Page 7 - A HANDBOOK OF ORGANIC ANALYSIS QUALITATIVE AND QUANTITATIVE

ORGANIC ANALYSIS

accumulate in two or more fractions, the boiling ranges of
which include the boiling-points of the pure components
under atmospheric pressure.
For temperatures materially above 1ooC. a stem-correc-
tion must be applied unless the entire thread of the ther-
mometer is surrounded by the vapour. A formula for this
purpose has been worked out, but as this involves a knowledge
of the average temperature of the exposed portion of the
thread (a value difficult to ascertain, depending upon the

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FIG. 1.
thermal conductivities of the mercury thread and the glass
wall of the thermometer), it is advisable to calibrate the
thermometer by direct comparison with short-stem standard
instruments.
When the liquid appears to be in any degree decomposed
at its boiling-point, fractionation must be carried out under
reduced pressure. A small trial sample should as before be