of the vapours in the liquid components.

(i.) If the vapour of A be readily soluble in the liquid B, and the
vapour of B readily soluble in the liquid A, there will exist a
mixture of A and B which will have a lower vapour pressure than any
other mixture. The vapour pressure composition curve will be convex to
the axis of compositions, the maximum vapour pressures corresponding
to pure A and pure B, and the minimum to some mixture of A and B. On
distilling such a mixture under constant pressure, a mixture of the
two components (of variable composition) will come over until there
remains in the distilling flask the mixture of minimum vapour
pressure. This will then distil at a constant temperature. Thus nitric
acid, boiling-point 68 deg., forms a mixture with water, boiling point
100 deg., which boils at a constant temperature of 126 deg., and
contains 68% of acid. Hydrochloric acid forms a similar mixture which
boils at 110 deg. and contains 20.2% of acid. Another mixture of this
type is formic acid and water.

(ii.) If the vapours be sparingly soluble in the liquids there will
exist a mixture having a greater vapour pressure than that of any
other mixture. The vapour pressure-composition curve will now be
concave to the axis of composition, the minima corresponding to the
pure components. On distilling such a mixture, a mixture of constant
composition will distil first, leaving in the distilling flask one or
other of the components according to the composition of the mixture.
An example is propyl alcohol and water. At one time it was thought
that these mixtures of constant boiling-point (an extended list is
given in Young's _Fractional Distillation_) were definite compounds.
The above theory, coupled with such facts as the variation of the
composition of the constant boiling-point fraction with the pressure
under which the mixture is distilled, the proportionality of the
density of all mixtures to their composition, &c., shows this to be
erroneous.

(iii.) If the vapour of A be readily soluble in liquid B, and the
vapour of B sparingly soluble in liquid A, and if the vapour pressure
of A be greater than that of B, then the vapour pressures of mixtures
of A and B will continually diminish as one passes from 100% A to 100%
B. The vapour tension may approximate to a linear function of the
composition, and the curve will then be practically a strai