86 SULPHIDES.
automatic apparatus, and collect the crystals which have separated
in the boiling-flask. The purified product melts at 275° to 276°,
and 0.508 part dissolves in 100 parts of boiling carbon bisulphide.
Thermoelectric Determination of the Boiling-point. Le Chate-
lier's pyrometer is used for the measurement of temperatures up
to 1600°. It consists of two wires, one of platinum and the other
of 10% rhodium - platinum
alloy, which, at one end, are
fused together in the oxyhy-
drogen flame and at the other
end are connected through a
suitable measuring instrument.
A difference between the
temperature at the junction
of the wires and that at the
free ends gives rise to an elec-
tric current, the feeble poten-
tial of which may be measured
by means of a sensitive volt-
meter. In order to economize
in the amount of platinum, the
free ends of the pyrometer are
connected to insulated copper
wires leading to the measuring
instrument. The two parts of
the circuit where the connec-
tion with the copper wires is
made are inserted in glass test-
tubes, and the latter are closed
by cork stoppers. The test-
tubes are placed in a beaker
FIG. 18. filled with water to serve as athermostat, and the beaker is
enveloped in asbestos paper. In the voltmeter which is to be used
for pyrometric work, a second scale is graduated to give directly the
temperature readings, each division of the scale representing 10
degrees. In order to find the true temperature of the junction, the
reading must be corrected by adding half the temperature of the
thermostat. If the latter is maintained at 0°, by filling the beaker