THE VALENCY AND SPECIFIC HEAT OF THE METALS 585 



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. 



Mrtitllic 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) ; 

 but Davy and Bussy first obtained metallic magnesium by acting OIL 

 magnesium chloride with the vapours of potassium. At the present 

 time (Deville's process) magnesium is prepared in rather consider- 

 able quantities by a similar process, only the potassium is replaced by 

 sodium. Anhydrous magnesium chloride, together with sodium chloride 

 and calcium fluoride, is fused in a closed crucible. The latter sub- 

 stances only serve to facilitate the formation of a fusible mass before 

 and after the reaction, which is indispensable in order to prevent 

 the access and action of air. One part of finely-divided sodium to 

 five parts of magnesium chloride is thrown into the strongly-heated 

 molten mass, and after stirring the reaction proceeds very quickly, and 

 magnesium separates, MgCl 2 -fNa 2 = Mg + 2NaCl. In working on a 

 large scale, the powdery metallic magnesium is then subjected to distil- 

 lation at a white heat. The distillation of the magnesium is necessary, 

 because the undistilled metal is not homogeneous 14 and burns unevenly : 

 the metal is prepared for the purpose of illumination. Magnesium 



lc> 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 Nas + O (about 100 thousand 

 calories) ; and, in the second place, because magnesia is not fusible dt 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 

 suttir-ient 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 sodium and potassium, not only because their combination with chlorine evolves more 

 heat than the combination of chlorine and magnesium (Mg + Cl2 gives 150 and Naj + 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. 



14 Commercial magnesium generally contains a certain amount of magnesium nitride 

 (Deville and Caron), Mg 3 N 2 that is, a product of substitution of ammonia which is 

 directly formed when magnesium is heated in nitrogen. It is yellowish green powder, 

 which gives ammonia and magnesia with water, and cyanogen when heated with carbonic 

 anhydride. Perfectly pure magnesium may be obtained by the action of a galvanic 

 current. 



