ANALYSIS, REFINING, AND COMPOSITION. 15 



25 per cent of the distillate being nearly pure A and the last 25 per 

 cent being nearly pure B. 



FBACTIONATION BY DEPHLEGMATION. 



In order to obtain better separations of A from B in a single dis- 

 tillation some kind of a still head or dephlegmator is used (such as 

 the Hempel column shown in fig. 1) in which the ascending vapors 

 are partially condensed and flow downward in liquid form to the 

 distilling flask. The descending liquid comes in contact with more 

 ascending vapors, and there is an interchange of constituents, some 

 of the low boiling material in the liquid being vaporized and some 

 of the high boiling material in the vapors being condensed, with the 

 result that the vapors which finally leave the top of the dephlegmator 

 contain a larger proportion of the lower boiling material than the 

 vapors which enter the dephlegmator from the flask. In other words, 

 when the dephlegmator is used there is a more complete separation 

 of the component materials. Under such conditions the distillation 

 curve w r ould be more like curve II in figure 3, which indicates about 

 45 per cent practically pure A, about 10 per cent of a rapidly chang- 

 ing mixture, and the rest practically pure B. 



SUMMARY. 



This consideration of the boiling-point curve of a mixture of two 

 components may be summed up as follows : 



(1) The more complete the separation the more accurately do the 

 curves represent the composition of the mixture. 



(2) The more vertical curves indicate a more uniform substance in 

 the distillate. 



(3) Curves more nearly horizontal indicate a distillate rapidly 

 changing in composition. 



DISTILLATION CURVES OF A GUM TURPENTINE. 



There is an analogy between the distillation curves for a turpentine 

 and the theoretical curve in figure 3, but complications arise in the 

 interpretation because the mixture distilled is complex and the sepa- 

 ration of some of the constituents is incomplete. Specific gravity 

 and index of refraction are plotted, like the boiling point, against 

 percentage weight, and the three curves are considered together in 

 interpreting the results of the distillation. 



The curves in figure 2, which give the results of the distillation of 

 a gum turpentine, might be interpreted, in the light of the previous 

 discussion, as indicating a composition of about 95 per cent of some 

 substance a, with boiling point 158 0., specific- gravity 0.865, 



