1902 Encyclopedia > Sulphur

Sulphur




SULPHUR. The sulphur minerals, which are very numerous and varied, arrange themselves under three heads,—(1) metallic sulphates, of which hydrated sulphate of lime, CaS04. 2H20, gypsum, is the most abundant; (2) metallic sulphides, a numerous family, including the majority of metallic ores, of which, however, only iron pyrites serves as a source for sulphur; (3) elementary sulphur. In the organic world we meet with sulphur everywhere, this element forming an essential (though quantitatively subordinate) component of the albumenoids, a class of compounds contained in all vegetable and animal structures. Of organic materials rich in sulphur we may name animal hair (containing about 4 per cent.) and the essential oils of the onion, garlic, and mustard.

Elementary Sulphur. This occurs as a mineral chiefly in the Upper Miocene deposits and in the Flötz, associated in general with gypsum, massive limestone, and marl. Commercially im-portant deposits are found in Sicily (provinces of Caltanis-setta, Girgenti, Catania), Italy (Latera and Scrofano, pro-vince of Borne), Spain (Teruel and Arcos), France (dept. Vaucluse), Transylvania, Poland (Swoszowice near Cracow), and Germany (Lüneburg, in Hanover). The exhalations of volcanoes include, as a rule, sulphurous acid, S02, and sulphuretted hydrogen, H2S, which two gases, if moist, readily decompose each other into water and sulphur,—a circumstance which accounts for the constant occurrence of sulphur in all volcanic districts. Mt Purace in Colombia wears a cap of sulphur (derived from its own crater) which accumulates at the rate of about 2 feet per annum,— its superficial area amounting to 1435 square yards. The solfatara at Bahara Saphinque on the Bed Sea is said to yield 600 tons of sulphur annually. The molten sulphur discharged from the crater of the Alaghez in the Armenian highlands forms solid excrescences, which the natives dis-lodge from their inaccessible positions by means of rifle-shots. A sulphur deposit near the Borax Lake in California is estimated to contain 20,000 tons. Most of the sulphur Sicilian or brimstone of commerce comes from the rich fields of sulphur. Sicily, where in 1884 the annual production had almost reached 400,000 tons. The mode of mining there adopted is by a network of horizontal galleries (tunnels) driven through the deposit; the solid squares thus marked off are hewn out, a central pillar being left to support the roof. The total excavation is generally 100 feet high and from 25 to 50 wide; not unfrequently the whole collapses. Down to a comparatively recent date all the work used to be done by hand, boys of eight to ten years of age being employed to carry the ore to the shaft and thence to the surface; only where a mine has reached a depth of 325 feet or more is water-power, if available, resorted to. Since 1868, however, the ore at Grotta Calda at least has been raised by properly constructed shafts with the help of steam-power, and this system is spreading.

The Sicilian ores are customarily classified as follows :—_ Per 100 parts of ore Per 100 parts of ore Sulphur present. Sulphur recovered.

Pichest ores 80-40 20-25
Rich ores 25-80 15-20
Ordinary 20-25 10-15

The poor yield of actual sulphur is explained by the rather primi-tive method used for its extraction. A semicircular or semi-elliptical pit (calcarone) about 33 feet in diameter and 8 deep is dug into the slope of a hill, and the sides are coated with a wall of stone. The sole consists of two halves slanting against each other, the line of intersection forming a descending gutter which runs to the outlet. This outlet having been closed by small stones and sulphate of lime cement, the pit is filled with sulphur ore, which is heaped up considerably beyond the edge of the pit and covered with a layer of burnt-out ore. In building up the heap a number of narrow vertical passages are left to afford a draught for the fire. The ore is kindled from above and the fire so regulated (by making or unmaking air-holes in the covering) that, by the heat produced by the combustion of the least sufficient quantity of sulphur, the rest is liquefied. The molten sulphur accumulates on the sole, whence it is from time to time run out into a square stone receptacle, from which it is ladled into damp poplar-wood moulds and so brought into the shape of truncated cones weighing 110 to 130 lb each. These cakes are sent out into commerce. A calcarone with a capacity of 28,256 cubic feet burns for about two months, and yields about 200 tons of sulphur. The immense volumes of sulphurous acid evolved give rise to many complaints ; all the minor pits suspend work during the summer to avoid de-struction of the crops. A calcarone that is to be used all the year round must be at least 220 yards from any inhabited place and 110 from any field under cultivation.

