590 PRINCIPLES OF CHEMISTRY 



The nearest analogues of magnesium are able to give exactly similar 

 double salts, both in crystalline form (monoclinic system) and com- 

 position ; they, like this salt (see Chapter XV.), are easily able to 

 (140) part with all their water of crystallisation, and correspond with 

 the salts of sulphuric acid, whose type may be taken as magnesium 

 sulphate, MgSO 4 . 26 It occurs at Stassfurt as kieserite, MgS0 4 ,H. 2 O r 



on each other : MgNa 2 (SO 4 ).,,4H 2 O + 13H 2 O = Xa 2 SO 4 ,10H.p + MgS0 4 ,7H,O ; that is, they 

 do not solidify or give a mixture of sodium and magnesium sulphates. If a mixture (in 

 equivalent quantities) of solutions of these salts be evaporated, and crystals of astra- 

 khanite and of the individual salts capable of proceeding from it be added to the con- 

 centrated solution to avoid the possibility of a supersaturated solution, then at tempera- 

 tures above 22 astrakhanite is exclusively formed (this is the method of its production),, 

 but at lower temperatures the individual salts are alone produced. If equivalent amounts 

 of Glauber's salt and magnesium sulphate be mixed together in a solid state, then there is 

 no change at temperatures below 22, but at higher temperatures astrakhanite and water 

 are formed. The volume corresponding with XXoSO 4 ,10H 2 O in grams =8'22.T46 = 220'5 

 cubic centimetres, and of MgSO 4 ,7H 3 O = 246, T68 = 14(V4 ; hence their mixture in equivalent 

 quantities occupies a volume of 366'9 c.c. The volume of astrakhanite =334/2'22 = 150-5,. 

 and the volume of 18H 2 O = 234, hence their sum =380'5 c.c., and therefore it is easy to 

 follow the formation of the astrakhanite in a suitable apparatus (a kind of thermometer 

 containing oil and a powdered mixture of sodium and magnesium sulphates), and to see 

 by the variation in volume that before 22 it remains unchanged, and at higher tempera- 

 tures proceeds more quickly the higher the temperature. At the transition temperature 

 the solubility of astrakhanite and of the mixture of the component salts is one and the 

 same, whilst at higher temperatures a solution which is saturated for a mixture of the 

 individual salts would be supersaturated for astrakhanite, and at lower temperatures the 

 solution of astrakhanite will be supersaturated for the component salts, as has been 

 shown with especial detail by Karsten, Diacon, and others. Roozeboom showed that 

 there are two limits to the composition of the solutions which can exist for a double salt ; 

 these limits are respectively obtained by dissolving a mixture of the double salt with 

 each of its component simple salts. Van't Hoff demonstrated, besides this, that the 

 tendency towards the formation of double salts has a distinct influence on the progress 

 of double decomposition, because at temperatures above 31 the mixture 2MgSO 4 ,7H 2 O 

 + 2NaCl passes into MgNa2(SO 4 ) 2 ,4H 2 O + MgCL,6H 2 O 4- 4H 2 O, whilst below 31 there is 

 not this double decomposition, but it proceeds in the opposite direction, as may be 

 demonstrated by the above-described methods. 



From these examples on double salts we see that there is as close a dependence 

 between the temperature and the formation of substances as there is between the tem- 

 perature and a change of state. It is a case of the conceptions of Deville concerning 

 dissociation, but extended in the direction of the passage of a solid into a liquid. On 

 the other hand, we here see how essential a rule water plays in the formation of com- 

 pounds, and how the affinity for water of crystallisation is essentially analogous to 

 the affinity between salts, and hence also to the affinity of acids for bases, because 

 the formation of double salts does not in any essential point (except the measure of 

 affinity that is, from a quantitative aspect) differ from the formation of salts them- 

 selves. When sodium hydroxide with nitric acid gives sodium nitrate and water the 

 phenomenon is essentially the same as in the formation of astrakhanite from the salt* 

 Na 2 SO 4 ,10H 2 O and MgSO 4 ,7H 2 O. Water is disengaged in both cases, and he-m-r the 

 volumes are altered. 



26 This salt, and especially its crystallo-hydrate with 7H.,O, is generally known M 

 Epsom salts. It has long been used as a purgative. It is easily obtained from magnesia 

 and sulphuric acid, and it separates on the evaporation of sea water and of many saline 

 springs. When carbonic anhydride is obtained by the action of sulphuric acid on 



