PHOSPHORUS PENTASULPHIDE. 87
with ice, no correction is necessary. On account of the sensitive-
ness of the thermo-element toward chemical influences, the wires
are protected by a long, narrow porcelain tube, to the closed end of
which the thermoelectric junction is inserted. Insidethis tube, one wire is isolated from the other by being
placed inside a porcelain capillary (a, Fig. 19). The
protective tube should, to save space, be made as nar-
row as possible, and it may be of glazed or unglazed
porcelain: the latter will stand a higher heat, but is not
impervious to gases; at temperatures up to red heat, a
protective tube of Jena glass may be employed, which,
particularly if the wires are insulated with mica, can
be made of smaller bore. In setting up an apparatus
for the pyrometric measurements of temperatures,
special care should be taken to properly connect the
thermo-element with the terminals of the voltmeter, to
carefully isolate the pyrometer wires, and to see that the
voltmeter is properly adjusted and set to the correct zero point.
The pyrometer is best standardized by carrying out measure-
ments with a few pure metals, the melting-points of which are
accurately known.
1
Refer to the discussion, under Tin (No. 6,
p. 15), of melting-point determinations.
To determine the boiling-point of the phosphorus penta-
sulphide, fasten a Jena-glass test-tube, about 40 cm. long and
4 cm. in diameter, in a perpendicular position, and place in it
about 40 g. of the material. Around the middle part of the
tube above the substance, wrap several layers of asbestos paper
and secure the pyrometer so that the thermoelectric junction
hangs two or three centimeters above the surface of the melted
material. Heat the phosphorus pentasulphide to boiling with a
large flame, meanwhile passing a slow stream of carbon dioxide
into the upper part of the test-tube in order to prevent the
ignition of the hot vapors when they come in contact with the
(^1) The National Testing Bureau at Berlin recommends the following fixed
thermometry points. The values agree closely with those used by the Bureau
of Standards at Washington, D. C.
B.P. of Oxygen, -183.0° B.P. of Naphthalin, 217.96 M.P. of Antimony, 630
B.P. of Carbon Dioxide, -78.5 M.P. of Tin, 231.84 M.P. of Silver, 960.5
M.P. of Mercury, -38.89 B.P. of Benzophenone, 305.9 M.P. of Gold, 1063
M.P. of Ice, 0.00 M.P. of Cadmium, 320.9 M.P. of Copper, 1083
Transf. P. of Sodium Sulphate, 32.38 M.P. of Zinc, 419.4 M.P. of Palladium, 1557
B.P. of Water, 100.00 B.P. of Sulphur, 444.55 M.P. of Platinum, 1764