THE VALENCY AND SPECIFIC HEAT OF THE METALS 601 



water and magnesium sulphate there may exist several definite and 

 more or less stable degrees of equilibrium ; the double salt 

 MgS0 4 K 2 SO 4 ,6H 2 O may be regarded as one of these equilibrated 

 ystems, the more so since it contains 6H 2 0, whilst MgSO 4 forms its 

 jHost stable system with 7H 2 O, and the double salt may be considered 

 as this crystallo-hydrate in which one molecule of water is replaced 

 by the molecule K ft S<V 8 



The power of forming basic salts is a very remarkable peculiarity 

 of magnesia and other feeble bases, and especially of those corre- 

 sponding with polyvalent metals. The very powerful bases corre- 

 sponding with univalent metals like potassium and sodium do not 

 form basic salts, and, indeed, are more prone to give acid salts, whilst 

 magnesium easily and frequently forms basic salts, especially with feeble 

 acids, although there are some oxides as, for example, copper and 

 lead oxides which still more frequently give basic salts. If a cold 

 solution of magnesium sulphate be mixed with a solution of sodium 

 carbonate there is formed a gelatinous precipitate of a basic salt, 



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

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

 MgS0 4 ,24H 3 O, which will completely freeze into a crystallo-hydrate at 5'3. According 

 to Coppet and Rxidorff, 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 49) i = 1 for both the heptahydrated and the anhydrous salt, from which it is evident 

 that it is impossible to judge the state of combination in which a dissolved 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 MgS0 4 in the presence of the hexa- 

 hydrated salt, S4'67 MgS0 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 

 heptahydrated 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 

 Stcherbakoff, an alkaline reaction with litmus, and an acid reaction with phenol- 

 phthalein. 



The specific gravity of solutions of certain salts of magnesium and calcium reduced 

 to 15/4 (see my work cited, Chapter I., Note 19), are, if water at 4 =10,000, 



MgS0 4 : 

 MgCl a : 

 CaCl 2 l 



* Graham even distinguished the last equivalent of the water of crystallisation of 

 tb heptahydrated salt as that which is replaced by other salts, pointing out that doubld 

 salts like BtgJ s (S0 4 1 8 ,6H 2 lose all their water at 185, whilst MgS0 4 ,7H a O only parts 

 with 6H 2 0. 



