132 ON A PROCESS OF FRACTIONAL CONDENSATION. 



first indication of choking of the worm is a partial or entire stoppage of the stream of 

 liquid which normally flows steadily from the end of the worm into the retort. Any 

 interruption or unsteadiness of this flow would indicate too rapid ebullition. 



As a rule, other things being equal, the greater the difference between the tem- 

 perature of the bath and that of the retort, the slower the products will come off 



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and the more effectual will be the separation. I think it possible, however, that the 

 earlier fractionings may be conducted so slowly that the loss of time would more 

 than counterbalance what might be gained by more thorough separation, and that 

 equally good results may be more economically obtained by more frequent operations, 

 somewhat more rapidly conducted. 



A striking illustration of the advantage to be gained by this process is presented by 

 the fact that, during the first fractioning of a crude mixture, such as American petro- 

 leum or coal-tar naphtha, for example, the difference between the temperature of the 

 bath and that of the retort may sometimes be as much as 35° C, or even more. While, 

 as the products become purer, this difference between the temperatures of the bath 

 and retort proportionally decreases, till finally, in operating on a pure product, the 

 temperature of the bath must be brought to within a few degrees of that of the retort, 

 in order to bring the vapors through. But the amount of this difference is variable 

 for different bodies of equal purity. 



These first fractionings must necessarily be quite arbitrary ; for, as a general rule, 

 when operating on such mixtures as those just mentioned, neither the thermometer 

 nor -the quantities obtained for any given range of temperature will indicate any 

 decided preponderance of any one substance. On the contrary, the temperature rises 

 uniformly, and about the same quantity is generally obtained for the same number of 

 degrees of temperature throughout the operation. In other mixtures, in which certain 

 bodies may seem to be present in much larger proportion than others, or in which there 

 may be a greater difference between the boiling-points of the constituents than in the 



referred 



facts which would be indicated by the thermometer of the 



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and by the relative quantities of the products obtained, — there might be something 

 gamed by exercising discretion in taking off fractions according to these indications. 



In the second series of fractioning, the first or lowest fraction of the preceding series, 

 which is large enough to operate upon by itself, is transferred to the retort, and brought 

 into ebullition. The temperature of the bath is then adjusted as above described, and 

 the distillation continued, the fractions obtained being placed in their appropriate bot- 

 tles until the temperature of the retort shall have risen to, or somewhat above, the 

 point at which the second or next succeeding fraction of the first series may be sup- 