The yield of sulphur, as seen from the table given above, is miser-ably small, but the scarcity of fuel in Sicily almost prohibits the introduction of any more rational method. As sulphur fuses at 114° C, high-pressure steam at once suggests itself as a suitable medium of heating. In the sulphur-works of Latera, in the pro-vince of Rome, the following apparatus (constructed by Gritti) is being used with success. A vertical truncated perforated cone of thick sheet-iron serves for the reception of the ore. This cone is enclosed in a similar cone of iron, which terminates in a detachable deep iron basin below, and is provided with a tightly fitting lid. All the joints in this outer shell are steam-tight. The inner cone having been charged and the lid secured, steam of sufficient pressure to ensure a temperature of from 125° to 135° C. is blown into the apparatus, which soon causes the sulphur to melt and collect in the basin below. After from 30 to 50 minutes, reckoning from the time when the above temperature is reached, the operation is completed. The steam is then turned off and the sulphur made to run from the basin into a receptacle beside the apparatus, to be cast into sticks or cakes. The iron basin is then detached, and by turning aside an iron damper which held the ore in its place the exhausted ore is made to drop into a pit. Each charge of ore amounts to about from 25J to 26J cwts., containing about 385 tt> of sulphur. Of this some 360 lb are recovered as saleable sulphur, at the ex-pense of about 286 lb of oak-wood as fuel. Extrac- R. E. Bollmann in 1867 proposed to extract the sulphur by tion. means of bisulphide of carbon. The process, after having been tried at Bagnoli near Naples and given up as hopeless, was intro-duced in 1873 in Swoszowice near Cracow under the guidance of "Winkler and has proved a success. The apparatus is constructed so that the bisulphide used in the process of extraction is recovered by distillation ; the loss of bisulphide amounts only to one-half per cent., sometimes to less, and the sulphur produced is very pure. But by far the greater part of the purer qualities of commercial sulphur is produced from Sicilian calcarone sulphur by distillation, which removes the 3 per cent, or so of earthy impurities contained in it. The following apparatus (invented originally by Michel of Marseilles and improved subsequently by others) enables the manu-facturer to produce either of two forms of " refined " sulphur which commerce demands. It consists of a stone-built chamber of about 2825 cubic feet capacity, which communicates directly with two slightly slanting tubular retorts of iron, each of which holds about 660 lb of sulphur. The retorts are charged with molten sulphur from an upper reservoir, which is kept at the requisite temperature by means of the lost heat of the retort fires. The chamber has a safety valve at the top of its vault, which is so balanced that the least surplus pressure from within sends it up. The first puff of sulphur vapour which enters the chamber takes fire and converts the air of the chamber into a mixture of nitrogen and sulphurous acid. The next following instalments of vapour, getting diffused throughout a large mass of relatively cold gas, condense into a kind of "snow," known in commerce and valued as "flowers of sulphur" (flores sulphuris). By conducting the distillation slowly, so that the temperature within the chamber remains at a sufficiently low degree, it is possible to obtain the whole of the product in the form of "flowers." If compact (" roll") sulphur is wanted the distilla-tion is made to go on at the quickest admissible rate. The tempera-ture of the interior of the chamber soon rises to more than the fusing-point of sulphur (114° C.), and the distillate accumulates at the bottom as a liquid, which is tapped off from time to time to be cast into the customary form of rods of about 1J inches diameter.

