264 REPORT— 1901. 



In these three constituents are present in four phases, and two solid 

 equilibrators are necessary, e.g., NaCl and KCl. But it must be carefully 

 borne in mind that when a double salt can be formed there are two 

 possible cases of equilibrium — ^viz., that in which the double salt and one of 

 the single salts and that in which the double salt and the other single 

 salt are in contact with the liquid. 



Similai'ly, when one of the salts gives rise to two or more hydrates, 

 there are several possible cases of equilibrium, though in this case also the 

 presence of but two equilibrators at a time is possible, as a rule. More- 

 over, two hydrates of the same substance may act simultaneously as 

 equilibrators, even under conditions other than those obtaining at the 

 transition points, as another substance is present. A case of this kind is 

 afforded by the formation of solutions saturated with the two hydrates of 

 Magnesium sulphate in presence of Magnesium chloride. 



Case III. — Solutions saturated loith two salts, whose basic and acidic 

 radicles are different and which therefore can interact. 



Magnesium sulphate and Potassium chloride may be quoted in illus- 

 tration of this case. In solution these interact in the manner expressed 

 by the equation 



KaClj + MgSO^ Z K^SO. + MgClo, 



one or other couple being stable, according to the conditions ; such pairs 

 of salts are therefore conveniently spoken of as reciprocal salt pairs. 



A solution of two such salts may be supposed to consist of four sub • 

 stances — the solvent and three of the four possible salts — in five phases 

 and not of five substances in six phases as the rule would seem to require. 

 The fourth salt being always obtainable from the other three, from the 

 standpoint of the phase rule the four salts are derivable from only three 

 substances : thus the stable pair at a cei'tain temperature being, let us 

 say, K2Cl2 + MgS04, these will exist together with e?iAer K0SO4 or MgClg, 

 but not with both, as the two cannot be together without interacting to 

 form the stable pair. 



Although in the case of a reciprocal salt pair only three equilibrators 

 are essential to secure saturation, and this is the maximum number that 

 can act simultaneously, except at a transition point, the number of com- 

 binations of three which are possible may be considerable. In the case of 

 KCl and MgS04, which can give rise not only to K2SO4 and MgClg, but 

 also to various double salts and hydrates, experience indicates that (at 

 temperatures about 25°) in all seven substances may be formed — viz., 

 KCl, K0SO4, MgCU.eHjO, MgS04.7H20, MgS04.6HoO, Schonite 

 (K2SO4.MgSO4.6H2O) and Carnallite (MgCl2.KCl.6H2O). As each of 

 these should serve as an equilibrator, and there are mathematically thirty- 

 five ways of combining three out of seven substances, the problem at first 

 seems very complicated. In practice, however, it is found that, for example, 

 K2SO4 and MgS04 cannot exist together, but always form the double salt 

 Schonite ; and that in a similar manner MgCl2 and KCl give rise to 

 Carnallite, so that finally the number of possible sets of three equilibrators 

 is reduced by experiment to five. In the case of a mixture of KNO3, 

 NaNOg, KCl, and NaCI, as neither double salts nor hydrates are formed, 

 the conditions are simplified, and only four sets of three equilibrators can 

 be chosen. In practice the determination of the number of forms stable 

 under the conditions of experiment often gives rise to considerable diffi- 

 culty ; and it must not be forgotten that the problem can only be solved 



