172 173 Preparation. Sulphur is obtained by purifying native sulphur, or by roasting sulphide of iron or copper out of contact with air. Selenion and tellurium are prepared by tedious and indirect methods which cannot advantageously be considered here. Chemical properties. The three elements combine directly with hydrogen, oxygen, the halogens, and many other elements. The hydrides are feebly acidic; the oxides, as a class, are acidic; many oxyhaloid compounds are known; the elements do not interact with acids to form salts. The compositions of the more important compounds are expressed by the following formulae, where M = S, Se, or Te:-MH,, MO,, MO, MOH,, MOH, MX, MX (X = Cl, Br, I), MOX, MOX, MOX Sulphur and selenion exhibit differences in physical, and to some extent also in chemical, properties, according to the conditions under which they are prepared. 2 2° Ordinary native sulphur crystallises in rhombic octahedral forms. If a quantity of sulphur is melted in a crucible, and allowed to cool slowly until a crust forms on the surface, and if holes are then pierced in this crust and the still molten sulphur is poured out, it is found that the sulphur remaining in the crucible has crystallised in monoclinic prisms. These however soon change to octahedral forms. If sulphur is melted, heated to 400° or so, and then suddenly cooled, by pouring into cold water, a semi-pasty, soft, plastic, solid, having properties somewhat like caoutchouc, is formed: this soft solid is sulphur. Plastic sulphur soon becomes brittle and crystalline. If an acid is added to an aqueous solution of potassium tetra- or penta-sulphide (K„S or K,S)—obtained by fusing solid potash with sulphur-sulphur is precipitated in the form of a white amorphous solid. If an acid is added to a warm aqueous solution of sodium thiosulphate (Na2SO) a yellow solid is produced which is also amorphous sulphur. These five sulphurs differ in specific gravity, solubility in carbon disulphide, &c.; the more prominent differences are presented in the following table. The insoluble varieties of sulphur have not been obtained wholly free from the soluble. If ordinary sulphur, which is wholly soluble in carbon disulphide, is heated to 130° or 140° and quickly cooled no insoluble sulphur is produced; at 170° a considerable quantity of the insoluble variety is formed, but not much more at 230°. According to Berthelot, the soluble varieties of sulphur are electro-negative to the insoluble varieties. Selenion may be obtained by passing sulphur dioxide (SO,) into an aqueous solution of selenious acid (H,SeO3). If the reddish black solid thus produced is melted and allowed to cool, a black, amorphous, lustrous, solid, with a fracture like glass, is obtained: if this is heated to 95°-100° the temperature suddenly rises to 200° or 230°, and a reddish-grey, metal-like, crystalline, somewhat malleable, solid is obtained. The crystalline variety of selenion is about 4.8 times, and the amorphous glassy variety about 4-3 times, heavier than water. Other varieties of selenion appear to exist. Amorphous selenion is a non-conductor of electricity: crystalline selenion conducts electricity; the conductivity increases as the temperature rises, and also increases very markedly when the selenion is exposed to sunlight. The crystalline variety is usually called metallic selenion. Tellurium does not exhibit any changes of properties analogous to the change of one variety of sulphur, or selenion, into the other varieties. Tellurium is a very bad conductor of electricity, but the conductivity is slightly increased by raising the temperature considerably and also by exposure to sunlight. The specific gravities of sulphur and selenion as gases vary with the temperature; the specific gravity of tellurium gas is constant. Thus ; Specific gravities of gases; (air = 1). Temp. 450°-5000 7000 8500 11000 1160° 1420° 1390°-1440° Sulphur. 6.6-6.9 2.8 2.4 .2.1 2.1 Selenion. Tellurium. 7.67 6.3 5.6 9.0 The proof that each of the five varieties of sulphur, and each of the two varieties of selenion, is composed of M. E. C. 9 174 only sulphur, or only selenion, consists in the facts-(1) that a given mass of one variety is changeable into exactly the same mass of another variety; (2) that equal masses of the different varieties when oxidised produce each exactly the same mass of the same oxide. Thus when 1 gram of any variety of sulphur is wholly burnt in oxygen, 2 grams of sulphur dioxide (SO,) are produced. The change of a specified mass of any variety of sulphur other than the octahedral into the same mass of the octahedral variety is accompanied by the production of heat; the change of a specified mass of amorphous selenion into the same mass of crystalline selenion is accompanied by the production of heat; therefore there is less energy in a specified mass of octahedral sulphur, or crystalline selenion, than in the same mass of any other variety of sulphur, or selenion.