In some places sulphur is extracted from iron pyrites by one of two methods. The pyrites is subjected to dry distillation from out of iron or fire-clay tubular retorts at a bright red heat. One-third of the sulphur is volatilized—8FeS2 = Fe3S4 + S2—and obtained as a distillate. The second method is analogous to the calcarone method of liquation : the ore is placed in a lime-kiln-like furnace over a mass of kindled fuel to start a partial combustion of the mineral, and the process is so regulated that, by the heat generated, the unburnt part is decomposed with elimination of sulphur, which collects in the molten state on an inverted roof-shaped sole below the furnace and is thence conducted into a cistern. Such pyrites sulphur is usually contaminated with arsenic, and consequently is of less value than Sicilian sulphur, which is characteristically free from this impurity. Milk of The substance known as "milk of sulphur" (lac sulphuris) is sulphur, very finely divided sulphur produced by the following, or some analogous, chemical process. One part of quicklime is slaked by means of 6 parts of water, and the paste produced diluted with 24 parts of water ; 2'3 parts of flowers of sulphur are added ; and the whole is boiled for about an hour or longer, when the sulphur dissolves,—_ 3CaO + 12S = 2CaS3 + CaS203. The mixed solution of pentasulphide and thiosulphate of calcium thus produced is clarified, diluted more largely in a tub, and then mixed with enough of pure dilute hydrochloric acid to produce a feebly alkaline mixture; this shows that only the bulk of the pentasulphide is decomposed,—CaS5 + 2HC1 = CaCl2 + H2S + (4S of precipitated sulphur). The addition of more acid would produce an additional supply of sulphur (by the action of the H»S203 on the dissolved H2S); but this thiosulphate sulphur is yellow and compact, while the CaS5 part has the desired qualities, forming an extremely fine, almost white, powder. The precipitate is washed, collected, and dried at a very moderate heat. It is used as a medicine. If sulphuric acid is used instead of hydrochloric acid the preparation is apt to be contaminated with hydrated sulphate of lime. In the United Kingdom, indeed, precipitated sulphate of lime used to be added intentionally to produce what the public had got accustomed to ; but this practice has been rightly stopped by the authorities.

During the year 1875 the production of sulphur in Europe is Produc-
stated to have been as follows :— tion and
Tons. uses.
Italy 360,000
Spain 4,000
Austria-Hungary 3,750
German empire (including 5000 tons of regenerated 1 sulphur) 14,500
Belgium 450
Total 3S2.700

By far the greater part of all the sulphur produced in Sicily and elsewhere is used for the manufacture of sulphuric acid. Subjoined is an enumeration of some other applications. (1) The manufacture of gunpowder (see vol. xi. p. 320). (2) The taking of casts. (3) The making of cements : (a) a mixture of molten sulphur and ferric oxide is used to cement the isolating bells to telegraph posts ; (b) a mixture of iron filings (100), flowers of sulphur (3 to 20), and sal-ammoniac (3 to 5) made into paste with water is used to cement iron bars (fences, &c. ) into stone sockets ; (c) a mixture of molten sulphur with powdered quartz or glass has been recommended as an acid-proof material for sulphuric acid chambers ; (d) a mixture produced by the incorporation of powdered quartz and colouring matters, such as vermilion, &c., with molten sulphur is employed for ornamental articles. (4) The vulcanization of india-rubber (see vol. xii. p. 840 sq.). (5) Dusting vine-plants with flowers of sul-phur is said to keep off the fungus Oidium Tuckeri, which has caused such devastation in the vineyards in Eranco and elsewhere.

Sulphur Compounds.

