the application of heat distill together in a fixed proportion, and the 

 boiling point will remain unchanged as long as the two constituents 

 of the mixture are present. This boiling will take place at a tempera- 

 ture below the boiling point of either the oil or the water. This low- 

 ering of boiling point is characteristic of a mixture of two mutually 

 insoluble liquids, and because of this fact an oil with a high boiling 

 point may be distilled with water at a temperature below the boiling 

 point of the latter. The same conditions exist and the same results 

 follow when saturated steam is blown into the oil. In practice the 

 steam distillation is carried on either by mixing the water with the oil 

 in the still and heating the still externally from beneath, or by blowing 

 steam into the oil from a boiler; the latter process is easier to regulate. 



If, however, nearly equal quantities of two oils which are freely 

 miscible with one another 

 and have different boiling 

 points be mixed and dis- 

 tilled with steam, the pro- 

 portion of oil and water 

 constantly changes and the 

 distillation temperature of 

 such a mixture does not 

 remain constant, but will 

 vary between the approxi- 

 mate temperature at which 

 the separate oils distill with 

 steam. 



The separation of light 

 and heavy oils by steam 

 distillation offers the same 

 difficulties as were men- 

 tioned in the discussion of 

 direct heat fractioiiation. 



55! 



FIG. 1. Percentage weight-steam temperature curve for a 

 mixture of two oils distilled in the ordinary manner. 



To remove these difficulties the principles of dephlegmation can be 

 applied to steam distillation in the same manner as 'to ordinary dis- 

 tillation, but the still head must be of special construction. 



(3) In scientific work of this character it is often very difficult, even 

 with the data in hand, to interpret the results quickly and accurately. 

 This is due in part to the fact that the eye can not readily grasp the 

 meaning of a mass of figures. They are too detailed, and too much 

 calculation is necessary in order to make comparisons. Something 

 less complicated would greatly aid matters; a picture, as it were, 

 which represents the exact conditions. 



If in the case of an ordinary distillation without a well made still 

 head the maximum boiling points of the various fractions be plotted on 

 the horizontal axis, and the corresponding percentages by weight of 

 distillate on the vertical axis, a curve which approximates figure 1 will 



[Cir. 152] 



