arsenious acid, sesquisalts of iron to protosalts, tellurous and selenous acids to metallic tellurium and selenium, &c. Metallic zinc is dissolved by sulphurous acid without any evolution of gas, sulphite and hyposulphite of zinc being formed (Zn1 + 3SO3H2= SO3Zn2+S2O3Zn2+3OH2): if hydrochloric acid be present, hydrosulphuric acid is evolved, and may be detected by paper moistened with acetate of lead. By this reaction the smallest traces of sulphurous acid may be detected (in hydrochloric acid, &c.). Most of the following sulphur-acids exhibit the same reaction. Aqueous sulphurous acid is decomposed by hydrosulphuric acid, pentathionic acid being formed and sulphur deposited (5SH2+5SO3H2= S5O6H2+S5+90H2).

When a solution of sulphurous acid is heated with chloride of tin and hydrochloric acid, yellow sulphide of tin is gradually precipitated: this reaction is rendered more delicate by the addition of a drop of a soluble copper-salt, owing to the formation of black sulphide of copper. In this reaction a piece of paper moistened with acetate of lead should be held to the mouth of the tube, since the hydrosulphuric acid is frequently evolved in the free state. Sulphurous anhydride is readily and completely absorbed by peroxide of lead, sulphate of lead being formed (Pb2O2+ SO2SO4Pb2). If a glass rod moistened with solution of starch and iodate of potassium be introduced into a gaseous mixture containing sulphurous anhydride, blue iodide of starch is at once formed.

Most sulphites are decomposed by heat into a sulphate and a sulphide (4SO3K2=3SO1K2+SK): the earthy sulphites give off sulphurous anhydride, a metallic oxide being left.

Estimation of Sulphurous Acid.-Sulphurous acid is the only oxygen compound of sulphur which (completely in very dilute solutions not containing more than 0·04 p. c. of acid) is converted by iodine into sulphuric acid (SO3H2+H2O+I2=SO4H2+21H). This reaction serves not only for the detection, but, by the use of a standard iodine solution, for the volumetric estimation of sulphurous acid.

Sulphurous acid may also be oxidised into sulphuric acid, and estimated as sulphate of barium.

Blowpipe reactions (see p. 24).

f. Hyposulphurous Acid (Dithionous acid), S2O3H2.- Formed by the digestion of a sulphite with sulphur: by the oxidation of soluble sulphides in contact with the air: by dissolving sulphur in a solution of an alkaline hydrate, when a metallic sulphide is formed simultaneously. It is so unstable an acid that it cannot be obtained in the free state, for its aqueous solution is decomposed spontaneously (if dilute, slowly, if concentrated, immediately) into sulphurous acid and sulphur, the latter of which separates out (S2O3H2SO3H2+S).

Most hyposulphites are soluble in water: the barium-salt is difficultly soluble. Their solutions give with proto-salts of mercury, salts of lead and silver, white precipitates of metallic hyposulphites, which speedily become yellow, brown, and black, especially if heat be applied, owing to the decomposition of the hyposulphite and formation of a metallic sulphide, sulphuric acid remaining in solution (S2O3Ag2 + OH2= Ag2S+SO4H2). With chloride of tin they give a brown, with subnitrate of mercury a black, precipitate of metallic sulphide. Solutions of hyposulphites dissolve chloride of silver, subchloride of mercury, and sulphate of lead. (c) When heated with hydrochloric acid, hyposulphites evolve sulphurous anhydride and deposit sulphur, which in this case is yellow, not white, as it usually is when separated from solutions by chemical action. When treated with iodine they form a metallic iodide and a tetrathionate (2S2O3Ba2+I2=S1O¤Ba2+ 2IBa.) These two reactions distinguish hyposulphurous from sulphurous acid. When treated with hypochlorite of sodium, or chlorine, hyposulphites are completely oxidised into sulphates, even at the ordinary temperature (S2O3Na2+C18+5OH2=2SO1 NaH+8CIH). All hyposulphites are decomposed by heat: those of the alkaline metals into a polysulphide and a sulphate (4S203 K2=S5K2+3SO4K2): others into sulphides or sulphates, with. evolution of sulphurous anhydride, owing to the combustion of separated sulphur. With zinc and hydrochloric acid hyposulphites behave like sulphites.

