tt remains to notice briefly the processes of fractional distillation and distillation with a retb('x condenser kept at constant temperature. In fractional distillation, which is very fre quently employed by organic. chemists. 21 mixture is first divided into a series of 'fractions' by dis tillation; then the fractions are subdivided by further separate distillations: then sonic of the fractions are mixed together in pairs or threes, the resulting liquids again distilled separately, and so forth until the required separation has been effected. if a condenser is attached to the still in such a manner that any liquid forming in it may Clow back into the still, and if the tem perature in this condenser is kept constant, then the vapors passing out of it uncondensed are found to have the same relative composition, no matter what changes of composition arc taking place in the distilling liquid. In most eases ex amined (but not in mixtures of alcohol and water), the distillate thus obtained is found to boil exactly at the temperature at which the con denser has been kept. See ClIF.MISTRI ; DOILIN
POINT ; EVAPORATION: DISTILLED LIQUORS.
DF.sTRT*CTIVE DISTILLATION. This is quite dif ferent from the process described above. For while that process is purely physical and in volves no chemical change whatever, destructive distillation. as implied by the term, causes pro found chemical changes ill the materials sub jected to it. Examples of destructive distillation are: the heating of coal in gas-works at a red heat, when it resolves itself into coke, which is left in the retort, and coal-gas, naphtha. tar, etc., which distil over into suitable receivers; the treatment of coal at and below a red heat, when it yields much paraffin oil; and the distillation of wood in close vessels, at a red heat, when charcoal is left in the vessel, and wood-vine[mr, wood-spirit. tar. etc., pass over in vapor. and are condensed. See TAR: COAL-TAR: GAS. ILLU MINATING ; PARAFFIN; ACETIC D DIPPECS ANIMAL OIL; etc.