694 PRINCIPLES OF CHEMISTRY 



marl that is, with clay mixed with a certain quantity of calcium car 

 bonate, strata of which are found nearly everywhere 



From the soil the lime and magnesia (in a smaller quantity) pass 

 into the substance of plants, where they occur as salts. Certain of 

 these salts separate in the interior of plants in a crystalline form for 

 example, calcium oxalate. The lime occurring in plants serves as the 

 source for the formation of the various calcareous secretions which are 

 so common in animals of all classes The bones of the highest animal 

 orders, the shells of mollusca, the covering of the sea-urchin, and similar 

 solid secretions of sea animals, contain calcium salts , namely, the shells 

 mainly calcium carbonate, and the bones mainly calcium phosphate. 

 Certain limestones are almost entirely formed of such deposits. 

 Odessa is situated on a limestone of this kind, composed of shells. 

 Thus magnesium and calcium occur throughout the entire realm of 

 nature, but calcium predominates. 



As lime and magnesia form bases which are in many respects 

 analogous, they were not distinguished from each other for a long 

 time. Magnesia was obtained for the first time in the seventeenth 

 century from Italy, and used as a medicine , and it was only in the 

 last century that Black, Bergmann, and others distinguished magnesia 

 from lime. 



Metallic magnesium (and calcium also) is not obtained by heating 

 magnesium oxide or the carbonate with charcoal, as the alkali metals 

 are obtained, 13 but is liberated by the action of a galvanic current 

 on fused magnesium chloride (best mixed with potassium chloride) , 

 Davy and Bussy obtained metallic magnesium by acting on magnesium 



case, at all events, with ordinary soils which have long been under corn is based not 

 BO much on the need of plants for the lime itself as on those chemical and physical 

 changes which it produces in the fiofl, as a particularly powerful base which aids the 

 alteration of the mineral and organic elements of the soil. 



15 Sodium and potassium only decompose magnesium oxide at a white heat and very 

 feebly, probably for two reasons. In the first place, because the reaction Mg + O deve- 

 lops more heat (about 140 thousand calories) than K 2 + O or Na^ + O (about 100 thousand 

 calories); and, in the second place, because magnesia is not fusible at the heat of a 

 furnace and cannot act on the charcoal, sodium, or potassium that is, it does not pass 

 into that mobile state which is necessary for reaction. The first reason alone is not 

 sufficient to explain the absence of the reaction between charcoal and magnesia, because 

 iron and charcoal in combining with oxygen evolve less heat than sodium or potassium, 

 yet, nevertheless, they can displace them. With respect to magnesium chloride, it acts 

 on so'dium and potassium, not only because their combination with chlorine evolves more 

 heat than the combination of chlorine and magnesium (Mg + Cl 2 gives 150 and Na^ + CLj 

 about 195 thousand calories), but also because a fusion, both of the magnesium chloride 

 and of the double salt, takes place under the action of heat. It is probable, however, 

 that a reverse reaction will take place. A reverse reaction might probably be expected, and 

 Winkler (1890) showed that Mg reduces the oxides of the alkali metals (Chapter XIII., 

 Note 42). 



