THE VALENCY AND SPECIFIC HEAT OF Till- .MKTALS 591 



and grnonilly separates from solutions as a heptahydrated salt,. 

 MgSO 4 ,7H.,O, and from supersaturated solutions as a hexahydrated 

 salt, MgSO 4 ,6H.,O ; at temperatures below it crystallises out as a 

 dodecahydrated salt, MgSO 4 ,12H 2 O, and a solution of the com- 

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

 water and magnesium sulphate there may exist several definite and 

 more or less stable degrees of equilibrium, and the double salt 

 MgSO 4 K 2 SO 4 ,6H. 2 O may be regarded as one of these equilibrated 

 systems, all the more as it contains 6H 2 O, whilst MgSO 4 is able to 



, magnesium sulphate remains in solution. When dolomite that is, amixture 

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

 chloric acid until about half of the salt remains, then the calcium carbonate is chiefly 

 dissolved and magnesium carbonate is left, and, by treatment with sulphuric acid, gives 

 a solution of magnesium sulphate. 



- 7 The anhydrous salt, MgSO 4 (sp. gr., 2'61), attracts moisture (7 mol. H.,0) from 

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

 when heated with carbon is decomposed, according to the equation 2MgSO 4 + C = 2SO 2 

 + CO 2 + 2MgO. The monohydrated salt (kieserite), MgSO 4 ,HoO (sp. gr., 2'56), dissolves 

 in water with difficulty, and remains when the other crystallo-hydrates are heated to 135. 

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

 1 >ared, and cooled without access of crystals of the heptahydrated salt, then MgSO 4 ,6H 2 O 

 erystallises out in monochnic prisms (Loewel, Marignac), which are quite as un- 

 stable as the salt, NaSO 4 ,7H 2 O (p. 509) ; but 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 

 MgSO 4 ,6H 2 O are deposited on them as prisms of the cubic system (Lecoq de Boisbaudran).. 

 The common crystallo-hydrate, MgSO 4 ,7H 2 O, Epsom salts, belongs to the rhombic 

 system, and is obtained by crystallisation below 30. Its specific gravity is 1*69. In a 

 vacuum, or at 100, it loses 5H 2 O, at 132 6H 2 O, and at 210 all the 7H.,O (Graham). If 

 crystals of ferrous or cobaltic sulphate be placed in a saturated solution, then 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 

 split up at temperatures above 0. Guthrie showed that dilute solutions of magnesium 

 sulphate, when refrigerated, separate ice until the solution attains a composition 

 MgSO 4 ,24HoO, which will completely freeze into a crystallo-hydrate at 5'3 (p. 97). 

 According to Coppet and Riidorff, the temperature of the formation, of ice falls by 0'078 

 for every part by weight of the heptahydrated salt per 100 of water. This figure gives 

 (Chapter I. Note 40) i = l for both the heptahydrated and the anhydrous salt, from which 

 it is clearly seen that it is impossible to judge the state of combination in which a dis- 

 solved substance occurs by the temperature of the formation of ice. 



The solubility of the different crystallo-hydrates of magnesium sulphate, according to 

 Loewel, also varies, like those of sodium sulphate or carbonate (see Chapter XII. Notes 

 7 and 18). At 100 parts of water dissolves 40'75 MgSO 4 in the presence of the hexa- 

 hydrated salt, :-:4T.7 M^SO 4 in the presence of the hexagonal heptahydrated salt, and 

 only 26 parts of MgSO 4 in the presence of the ordinary heptahydrated salt that is, 

 solutions giving the remaining crystallo-hydrates will be supersaturated for the ordinary 

 heptahydnited salt. 



All this shows how many diverse aspects of more or less stable equilibria may exist 

 between water and a substance dissolved in it ; this has already been enlarged on in 

 Chapter I. 



Carefully purified magnesium sulphate in its aqueous solution gives, according to- 



