heated in water, if the temperature be as high as 150. One part 

 of calcium sulphate requires at 525 parts of water for solution, 

 at 38 466 parts, and at 100 517 parts of water. The maximum 

 solubility, which is at about 36, is nearly the same as that of sodium 

 sulphate. 49 



As lime is a more energetic base than magnesia, so calcium chloride, 

 CaCl.,, is not so easily decomposed by water, and its solutions only 

 disengage a small quantity of hydrochloric acid when evaporated, and 

 when the evaporation is conducted in a stream of hydrochloric acid it. 

 easily gives an anhydrous salt which fuses at 719 ; otherwise an 

 aqueous solution yields a crystallo-hydrate, CaCL 2 ,6H 2 0, which melts 

 at 28. 50 



49 As Marignac showed, gypsum especially desiccated at 120 easily gives supersatu- 

 rated solutions with respect to CaSO4,2H 2 O, which contain as much as 1 part of CaSC>4 

 to 110 parts of water. Boiling dilute hydrochloric acid dissolves gypsum, forming cal- 

 cium chloride. The behaviour of gypsum towards the alkali carbonates has been de- 

 scribed in Chap. X. Alcohol precipitates gypsum from its aqueous solutions, because, 

 like the sulphates in general, it is sparingly soluble in alcohol. Gypsum, like all the sul- 

 phates, when heated with charcoal, gives up its oxygen, forming the sulphide, CaS. 



Calcium sulphate, like magnesium sulphate, is capable of forming double salts, but 

 with difficulty, and they are chemically less stable. They contain, as is always the case 

 with double salts, less water of crystallisation than the component salts. Rose, Phillips, 

 Schott, Stchefarovitch, Struve, Ditte, and others obtained the salt CaK 2 (SO 4 )2,H 2 O ; 

 a mixture of gypsum with an equivalent amount of potassium sulphate and water 

 solidifies into a homogeneous mass. Fritzsche obtained the corresponding sodium salt 

 in a hydrated and anhydrous state, by heating a mixture of gypsum with a saturated 

 solution of sodium sulphate. The anhydrous salt occurs in nature as glanberite. 

 Fritzsche also obtained gaylussite, Na-jCa^Os^SHjjO, by pouring a saturated solution 

 of sodium carbonate over freshly-precipitated calcium carbonate. 



50 Calcium chloride has a specific gravity 2'20, or, when fused, 2'12, and the sp. gr. of 

 the crystallised salt CaCl 2 ,6H 2 O is T69. If the volume of the crystals at 0=1, then at 

 29 it is r020, and the volume of the fused mass at the same temperature is 1'118 (Kopp) 

 (specific gravity of solutions, see Note 27). The solution containing 50 p.c. CaCl 2 boils 

 at 180, 70 p.c. at 158. Superheated steam decomposes calcium chloride with more dim- 



