Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/513

Rh SULPHUR GROUP.] CHEMISTRY 501 quently sulphides to which there are no corresponding oxides. All sulphides are decomposed more or less readily by hydrochloric acid gas, hydrogen sulphide and a chloride being produced, and in some cases sulphur is also liberated. Many of the sulphides which are decomposed when heated in an atmosphere of hydrochloric acid gas are little, if at all, affected by a boiling solution of the acid. Hydrogen selenide and telluride closely resemble hydro gen sulphide in properties, but are far less stable com pounds. The former has a most offensive acrid odour, impairing or even destroying the sense of smell for several hours, and producing inflammation of the eyes. Their aqueous solutions are decomposed on exposure to the air with separation of selenium and tellurium respectively; and with solutions of salts of most metals they produce precipi tates of t?ie corresponding seleuhydrates or selenides and tellurhydrates or tellurides. The selenides and the tellu- rides, like the sulphides, may be formed by the direct combination of their elements, and are usually so prepared; in their general behaviour they resemble the corresponding sulphides. A higher sulphide of hydrogen is known of which the composition has not yet been satisfactorily determined, owing to the difficulty of obtaining it in a pure state ; but as it is obtained on adding a solution of potassium penta- sulphide, K. 2 S 5, to a dilute solution of hydrochloric acid, and is formed without evolution of hydrogen sulphide or separation of sulphur, it appears most probable that it is the pentasulphide H 2 S 5. It separates as an oily liquid, heavier than water, possessing a peculiar sulphurous dis agreeable odour ; it is soluble in water. It is a very unstable substance, undergoing decomposition into hydrogen sulphide and sulphur with great facility ; this decomposi tion is instantaneous under the influence of substances such as finely divided platinum, gold, and charcoal. Its stability is increased by the presence of moderately strong acids, but diminished by alkalies. With an alcoholic solu tion of potassium hydrosulphide it furnishes potassium pentasulphide, hydrogen sulphide being evolved This reaction is analogous to that between sulphuric acid and potassium hydroxide : H 2 SO 4 + 2KHO = K 2 SO 4 + 2H 2 ; hydrogen pentasulphide may, in fact, be regarded as the analogue of sulphuric acid. Sulphur, selenium, and tellurium form compounds with chlorine, bromine, and iodine, similar in composition, but differing greatly in stability. &quot;With chlorine, sulphur forms the three compounds S 2 C1 2, SC1 2 , and SC1 4 , all of which are liquid. The chloride S 2 C1 2 is obtained by passing chlorine over sulphur, which is gently heated; it is a mobile reddish-yellow liquid, having a peculiar, pene trating, and most disagreeable odour. It boils without suf fering decomposition at 137 C. It is slowly decomposed by water, yielding hydrogen chloride, sulphur, and thio- sulphuric acid 2S 2 C1 2 + 30H 2 = 4HC1 + 2S + H 2 S 2 3. When saturated with chlorine at about 10 C. it is con verted into the dichloride SC1 2, but if saturated with chlo rine at about - 22 C. it furnishes the tetrachloride SC1 4. Both of these compounds are so unstable, however, that they are resolved into the lowest chloride S 2 C1 2 and chlo rine when very slightly heated. The chlorides of selenium and tellurium are also obtained by the direct action of chlorine on the elements. Two chlorides of selenium are known, Se 2 Cl 2 and SeCl 4, the former being liquid and the latter a white crystalline solid. Tellurium forms the two chlorides TeCl 2 and TeCl 4, both of which are solid. These chlorides of tellurium, as well as selenium tetrachloride, appear to volatilize without de composition. Very little is known of the bromides of sulphur and sele nium, but from the observations which have been made there is no doubt that the sulphur bromides are much less stable than the chlorides ; sulphur, therefore, like oxygen, appears to have less affinity for bromine than for chlorine. Tellurium furnishes two crystalline bromides, TeBr 2 and TeBr 4, both of which may be sublimed without decomposi tion. The only iodide of sulphur which is known has the composition S 2 I 2, and is obtained by the direct union of its elements ; it is a black crystalline solid, insoluble in water, and readily decomposes when heated. Iodides of selenium are not known with certainty, but two tellurium iodides have been prepared, TeI 2 and TeI 4 ; both are black crystalline bodies, which give off iodine when heated. The chlorides and bromides of selenium and tellurium, like the chlorides and bromides of sulphur, are decomposed by water, but the iodides of these three elements are com paratively stable and are scarcely affected unless heated with water. Sulphur, selenium, and tellurium burn in oxygen or air, forming the dioxides S0 2, Se0 2 , Te0 2. Under certain conditions, sulphur dioxide takes up an additional atom of oxygen, and is converted into the trioxide, S0 3 ; a tellurium trtoxide, TeO 3, may also be obtained, but the corresponding oxide of selenium is not known. A third oxide of sulphur, S 2 3, has recently been described. The dioxides and trioxides of sulphur, selenium, and tellurium have the property in common of forming corre sponding acids when combined with water : H 2 S0 3 H 2 Se0 3 H 2 Te0 3 Sulphurous acid. Selenious cid. Tellurous acid. H 2 SO 4 H 2 Se0 4 Sulphuric acid. Seleuic acid. These compounds differ greatly in stability and in their properties generally the sulphur and selenium compounds being closely related, whilst the tellurium compounds are widely different from them in most respects. ,, : Sulphur dioxide or sulphurous anhydride is a colourless gas, of pungent suffocating odour; by a pressure of three atmospheres, or a refrigerating mixture of ice and salt, it is readily condensed to a colourless mobile liquid, which boils at about - 10 C. The liquid anhydride freezes at - 76 C., forming a transparent colourless crystalline solid, which melts at about - 79 C. Water at C. dissolves about 68 times its bulk of the gas, but only 32 times its bulk at 24, forming a solution of sulphurous acid, H 2 S0 3 , which is readily decomposed by heat. Selenium dioxide or selenious anhydride is a white infusible substance, which volatilizes at a temperature below redness ; its vapour condenses in snow white deli quescent prisms, which dissolve in water forming selenious acid, H 2 SeO 3. Tellurium dioxide, however, is only very slightly soluble in water, and the solution does not exhibit an acid reaction ; the attraction of this oxide for water, in fact, is so slight, that when a solution of tellurous hydrate (tellurous acid), H 2 Te0 3, is heated to about 40 C. the oxide separates. It fuses readily and volatilizes, and the fused oxide, which is a transparent, deep yellow liquid, solidifies on cooling to a white crystalline mass. The difference in physical properties between sulphur H 2 Te0 4. Telluric acid.
 * md selenium dioxides is so great as to suggest that the