lower boiling liquid, and each of the last a larger proportion of the 

 higher boiling liquid; all the fractions will contain both liquids in 

 varying proportions, and it will be impossible to separate the two 

 liquids completely by one such distillation. By repeated distillations 

 of the several fractions, however, and by combination of those frac- 

 tions which distill between the same temperatures, a fairly complete 

 separation can eventually be made. This is usually a long and tedious 

 operation and not readily applicable to an analytical method. 



By taking advantage of the well-known principle of dephlegmation 

 these difficulties are largely removed and in many cases one operation 

 will suffice to separate two liquids in practically pure condition. 

 This principle is applied by placing between the distilling flask and 

 the condenser a new form of column still head or dephlegmator (fig. 

 4). This is of such construction that the vapors are partly condensed 

 before they reach the condenser, and the uprising vapors are therefore 

 washed by the reflowing liquid. When a mixture of light and heavy a 

 oils, such as might occur in turpentines, is distilled through this appara- 

 tus, the tendency is for the vapors of the light oil to pass freely out into 

 the condenser, while the vapors of the heavy oil are condensed and 

 return to the still, to be carried over only after all the light oil has 

 been distilled. The results are the same as though repeated distilla- 

 tions were made. 



(2) When distilling high boiling oils over a direct flame, it is 

 difficult to regulate the temperature, and there is always danger of 

 superheating. It is true that this superheating may be slight when 

 the distillation is carefully carried on, but if the operation is hastened 

 the danger may be greatly increased. In the case of turpentine and 

 other liquids which contain unstable substances, like terpenes, 

 superheating may cause decomposition or alteration of the oil, and 

 for this reason a study of the properties of the distillate may not clearly 

 show the composition of the original oil. 



Not only does superheating tend to cause decomposition of the 

 turpentine, but it introduces errors in temperature readings. Fur- 

 ther, the rate of distillation may not be constant. These several 

 sources of error involved in an analysis by ordinary direct heat dis- 

 tillation can be largely eliminated by distillation with steam, since 

 the process can be carried on at a much lower temperature and with a 

 more certain regulation. 



The fundamental principles of distillation with steam may be out- 

 lined as follows: If a pure liquid, such as benzine, which does not dis- 

 solve in water be placed in a flask and water added, the two will upon 



"Light" and " heavy" are terms used synonymously with low boiling and high 

 boiling. The terms arise from light and heavy weight that is, low and high specific 

 gravity which usually accompany low and high boiling points respectively. 

 [Cir. 152] 



