Properties of Liquid Mixtures. 285 



that the pressure at the critical point should be below one 

 atmosphere, and that limits very much the choice of liquids. 

 As a rule, two liquids either mix completely cold, or if they 

 do not do that, raising to the boiling-point does not suffice to 

 make them mix ; two or three cases are all that I have been 

 able to rind in which the point of complete mixture can be 

 arrived at by boiling, and consequently corresponds to a 

 vapour-pressure below the atmospheric. Many other pairs 

 of incompletely miscible liquids have been studied by 

 Alexejew and others, but to arrive at their critical points it 

 was necessary to raise them to a high temperature in sealed 

 tubes. A recent paper by Victor Rothmund * contains new 

 observations on the relation between concentration and tem- 

 perature, including the concentration and temperature of 

 the critical point, made by Alexejew's method. That paper 

 contains a long account of previous work on the subject, 

 which makes it the less necessary for me to go over the same 

 ground. I will therefore mention only what has been done 

 on vapour-pressures, as Rothmund does not touch on that 

 side of the subject, merely adding two remarks to his paper. 

 First, it does not seem to have been noticed that normal 

 organic liquids always mix completely : I hoped to have 

 found a normal pair to study first, in order to avoid the com- 

 plication due to the abnormality supposed to be molecular 

 ao-o-regation in the liquid ; I have not succeeded in finding 

 such a pair. All the incompletely miscible pairs of liquids 

 so far noted include water, methyl alcohol, or a low fatty 

 acid as one member. To those with accessible critical 

 points mentioned by Rothmund, I have only one pair to add, 

 viz., ethylene dibromide and formic acid ; these mix on 

 boiling and separate into two layers when cold. I have not 

 yet gone further with this couple ; the vapour-pressure 

 observations below refer to the well-known cases of phenol 

 and water, aniline and water. 



An account of previous experiments on the vapour-pressure 

 of incompletely miscible liquids will indeed not take up much 

 space, since, so far as I know, there is only one to record, 

 viz., Konowalow's f measurements on isobutyl alcohol- water 

 mixtures. His observations (made by a static method) give 

 some points on the vapour-pressure curve up to 100° for 

 (1) pure isobutyl alcohol (100 °/ ) : (2) mixtures containing 

 94*05 °/o an d 6*17 °/o>both clear; (3) an undetermined mixture 

 which separated into two layers. He unfortunately did not 

 measure the solubility of the alcohol in water, or water in 



* Zeitschr.f.phys. Chem. xxvi. p. 433 (1898). 

 f Wied. Ann. xiv. p. 43. 



Phil. Mag. S. 5. Vol. 47. No. 286. March 1899. X 