Sulphuretted hydrogen, H2S (see CHEMISTRY, vol. v. p. 499 sq.), Sulphur-is used largely as such, or as sulphide of ammonium, (NH4)2S etted = 2NH3 + H2S, for the detection, discrimination, and separation of hydro-metals. To give an example : the least quantity of lead dissolved gen. in water as (say) nitrate can be detected by the addition of sulphur-etted hydrogen, which brings down the lead as a black precipitate of sulphide of lead,—Pb(N03)2 + H2S = PbS + 2HN03. The presence of a moderate quantity of mineral acid in the original solution does not interfere with the test. What we said of solution of salts of lead holds substantially of those of the following groups of metals. The formulas and the colours of the sulphides are given in brackets. A. Lead (black, PbS), silver (black, Ag2S), mercury as mercurous or mercuric salt (black, HgS + Hg or HgS respectively), copper (greenish black, CuS), bismuth (brown, Bi2S3), cadmium (yellow, CdS). B. Arsenic (yellow, As2S3), antimony (orange-red, Sb2S3), tin as stannic salt (yellow, SnS2). The sulphides A are insoluble ; the sulphides B are soluble in sulphide of ammonium solution, and the latter, from this solution, can be reprecipitated by acidification with dilute sulphuric or hydrochloric acid. The brown SnS pre-cipitated from stannous salts is insoluble in the (colourless) solution of (NH4)2S, but soluble in the yellow solution of the polysulphide (NH4)2S2, as SnS2. C. The following metals are not precipitated from their salt solutions if these are acidified sufficiently by added mineral acid ; but they are precipitated from their neutral or alkaline solutions by sulphide of ammonium :—iron (black, FeS), nickel (black, NiS), cobalt (black, CoS), manganese (flesh-coloured, MnS), zinc (white, ZnS). Aluminium and chromium, given as salts of their oxides, R203, are precipitated by sulphide of ammonium as hydrated oxides (A1203.aH20, colourless; Cr203.a;H20, green or violet). The reagent acts on these as ammonia, NH3, the H2S being liberated, and behaves in a similar way to acid solutions of certain salts, e.g., the phosphates, of the following group D, these salts, e.g., Ca3P208, being precipitated as such. The ordinary salts of group D (barium, strontium, calcium, magnesium), and the salts of the alkali metals E (potassium, sodium, &c.) generally, give no precipitate with either sulphuretted hydrogen or sulphide of ammonium. It is easy to translate what we have stated into a method for the separation of groups A, B, C (D and E), from one another.





Of the three chlorides treated of in CHEMISTRY (vol. v. p. 501) Chlor-only the lowest, S2C12, is of industrial importance. It is prepared ides, by passing perfectly dry chlorine gas over heated sulphur contained in a retort, the retort being connected with a condenser constructed so that the uncondensed vapours are led away into the chimney. The two elements unite readily, and chloride of sulphur, S2C12, distils over, contaminated, however, by more or less of surplus chlorine present as higher chlorides. To remove (or decompose) these the crude product is subjected to fractional distillation ; the thermometer rises rapidly and soon becomes constant (at about 136° under 758 mm. pressure). What afterwards distils over, at the constant boiling-point, is collected as pure S2C12,—a yellowish red liquid of l-68 sp. gr. at 16°'7 C. and D7055 at 0° (Kopp), which emits fumes of hydrochloric acid in moist air. Its smell is characteristic and unpleasant. Chloride of sulphur is decomposed by water, alcohol, ether (see CHEMISTRY) ; and benzol and bisulphide of carbon mix with it in all proportions without decomposition. A mixture of 100 parts of bisulphide of carbon and some 2-5 of chloride of sulphur is used for the vulcanization of (chiefly sheet) india-rubber. The mixture is readily imbibed by the rubber, which when allowed to dry (at from 22° to 25° C.) gives up the bisulphide of carbon and the chlorine of the reagent, the latter as HC1, but, retains its sulphur in a state of chemical combination. Sulphur- The gas S02 (see CHEMISTRY, vol. v. p. 501), produced extempore ous acid, by the combustion of sulphur, is used for the bleaching of silk, wool, straw, and wicker work, also for the disinfection of rooms and of wine-casks (to prevent acetous fermentation). A solution of the gas in water is manufactured industrially, for use chiefly in the manufacture of sugar. It is added to the beetroot or cane juice to prevent its fermentation while awaiting concentration. A solution of "bisulphite of lime" (produced by saturating milk of lime with sulphurous acid gas) is much used as an antiseptic generally. Liquefied sulphur dioxide has found an application as a frigorilic for the manufacture of ice. The apparatus used is so constructed that the volatilized sulphur dioxide is all caught and recondensed. Sulphurous acid when required as such or for the making of sulphites is always produced, even industrially, from oil of vitriol, by reduction with either sulphur or charcoal. In the heat the reactions are 2803 + 8 = 3803 and 2S03 + C = C02 + 2S02 respectively, and either can be (and is) executed practically in cast-iron vessels. The presence of carbonic acid in the gas produced by the charcoal process does not interfere with the preparation of sulphites.

