[18] The term "degree of freedom" employed here must not be confused with the same term used to denote the various movements of a gas molecule according to the kinetic theory. [19] Trevor, _Jour. Physical Chem._, 1902, 6. 136. [20] Ostwald, _Principles of Inorganic Chemistry_, translated by A. Findlay, 2nd edit., p. 7. (Macmillan, 1904.) [21] See the volume in this series on _Thermodynamics_ by F. G. Donnan. [22] _Pogg. Annalen_, 1844, 61. 225. [23] _Mémoires de l'Acad._, 26. 751. [24] _Phil. Trans._ 1884, 175. 461; 1892, A, 183. 107. [25] _Bihang Svenska Akad. Handl._ 1891, 17. I. 1. [26] Abh_andl. physikal.-tech. Reichsanstalt_, 1900, 3. 71. [27] Ostwald-Luther, _Physiko-chemische Messungen_, 2nd edit., p. 156. [28] _Annales chim. et phys._, 1892 [6], 26. 425. [29] The vapour pressure of water at 0° has recently been very accurately determined by Thiesen and Scheel (_loc. cit._), and found to be 4.579 ± 0.001 mm. of mercury (at 0°), or equal to 0.006025 atm. [30] Juhlin, _Bihang Svenska Akad. Handl._, 1891, 17. I. 58. See also Ramsay and Young, _loc. cit._ [31] _Trans. Roy. Soc. Edin._, 1849, 16. 575. [32] _Proc. Roy. Soc. Edin._, 1850, 2, 267. [33] _Annalen der Physik_, 1899 [3], 68. 564; 1900 [4], 2. 1, 424. See also Dewar, _Proc. Roy. Soc._, 1880, 30. 533. [34] The pressure of 1 atmosphere is equal to 1.033 kilogm. per sq. cm.; or the pressure of 1 kilogm. per sq. cm. is equal to 0.968 atm. [35] Tammann, _loc. cit._, 1900, 2. 1, 424; cf. Goossens, _Arch. néerland_, 1886, 20. 449. [36] J. Thomson, _Proc. Roy. Soc._, 1874, 22. 28. [37] A field is "enclosed" by two curves when these cut at an angle less than two right angles. It may be useful to remember that an invariant system is represented by a _point_, a univariant system by a _line_, and a bivariant system by an _area_. [38] _Phil. Trans._, 1724, 39. 78. [39] Juhlin, _loc. cit._, p. 61; cf. Ramsay and Young, _loc. cit._: Thiesen and Scheel, _loc. cit._ [40] This small difference is due to experimental errors in the determination of the vapour pressures; a differential method betrayed no difference between the vapour pressure of ice and of water at 0°. [41] _Phil. Mag._, 1874 [4], 47. 447; _Proc. Roy. Soc._, 1873, 22. 27. [42] _Pogg. Annalen_, 1858, 103, 206. [43] See _Phil. Trans._, 1884, 175, 461. [44] This phenomenon of distillation from the supercooled liquid to the solid has been very clearly observed in the case of furfuraldoxime (V. Goldschmidt, _Zeitschr. f. Krystallographie_, 1897, 28. 169). [45] _Annalen der Physik_, 1900 [4], 2. 1, 424. [46] A similar triple point has been determined by Tammann in the case of phenol (_Annalen der Physik_, 1902 [4], 9. 249). [47] _Annales chim. et phys._, 1821, 19. 414. [48] Lehmann, _Molekularphysik_, I. 153.; Arzruni, _Physikalische Chemie der Krystalle_. (Graham-Otto, _Lehrbuch der Chemie_, I. 3.) [49] Brodie, _Proc. Roy. Soc._, 1855, 7. 24.