582 PRINCIPLES OF CHEM1STKY 



alkalis, decompose water (without the addition of acids), although not 

 so easily as the latter metals. The process of the decomposition is 

 essentially one and the same ; for example, Ca + 2H. 2 O=CaH 2 O 2 + H 2 

 that is, hydrogen is liberated and a hydroxide of the metal formed. 

 These hydroxides are bases which saturate nearly all acids. How- 

 ever, the hydroxides RH 2 O 2 of calcium and magnesium are in no 

 respects so energetic as the hydroxides of the true metals of the 

 alkalis ; thus when heated they lose water, are not so soluble, develop 

 less heat with acids, and form various salts, which are less stable 

 and more easily decomposed by heat than the corresponding salts 

 of sodium and potassium. Thus calcium and magnesium carbonates 

 easily part with carbonic anhydride when ignited ; the nitrates are 

 also very easily decomposed by heat, calcium and magnesium oxides, 

 CaO and MgO, being left behind. The chlorides of magnesium 

 and calcium, when heated with water, evolve hydrogen chloride, 

 forming the corresponding hydroxides, and when ignited the oxide 

 itself. All these points already evince a weakening of the alkaline 

 properties. 



These metals have been termed the metals of the alkaline earths, 

 because they, like the alkali metals, form energetic bases. They are 

 called alkaline earths because they are met with in nature in a state of 

 combination, forming the insoluble mass of the earth, and because as 

 oxides, RO, they themselves have an earthy appearance. Not a few 

 salts are known of these metals which are insoluble in water, whilst 

 the corresponding salts of the alkali metals are generally soluble for 

 example, the carbonates, phosphates, borates, and other salts of the 

 alkaline earth metals are nearly insoluble. This serves to distinguish 

 the metals of the alkaline earths from the metals of the alkalis. For 

 this purpose a solution of ammonium carbonate is added to a mixed 

 solution of salts of both metals, when by a double decomposition the 

 insoluble carbonates of the metals of the alkaline earths are formed 

 and pass into a precipitate, whilst the metals of the alkalis remain in 

 solution : RX 2 4 Na 2 C0 3 = RCO 3 + 2NaX. 



We may here remark that the oxides of the metals of the alkaline 

 earths are frequently called by special names : MgO is called magnesia 

 or bitter earth ; CaO, lime ; SrO, strontia ; and BaO, baryta. 



In the primary rocks the oxides of calcium and magnesium are 

 combined with silica, sometimes in variable quantities, so that in some 

 cases the lime predominates and in other cases the magnesium. Both 

 oxides, being analogous to each other, replace each other in equivalent 

 quantities. The various forms of augite, hornblende or amphibole, and 

 of similar minerals, which enter into the composition of nearly all rocks, 



