Page:Encyclopædia Britannica, Ninth Edition, v. 21.djvu/658

632 gr. 4'5 (isomorphous 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 1 25 and 1 80 C. Fused selenium when cooled down suddenly hardens into a very dark-coloured glass of 4 '2 8 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 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 Se 2 &#61; 1 molecule, demands 5 '47.

Elementary Tellurium.—This, the compact form, is a silver -white resplendent metal of markedly crystalline structure; the crystals are rhotnbohedra, 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&#61;1), corresponding to Te 2 &#61; 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 what it was at 200. The numerical value at 200" (silver &#61; 100) was found equal to 0'0035 to O'OOSl (F. Exner).

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1em 1em  SELEUCIA, or (Σελεύκεια). Of the numerous ancient s of this name the most famous are—(1) the great on the  founded by  (see vol. xviii. p. ), of the greatness and decay of which an account has been given in vol. xviii. p. ; (2) a on the northern frontier of  towards, some miles north of the mouth of the, also founded by , and forming with , , and  the n. It served as the of  (Acts xiii. 4). Considerable ruins are still visible, especially a great cutting through solid rock, about two-thirds of a mile long, which speaks of as the road from the city to the sea.