[382] A very useful summary of the investigations carried out by van't Hoff and his pupils on the formation of the Stassfurt salt-beds is given by E. F. Armstrong, in the _Reports of the British Association for 1901_, p. 262. See also van't Hoff, _Zur Bildung der ozeanischen Salzablagerungen_ (Brunswick, 1905). [383] See especially Meyerhoffer, _Silzungsber. Wien. Akad._, 1895, 104. II. _b_, 840; Meyerhoffer and Saunders, _Zeitschr. physikal. Chem._, 1899, 28. 453; 31. 370. The investigation of the equilibria between reciprocal salt-pairs alone (three-component systems) is of great importance for the artificial preparations of minerals, as also in analytical chemistry for the proper understanding of the methods of conversion of insoluble systems into soluble by fusion (see Meyerhoffer, _Zeitschr. physikal. Chem._, 1901, 38. 307). [384] See Meyerhoffer, _Zeitschr. physikal. Chem._, 1899, 28. 459. [385] Compare the reciprocal salt-pair NaCl--NH_{4}HCO_{3} (p. 321). In this case the upper limit of the transition interval was found by extrapolation of the solubility curve for NaHCO_{3} + NH_{4}Cl + NH_{4}HCO_{3} and NaHCO_{3} + NH_{4}Cl + NaCl to be 32° (Fedotieff, _Zeitschr. phys. Chem._, 1904, 49. 179). [386] Löwenherz, _Zeitschr. physikal. Chem._, 1894, 13. 464. [387] Meyerhoffer and Saunders, _Zeitschr. physikal. Chem._, 1899, 28. 479. [388] As the quantities of the salts are expressed in _equivalent_ gram-molecules, the molecule of sodium and potassium chloride must be doubled in order to be equivalent to sodium sulphate and potassium sulphate. [389] _Sitz-Ber. der kgl. preuss. Akad. der Wiss._, 1903, p. 359. Van't Hoff, _Zur Bildung der ozeanischen Salzablagerungen_, I. p. 34 (Brunswick, 1905). [390] _Zeitschr. für Kristallographie_, 1904, 39. 155. [391] Meyerhoffer and Saunders, _Zeitschr. physikal. Chem._, 1899, 28. 479. [392] _Zeitschr. physikal. Chem._, 1904, 49. 162. [393] Another commercial process, in the study of which good service is done by the Phase Rule, is the caustification of the alkali salts (G. Bodländer, _Zeitschr. für Elektrochem._, 1905, 11. 186; J. Herold, _ibid._, 418). [394] _Zeitschr. physikal. Chem._, 1900, 35. 32. [395] Mention may also be made here of the equilibria between magnesium carbonate and potassium carbonate, although these do not form a reciprocal salt-pair (Auerbach, _Zeitschr. für Elektrochem._, 1904, 10. 161). [396] O. N. Witt and K. Ludwig, _Ber._, 1903, 36. 4384; Meyerhoffer, _ibid._, 1904, 37. 261, 1116. [397] _Zeitschr. physikal. Chem._, 1905, 53. 513. Compare also, _ibid._, 1903, 38. 307. [398] See Schwarz, _Beiträge zur Kenntnis der umkehrbaren Umwandlungen polymorpher Korper_ (Göttingen, 1892); or, Roozeboom, _Heterogen. Gleichgewicht_, I. p. 125. Also Barnes and Cooke, _Journ. Physical Chem._, 1902, 6. 172. [399] Van't Hoff and van Deventer, _Zeitschr. physikal. Chem._, 1887, 1. 173. [400] Reicher, _Zeitschr. für Krystallographie_, 1884, 8. 593.