processes. This method is particularly successful in the case of
substances which cannot be distilled at their ordinary boiling-points
(it will be seen in the following section that distilling with steam
implies a lowering of boiling-point), and which can be readily
separated from water. Instances of its application are found in the
separation of ortho- and para-nitrophenol, the o-compound distilling
and the p- remaining behind; in the separation of aniline from the
mixture obtained by reducing nitrobenzene; of the naphthols from the
melts produced by fusing the naphthalene monosulphonic acids with
potash; and of quinoline from the reaction between aniline,
nitrobenzene, glycerin, and sulphuric acid (the product being first
steam distilled to remove any aniline, nitrobenzene, or glycerin, then
treated with alkali, and again steam distilled when quinoline comes
over). With substances prone to discolorization, as, for example,
certain amino compounds, the operation may be conducted in an
atmosphere of carbon dioxide, or the water may be saturated with
sulphuretted hydrogen. Liquids other than water may be used: thus
alcohol separates [alpha]-pipecoline and ether nitropropylene.

5. _Theory of Distillation._--The general observation that under a
constant pressure a pure substance boils at a constant temperature
leads to the conclusion that the distillate which comes over while the
thermometer records only a small variation is of practically constant
composition. On this fact depends "rectification or purification by
distillation." A liquid boils when its vapour pressure equals the
superincumbent pressure (see VAPORIZATION); consequently any process
which diminishes the external pressure must also lower the
boiling-point. In this we have the theory of "distillation under
reduced pressure." The theory of fractional distillation, or the
behaviour of liquid mixtures when heated to their boiling-points, is
more complex. For simplicity we confine ourselves to mixtures of two
components, in which experience shows that three cases are to be
recognized according as the components are (1) completely immiscible,
(2) partially miscible, (3) miscible in all proportions.

When the components are completely immiscible, the vapour pressure of
the one is not influenced by the presence of the other. The mixture
consequently distils at the temperature at which the sum of