068 



Scientific Proceedings, Royal Dublin Society. 



In 1903 four pairs of closely related liquids were examined by Miss E. C. 

 Fortey and the author (Trans. Chem. Soc, Ixxxiii, 45). The boiling points of 

 several mixtures of each pair of liquids were deteimined under pressures 

 from about 660 to 840 mm., and by interpolation the boiling points of these 

 mixtures under the pressures 700, 760, and 820 mm. were ascertained. 

 Finally, the values of P in the formula P = mP a + (1- m)P b were calculated from 

 the known vapour-pressures of the pure substances at these temperatures, 

 and it was found that the differences between the calculated and observed 

 pressures were very small, although not negligible. The substances examined 

 were ethyl acetate and ethyl propionate, toluene and ethyl benzene, w-hexane 

 and ra-octane, and benzene and toluene. It had previously been found by the 

 author {ibid., lxxxi, 768 (1902)] that the vapour-pressure formula is strictly 

 applicable to mixtures of chlorobenzene and bromobenzene. 



Unfortunately, in none of these cases has the relation between the 

 composition of the liquid and that of its vapour been experimentally deter- 

 mined, but for benzene and toluene indirect evidence is now available. 



The boiling points of mixtures of benzene and toluene observed by Miss 

 Fortey and the author are given below : — 



The pressures P, calculated from the formula 



mP a + (10 -m)P b 

 100 



where M is the molar percentage of benzene and P A its vapour-pressure, are 

 compared with the actual pressure in the table below : — 