Thiosul- The soda salt Na2S203 + 5H20, known commercially as hypo-phates. sulphite of soda, is used industrially for chiefly two purposes, namely, (1) as a solvent for chloride of silver in photography (se PHOTOGRAPHY),—AgCl + Na2S203=NaCl + AgNaS203,—and (2) as an '' antichlor " in paper-making, to destroy the remnants of chlo-rine in bleached paper pulp. To understand its action we need only know that chlorine and water in such cases act like oxygen,— Cl2 + H20 = 2HCl + 0 ; every 4x0 thus produced converts one S202 of iSTa20S202 into 2S03 of sulphuric acid. For the preparation of this salt a great many methods have been invented. The simplest to explain is the treatment of a solution of normal sulphite of sodium with sulphur,—S03Na2 + S = S203iSra2. Instead of adding free sulphur, Liebig prepares a solution of polysulphide of sodium (by dissolving sulphur in caustic-soda ley) and adds it to the sulphite. The surplus sulphur combines with the sulphite ; besides, the poly-sulphide contains thiosulphate from the first. Another method is to pass sulphurous acid through a solution of sulphide of sodium. Here, by first intention, if we may say so, sulphite of sodium and H2S are produced ; but the H2S and the excess of S02 give water and sulphur, and two-thirds of this sulphur unite with the sulphite first formed into thiosulphate. The crude sulphide of calcium, which is produced so largely in the Le Blanc process (see SODIUM, supra, p. 243), when exposed to the air gets oxidized, with forma-tion of calcium thiosulphate, which can be extracted by means of water and converted into sodium salt by double decomposition with carbonate or sulphate of soda. Pure thiosulphate of soda forms large transparent monoclinic prisms, which lose no water on exposure to ordinary air in the cold. At about 48° C. they fuse into a liquid, which may remain liquid on cooling, but solidifies sud-denly when a fragment of the solid salt is dropped in. 100 parts of water dissolve
at 16° 25° 35° 45° C.
65 75 89 109 parts of the salt (Mulder).

The solution is not subject to oxidation in the air.

Sulphuric acid.

The anhydride S03 is used largely in the manufacture of tar colours. Oil of vitriol is decomposed by dropping it on a mass of platinum scrap kept at a bright red heat within a fireclay retort, —S04H2=H20 + S02 + -J02; and, after removing the water—the bulk by partial condensation and the rest by means of vitriol—the sulphur dioxide and the oxygen are made to recombine by passing them over platinized asbestos at a dull red heat. The fumes of S03 formed are condensed in a dry receiver by application of cold from without (Winkler's process).

