ON A PROCESS OF FRACTIONAL CONDENSATION. 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 as before. In the same manner, 
I proceed with the remaining fractions of the first series. 
All subsequent fractionings are similarly conducted. As the work progresses, 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 
fractioning for every degree, it is important to use every precaution to protect the 
thermometer from external influences, and to carefully apply the corrections for varia- 
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, whi@h still continue to change more or less in each succeed- 
ing operation. When the fracti s of constant boiling-point have once been obtained, 
if it were not important. td%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 purpdses 4 fequired. 
