DOUBLE SALTS 145 



devoted to the investigation of solubility relations. The 

 temperature-concentration curves (Table IV.) for the three 

 condensed 3-phase systems, viz. : (1) solution + simple salt 

 MA + simple salt M'A, (2) solution + double salt + simple salt 

 MA, (3) solution + double salt + simple salt M'A, are perfectly 

 analogous to the corresponding temperature-pressure curves 

 (Table III.), in that their intersection marks the transition point, 

 i.e. the temperature at which the concentration of the different 

 solutions is the same and at which the three solid phases, 

 double salt and its two constituent simple salts, can co-exist. 

 The graphic representation of these solubility relations has to 

 take account of three variables, (i) the temperature and (ii and 

 iii) the amount present in the saturated solution of each of the 

 components MA and M'A. The most satisfactory manner of 

 doing this is by means of space models with three rectangular 

 axes, 1 though much may be accomplished by means of figures 

 in a plane, such as those reproduced in Table IV. In the 

 projection there adopted 2 the temperature axis is placed verti- 

 cally, and the values found at different temperatures for the 

 concentration of the two components MA and M'A (expressed 

 as the number of molecules present in one hundred molecules 

 of water) are represented by ordinates on both sides of the 

 temperature axis. Hence two corresponding points such as 

 G and G', M and M', etc., always belong together and refer to 

 the same solution, whose concentration with respect to MA and 

 M'A they represent for a special temperature. 



The determination over a certain temperature range of such 

 solubility curves, together with an extensive use of graphic 

 methods for their representation, have made it possible to get 

 complete knowledge of the equilibrium condition of the system 



makers, " Gleichgewicht des Doppelsalzes von Jodblei und Jodkalium mit 

 wassriger Losung," ibid. 9, 1892, p. 57; and 10, 1892, p. 467; " Graphische 

 Ableitungen aus den Losungsisothermen eines Doppelsalzes und seiner Kom- 

 ponenten und mogliche Formen der Umwandlungscurve," ibid., 1 1, 1893, pp. 55-1 10 ; 

 " Theoretische und experimentelle Untersuchung liber kryohydratische Tempera- 

 turen bei Systemen von zwei Salzen mit oder ohne Doppelsalzbildung," ibid., 

 12, 1893, p. 73. 



1 Of such representations in space, that best known is the carnallite model, 

 Van't Hoff and Meyerhoffer, Zs.physik. Chem. 30, 1899, pp. 86 et seq. See also 

 Finlay, The Phase Rule, pp. 275 et seq. 



2 Koppel, " Die Bildungs- und Loslichkeitsverhaltnisse analoger Doppelsalze," 

 Zs.physik. Chem. 52, 1905, p. 385. 



10 



