THE VALENCY AND SPECIFIC HEAT OF THE METALS 593 



termed limestone. Some common flagstones 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 car- 

 bonic anhydride so easily as the limestones under the action of acids. 

 Dolomites u sometimes contain an equal number of molecules of calcium 

 oarbonate and magnesium carbonate, and they also sometimes appear 

 in a crystalline form, which is easily intelligible, because calcium car- 

 bonate itself is exceedingly common in this form in nature, and is, then 

 known as cede spar, whilst natural crystalline magnesium carbonate is 

 termed magnesite. The formation of the crystalline varieties of the 

 insoluble carbonates is explained by the possibility of a slow deposition 

 from solutions containing carbonic acid. Besides which (Chapter X.) 

 calcium and magnesium sulphates are obtained from sea water, and 

 therefore they are met with both as deposits and in springs. It must 

 be observed that magnesium is held in considerable quantities in 

 sea water, because the sulphate and chloride of magnesium are very 

 soluble in water, whilst calcium sulphate is but little soluble, and is 

 used in the formation of shells ; and therefore if the occurrence of con- 

 siderable deposits of magnesium sulphate cannot be expected in nature, 

 still, on the other hand, one would expect (and they do actually occur) 

 large masses of calcium sulphate or gypsum t CaSO^HjO. Gypsum 

 sometimes forms strata of immense size, which extend over many 

 hectometres for example, in Russia on the Volga, and in the Donetz 

 and Baltic provinces. 



Lime and magnesia also, but in much smaller quantities (only to 

 the amount of several fractions of a per cent, and rarely more), enter 

 into the composition of every fertile soil, and without these bases the 

 soil is unable to support vegetation. Lime is particularly important 

 in this respect, and its presence in a larger quantity generally improves 

 the harvest, although purely calcareous soils are as a rule infertile. 

 For this reason the soil is fertilised both with lime la itself and with 



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

 ft magnesium salt acts on calcium carbonate. Magnesium carbonate may be formed 

 by double decomposition, and it must be supposed that this process teases at -a certain 

 limit (Chapter XL), when we shall obtain a mixture of the carbonates of calcium 

 and magnesium. Haitinger heated a mixture of calcium carbonate, CaCO 8 , with a solu- 

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

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

 bonate, MgCO 5 , and a portion of the lime was converted into gypsum, CaSO*. Lubavin 

 (1898) showed that MgC0 5 is more soluble than CaCO 3 ia salt water, which is of some 

 if nificance in explaining the composition of sea water. 



" Th undoubted action of lime in increasing the fertility of eoila if not in every 



