614 PRINCIPLES OF CHEMISTRY 



in exactly the same manner for calcium, Ca = 40 (and magnesium, 

 Mg = 24), there is another analogue of lighter atomic weight, 

 beryllium, Be = 9, besides the near analogues strontium, Sr = 87, and 

 barium, Ba = 137. As rubidium and caesium are more rarely met 

 with in nature than potassium, so also strontium and barium are rarer 

 than calcium (in the same way that bromine and iodine are rarer than 

 chlorine). Since they exhibit many points of resemblance with calcium, 

 strontium and barium may be characterised after a very short acquain- 

 tance 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 sulphate, BaSO 4 , which forms anhydrous crystals of the rhombic 

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

 and 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 ; for the carbonate, although more easily transformed 

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

 carbonic anhydride), is a comparatively rare mineral (BaC0 3 forms 

 the mineral witheriie ; 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 treated 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, when BaSO 4 

 -f 4C=BaS + 4CO. The residue is then treated with water, in which 

 the barium sulphide is soluble. 62 "When boiled with hydrochloric acid, 



crystallo-hydrate CaCl 2 ,6H 2 O separates, and that a solution of the above composition 

 (CaCl 2 ,14H 2 O requires 44'0 parts calcium chloride per 100 of water) solidifies as a cryo- 

 hydrate at about -55. 



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

 p. 487. 



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 

 sulphate. The hydrogen sulphide is sometimes removed from the solution by boiling 

 with the oxides of copper or zinc. If sugar be added to a solution of' barium sulphide> 

 barium saccharate is precipitated on heating ; it is decomposed by carbonic anhy 



