TIIK YALF.NCY .VXD SPECIFIC HEAT OF THE 3IKTALS 598 



carbonate in the presence of water. If this solution be left to 

 evaporate spontaneously the normal salt separates in a hydrated form, 

 but in the evaporation of a heated solution, through which a stream of 

 carbonic anhydride is passed, the anhydrous salt is formed as a 

 crystalline mass, which remains unaltered in the air, like the natural 

 mineral. 29 The decomposing influence of water on the salts of mag- 

 nesium, which is directly dependent on the feeble basic properties of 

 magnesia, 30 is most clearly seen in magnesium chloride, MgCl 2 . This 

 salt is contained, 31 as we have already seen (Chapter X.), in sea water, 

 and remains in the last mother liquors of its evaporation. On cooling 

 a sufficiently concentrated solution, the crystallo-hydrate, MgCl 2 ,6H 2 O, 

 separates; 32 but if it be 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 mag- 

 nesium chloride. 33 From what has been said it is evident that anhy- 

 drous 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 evolution of hydrochloric acid does not 

 take place, and after complete evaporation the remaining mass is 

 entirely soluble in water. This renders it possible to obtain anhydrous 



39 The crystalline form of the anhydrous salt obtained in this manner is not the same 

 as that of the natural salt. The former gives rhombohedra, like those in which calcium 

 carbonate appears as calc spar, whilst the natural salt appears as rhombic prisms, like those 

 sometimes presented by the same carbonate, as aragonite, which will soon be described. 



50 Magnesium sulphate enters into certain reactions which are proper to sulphuric 

 acid itself. Thus, for instance, if a carefully-prepared mixture of equivalent quantities 

 of hydrous magnesium sulphate and sodium chloride be heated to a red heat, the evolu- 

 tion of hydrochloric acid is observed just as in the action of sulphuric acid on common 

 salt, MgSO 4 + 2NaCl + H.,O = Na. 2 SO 4 + MgO + 2HC1. Magnesium sulphate acts in a 

 similar manner on nitrates, with the evolution of nitric acid. A mixture of it with 

 common salt and manganese peroxide gives chlorine. Sulphuric acid is sometimes 

 replaced by magnesium sulphate in galvanic batteries for example, in the well-known 

 Mridinger battery. In the above-mentioned reactions we see a striking example of how 

 alike the reactions of acids and salts are, especially of salts which are formed by such 

 feeble bases as magnesia. 



"' As sea water contains many salts, MCI and MgX._>, it follows, according to Ber- 

 thollet's teaching, that MgCL is also present. 



"- As the crystallo-hydrates of the salts of sodium often contain 10H. 2 O, so many of 

 the salts of magnesium contain 6H.>O. 



" This decomposition is most simply defined as the result of the two reverse reactionp 

 MgCl 3 + H. 3 = MgO + 2HC1 and MgO + 2HC1 = MgCl 2 + H 2 O, or as a distribution 

 between O and C1 2 on the one hand and H 2 and Mg on the other. It is then clear that 

 according to Berthollet's doctrine, the mass of the hydrochloric acid converts the mag- 

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

 into oxide. The crystallo-hydrate, MgCl 2 ,6HoO, forms the limit of the reversibility. But 

 an intermediate 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 tempera- 

 ture (see Zinc). 



VOL. I. Q Q 



