SULPHUR. —Sulphurous Acid.

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bismuth, and copper, black precipitates; antimony,orange; arsenic, tin, and cadmium, yellow. The follow-ing metals, though not precipitated from their acidi-fied solutions, are thrown down as sulphides whensulphuretted hydrogen is passed through theiralkaline solutions—nickel, iron, cobalt, yellow; man-ganese, flesh-coloured; zinc, white. The sulphides ofbarium, strontium, calcium, lithium, sodium, andpotassium, are soluble in water.

Compounds of Sulphur and Oxygen .—Sulphur formstwo compounds with oxygen — sulphurous oxide,S0 2 , and sulphuric oxide, S0 3 . The latter will betreated in the article on Sulphuric Acid .

Sulphurous Oxide, Sulphur Dioxide , Sulphurous Anhy-dride, Sulphurous Acid , S0 2 .—The irritating vapourOr gas given off by burning sulphur. It is atransparent colourless gas, thirty-two times heavierthan hydrogen; it is capable of assuming theliquid form, like carbonic anhydride, sulphurettedhydrogen, and sundry other gases, at an increasedpressure with a very low temperature. Faradayliquefied it with a pressure of from three to fiveatmospheres. Its boiling point is —10° C. (14°Fahr.). The rapid evaporation of the liquid anhydrideunder an air-pump generates enough cold to freezeit. Solid sulphurous anhydride is heavier than theliquid, and melts at about ■—79° C. Sulphurousanhydride may be produced either by the oxidationof sulphur or the abstraction of oxygen from sul-phuric acid.

When sulphur is made to burn in oxygen gas orcommon air sulphurous anhydride is obtained. Ifit is desired to pass a stream of the gas throughany liquid intended to absorb it, the sulphur mustbe burnt in a close vessel or apparatus, which has astrong aspirator attached; the vessel containingthe liquid to be treated being placed betweenthe gas generator and the aspirator. The methodgenerally adopted for procuring sulphurous oxide isthat of depriving sulphuric acid of one equivalentof oxygen by any convenient reagent. A mixtureof strong oil of vitriol and sulphur heated in aflask, which has a delivery tube attached, servesvery well.

Sulphurous anhydride as nearly pure as possible,in fact, quite pure if made to traverse a little waterto take up any sulphuric acid carried over, may beobtained by heating sulphuric acid with one-third itsweight of copper turnings or clippings. The re-action is as follows—•

2H a S0 4 + Cu = CuS0 4 + 2H s O + S0 2 .

Mercury may be substituted for the copper, whena like reaction takes place. As water dissolves atthe ordinary temperature thirty-three times its bulkof sulphurous anhydride, the gas cannot be collectedover water. Its high specific gravity, more thandouble that of air, allows the method of displacementto be employed, and it may also be collected bymeans of the mercurial trough.

Sulphurous acid, II 2 S0 3 , has been obtained incrystals by cooling a saturated solution of the acidobtained by passing the anhydride into water. A

hydrated sulphurous acid is also known. The acidis considered to exist in all solutions of sulphurousanhydride in water. The acid and the anhydrideare powerful reducing agents. Acid solutions ofthe manganates and chromates are decolorized bythem—in the latter case green chromous salts areformed. For analyzing the commercial sulphites,it is only necessary to weigh out a stated quantityof the sulphite or hyposulphite to be examined,dissolve it in water, make it acid with hydrochloricacid, and then to note the number of measures of anormal solution of potassium bichromate required tobe added to convert the sulphurous into sulphuricacid, and data are obtained, from which the amountof sulphurous acid in the sample may be easilycalculated. This reducing property of sulphurousacid has been turned to advantage by Bunsen inthe indirect analysis of various bodies ; for instance,in the valuation of manganese. He places themanganese in a flask, treats it with hydrochloricacid, and passes the evolved chlorine into a solutionof potassium iodide; each atom of chlorine sets freean atom of iodine. When the operation is over, hedisconnects the vessel containing this iodine, andadds from a burette a normal solution of sulphurousacid until the precipitated iodine has disappeared.Two equivalents of iodine convert an equivalent ofsulphurous into sulphuric acid, as follows—■

II 2 S0 3 + HjO + 21 = H 2 S0 4 + 2 HI.

Thus, tracing the reaction back, one equivalent ofsulphurous acid represents an equivalent of binoxideof manganese. This principle is applied in a varietyof ways. The deoxidising power of sulphurous acidis taken advantage of by the chemist in the estimationof arsenic. When arsenic exists as arsenic acid, it isvery slowly thrown down by sulphuretted hydrogen,whereas arsenious acid is easily precipitated. Sul-phurous acid possesses the power of abstractingoxygen from arsenic acid, and reducing it toarsenious acid, so that if a stream of sulphurousanhydride be passed through the arseniate to beanalyzed, after making it acid with hydrochloricacid, it is converted into arsenious acid, whichfacilitates its precipitations when one comes toemploy sulphuretted hydrogen.

Sulphurous anhydride passed through a dilutesolution of ferric chloride quickly reduces it to theferrous salt—

Fe 2 Cl 6 + S0 2 + 2H 2 0 = 2FeCl 2 + H 2 S0 4 + 2HC1.

Sulphurous acid reduces salts of gold, throwingdown the gold in the metallic state. It is capableof taking up the second equivalent of oxygen fromalmost every metallic binoxide.

Sulphites .—-Acid sodium sulphite was at one timean article of commerce under the half-fanciful andhalf-true appellation of antichlore. It is now almostsuperseded as such by sodium hyposulphite. Thename antichlore is derived from the use to which it isapplied, viz., to correct the evil effects of an over-dose of chlorine. Iu the bleaching of paper pulp, ifany excess of chlorine or hypochlorite has been uspd,