Sulphur, however, forms numerous oxidized acid compounds: two of them (sulphurous acid and sulphuric acid) have been long known and employed on a large scale in the arts; the others are less important, and of comparatively recent discovery. Some of these acids of sulphur are interesting, inasmuch as they exhibit a combining ratio different from any which we have as yet considered, and they show the application of the law of multiple proportions to the case of the sulphur, as well as to that of the oxygen which they contain.

The following table exhibits the composition of the various oxyacids of sulphur, the existence of which is at present known. The five compounds which stand last on the list are often spoken of as constituting the polythionic series (from Toλù, many, Oɛov, sulphur), in allusion to the multiple proportion in which the sulphur enters their composition.

We will examine first the sulphurous acid, then the sulphuric acid, and will pass slightly over the other acids, the compounds of which, with the exception of some of the hyposulphites, have as yet received no practical applications.

(410) SULPHUROUS ANHYDRIDE, or Sulphurous Acid : SO2=64; Mol. Vol., (or SO, = 32); Theoretic Sp. Gr. of Gas, 2'2112; Observed, 2247; of Liquid, 138, at 60°; Melting-pt.-105°; Boiling-pt., 13'9 (Regnault; 14° Faraday).-Sulphur burns in oxygen with a lilac-coloured flame, and produces a permanent gas; after the combustion has terminated, and the gas has been allowed to regain its original temperature, the bulk of the gaseous products is found to be the same as before the experiment, but the density of the gas is doubled. This experiment furnishes an easy proof of the composition of the gas; for it is thus shown to contain equal weights of sulphur and oxygen. Sulphurous anhydride is the sole product if the oxygen be dry.

The composition of sulphurous anhydride may be represented in the following way :—

Sp. gr.
1*1056

ΙΟ = 1*1056

By weight.
= 32 or 50

I

By volume.
or o'5 =

=

32

2

64

100 2

SULPHUROUS ANHYDRIDE-MODE OF PREPARATION.

173

Properties.-This gas has a pungent suffocating odour, like that of burning sulphur, and in a concentrated form it is quite irrespirable; but if breathed in a diluted form it produces the symptoms of ordinary catarrh. It is not inflammable, but quickly extinguishes the flame of burning bodies. Sulphurous anhydride combines with water immediately, and becomes converted into sulphurous acid, H2SO3. This compound has never been isolated in a pure state, and a very gentle heat is sufficient to occasion the decomposition of its solution into the anhydride and water. The liquid has a taste and smell similar to that of the gas; the solution gradually absorbs oxygen from the air, and becomes converted into sulphuric acid. A crystalline hydrate of sulphurous acid (SO,, 15 H,O; Schönfield); or (SO,,9 H2O, Pierre) may also be obtained at a low temperature; at 40° this hydrate melts and is decomposed. Water, according to Bunsen, takes up, at 32°, 68.8 times its bulk of the gas; 43'5 times its bulk at 59°; and 32 at 75°. Owing to the solubility of sulphurous anhydride in water, the gas must always be collected either over mercury, or in dry bottles by displacement: from the high density of the gas (double that of oxygen), the latter method is easily applied.

Preparation.-1. When required in a pure state sulphurous anhydride is always prepared by depriving oil of vitriol of part of its oxygen. In order to effect this, two or three ounces of sulphuric acid in a concentrated form may be boiled in a glass retort upon half an ounce of copper clippings or of mercury. The reaction in the case of copper may be seen in the following equation :

The gas must be washed by allowing it to bubble up through a bottle containing a small quantity of water, which retains sulphuric acid and any impurities which might be mechanically suspended in the gas.*

2.-Sulphuric acid may be more economically deoxidized by means of charcoal or dry sawdust, but the gas in this case is accompanied by one-half its volume of carbonic anhydride; €+2 H2SO1 = 2 SO2+ €Ð ̧+ 2 H2O. For most purposes, however, such as the preparation of the alkaline sulphites, the presence of carbonic anhydride is unimportant.

4

According to Maumené, a certain quantity of the subsulphide of copper, Eu,S, is also produced during this operation, after which a mixture of sulphide and oxysulphide of copper is also formed.

174

SULPHUROUS ANHYDRIDE.

3.-Sulphurous anhydride may also be procured readily by a process of oxidation: for example, by heating in a flask an intimate mixture of 4 parts of flowers of sulphur and 5 of finely powdered peroxide of manganese, sulphurous anhydride and sulphide of manganese are produced; S, + MnO, SO, + MnS. A result somewhat similar is obtained by heating a mixture of 3 parts of black oxide of copper with 1 part of sulphur; 2 ¤u✪ + S2 becoming Eu2S + SO2.

=

2

Sulphurous anhydride is emitted abundantly from the craters of volcanoes, and it is occasionally met with in solution in the springs of volcanic districts.