The fact that finely divided platinum, in virtue of its power of condensing oxygen, induces the union of S02 and J02 into S03 has been known for a long time ; but all attempts to utilize the reaction for the production of sulphuric acid from a mixture of sulphur dioxide, air, and nitrogen produced by the combustion of sulphur or pyrites in air have failed. The platinum acts too feebly in the presence of the unavoidably large mass of nitrogen, and soon loses its efficacy altogether owing to the accumulation on it of particles of incombustible matter from the kiln gases. Oxide of chromium, Cr203, and oxide of iron, Fe203, act like platinum, through transi-tory formation of the respective sulphates—the gases produced in pyrites kilns include a considerable quantity of ready-made S03 —but they also are not available practically for the making of sul-phuric acid. In short, all attempts to produce this reagent other-wise than by means of the old Nbrdhausen or the chamber process have so far been unqualified failures industrially. In regard to the chamber process we may add a few notes to what has been said under CHEMISTRY (vol. v. p. 503 sq.). As stated in that article, of pro-
nitrous acid, N203, when brought into contact with sufficiently duction.
strong vitriol unites with it, giving rise to bodies similar to chamber
crystals,— N203 + H20 + 2S03 = 2S02^Q ;
or, what comes practically to the same,
N208 + S03(out of the vitriol) = S02^°2.





In the presence of sufficient water this union does not take place, because the water causes the product to break up as shown by the equation if read from right to left. These facts explain wdiy a stronger acid than one containing some 60 per cent, or so of real H2S04 cannot be produced directly in the chamber. This inconvenience has led, in the hands of Gay - Lussac, to an important improvement on the original process. He inserts between the chamber outlet and the chimney a tower made of acid-proof stone and filled with pieces of coke, over which concentrated oil of vitriol is made to trickle down wdiile the chamber gases ascend through the tower on their way to the chimney. The vitriol absorbs all or most of the N203, which would otherwise be lost. But the practical reliberation of the N203 was beset with very great difficulties, which have been fully overcome only by a more recent invention of Glover's. He places between the kiln and the entrance side of the chamber a tower similar in construction to Gay-Lussac's, which the kiln gases have to traverse before they get into the latter. Through the tower he runs at the same time a stream of nitrated (Gay-Lussac) acid and one of ordinary chamber acid. The latter acts on the nitrated acid as water ; at least it virtually sets free the combined nitrous acid, so that it is reduced by the sulphurous acid coming from the kiln to nitric oxide, which travels into the chamber with the rest of the gases to do duty there in the well-known manner. As the kiln gases are very hot, a considerable quantity of the water which goes through a Glover tower (as chamber acid) is volatilized and thus made to supply part of the steam necessary for the process. The Glover tower, besides fulfilling its primary object, serves to concen-trate part of the chamber acid and to supply part of the neces-sary steam without expense for fuel. The expenditure of nitrate of soda, which before the introduction of the two towers used to amount to from 8 to 13 parts per 100 of sulphur burned, has been reduced to from 3'5 to 6'5. The actual loss of nitrous acid of course is the less, cxterisparibus, the larger the chamber, and (for a given chamber) the greater the care with which the process is conducted. But even under the most skilful management more nitrous acid is lost than can be accounted for by the unavoidable imperfections in the apparatus and in the mode of working them. From the in-vestigations of Weber and of Fremy it appears that, in the presence of relatively much water more especially, part of the nitrous acid suffers reduction, not to nitric, but to nitrous oxide, K20, which, being unsusceptible of direct oxidation, is lost for the process.

For a great many purposes (e.g., the manufacture of " superphosphate" from bones or mineral phosphate of lime) the 60 to 64 pertion of cent, acid which comes out of the chamber can be used as it is ; strong but it is not strong enough for all purposes. In the production of acid, stronger (from chamber) acid the first step always is to run the acid into long, very shallow lead pans and to simply boil it down in these, either by the application of heat from below, in which case the bottoms of the pans must be protected by making them rest on plates of iron, or by enclosing the pans in a vault and causing the hot gases of a furnace fire to strike along the surface of the acid. The result in either case is that, while more and more water goes away as steam, the residual acid of course gets stronger and stronger. But with the strength the boiling-point rises, and, as necessary consequences, the extent to which the acid attacks the lead (with formation of sulphate and sulphurous acid) and the danger of melt-ing down the pans by local overheating become greater and greater. When the acid has come up to about from 78 to 80 per cent, (corresponding to a specific gravity of 1'7 after cooling), it is not safe to push the concentration any further, quite apart from the fact that an acid of 80 per cent, when boiled down emits a very appreciable proportion of acid along with the volatilized water. An acid of 17 indeed is amply strong enough for a variety of applications, such as, for instance, the conversion of salt into sul-phate. If a stronger acid is wanted the concentration must be continued in glass or platinum retorts.

