FRACTIONAL 



133 



posed, or has been found by experiment, to boil. This fraction is then added to the 

 residue in the retort, and the distillation is continued m before. In the Mime manner, 

 I proceed with the remaining fractions of the first series. 



All subsequent fractionings are similarly conducted. As the work progn -<•*, how- 

 ever, the fractions are taken for a gradually decreasing number of degrees of tempera- 

 ture, until finally it becomes necessary, for the attainment of absolute constancy of 

 boiling-point, to take off a fraction for every degree, centigrade; and to continue thus 

 to operate on these fractions, each representing one degree of temperature, until the 

 desired end is attained. 



The operator will observe that, in each series of fractions, in which each fraction has 

 been taken for the same range of temperature, the difference between the boiling-points 

 of any two contiguous fractions is nearly the same as the difference between any other 

 two contiguous fractions, — in other words, that the difference referred to approximates 

 to a common difference throughout the same series. Once ascertained, this difference 

 serves as a valuable guide in determining with sufficient accuracy when to add the 

 next fraction to the retort. By observing this systematic course, irregularities, from 

 the improper mixture of products, may be avoided, and time thus economized. 



After a few series of fractionings, — sometimes after two or three, variable in num- 

 ber, according to the nature or complication of the mixture, — it will be found that 

 some of the fractions are considerably larger than others for the same range of tem- 

 perature, indicating approximately the boiling-points of the several constituents. But 

 fractions of constant boiling-point, or those, the boiling-points of which cannot be sen- 

 sibly changed by further fractional condensation, are not obtained, as already men- 

 tioned, till after repeated careful fractioning for every degree of temperature. When 



for every degree, it is important to use every precaution to prot ct the 

 thermometer from external influences, and to carefully apply the corrections for varia- 



o 



tions in the atmospheric pressure. This may even be desirable earlier; but it is of so 

 much importance in the case specified, that, if omitted, the operator would be liable 

 one day to mix products which he had separated the day previous. 



In this way, certain larger fractions are obtained, which are not susceptible of further 

 alteration in their boiling-points ; but there are yet considerable quantities of liquid in 

 the intermediate fractions, which still continue to change more or less in each succeed- 

 ing operation. When the fractions of constant boiling-point have once been obtained, 

 if it were not important to test for other bodies in the intermediate fractions the 

 operation might here be suspended, provided the pure products already obtained should 

 be large enough for the purposes required. 



