36 CHANGES OF CONDITION.
The Transition Temperature from the Heating Curve. The cool-ing curve of sulphur does not, when determined in the simplest
manner, show an exact point of solidification, because this is
dependent on the temperature to which the material has pre-
viously been heated; nor does it indicate sharply the transition
temperature, on account of the ability already mentioned of
monoclinic sulphur to persist in the unstable condition. Both
values, however, can be established within 5° if the rate is
measured at which the temperature of rhombic sulphur rises with
a uniform application of heat.
Melt about 7 g. of sulphur in a test tube, place the bulb of
a thermometer in the liquid, let the latter solidify, and after
moistening it with a little carbon bisulphide allow it to stand until
the next day. It is well also to prepare one or two duplicate
tubes in the same manner. Heat one of the tubes in a sulphuric-
acid bath in such a manner that the temperature of the bath
keeps constantly about 15° in advance of that shown by the
thermometer in the tube, and never rises more rapidly than 1° in
10 seconds. Read the thermometer every 10 seconds between
60° and 120°, and plot the corresponding temperatures and times
on coordinate paper. The rise in temperature is retarded
between 95° and 100° (the transition point) in consequence of
the amount of heat absorbed in the transformation. In the
further course of the curve there is only a moderately rapid rise
in temperature, — partly because the transition is not completed,
partly because melting begins — until, above a point between
115° and 120° (the melting-point), the temperature again rises
rapidly.
Allow the melt to cool again, and induce crystallization
between 110° and 100° by means of a glass thread, or by seeding
with a minute monoclinic crystal; then allow to cool further to
60°, and repeat the experiment. This time, since only mono-
clinic sulphur is present, there is no indication of a transition
point; the melting-point, however, is shown, although, as in the
preceding experiment, it is not sharply defined.
According to recent investigations, it seems probable that the peculiar
changes in consistency and color which sulphur undergoes on further heating
are due to two different liquid modifications: sX the thin liquid and sn the
viscous form.^1(^1) Smith and Holmes, Z. physik. Chem. 43, 469 (1903); 53, 602 (1905);