THE YALKNCY AND SPECIFIC HEAT <>K THE METALS 583 



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 sea water. The carbonates, CaCO 3 and 

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

 ii-itt, , xnf a rated with carbonic anhydride, 10 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, that which would 

 be exhibited by a gradually accumulating deposit on the bottom of 

 the sea. Furthermore, the remains of sea organisms, plants, ferns, 

 <fcc., are frequently found amongst these deposits. These deposits of 

 calcium and magnesium carbonates are the most important sources of 

 these metals. Lime generally predominates, because it is present in 

 rocks and stream water in greater quantity than magnesia, and in this 

 case these sedimentary rocks are termed limestone. Some common flag- 

 stones used for paving, &c., and chalk may be taken as examples of this 

 kind of formation. Those limestones in which a considerable portion 

 of the calcium is replaced by magnesium are termed dolomites. The 

 dolomites are distinguished by their hardness, and by their not parting 

 with the whole of their carbonic anhydride so easily as the limestones 

 under the action of acids. Dolomites 1 1 sometimes contain an equal 

 number of molecules of calcium carbonate and magnesium carbonate, 

 and they also sometimes appear in a crystalline form, which is easily 

 understood, because calcium carbonate itself is exceedingly common in 

 this form in nature, and is then known as calc spar, whilst natural 



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

 soluble "than the normal ; here, on the contrary, with an excess 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 . 



11 The formation of dolomite may be explained, if only we imagine that a solution of 

 a magnesium salt acts on calcium carbonate. Magnesium carbonate may be formed by 

 double decomposition, and it must be supposed that this process is able to cease at a 

 certain limit (Chapter XII.), when we shall obtain a mixture of the carbonates of calcium 

 and magnesium. Haitinger heated a mixture of calcium carbonate, CaCO 5 , with a solu- 

 tion of an equivalent quantity of magnesium sulphate, MgSO 4 , in a closed tube at 200, 

 and then a portion of the magnesia actually passed into the state of magnesium car- 

 bonate, MgCO 3 , and a portion of the lime was converted into gypsum, CaSO 4 . 



