[211] Richards, _Zeitschr. physikal. Chem._, 1898, 26. 690; Richards and Wells, _ibid._, 1903, 43. 465. This temperature is not quite the same as that of the _quadruple point_ anhydrous salt--hydrated salt--solution--vapour, because the latter is the temperature at which the system is under the pressure of its own vapour. Since, however, the influence of pressure on the solubility is very slight (p. 107), the position of the two points will not be greatly different. The quadruple point was found by Cohen (_Zeitschr. physikal. Chem._, 1894, 14. 90) to be 32.6° and 30.8 mm. of mercury. [212] Van't Hoff and van Deventer, _Zeitschr. physikal. Chem._, 1887, 1. 185. Cf. Cohen, _ibid._, 1894, 14. 88. [213] Debray, _Compt. rend._, 1868, 66. 194. [214] Richards, _Zeitschr. physikal. Chem._, 1898, 26. 690. A number of other salt hydrates, having transition-points ranging from 20° to 78°, which might be used for the same purpose, have been given by Richards and Churchill, _ibid._, 1899, 28. 313. [215] _Zeitschr. physikal. Chem._, 1903, 46. 818. [216] Van't Hoff, _Lectures on Physical Chemistry_, I. p. 67. [217] Cohen, _Zeitschr. physikal. Chem._, 1894, 14. 90. [218] Ziz, _Schweigger's Journal_, 1815, 15. 166. See Ostwald, _Lehrbuch_, II. 2. 717. [219] See, for example, the solubility determinations published in _Wissenschaftliche Abhandl. der physikalisch-technischen Reichsanstalt_, Vol. III., or in the _Berichte_, for the years 1897-1901. [220] Meusser, _Ber._, 1901, 34. 2440. [221] Mylius and von Wrochem, _Ber._, 1900, 33. 3693. [222] Walker and Fyffe, _Jour. Chem. Soc._, 1903, 83. 180. [223] _Monatshefte_, 1887, 8. 601. [224] The equilibria between calcium chloride and water have been most completely studied by Roozeboom (_Zeitschr. physikal. Chem._, 1889, 4. 31). [225] Hammerl, _Sitzungsber. Wien. Akad._, 2^{te} Abteil, 1878, 78. 59. Roozeboom, _Zeitschr. physikal. Chem._, 1889, 4. 31. [226] Lidbury, _Zeitschr. physikal. Chem._, 1902, 39. 453. The curvature at the melting point is all the greater the more the compound is dissociated into its components in the liquid state. If the compound is _completely undissociated_, even in the vapour phase, the two branches of the curve will _intersect_, (_e.g._ pyridine and methyl iodide; Aten, _Versl. Konink. Akad. Wetensch. Amsterdam_, 1905, 13. 462). The smaller the degree of dissociation, therefore, the sharper will be the bend. (See Stortenbeker, _Zeitschr. physikal. Chem._, 1892, 10. 194.) From the extent of flattening of the curve, it is also possible, with some degree of approximation, to calculate the degree of dissociation of the substance in the fused state. (See Roozeboom and Aten, _Zeitschr. physikal. Chem._, 1905, 53. 463; Kremann, _Zeitschr. Elektrochem._, 1906, 12. 259.) [227] See Roozeboom, _Zeitschr. physikal. Chem._, 1889, 4. 31. [228] Tammann, _Wied. Annalen_, 1899, 68. 577. [229] Duhem, _Journ. Physical Chem._, 1898, 2. 31.