602 PRINCIPLES OF CHEMISTRY 



Mg(HO) 2) 4MgC0 3 ,9H 2 O ; bub all the magnesia is aot precipitated in 

 this case, as a portion of it remains in solution as an-acid double salt. 

 If sodium carbonate be added to a boiling solution of magnesium 

 sulphate a precipitate of a still more basic salt is formed, 4MgSO 4 

 + 4Na. 2 C0 3 + 4H 2 = 4Na 2 SO 4 + C0 2 + Mg(OH),,3MgCO 3 ,3H 2 O. 

 This basic salt forms the ordinary drug magnesia (magnesia alba), in 

 the form of light porous lumps. Other basic salts are formed under 

 certain modifications of temperature and conditions of decomposition. 

 But the normal salt, MgC0 3 , \hich occurs in nature as magnesite 

 in the form of rhombohedra of specific gravity 3'056, cannot be 

 obtained by such a method of precipitation. In fact, the formation of 

 the different basic salts shows the power of water to decompose the 

 normal salt. It is possible, however, to obtain this salt both in an 

 anhydrous and hydrated state, A solution of magnesium carbonate in 

 water containing carbonic acid is taken for this purpose. The reason 

 for this is easily understood carbonic anhydride is one of the pro- 

 ducts of the decomposition of magnesium 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 magnesium, 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 in the last mother-liquors of 

 the evaporation of sea-water. On cooling a sufficiently concentrated 

 solution, the crystallo -hydrate, MgCl 2 ,6H 2 O, separates ; 32 but if it be 



29 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 shortly 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 hydrated magnesium sulphate and sodium chloride be heated to redness, the evolu- 

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

 salt, MgSO 4 + 2NaCl + H 2 O = Na2SO 4 + MgO + 2HCl. 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 

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

 similarity of the reactions of acids and salts, especially of, salts which contain such feeble 



51 As sea-water contains many salts, MCI and MgX 2 , it follows, according to Ber- 

 thollet's teaching, that MgCl 2 is also present. 



32 As the crystallo-hydrates of the salts of sodium often, contain 10H 8 O, so many of 

 the salts of magnesium contain 6H 2 O. 



