Till-: VALKNVY AND SPECIFIC HEAT OF THE METALS 605 



than calcium (as also bromine and iodine are rarer than chlorine). 

 Whilst exhibit inu' many points of resemblance with calcium, strontium 

 and barium may be characterised after a very short acquaintance with 

 their chief compounds ; this shows the important advantages gained by 

 distributing the elements according to their natural groups, to which 

 matter we shall turn our attention in the next chapter. 



Among the compounds of barium met with in nature the commonest 

 is the sHlplmti', P>aSO 4 , which forms anhydrous crystals of the rhombic 

 system, which are identical in their crystalline form with anhydrite, 

 and which generally occur as transparent and semi-transparent masses 

 of tabular crystals having a high specific gravity, namely 4'45, for which 

 reason this salt bears the name of heavy spar or barytes. Analogous to 

 it is celestine, SrSO 4 , which is, however, more rarely met with ; heavy 

 spar frequently forms the gangue separated on dressing metallic ores 

 from the vein stuff. This mineral is the source of all other barium 

 compounds, because the carbonate, although more easily transformed 

 into the other compounds (because acids act directly on it, evolving 

 carbonic anhydride), is, however, a comparatively rare mineral (BaCO 3 

 forms the mineral witherite ; SrCO 3 , strontianite ; both are rare, the 

 latter is found at Etna). The treatment of barium sulphate is 

 rendered difficult from the fact that it is insoluble both in water and 

 acids, and has therefore to be conducted by a method of reduction. 51 

 Like sodium sulphate and calcium sulphate, heavy spar when heated 

 with charcoal parts with its oxygen and forms barium sulphide, BaS. 

 For this purpose a pasty mixture of powdered heavy spar, charcoal, 

 and tar is subjected to the action of a strong heat, in which case 

 BaSO 4 + 40 = BaS + 4CO. The residue is then treated with water, in 

 which the barium sulphide is soluble. 52 When boiled with hydrochloric 



in millimetres, are able to exist together in stable equilibrium, according to Roozeboom's 



Solutions of calcium chloride may serve as a convenient example for the study of the 

 supersaturated state, which easily comes about in this case, because different hydrates 

 are formed. Thus at 25 solutions containing more than 88 parts of anhydrous calcium 

 chloride per 100 of water will be supersaturated for the hydrate CaCl 2 ,6H 2 O. 



51 The action of barium sulphate on sodium and potassium carbonates is given on 

 p. 427. 



52 Barium sulphide is decomposed by water, BaS + 2H 2 O = H 2 S + Ba(OH) 2 (the reac- 

 tion is reversible), but both substances are soluble in water, and their separation is com- 

 plicated by the fact that barium sulphide absorbs oxygen and gives insoluble barium 



