600 PRINCIPLES OF CHEMISTRY 



position ; they, like this salt (see Chapter XV.), are easily able (at 140) 

 to 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 0, 

 and generally separates from solutions as a heptahydrated salt, 

 MgS0 4 ,7H 2 O, and from supersaturated solutions as a hexahydrated 

 ealt, MgS0 4r 6H 2 O ; at temperatures below it crystallises out as a 

 dodecahydrated salt, MgS0 4 ,12H 2 0, and a solution of the com- 

 position MgSO 4 ,2H 2 solidifies completely at 5. 27 Thus between 



double decomposition, but it proceeds in the opposite direction, as may be demonstrated 

 by the above-described methods. Van der Heyd obtained a potassium astrakhanite, 

 K 2 SO 4 MgSO 4 ,4H,jO, from solutions of the component salts at 100 



From these experiments 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 

 teihperature and a change of state. It is a case of Deville's principles of dissociation, 

 extended in the direction of the passage of a solid into a liquid. On the other 

 hand, we see here how essential a rdle water plays in the formation of compounds, 

 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- differ hi any essential point (except the degree of 

 affinity that is, from a quantitative aspect) from the formation of salts themselves. 

 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 salts NaaSO^lOlLjO 

 and MgS0 4 ,7H 2 0. Water is disengaged in both cases, and hence the volumes are altered. 



86 This salt, and especially its crystallo-hydrate with 7H 2 O, is generally known as 

 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 

 magnesite, magnesium sulphate remains in solution. When dolomite that is, a mixture 

 of magnesium and calcium carbonates is subjected to the action of a solution of hydro* 

 chloric acid until about half of the salt remains, the calcium carbonate is mostly dis- 

 solved and magnesium carbonate is left, which by treatment with sulphuric acid gives ft 

 solution of magnesium sulphate. 



87 The anhydrous salt, MgSO 4 (sp. gr. 2'61), attracts moisture (7 mol. H 2 O) from 

 moist air ; when heated in steam or hydrogen chloride it gives sulphuric acid, and when 

 heated with carbon it is decomposed according to the equation 2MgS0 4 + C = 2SO<, + CO a 

 + 2MgO. The monohydrated salt (kieserite), MgSO 4 ,H 2 O (sp. gr. 2'56), dissolves in 

 water with difficulty ; it is formed by heating the other crystallo-hydrates to 185. 

 The hexahydrated salt is dimorphous. If a solution, saturated at the boiling-point, 

 pe prepared,, and cooled -without access ^of crystals of the heptahydrated salt, then 

 ftJgSO 4 ,6H.^O crystallises out in monoclinic prisms (Loewel, Marignac), which are quite 

 AS unstable as the salt, Na.jSO 4 ,7H 2 O;.butr if prismatic crystals of the cubic system 

 of the copper-nickel salts of the composition MSO 4 ,6H 2 O be added, then crystals of 

 MgS0 4 ,6H a C) are deposited on them as prisms of the cvbic system (Lecoq de Boisbau- 

 dran). The common crystallo-hydrate, MgSO^THgO, Epsom salts, belongs to the rhombio 

 system, and is obtained by crystallisation below 80. Its specific gravity is 1-69. In a 

 vacuum, or at 100, it loses 5H 2 0, at 182 CH 2 O, and at 210 all the 7H 3 O (Graham). If 

 crystals of ferrous or coboltic sulphate be placed in a saturated solution, hexagonal 

 crystals of the heptahydrated salt are formed (Lecoq de Boisbaudran) ; they present an 

 unstable state of equilibrium, and soon become cloudy, probably owing to their trans* 

 formation into the more stable common form. Fritzsche, by cooling saturated solutions 

 below 0. obtained a mixture of crystals of ice and of a dodecahydrated salt, which easily 