The vitriol maker's glass retort, as a rule, consists of two detachable parts, namely, a pear-shaped body about 31 feet high and nearly 2 feet in diameter, and a glass alembic whose wider end fits glass the mouth of the pear, while its narrower outlet end points downwards and terminates within a slightly slanting lead-pipe, which conveys the distillate to a leaden tank. The retort rests on a layer of sand contained in a closely fitting iron basin, and the lateral space between the two is filled completely with sand. The iron basin is suspended within a furnace in such a way that only it, and not any part of the retort, is touched directly by the flame. As a rule, some twelve retorts stand side by side, each in its own sandbath, and are heated by the same fire. As the temperature of the boiling liquid and of the vapour rises at the end to beyond 300° C, a sudden draught of cold air might cause rupture of a retort; the apparatus is therefore placed in a special room accessible only through double doors, and the inner door is not permitted to be opened before the outer has been shut. The acid, as it is boil-ing down, gets stronger and stronger, because, although the vapour is very strongly acid from the first, its percentage p' of real H2S04 at any given stage is less than the value p, which obtains in the boiling liquid as it is at the time, p' at a given barometric pres-sure is a fixed function of^> only, and increases asp increases ; the difference p -p' accordingly gets less and less. It becomes nil, not when the acid has become pure H2S04, but when it has come up to the composition 12S03 +13H20 (Marignac). This particular hydrate only boils without change of composition; even pure H2S04 when distilled, by giving up more than ISO., for 1H20, becomes reduced to that hydrate 12S03 + 13H0, which then boils without further change of composition. A stronger acid than "Marignac," as we may call it, cannot be produced by the concentration of weaker acid, and even its production (from 1'7 acid) involves a very con-siderable loss of acid as distillate. Hence practically the process is stopped when the acid in the retorts has come up to some 96 per cent, of H2S04, which is ascertained by the specific gravity of the last runnings being at a certain value. As soon as this point is reached the retorts are allowed to cool till the contents can be withdrawn with safety by means of lead siphons into glass carboys. This, however, means a considerable loss of time and fuel; besides, the process of distilling from out of glass vessels is not free from danger, and for these reasons it is preferred in many establishments to con-centrate the pan acid in large platinum stills, although these are extremely expensive. The great advantage of the platinum still is that it admits of continuous working ; while pan acid (containing say 1 lb of water per AT lb of full strength—96 per cent, or so—acid) runs in, and a far weaker acid (containing for the same period of time 1 lb of water and n tb of full strength acid) is distilling over, the balance N-n lb of finished acid is being withdrawn by means of a platinum siphon. The outer limb of the siphon in its middle portion divides into a system of four narrower tubes and is cooled down by means of a cold-water jacket surrounding it, so that the acid can be run directly into carboys. Platinum The platinum retort in its latest form has a large undulating retort. bottom made of strong metal, on which a rapidly converging low body joins, made of thinner metal because it is not so directly ex-posed to the flame. Along with this still a fiat platinum pan is used with an undulating bottom similar to that of the still for the preliminary concentration of the acid. As platinum is not liable to fuse or be attacked by any strength of boiling acid, a relatively small platinum pan does as much work as a far larger one made of lead.

Sulphates.

