In order to cleanse the apparatus--which should be performed after each working--the still _A_ is emptied of water by opening the cock _Q_. The contents of the condenser are then emptied in like manner by opening the cock _J_, through which they flow upon the plates in the column _B_, and wash out essential oils which remain in them. These two cocks are then closed, and the door _U_ in the still head is removed. The water in the cooler _G_ is then run by means of pipe into the still _A_, so as partially to cover the steam-coil in the latter. After again securing the door _U_, a strong heat is applied, and the water in the still is well boiled, the steam evolved thoroughly cleansing all the different parts of the apparatus; this is continued for 15 or 20 minutes, when the heat is withdrawn and the still left to cool gradually. In the intermittent rectifying still above described the impure products are distilled with the next charge. In the apparatus as perfected and used in large distilleries or rectification plants, the division of the several products composing the phlegm or raw spirit is made at one time and continuously on the principle now to be described. It was stated in the beginning of this chapter that the various impurities in alcohol, the ethers, the water and the fusel oils, have each their own vaporizing point and each their own condensing point. As this is so, they may be separated from each other and from the alcohol on the same principle as we have seen that water is separated from the mixture of pure alcohol and water; that is, by fractionation, as it is termed, or by "sifting out" one body from another. [Illustration: FIG. 37.--Fractional Distilling Apparatus.] Thus in fractional distillation, each condenser or retort in the apparatus shown in Fig. 37, above acts as a sieve or trap, letting pass the most volatile substances but retaining those of a less degree of volatility. By passing the mixed vapor together through a good condensing medium the temperature of which is lower than the boiling point of the less volatile, but not so low as the boiling point of the more volatile the vapors of the less volatile liquid will be condensed, while the more volatile will retain their gaseous form. Thus by having a number of condensing mediums each one slightly lower in temperature than the other, the various vapors with their various points of volatilization will be successively condensed, allowing the passage of the more volatile vapors over to the condenser beyond.