612 PRINCIPLES OF CHEMISTRY 



which an anhydrous calcium sulphate, CaSO 4 , called anhydrite (specific 

 gravity 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 ignit- 

 ing gypsum. If gypsum be very strongly heated it shrinks and loses 

 its power of combining with water. 48 bis One part of calcium sulphate 

 requires at 525 parts of water for solution, at 38 466 parts, and at 100 

 57 1 parts of water. The maximum solubility of gypsum is at about 36, 

 which is nearly the same temperature as that at which sodium sulphate 

 is most soluble. 49 



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

 CaCl 2 , 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 O, which melts 

 at 30 50 



48 bts According to MacColeb, gypsum dehydrated at 200 has a specific gravity 

 2'577, and heated to its point of fusion, 2'654. Potilitzin (1894) also admits the two 

 above-named modifications of anhydrous gypsum, which, moreover, always contain the 

 semi-hydrated hydrate (Note 47), and he explains by their relation to water the 

 phenomena observed in the solidification of a mixture of burnt gypsum and water. 



49 As Marignac showed, gypsum, especially when desicated at 120, easily gives super- 

 saturated solutions with respect to CaSO 4 ,2H 2 O, which contain as much as 1 part of 

 CaSC>4 to 110 parts of water. Boiling dilute hydrochloric acid dissolves gypsum, 

 iorming calcium chloride. The behaviour of gypsum towards the alkaline carbonates 

 has been described in Chapter X. Alcohol precipitates gypsum from its aqueous 

 solutions, because, like the sulphates in general, it is sparingly soluble in alcohol. 

 Gypsum, like all the sulphates, 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, Struve, 

 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 glaulerite. Fritzsche also obtained gaylussite, No-jCa^C-s^SH-jO, 

 "by pouring a saturated solution of sodium carbonate on to freshly-precipitated calcium 

 carbonate. Calcium also forms basic salts, but only a few. Veeren (1892) obtained 

 Ca(NO 3 ) 2 Ca(OH) 2 ,2ffi 2 O by leaving powdered caustic lime in a saturated solution of 

 Ca(NO 3 ) 2 until it solidified. This salt is decomposed by water. 



60 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 = 1, then at 

 29 it is 1'020, 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 diffi- 

 culty than magnesium chloride and with greater ease than barium chloride (Kuhnheim). 

 Sodium does not decompose fused calcium chloride even on prolonged heating (Lies- 

 Bodart), but an alloy of sodium with zinc, lead, and bismuth decomposes it, forming an 



