18 Messrs. Barrell, Thomas, and Young on the 



reduce the quantitities of A and C in this fraction until they 

 both disappeared from the middle portion of the distillate, 

 and an idea of the difficulty of attaining such a result may be 

 gathered from the diagram P. 



With substances whose boiling-points are not very far 

 apart the rise of temperature during such a distillation would 

 be almost imperceptible, and in the fractionation it would be 

 necessary to collect a large number of fractions above and 

 below the true boiling-point of B with very small tempera- 

 ture-ranges. It would be necessary also to read the tempera- 

 ture with extreme care, and to make very accurate corrections 

 for changes of the barometer (which would be impossible 

 unless the vapour-pressures of the substance had previously 

 been determined). 



It has been pointed out, however, that after the mixture 

 has been fractionated a few times, the first portion of the 

 first distillate is free from C ; while the residue from the last 

 distillate is free from A. If, therefore, after the fifth or sixth 

 fractionation we remove these first and last portions each 

 time, we shall gradually accumulate two quantities of liquid, 

 one (the first portions) containing only A and B, the other 

 (the residues) only B and C, and these may be separately 

 fractionated in the ordinary way. 



It is to be noticed also that as a quantity of liquid is 

 removed each time, the total amount to be distilled, and 

 therefore the time required for the distillation, gradually 

 diminishes. 



In arranging the fractions — as regards temperature — it is 

 advisable, if the true boiling-points of the liquids are known, 

 to take these temperatures and also the middle temperatures 

 between the boiling-points of A and B and of B and G as 

 definite points, and to arrange the fractions as far as possible 

 symmetrically about these points. There will thus be frac- 

 tions of small and diminishing temperature-range above and 

 below the boiling-point of B — and at the boiling-points of A 

 and C after a few fractionations and until these substances 

 are to a large extent eliminated — and fractions of large and 

 increasing temperature -range above and below the two middle 

 temperatures. 



As the preliminary fractionation proceeds, more and more 

 of A and will be eliminated, so that after a time the boiling- 

 point of the first portion of the lowest fraction will gradually 

 rise, while that of the residue from the last fraction will 

 gradually fall until either A or C disappear, or, if we proceed 

 still further, until we have nothing but B left. Much time 