Sulphurous anhydride, by transmission through a tube sur rounded by a mixture of ice and salt, may be condensed to a colourless, transparent, limpid liquid, which dissolves bitumen; it freezes at -105°, forming a transparent, colourless, crystalline solid, heavier than the liquid; in closed tubes, at 60° it exerts a pressure of 2.54 atmospheres. Fig. 302 shows a method of lique

FIG. 302.

D

FIG. 303.

fying sulphurous anhydride. The gas is generated in the flask, A, washed and dried by means of concentrated sulphuric acid placed in the bottle, B, transmitted through the pewter worm, c, which is surrounded by a freezing mixture of ice and salt, and collected in the receiver, D, which is also cooled by a freezing

INDUSTRIAL APPLICATIONS-SULPHITES.

175

mixture; the liquefied compound is stored up for use in small tubes, one of which is shown at E, fig. 303: the tube having been placed in the freezing mixture, the acid is poured into it through a small tube funnel, and the liquid is preserved by drawing off and sealing the tube at the narrow portion in the flame of the blowpipe, whilst the receiver still remains in the freezing mixture.

Uses.-Sulphurous acid possesses considerable bleaching powers, and is extensively employed in bleaching straw and wool, as well as silken goods, isinglass, sponge, and other articles which would be injured by chlorine. The articles to be bleached are moistened, and suspended in closed chambers in which sulphur is burned in an open dish; the sulphurous anhydride is absorbed by the damp goods, and their colour is discharged. The acid appears to act by forming colourless compounds with certain colouring matters. It does not, like chlorine, decompose the colouring matter; for the sulphurous acid may either be expelled by a stronger acid, or it may be neutralized by an alkali, and the colour will be restored: the reproduction of the yellow colour in new flannel, when it is washed with an alkaline soap for the first time, affords a practical illustration of the effect of an alkali upon goods which have been bleached by sulphurous acid, Sulphurous acid is also highly valuable as a disinfecting agent. It is a powerful antiseptic. Meat which has been exposed to the action of the gas, and then sealed up in metallic canisters filled with nitrogen to which a little nitric oxide has been added to remove the last traces of oxygen, may be preserved fresh for years (Hands).

It is, however, principally as a preliminary step in the manufacture of oil of vitriol that sulphurous anhydride is made upon the large scale, and in this case it is always obtained by burning sulphur, or a metallic sulphide, in air.

(411) Sulphites.-Sulphurous acid is a weak dibasic acid. With the alkalies it forms two kinds of salts, one of which is represented by the ordinary sulphite of sodium (Na2SO3, 7 H ̧Ð), while the other class is represented by the acid sulphite of potassium (KHSO1) often called the bisulphite. The sulphites of the alkaline metals are the only ones which are freely soluble in water; but those of barium, strontium, and calcium, are dissolved to some extent by an aqueous solution of sulphurous acid.

The following table shows the composition of some of the sulphites :

Many of the sulphites are decomposed by a strong heat, the acid being gradually expelled. They are also decomposed by sulphuric or by hydrochloric acid, with extrication of sulphurous acid, which is known by its peculiar and pungent odour. The best test for detecting small traces of sulphites consists in the addition of a fragment of zinc and a drop or two of hydrochloric acid to the solution; the sulphurous acid is deoxidized, the sulphur combines with hydrogen, and sulphuretted hydrogen is given off; the gas last named may be detected by suspending a piece of paper moistened with a solution of acetate of lead, in the upper part of the vessel, which should be closed by a glass plate. Salts of silver in solution give a white precipitate with solutions of the soluble sulphites; the precipitate is soluble in excess of the sulphite, and it is partially reduced to metallic silver when the liquid is boiled: a characteristic reaction is the formation with chloride of barium of a white precipitate of sulphite of barium, which is soluble in hydrochloric acid, but the solution thus obtained gives a white precipitate of sulphate of barium on the addition of a solution of chlorine, of iodine, or of bleaching powder. The sulphites, when moist, absorb oxygen from the air; and solutions of these salts are often used as deoxidizing agents: for example, the ferric salts are reduced by them to ferrous salts. Gold, selenium, and tellurium, are precipitated by them, from solutions containing excess of hydrochloric acid, in the reduced or metallic form; arsenic acid is reduced to arsenious acid, and chromic acid to a green salt of chromium.

Sulphurous acid dissolves and is decomposed by the metals which, like zinc, iron, tin, and cadmium, evolve hydrogen with hydrochloric acid. Iron, for example, is rapidly dissolved by sulphurous acid if heated with its solution, sulphite and hyposulphite of iron being formed, whilst the hyposulphite is speedily resolved into sulphide and tetrathionate of the metal, and the tetrathionate in its turn is converted into sulphate of iron and