Several of these are treated of under the heads of the respective bases. Thus, for the sulphates of ammonia, see NITROGEN, vol. xvii. p. 515 sq.; for POTASSIUM and SODIUM, see these articles ; for calcium, see LIME (vol. xiv. p. 648) and GYPSUM (vol. xi. p. 351); for barium, see BARYTES (vol. iii. p. 406); for magnesium, see ErsoM SALTS (vol. viii. p. 496) and MAGNESIUM (vol. xv. p. 217); and for iron, see COPPERAS (vol. vi. p. 352).

Sulphate of aluminium, A12(S04)3 +18H20, the active ingredient of ALUM (vol. i. p. 643), is now being produced industrially in a state of perfect freedom from iron, and is more and more taking the place of alum. Paper-makers, at least, no longer use anything else for the production of alumina soap, wdiich in machine-made paper serves as the principal ingredient of the size. The crude salt is easily produced by treatment of relatively pure bauxite (native hydrated alumina) or china clay with chamber acid at a suitable temperature. The resulting mass is dissolved in water, the undissolved matter (silica, &c.) allowed to settle, the clear liquor drawn off, and from it an apology for what is wanted is obtained by evaporation'to a small volume and allowing to crystallize. But the salt thus obtained is always contaminated with a variety of foreign sulphates, including sulphate of iron, and this last-named impurity, for the majority of applications, cannot be suffered to remain. One of the best methods for its removal, if not the best, is that discovered by Semper and Eahlberg : the solution, which must contain all its iron as ferric salt and contain somewhat less than the normal proportion of sulphuric acid, is digested with hydrated binoxide of lead. In the course of about a week all the iron is completely precipitated. The better qualities of sulphate of alumina nowadays have at most only a few thousandths per cent, of iron.

Sulphate of copper (blue vitriol) is made technically in chiefly two ways. One method is to heat metallic copper to redness in until it is almost completely oxidized, and to dissolve the oxide by means of dilute sulphuric acid. The Cu20 present behaves like a mixture of metal and CuO. Another process starts from the sub-sulphide Cu2S (produced metallurgically as "mat," or perhaps ex-pressly from its elements), and converts this into sulphate and oxide by careful roasting. The product is dissolved in dilute sul-phuric acid. Large quantities of blue vitriol are produced incident-ally in the " parting " of auriferous silver (see GOLD, vol. x. p. 749) by means of oil of vitriol. Sulphate of copper crystallizes from its aqueous solution in large transparent blue crystals of the triclinic system ; their composition is CuS045H20. The crystals are stable in the air. At 100° C. they lose 4H20, the last H20 requiring a temperature of 200° C. for its expulsion. The anhydrous salt is dirty white ; it readily reunites with water, and consequently is available as a dehydrating agent, for instance, for the preparation of absolute alcohol from spirit of wine. 100 parts of water dissolve at 0° 10° 20° 50° 100° C.
31-6 37-0 42-3 65-8 203-3 parts of crystallized salt (Boggiale). The salt is insoluble in alcohol. Blue vitriol is used largely in electrotyping and for many other purposes.

Subjoined are two general tests for sulphur. (1) All sulphur Analysis, compounds when brought in contact at a red heat with a mixture of nitre and carbonate of soda (or some other equivalent alkaline oxidizing mixture) are changed so that the sulphur assumes the form of alkaline sulphate, which can be extracted by means of water. From the (filtered) solution the S03 is precipitated by addition of chloride of barium as BaS04,—a white powdery precipitate characteristically insoluble in water and in dilute acids. (2) Any non-volatile sulphur compound, when heated on charcoal in a reducing flame with carbonate of soda, yields sulphide of sodium (" hepar "), which, when moistened with water on a silver coin, produces a black stain of metallic sulphide. (Compare SELENIUM, vol. xxi. pp. 631-632.) (W. D.)


Footnotes

This chemical element has already been treated in its scientific aspects under CHEMISTRY (vol. v. p. 498 sq.). The present article is intended to supplement what is there given, in the direction chiefly of practical applications.

1 See SODIUM, "Le Blanc process for making soda-ash," p. 243 above.




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