608 PRINCIPLES OF CHEMISTRY 



solution of ordinary ammonium carbonate dissolves magnesium 

 oxide or carbonate, and precipitates crystals O f a double salt, 

 Mg(NH 4 ) 2 (CO 3 ) 2 ,4H 2 O, from which water extracts the ammonium 

 carbonate. With an excess of an ammonium salt the double salt passes 

 into solution,'' 1 and therefore if a solution contain a magnesium salt 

 and an excess of an ammonium salt for instance, sal-ammoniac- 

 then sodium carbonate will no longer precipitate magnesium carbonate. 

 A mixture of solutions of magnesium and ammonium chlorides, on 

 evaporation or refrigeration, gives a double salt, Mg(NH 4 )Cl 3 ,6H 2 0. 29 

 The salts of potassium, like those of ammonium, are able to enter into 

 combination with the magnesium salts. 23 For instance, the double 

 salt, MgKCl 3 ,6H 2 O, which is known as carnallite^ and occurs in the 

 salt mines of Stassfurt, may be formed by freezing a saturated solution 

 of potassium chloride with an excess of magnesium chloride. A satu- 

 rated solution of magnesium sulphate dissolves potassium sulphate, and 

 solid magnesium sulphate is soluble in a saturated solution of potassium 

 sulphate. A double salt,- K 2 Mg(S0 4 ) 2 ,6H 2 O, which closely resembles 

 the above-mentioned ammonium salt, crystallises from these solutions. 28 



il This is an example of equilibrium and of the influence of mass ; the double salt is 

 decomposed by water, but if instead of water we take a solution of that soluble part which 

 is formed in the decomposition of the double salt, then the latter dissolves as a whole. 



22 If an excess of ammonia be added to a solution of magnesium chloride, only hall 

 the magnesium is thrown down in the precipitate, 2MgCl 2 + 2NH 4 .OH -= Mg(OH) a 

 4- Mg.NH 4 Cl3-f NH 4 C1. A solution of ammonium chloride reacts with magnesia, evolving 

 ammonia and forming a solution of the same salt, MgO4SNH 4 Cl = MgNH 4 Cl.^-f H 2 O 



Among.the double' salts of ammonium and magnesium, the phosphate. MgNH 4 P(>4,6H 2 O, 

 is almost insoluble in water (0*0? gram is soluble in a litre), even in the presence of 

 ammonia. Magnesia is very frequently precipitated as this salt from solutions in which 

 it is held by ammonium salts. As lime is not retained in solution by the presence of 

 ammonium salts, but is precipitated nevertheless by sodium carbonate, &c., it is very 

 easy to separate calcium from magnesium by taking advantage of these properties. 



25 In order to see the nature and cause of formation of double salts, it is sufficient 

 (although this does not embrace the whole essence of the matter) to consider that one of 

 the metals of such salts (for instance, potassium) easily gives acid salts, and the other 

 (in this instance, magnesium) basic salts ; the properties of distinctly basic elements pro* 

 dominate in the former, whilst in the latter these /properties are enfeebled, and the 

 salts formed by them bear the Character of acids for example, the salts of aluminium 

 or magnesium act in many cases like acids. By their mutual combination these two 

 opposite properties of the salts are both satisfied 



84 Carnallite has been mentioned in Chapter X. (Note 4) and m Chapter XIII. These 

 deposits also contain much kaimte, KMgCl(SO 4 ),8H,O (sp. gr. 2'18 ; 100 parts of water 

 dissolve 79-6 parts at 18). This double salt contains two metals and two haloids. Feit 

 (1889) also obtained a bromide corresponding to camallite. 



24 The component parts of certain double salts diffuse at different rates, and as th 

 diffused solution contains a different proportion of the component salts than the 

 solution taken of the double salt, it shows that such salts are decomposed by water. 

 According to Riidorff, the double salts, like carnallite, MgK 2 (SO 4 ) 2 ,6H 3 O, and the alums, 

 all belong to this order (1888). But such salts as tartar emetic, the double oxalates, and 



