Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/511

Rh SULPHUR GROUP.] CHEMISTRY 499 lower specific gravity, viz., 1 98, but of higher melting point (120 C.) ; in the course of a few days, although the crystals maintain their prismatic form, they become opaque, and, on examination, are found to be converted into aggre gations of minute octahedrons. This change of prismatic into octahedral sulphur is attended with the development of a considerable amount of heat, which is especially notice able when the conversion is caused to take place rapidly by scratching the crystals. The transformation of the octahe dral variety into the prismatic takes place at about the melt ing point of the former, a transparent crystal of octahedral sulphur being converted into an opaque mass of prismatic crystals when heated for some time to a temperature of 105 3 -115 C. ; also, when a saturated solution of sulphur in hot turpentine is allowed to cool, crystals of prismatic sul phur are at first deposited, but after a time, when the liquid has become comparatively cool, the crystals which separate are octahedral. Prismatic sulphur is soluble in carbon disulphide and other menstrua which dissolve the octahedral modification ; another modification, which is also soluble, but destitute of crystalline form, is obtained on addition of acids to solu tions of alkaline polysulphides K 2 S 5 + 2HC1 = 4S + H 2 S + 2KC1. Potassium Hydrogen oni^t-nr Hydrogen Potassium pentasulphide. chloride. sulphide. chloride. It has a greenish-white colour, and constitutes ordinary milk of sulphur ; by keeping, it becomes gradually con verted into octahedral sulphur. Sublimed sulphur, or flowers of sulphur, is probably closely allied to this form, but always contains a small proportion of insoluble sul phur. When sulphur is heated to 260-300 C. and then poured in a thin stream into cold water, it is converted into tha so-called plastic modification, and is obtained as a soft yellowish-brown semitransparent mass, capable of being drawn out into fine elastic threads. In this state sulphur has the specific gravity 1 95, and is insoluble in carbon disulphide ; in the course of a few hours, however, it again becomes brittle and almost entirely reconverted into the octahedral modification, the change being accompanied by the development of heat. When the chloride of sulphur, S 2 C];,, is decomposed by water, it furnishes sulphur, hydrochloric acid, and thio- sulphuric acid ; the sulphur thus obtained is amorphous and insoluble in carbon disulphide, but is converted into octahedral sulphur by fusion, or by exposure for some time to a temperature of 100 C. Thiosulphuric acid also gra dually decomposes into sulphur and sulphurous acid, and if a solution of a thiosulphate is decomposed by hydro chloric acid Xa.,S.,0 3 2HC1 = S Sulphur. H 2 SO 3 Suiphurou, 2NaCl Sodium the sulphur which separates is soluble in carbon disul phide, but is obtained on evaporation of the solution in an amorphous condition, and cannot then be again dis solved in carbon disulphide; it is stated, however, that if the hyposulphite is decomposed by dilute sulphuric acid, the sulphur which separates is insoluble in carbon di sulphide. When a solution of ordinary sulphur in carbon disulphide is exposed to sunlight, an amorphous insoluble modification separates ; the reconversion of this modifica tion into octahedral sulphur is attended with the absorp tion of heat. Several minor modifications of sulphur have been de scribed, but it is not known whether the differences which they exhibit are inherent, or whether, as is not improbable, they are due to the presence of impurity. There appears to be a relation between the modification formed in - 1 reaction and the compound or compounds from which it is obtained, since, generally, the sulphur separated from compounds in which it is associated with positive elements is soluble, whilst that separated from compounds in which it is associated with negative elements is insoluble in carbon disulphide ; but the conditions under which the sulphur is separated doubtless exercise an important in fluence. At present we have no knowledge as to the manner in which the various allotropes are related to each other ; it has been suggested, however, that the differences between them are at least in some cases due to differences in mole cular composition. Several allotropic modifications of selenium are also known, but a tendency to form allotropes has not been observed in the case of tellurium. Three principal modifications of selenium, corresponding to the three principal modifications of sulphur, may be distinguished, viz., (1), black crystalline selenium, of specific gravity about 4 80, which may be placed by the side of the octahedral variety of sulphur, being the form in which selenium separates when solutions of metallic selenides are exposed to the air, and since all other modifications are converted into it when heated to about 150 C. ; (2), red crystalline selenium, of specific gravity 4 46 to 4 51, which probably corresponds to monoclinic sulphur, as it has the same crystalline form, and may be obtained by rapidly cooling melted selenium ; and (3), red amorphous selenium, of specific gravity about 4 3. This last corresponds to the amorphous allotrope of sulphur, and, as in the case of sulphur, it occurs in two modifications, the one soluble and the other insoluble in carbon disulphide. Black crystal line selenium, unlike octahedral sulphur, however, is in soluble in carbon disulphide, but the red crystalline allo trope is soluble, although to a much less extent than sulphur. The conversion of these modifications of selenium into each other is attended with development or absorption of heat, just as in the case of sulphur, and the melting points of the several modifications are different. Thus, if amorphous selenium, which melts a few degrees above 100 C., be heated to about 96 C., it quickly becomes crys talline, the change being attended with considerable rise of temperature, and the melting point rises to 217 C. The electrical conductivity of selenium is found to vary in a remarkable mariner with the temperature, and is also in fluenced by light. Amorphous selenium is a non-conductor up to 80 C., but from this temperature up to 210* Q. its conductivity gradually increases, after which it again diminishes. Selenium which has been kept for several hours at 210 C. and then gradually cooled is especially sensitive to the influence of light, its conductivity increas ing with the intensity of the light. Sulphur, selenium, and tellurium form gaseous com pounds with hydrogen, analogous in composition to water: H 2 S Hydrogen sulphide. , H 2 vSe Hydrogen selenide. H 2 Te Hydrogen telluride. Hydrogen sulphide may be produced directly from its elements by passing hydrogen gas into boiling sulphur, but it is always prepared by the action of a solution of hydro chloric or sulphuric acid on a metallic sulphide, that of iron being commonly employed FeS + 2HC1 = H 2 S + F*C1. Ferrous sulphide. Hydrogen chloride. Hydrogen sulphide. Ferrous chloride. The compounds of selenium and tellurium with hydrogen are obtained in a similar manner from metallic selenides and tellurides. Hydrogen and selenium also directly enter into reaction ; the quantity of hydrogen selenide formed is apparently a function of the temperature ; it increases from 250 to 260 C., and decreases regularly from the latter point to 700 C.