[343] The influence of pressure on the transition point in the case of tachydrite has been determined by van't Hoff, Kenrick, and Dawson (_Zeitschr. physikal. Chem._, 1901, 39. 27, 34; van't Hoff, _Zur Bildung der ozeanischen Salzablagerungen_, I. p. 66--Brunswick, 1905). This salt is formed from magnesium chloride and calcium chloride at 22°, in accordance with the equation-- 2MgCl_{2}.6H_{2}O + CaCl_{2}.6H_{2}O = Mg_{2}CaCl_{6}.12H_{2}O + 6H_{2}O Increase of pressure raises the transition point, because the formation of tachydrite is accompanied by increase of volume; the elevation being 0.016° for an increase of pressure of 1 atm. The number calculated from the theoretical formula (p. 57) is 0.013° for 1 atm. If one calculates the influence of the pressure of sea-water on the temperature of formation of tachydrite (which is of interest on account of the natural occurrence of this salt), it is found that a depth of water of 1500 metres, exerting a pressure of 180 atm., would alter the temperature of formation of tachydrite by only 3°. The effect is, therefore, comparatively unimportant. [344] Roozeboom, _Zeitschr. physical. Chem._, 1887, 1. 227. [345] _Zeitschr. physical. Chem._, 1887, 1. 227. [346] Van't Hoff and Müller, _Ber._, 1898, 31. 2206. [347] Van't Hoff and van Deventer, _Zeitschr. physikal. Chem._, 1887, 1. 165. [348] For a full discussion of the solubility relations of sodium ammonium racemate, see van't Hoff, _Bildung und Spaltung von Doppelsalzen_, p. 81. [349] _Annales chim. phys._, 1848 [3], 24. 442. [350] See Van't Hoff and van Deventer, _Zeitschr. phys. Chem._, 1887, 1. 165. [351] Meyerhoffer, _Zeitschr. physikal. Chem._, 1890, 5. 121. [352] Roozeboom, _Zeitschr. physikal. Chem._, 1888, 2. 518. [353] Meyerhoffer, _Zeitschr. physikal. Chem._, 1890, 5. 109. On the importance of the transition interval in the case of optically active substances, see Meyerhoffer, _Ber._, 1904, 37. 2604. [354] In connection with this chapter, see, more especially, van't Hoff, _Bildung und Spaltung von Doppelsalzen_, p. 3, _ff._; Roozeboom, _Zeitschr. physikal Chem._, 1892, 10. 158; Bancroft, _Phase Rule_, p. 201; 209. [355] The same restriction must be made here as was imposed in the preceding chapter, namely, that the two salts in solution give a common ion. [356] For example, addition of ammonium chloride to solutions of ferric chloride (Roozeboom, _Zeitschr. physikal. Chem._, 1892, 10. 149). [357] It must, of course, be understood that the temperature is on that side of the transition point on which the double salt is stable. [358] Excess of the double salt must be taken, because otherwise an unsaturated solution might be formed, and this would, of course, not deposit any salt. [359] Meyerhoffer, _Ber._, 1904, 37. 2605. [360] Meyerhoffer, _Ber._, 1897, 30. 1809. [361] Meyerhoffer, _Ber._, 1904, 37. 2604.