the Mutual Solubility of Liquids. 181 



other, that these two substances are mutually almost insoluble, 

 in contrast to liquids like ether and water. There is, however, 

 no reason to make, as Regnault did, this difference the basis 

 of a division into two groups, because it is now known that 

 almost every degree of mutual solubility occurs, and that the 

 solubility sometimes changes rapidly with the temperature. 

 Regnaulfs measurements of the vapour-pressures of mixtures 

 of ether and water tended to show that what we shall call the 

 pressures of the three phases were practically equal to those 

 of pure ether between 15° C. and 33° 0. The same conclusion 

 is arrived at from some determinations, due to Marchis *, 

 of the boiling-points of these mixtures. Later experiments 

 by Linebargerf, KonowalowJ, Beckmann§, and others prove, 

 however, that up to the boiling-point the pressure of the three 

 phases exceeds by a few millimetres the pressure of ether. 

 For this pair of liquids, therefore, the equilibrium-pressure 

 was higher than that of either substance, though much less 

 than the sum of the vapour-pressures of the pure substances. 



Konowalow, in his investigation on mixtures, found a com- 

 bination of partial miscibility — water and isobutyl alcohol ||. 

 The pressures of the three phases were in this case also higher 

 than for water, the more volatile of the two components, and 

 nearly equal to the sum of the pressures of the pure sub- 

 stances, this combination approaching the case of carbon 

 disulphide and water. For bromine and water, a combination 

 investigated by Bakhuis Roozeboom % the pressures are 

 higher than for pure bromine. Konowalow** was also the first 

 to point out the theoretical possibility of a second case, in which 

 the pressure of the three phases would be between the pres- 

 sures of the components, and the impossibility of the three- 

 phase pressure being less than the vapour-pressure of either 

 component. 



The first instance of Konowalow's second group was 

 discovered by Bakhuis Roozeboom ff — sulphur dioxide and 

 water. Bancroft in his book, ' The Phase Rule/ gives two 

 more instances — sulphur and xylene, and sulphur and toluene. 



We are aware of no other experiments regarding the 

 equilibrium-pressure of three phases ; all other investigations 



* Marchis, Compt. Rend. cxvi. p. 3s8. 



t Linebarger, Chem. News, vol. lxx. p. 52. 



% Konowalow, Wied. Ami. xiv. p. '223. 



§ Beckmann ; comp. Nernst, Zeitschrift Phys. Chem. viii. p. 134. 



|| Konowalow, Wied. Ann. xiv. p. 42. 



II Bakhuis Roozeboom, Zeitschrift Phys. Chem. ii. p. 453. 



** Konowalow, Wied. Ann. xiv. p. 222. 



ft Bakhuis Roozeboom, Zeitschrift Phys, Chem. ii. p. 450. 



Phil. Mag. S. 5. Vol. 48. No. 291. Aug. 1899. O 



