Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

ALLOTROPIC MODIFICATIONS OF SULPHUR. 35

the experiment. Read the temperature every 10 seconds and

plot the results (Fig. 10).

When the temperature has fallen to about 130°, proceed to

obtain the heating curve by warming the bath so that its tem-

perature keeps constantly 20° or 30° above that of the sulphide

(Fig. 11); the continuous curve gives the bath temperatures, the

dotted one the readings for the silver sulphide. The point at

which the transformation is finished, and the temperature begins

to rise-rapidly is particularly sharp.

16. Allotropic Modifications of Sulphur.
    Monoclinic sulphur has a density of 1.96 and a melting-point of 119.25°;
    rhombic sulphur a density of 2.06 and a melting-point of 112.8°. When sul-
    phur crystallizes from a solution in carbon bisulphide, it is obtained in the
    rhombic modification, which at room temperature can be preserved unchanged
    for an indefinite time. If on the other hand melted sulphur is allowed to
    solidify, the crystals which form are not of the rhombic, but are commonly of
    the monoclinic, modification; this modification persists for some time, even
    below the transition temperature, as an unstable form, for the reason that the
    transition takes place slowly. In addition there have been six or seven other
    allotropic forms of sulphur shown to exist by crystallographic optical methods.

Monoclinic Sulphur. Transformation into Rhombic Sulphur.

Heat about 7 g. of sulphur in a test tube until it has melted and

begun to turn dark-colored (140° to 150°); dip a thermometer

in the liquid and clamp it in position. Allow the melted sulphur

to cool in a sulphuric-acid bath at 80° to 90°, and when it is at
110° arrest the supercooling by dipping a glass thread into the
melt. The sulphur crystallizes to a wax-yellow mass of trans-

parent monoclinie needles, while the liberated heat of solidifica-

tion raises the temperature several degrees. By the next day,

or more quickly on moistening the mass with carbon disulphide,
the crystals become light-yellow and opaque, changing thereby

into an aggregate of rhombic crystals.

Transformation of Rhombic into Monoclinic Sulphur. Heat a

few clear crystals of rhombic sulphur for two or three hours in a

test tube which dips in a bath of a boiling, concentrated solution
of common salt (temperature 108° to 112°), replacing when neces-

sary the water evaporated from the bath. The sulphur crystals

gradually become clouded, and change finally to a friable, light-

yellow mass of monoclinic sulphur.