THE VALENCY AND SPECIFIC HEAT OF THE METALS 589 



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

 combination with the magnesium salts.- 3 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(8O 4 ) 2 ,6H 2 O, which closely resembles 

 the above-mentioned ammonium salt, crystallises from these solutions. 25 



ammonia salts, but is then precipitated by sodium carbonate, &c., it is therefore very 

 easy to separate calcium from magnesium on the basis of these properties. 



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

 to regard them from the aspect (although this does not embrace the whole essence of the 

 matter) 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 predominate in the former, whilst in the latter these properties are 

 enfeebled, and the salts formed by them bear the character of acids for instance, the 

 salts of aluminium or magnesium act in many cases like acids. By their mutual combi- 

 nation both these properties of the salts are satisfied. 



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

 deposits also contain much kainite, KMgCl(SO 4 ),3H 2 O (sp. gr., 2'13 ; 100 parts of water 

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



25 The component parts of certain double salts diffuse at different rates, and as the 

 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 ).>,6H 2 O, and the alums, 

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

 double cyanides are not separated by diffusion, which in all probability depends both on 

 the relative rate of the diffusion of the component salts and on the measure of affinity 

 acting between them. Those complex states of equilibrium which exist between water, 

 the individual salts MX and NY, and the double salt MNXY, have been already partially 

 analysed (as will be shown hereafter) in that case when the system is heterogeneous 

 (that is, when something separates out in a solid state from the liquid solution), but in 

 the case of equilibria in a homogeneous liquid medium (in a solution) the phenomenon is 

 not so clear, because it concerns that very theory of solution which cannot yet be 

 considered as established (Chapter I. Note U, and others). As regards the heterogeneous 

 decomposition of double salts, it has long been known that such salts as carnallite and 

 K 2 Mg(SO 4 ).,> give up the more soluble salt of magnesium if an insufficient quantity of 

 water for their complete solution be taken. The complete saturation of 100 parts of water 

 requires at 14*1, at 20 25, and at 60 50'2 parts of the latter double salt (anhydrous), 

 while 100 parts of water dissolve 27 parts of magnesium sulphate at 0, 86 parts at 20, 

 and 55 parts at 60, of the anhydrous salt taken. 



Of all the states of equilibrium exhibited by double salts the most fully investigated 

 as yet is the system containing water, sodium sulphate, magnesium sulphate, and their 

 double salt, Na^Mg^O.^, which crystallises with 4 and 6 mol. OH 2 . The first crystallo- 

 hydrate, MgNa 2 (SO 4 ) 2 ,4H 2 O, occurs at Stassfurt, and as a sedimentary deposit in many 

 of the salt lakes near Astrakhan, and is therefore called astrakhanite. The specific 

 gravity of the monoclinic prisms of this salt is 2'22. If this salt, in a finely-divided state, 

 be mixed with the necessary (according to the equation) quantity of water, then the 

 mixture solidifies like plaster of Paris into a homogeneous mass if the temperature be 

 below 22 (Van't Hoff und Van Deventer, 1886 ; Bakhius Roozeboom, 1887) ; but if the 

 temperature be above this transition-point then the water and double salt do not react 



