Encyclopaedia Britannica > Volume 21, ROT-Siam

632
S E L-
gr. 4'5 (isomorplious with monoclinic sulphur), which retain
their solubility in bisulphide of carbon up to 100° C. At
110° C. or higher temperatures they pass into the metallic
modification (see below) with evolution of heat. With
the amorphous kind a similar change sets in at or above
80° C. and attains its maximum of rapidity at a point be¬
tween 125° and 180° C. Fused selenium when cooled down
suddenly hardens into a very dark-coloured glass of 4'28
sp. gr., soluble in bisulphide of carbon; on gradual cool¬
ing it becomes more or less completely “ metallic.” (2)
Metallic selenium is a dark grey or black solid of 4‘8 sp.
gr.; it exhibits metallic lustre, stretches perceptibly under
the hammer, and its fracture is similar to that of grey cast
iron. It is insoluble in bisulphide of carbon. Its fusing
point is sharply defined and lies at 217° C. At the ordi¬
nary temperature it conducts electricity, while the non-
metallic modification does not; at higher temperatures, or
after temporary exposure to higher temperatures, the con¬
ductivity on either side becomes an eminently variable
quantity. According to Draper and Moss, glassy selenium
begins to conduct electricity at 165° to 175° C., and the
conductivity increases regularly as the temperature rises
to near the boiling-point. With metallic selenium, which
behaves similarly, the increase of conductivity is propor¬
tional to the increase of temperature to near the fusing
point (217° C.); but from this point upwards it decreases
rapidly and attains its minimum at 250° C. According
to W. Siemens, however, selenium by long exposure to
200° C. becomes what one may call electrically metallic;
the conductivity then decreases when the temperature
rises, just as it does with ordinary metals. But this electro-
metallicity is not permanent; on continued exposure to a
lower temperature it vanishes gradually, until the propor¬
tion of quasi-metal has fallen to a limit-value depending
on that temperature. Very surprising is the observation
of Sale that the electric conductivity of metallic selenium
increases on exposure to the light; the red and ultra-red
rays, as he found, act most powerfully. The effect of
insolation is almost instantaneous, but on re-exposure to
darkness the original condition is re-established only very
gradually. W. Siemens found that his electro-metallic
selenium (as produced at 200° C.) is more sensitive to
light than any other kind. The conductivity of such
selenium starting from darkness is raised twofold by dif¬
fuse and tenfold by direct sunlight. The specific heat of
selenium, according to Regnault, is 0'0746 both in the
glassy and in the metallic modification. Selenium (of any
kind) boils at 700° C. (Mitscherlich). The vapour has
an intense colour intermediate between that of chlorine
and that of sulphur. According to Deville and Troost,
at 880° C. it is 7'67 times, and at 1420° is 5-68 times, as
heavy as air; theory, for Se2 = l molecule, demands 5'47.
Elementary Tellurium.—This, the compact form, is a
silver-white resplendent metal of markedly crystalline
structure; the crystals are rhombohedra, and the ingot
consequently is very brittle. Specific gravity 6-2. The
metal fuses at about 500° C., and is distillable at very high
temperatures. Its vapour is golden yellow and has a very
brilliant absorption-spectrum. The vapour density, accord¬
ing to Deville and Troost, is 9-08 at 1439° C. (air = l),
corresponding to Te2 = 1 molecule. A bar of tellurium be¬
comes feebly electrical when rubbed with a woollen cloth.
The electric conductivity, like that of selenium, is largely
influenced by the temperature and previous exposure to
heat, and it increases after exposure to light, though not
to the same extent as selenium does. Starting from the
ordinary temperature the conductivity decreases up to some
point between 90° and 145° C.; it then increases up to
200° C. (the highest temperature tried); on cooling it de¬
creases steadily, and finally is only one-fifth or one-sixth of
-SEE
what it was at 200°. The numerical value at 200° (silver =
100) wras found equal to O'OOSS to O'OOSl (F. Exner).