* Sulphur and selenion are said to exhibit allotropy; the change from one variety of sulphur, or selenion, to another is called an allotropic change. Tellurium does not exhibit allotropy. We shall consider allotropic changes more fully hereafter; meanwhile the student should compare what has been said regarding the existence of more than one variety of the same element with what he learned about the composition of elements in chap. 11. par. 32. Compounds of sulphur, selenion, and tellurium, with hydrogen. MH. These compounds are gases under ordinary conditions; they have all an extremely powerful and disagreeable smell, and are very poisonous. They are obtained (1) by passing hydrogen over the molten elements, (2) by the interaction of acids in aqueous solution on various metallic sulphides, selenides, or tellurides; thus ZnM + H2SO Aq = ZnSO2Aq + MH2. The hydrides MH, dissolve in water; the solutions of the sulphur and selenion compounds redden litmus and react with alkalis as acids do; thus An 2 H,SAq+ KOHAq = KHSAq + H2O; aqueous solution of tellurium hydride does not exhibit acidic functions. The aqueous solutions absorb oxygen from the air and are changed to water and the element M; in the case of H,S, s. further Chap. xiv. * SO,Aq is also formed, and this absorbs more oxygen and becomes SO,Aq. These hydrides are all decomposed by heat, thus MH, = M + H2; the stability towards heat decreases as the combining weight of M increases. The composition of these hydrides is determined by heating a measured volume of each with a weighed quantity of zinc, iron, or copper: zinc &c. sulphide, selenide, or telluride, and hydrogen, are thus produced; the volume of hydrogen is the same as the volume of the hydride before the change; the increase in the weight of zinc, iron, or copper, represents the mass of sulphur, selenion, or tellurium, formerly combined with hydrogen. The specific gravity of each gaseous hydride is determined. From the data thus obtained the composition of the hydrides is calculated. To take an example: 500 c.c. of hydrogen sulphide, measured at 0° and 760 mm., were heated in a closed glass tube with 3 grams of powdered zinc; when the change was completed the gaseous contents were cooled to 0° and measured, 500 c.c. of hydrogen were obtained; the mixture of zinc and zinc sulphide in the tube weighed 3.7149 grams. The specific gravity of hydrogen sulphide was found to be 17 (hydrogen = 1); as 1000 c.c. of hydrogen measured at 0° and 760 mm. weigh 08936 gram, it follows that the 500 c.c. of hydrogen sulphide used weighed The experimentally determined data are then as follows::500 c.c. hydrogen sulphide, weighing 75956 gram, are composed of 500 c.c. of hydrogen, weighing 04468 gram, and 7149 gram of sulphur. But the combining weight of sulphur is 32. What mass of hydrogen is combined with one combining weight of sulphur? 7149 0446832: 2. Therefore the simplest formula which expresses the composition of hydrogen sulphide is H.S. But does this formula express the composition of one reacting weight of hydrogen sulphide? If the volume occupied by 1 gram of hydrogen is called one volume, then 34 grams of hydrogen sulphide (i.e. the mass, taken in grams, represented by the formula H,S) occupy 2 volumes. But we have already learned that the reacting weights of gaseous compounds are those masses of them which occupy two volumes (s. par. 88.); hence the formula H S expresses the composition of a reacting weight of hydrogen sulphide. Further study of the reactions of this compound confirms this conclusion; all the interactions of 175 hydrogen sulphide with other compounds and with elements can be simply expressed as occurring between 34, or a whole multiple of 34, parts by weight of hydrogen sulphide. Compounds with oxygen. MO, and MO3. When sulphur, selenion, or tellurium, is burnt in oxygen, a dioxide (MO) is produced. When a mixture of sulphur dioxide gas and oxygen is passed over hot very finely divided platinum sulphur trioxide (SO) is formed. When solid telluric acid (H2TeO) is heated tellurium trioxide (TeO,) and water are formed. Selenion trioxide has not yet been isolated. A better yield of tellurium dioxide is obtained by heating tellurium with concentrated nitric acid. When concentrated nitric acid is heated a portion of it is changed to water, oxygen, and nitric oxide; thus 2HNO, (heated) = HO+30 +2NO; if some fairly easily oxidised substance is present in contact with the changing nitric acid, that substance combines with the whole or a part of the oxygen, and is thereby oxidised. In the present instance the oxidisable body is tellurium; the tellurium is oxidised to tellurium dioxide, and simultaneously the nitric acid is deoxidised or reduced to water and nitric oxide. Compounds and elements which do not directly combine with oxygen may frequently be oxidised by bringing them into contact with another substance which under the conditions of the experiment is evolving oxygen, or into contact with other substances which are interacting and producing oxygen. We have already had examples of such processes of oxidation (s. chap. VIII. par. 121). Another example is given by one of the processes for forming selenic acid (H.SeO). Selenion is suspended in water and chlorine is passed into the water; the chlorine and water interact to produce hydrochloric acid and oxygen, and the oxygen combines with the selenion to produce selenion dioxide, which again interacts with a portion of the water to produce selenious acid; these changes occur simultaneously; the initial and final compositions of the whole interacting system may be expressed by the equation Se + 6C1+ 4H,O+ Aq= H SeO Aq + 6HCIAq. Compounds other than those of oxygen are frequently produced by arranging a chemical change so that the constituents of the specified compound are produced in contact with each other; thus hydrogen does not interact with an aqueous solution of sodium sulphite (Na,SO), but if this solution is brought into contact with zinc and dilute sulphuric acid a portion of the sodium sulphite is deoxidised, or reduced, to hydrogen sulphide and water. We know that zinc and dilute |