THE VALENCY AND SPECIFIC HEAT OF THE METALS 603 



further heated (above 106) to remove the water, then hydrochloric 

 acid passes off together with the latter, so that there ultimately remains 

 magnesia with a small quantity of magnesium chloride, 33 From what 

 has been said it is evident that anhydrous magnesium chloride cannot 

 be obtained by simple evaporation. But if sal-ammoniac or sodium 

 chloride be added to a solution of magnesium chloride, then the evolu,- 

 tion of hydrochloric acid does not take place, and after complete 

 evaporation the residue is perfectly soluble in water. This renders it 

 possible to obtain anhydrous magnesium chloride from its aqueous 

 solution. Indeed the mixture with sal-ammoniac (in excess) may be 

 dried (the residue consists of an anhydrous double salt, MgCl 2 ,2NH 4 Cl) 

 and then ignited (460), when the sal ammoniac is converted into 

 vapour and a fused mass o anhydrous magnesium chloride remains 

 behind. The anhydrous chloride evolves a very considerable amount of 

 heat on the addition of water, which shows the great affinity the salt 

 hag for water. 34 Anhydrous magnesium chloride is not only obtained 

 by the above method, but is also formed by the direct combination of 

 chlorine and magnesium, and by the action of chlorine on magnesium 

 oxide, oxygen being evolved ; this proceeds still more easily by heating 

 magnesia with charcoal in a stream of chlorine, when the charcoal serves 

 to take up the oxygen. This latter method is also employed for the pre- 

 paration of chlorides which are formed in an anhydrous condition with 

 still greater difficulty than magnesium chloride. Anhydrous magnesium 

 chloride forms a colourless, transparent mass, composed of flexible 

 crystalline plates of a pearly lustre. It fuses at a low red heat (708) 

 into a colourless liquid, remains unchanged in a dry state, but under 

 the action of moisture is partially decomposed even at the ordinary 

 temperature, with formation of hydrochloric acid. When heated in the 

 presence of oxygen (air) it gives chlorine and the basic salt, which 



53 This decomposition is most simply defined as the result of the two reverse reactions, 

 MgCl 2 + H 2 = MgO + 2HC1 and MgO + 2HC1 = MgCl 2 + H 2 0, or as a distribution 

 between O and C1 2 on the one hand and H 2 and Mg on the other. (With O, MgCl 2 gives 

 chlorine, see Chapter X., Note 88. and Chapter II., Note 3 bis and others, where the 

 reactions and applications of MgCl 2 are given.) It is then clear that, according to 

 Berthollet's doctrine, the mass of the hydrochloric acid converts the magnesium oxide 

 into chloride, and the mass of the water converts the magnesium chloride into oxide. 

 The crystallo-hydrate, MgCl 2) 6H 2 0, forms the limit of the reversibility. But an inter- 

 mediate state of equilibrium may exist in the form of basic salts. On mixing ignited 

 magnesia with a solution of magnesium chloride of specific gravity about 1*2, a solid 

 mass is obtained which is scarcely decomposed by water at the ordinary temperature 

 (see Chapter XVI., Note 4). A similar means is employed for cementing sawdust into 

 & solid mass, called cylolite, used for flooring, &c. 



We may remark that MgBr 2 crystallises not only with 6H 2 O (temperature of fusion 

 152), but also with 10H 2 (temperature of fusion + 12 b , formed at - 18. Panfiloff , 1804). 



54 According to Thomson, the combination of MgCl a with 6HjO evolves 88,000 calories, 

 and its solution in an excess of water 36,000. 



