ght line.
On distilling such a mixture pure A will come over first, followed by
mixtures in which the quantity of B continually increases;
consequently by a sufficient number of distillations A and B can be
completely separated. Examples are water and methyl or ethyl alcohol.

[Illustration: FIG. 4.]

Van't Hoff (_Theoretical and Physical Chemistry_, vol. i. p. 51)
illustrates the five cases on one diagram. In fig. 4 let AB be the
axis of composition, AP be the vapour pressure of pure A, BQ the
vapour pressure of pure B. For immiscible liquids the vapour pressure
curve is the horizontal line ab, described so that aP = QB and bQ =
AP. For partially miscible liquids the curve is Pa1b1Q. The horizontal
line a1b1 corresponds to the two layers of liquid, and the inclined
lines Pa1Qb1 to solutions of B in A and of A in B. The curves Pa4Q,
having a minimum at a4, Pa3Q, having a maximum at a3, and Pa5Q, with
neither a maximum nor minimum, correspond to the types i., ii., iii.
of completely miscible mixtures.

6. _Dry Distillation._--In this process the substance operated upon is
invariably a solid, the vapours being condensed and collected as in
the other methods. When the substance operated upon is of uncertain
composition, as, for example, coal, wood, coal-tar, &c., the term
destructive distillation is employed. A more general designation is
"pyrogenic processes," which also includes such operations as leading
vapours through red-hot tubes and condensing the products. We may also
consider here cases of sublimation wherein a solid vaporizes and the
vapour condenses without the occurrence of the liquid phase.

Dry distillation is extremely wasteful even when definite substances
or mixtures, such as calcium acetate which yields acetone, are dealt
with, valueless by-products being obtained and the condensate usually
requiring much purification. Prior to 1830, little was known of the
process other than that organic compounds generally yielded tarry and
solid matters, but the discoveries of Liebig and Dumas (of acetone
from acetates), of Mitscherlich (of benzene from benzoates) and of
Persoz (of methane from acetates and lime) brought the operation into
common laboratory practice. For efficiency the operation must be
conducted with small quantities; caking may be prevented by mixing the
substance with sand or powdered pumice, or, better, with iron filings,