io PRACTICAL ORGANIC AND BIO-CHEMISTRY 



porous earthenware have been added, is heated over a gauze at such a 

 rate that the condensed liquid comes over drop by drop. Fractions 

 are collected at 5 or 10 ranges of temperature. 



The redistillation of each fraction is carried out as follows : In 

 the apparatus which has been washed out and dried fraction I is put 

 which boiled, say, at 90-95, and distilled till the thermometer shows 

 95, when the distillation is stopped and fraction II, 95-100, is added 

 to the remainder in the flask. On distilling, fractions below 95 are 

 collected in the first receiver and the second fraction is distilled till 

 the temperature reaches 100. Distillation is stopped and the next 

 fraction added. The process is continued until all the fractions have 

 been redistilled. Finally the main fractions boiling at the extremes 

 will be obtained. 



A mixture of benzene, toluene and xylene, such as occurs in coal 

 tar, may be taken in illustration. 



Constant Boiling Mixtures. 



It frequently happens that two liquids form a mixture which has 

 a constant boiling-point and behaves like a single liquid. Such a 

 mixture cannot be separated by fractional distillation, although more 

 or less separation may be possible by distilling at a different pressure. 



Such a mixture may have a boiling-point which is lower than 

 either of its constituents, or higher. 



Excess of either constituent in the mixture beyond that forming 

 the constant boiling mixture can be separated by fractional distilla- 

 tion and it will distil over either before or after the mixture according 

 to the boiling-points. Such constant boiling mixtures are mixtures 

 of ethyl alcohol and water, methyl alcohol and acetone, benzene and 

 alcohol, pyridine and water, water and formic acid, chloroform and 

 acetone. They can only be separated by chemical means. 



Fractional Distillation in Vacuo. 



The same apparatus as described above for distillation in vacua can be 

 used for fractional distillation in vacuo, but as each fraction distils the 

 apparatus must be disconnected so as to insert a new receiver. To avoid 

 releasing the vacuum and disconnecting the apparatus, and to allow fractional 

 distillation to proceed continuously several contrivances have been suggested. 

 Usually the vapours of liquids distilled in vacuo condense without being cooled 

 by water or ice, and an apparatus as in Fig. 5 may be used. The receivers 

 are turned into position when necessary. A very simple form is shown in 



