592 PRINCIPLES OF CHEMISTRY 



and fall as a precipitate, whilst the metals of the alkalis remain in 

 solution : RX 2 + Na 2 CO 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. The two 

 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, 

 contain lime and magnesia and silica. The majority of the primary 

 rocks also contain alumina, potash, and soda. These rocks, under 

 the action of water (containing carbonic acid) and air, give up lime 

 and magnesia to the water, and therefore they are contained in all 

 kinds of water, and especially in sea- water. The carbonates CaCO 3 and 

 MgCO 3 , frequently met with in nature, are soluble in an excess of 

 water saturated with carbonic anhydride, and therefore many natural 

 waters contain these salts, and are able to yield them when evaporated. 

 However, one kilogram of water saturated with carbonic anhydride 

 does- not dissolve more than three grams of calcium carbonate. By 

 gradually expelling the carbonic anhydride from such water, an in- 

 soluble precipitate of calcium carbonate separates out. It may confi- 

 dently be stated that the formation of the very widely distributed 

 strata of calcium and magnesium carbonates was of this nature, because 

 these strata are of a sedimentary character that is, such as would 

 be exhibited by a gradually accumulating deposit on the bottom of 

 the sea, and, moreover, frequently containing the remains of marine 

 plants, and animals, shells, <fcc. It is very probable that the presence 

 of these organisms in the sea has played the chief part in the pre- 

 cipitation of the carbonates from the sea water, because the plants 

 absorb C0 2 , and many of the organisms CaCO 3 , and after death give 

 deposits of carbonate of lime ; for instance, chalk, which is almost 

 entirely composed of the minute remains of the calcareous shields of 

 such organisms. These deposits of calcium and magnesium carbonates 

 are the most important sources of these metals. Lime generally pre- 

 dominates, because it is present in rocks and running water in greater 

 quantity than magnesia, and in this case these sedimentary rocks are 



10 Sodium carbonate and other carbonates of the alkalis give acid salts which are les 

 soluble than the normal; here, on the contrary, with an excels of carbonic anhydride, a 

 salt is formed which is more soluble than the normal, but this acid salt is more unstable 

 than sodium hydrogen carbonate, NaHCO 5 . 