Extraction of the Elementary Substances.—If seleniferous sulphur
or pyrites is used for the manufacture of oil of vitriol by the
chamber process, most of the selenium accumulates as such in the
“chamber mud,” from which it may be extracted by the following
method of Wohler’s. The mud, after having been thoroughly
washed and dried, is fused with alkaline nitrate and carbonate, to
convert the selenium into selenate (Se04K2 or Na2), which is ex¬
tracted by means of water. The filtered solution" is boiled with
hydrochloric acid to convert the selenic into selenium acid (SeO,
+ 2HC1 = C12 + H20 + Se02), and this last is then reduced by addi¬
tion of sulphurous acid and heating, when the selenium comes down
as a red precipitate (Se02 + 2S02 = 2SOs + Se). A richer material than
chamber mud is seleniferous ore-smoke as produced in Mansfeld,
which likewise contains free selenium. Its extraction, according
to O. Pettersen and F. Nilson, is best effected by digestion with con¬
centrated solution of cyanide of potassium at 80° C., which converts
the selenium into selenocyanide (SeNCK), easily extractable by water.
The filtered solution is acidified with hydrochloric acid and allowed
to stand, when the selenium (through the spontaneous decomposi¬
tion of the SeNC. H into NCH and Se) comes down as a precipitate.
Tellurium is generally prepared from Transylvanian gold ore.
The powdered ore is oxidized by means of hot nitric acid and the
least sufficiency of hydrochloric acid, the excess of nitric acid being
chased away by evaporation, and the residue mixed with sulphuric
acid (to convert the lead into insoluble sulphate), and with some
tartaric acid to prevent precipitation of tellurious acid (Te02) in
the subsequent treatment with water. From the filtered aqueous
solution the gold is removed by addition of ferrous sulphate and
by filtration. The filtrate is treated with sulphurous acid to reduce
the tellurious acid to tellurium, which separates out as a black
precipitate. The precipitated metal is fused down and then sublimed
at a very high temperature, in a porcelain tube, in a current of
hydrogen, to remove non-volatile impurities and eliminate the last
trace of selenium (SeH2).
Chemical Relations.—Selenium and tellurium are similar in their
chemical character to sulphur; the gradation of properties within
the triad is in the order of the atomic weights, which are 8 = 32-06,
Se = 79,07, Te = 128 (0 = 16). In oxygen or air the elementary sub¬
stances burn readily into (solid) dioxides (Se02, Te02), in the case
of selenium with production of a characteristic stench of putrid
radish, owing probably to the formation of a trace of hydride, SeH2.
Nitric acid, in the heat, converts sulphur directly into sulphuric
acid. In the case of the two rare elements the oxidation stops at
the stage corresponding to sulphurous acid. The acids Se03H2 and
Te03H2 are not liable to further oxidation by any of the wet-way
reagents (HNO.s, H20 and Cl2, Br2, I2, &c.) which convert sulphur¬
ous into sulphuric acid.
By fusion with nitre and alkaline carbonate the three elements,
in their elementary or less oxygenated forms, are readily converted
into salts, R2204 (sulphates, &c., 2 = S, Se, or Te). Selenic and
telluric acids (H2204), unlike sulphuric, when boiled with aqueous
hydrochloric acid, are gradually reduced to the lower acids (Se or
Te)03H2, with evolution of chlorine ; and the lower acids are readily
reduced to (precipitates of) elementary selenium and tellurium re¬
spectively by the action of sulphurous acid in the heat. Chlorine
combines readily with elementary selenium and tellurium into
dichlorides (Se or Te)Cl2, which, however, on continued chlorina¬
tion are at last completely converted into the tetrachlorides (Se or
Te)Cl4. These last, unlike the corresponding sulphur compound,
are distillable without decomposition. Metals capable of uniting
directly with sulphur as a rule unite also with selenium and tellurium
into corresponding compounds. Hydrogen unites with elementary
selenium and tellurium in the heat into gaseous hydrides (Se or
Te)H2 closely similar to sulphuretted hydrogen. But, as these
hydrides are liable to dissociation, the pure compounds must be
prepared by the decomposition of the zinc compounds Zn2 with
hydrochloric acid. For the description of individual compounds
reference must be made to the handbooks of chemistry. (W. D.)
SELEUCIA, or Seleuceia (EeA £vk€lo). Of the numer¬
ous ancient towns of this name the most famous are—(1)
the great city on the Tigris founded by Seleucus I. Nicator
(see vol. xviii. p. 587), of the greatness and decay of which
an account has been given in vol. xviii. p. 601; (2) a city
on the northern frontier of Syria towards Cilicia, some
miles north of the mouth of the Orontes, also founded
by Seleucus I., and forming with Antioch, Apamea, and
Laodicea the Syrian Tetrapolis. It served as the port of
Antioch (Acts xiii. 4). Considerable ruins are still visible,
especially a great cutting through solid rock, about two-
thirds of a mile long, which Polybius speaks of as the road
from the city to the sea.