In order to detect hyposulphurous acid in presence of hydrosulphuric acid or a soluble sulphide, the solution is made neutral

and precipitated by a zinc salt, the sulphide of zinc filtered off, and the filtrate tested for hyposulphurous acid.

Blowpipe reactions (see p. 24).

g. Hyposulphuric Acid (Dithionic acid), S206 H2.- Formed by the action of binoxide of manganese on an aqueous solution of sulphurous acid, a sulphate being always formed simultaneously (2 Mn2O2 + 3SO3H2 = S2O‘Mn2 + SO1Mn2 + 3OH2).

All hyposulphates are soluble in water; hence, their solutions are not precipitated by any reagents. When treated with hypochlorite of sodium in the cold, their solutions are not oxidised into sulphates; but when heated with nitric acid, or hydrochloric acid and chlorate of potassium, they are completely oxidised into sulphates. When heated with a non-oxidising mineral acid (sulphuric acid is best) they are decomposed, a sulphate being formed and sulphurous anhydride evolved, but no sulphur separated: with solid hyposulphates, concentrated sulphuric acid effects this decomposition in the cold. In the dry way, they are decomposed by heat in precisely the same manner.

Hyposulphuric acid is distinguished from sulphuric and sulphurous acids, by the solubility of its barium-salt; from hyposulphurous and the other polythionic acids, by its not separating sulphur when decomposed by acids, and by its giving no precipitate with salts of silver or mercury.

h. Besides those above enumerated, there are other acids formed by the combination of sulphur, oxygen, and hydrogen. They are trithionic acid, S3O6H2; tetrathionic acid, S4O6H2; and pentathionic acid, S5O6H2. These acids resemble each other considerably in their reactions; but as they are very rare acids, that would never be met with in actual analysis, it is not necessary, in a practical work, to do more than indicate their existence. These acids, together with dithionous and dithionic acids, form the series called polythionic acids, i. e. acids containing more than one atom of sulphur.

All polythionates are decomposed when heated with cyanide of mercury, hydrocyanic acid being evolved. In the case of dithionites (hyposulphites), 1 atom of sulphide of mercury is formed to 1 atom of sulphuric acid; in the case of all the rest, I

atom of metallic sulphide is formed to 2 atoms of sulphuric acid. With tetrathionates, 1 atom, with pentathionates, 2 atoms, of sulphur are mixed with the metallic sulphide.

Detection of Sulphur in the dry way, and before the blowpipe.Pure sulphur, when heated on platinum foil, melts and volatilises entirely, burning with a blue flame, and being converted into sulphurous anhydride, SO2, which is easily recognised by its smell. All the foregoing sulphur compounds, without exception, when heated on charcoal in the inner blowpipe flame with carbonate of sodium, or a mixture of 1 part borax and 2 parts carbonate of sodium, form sulphide of sodium, which is easily recognised by dissolving the fused mass in water, and testing the solution with nitroprusside of sodium; by treating the fused mass with a strong acid, when hydrosulphuric acid is evolved; or by moistening the fused mass and placing it on a clean piece of metallic silver, when the surface of the silver is marked with a black stain of sulphide of silver. Any sulphur compound, when heated on a platinum-wire, in the inner or reducing flame, with a bead of carbonate of sodium saturated with silica, gives a yellowish or reddish-brown transparent bead, according to the amount of sulphur contained in the compound.

For the detection of sulphur in organic compounds, the substance under examination is fused with solid caustic potash on a silver plate, on which, after the fused mass is moistened with water, a black stain remains if sulphur be present. Or the compound is decomposed by nitric acid, hydrochloric acid and chlorate of potassium, or by fusion with a mixture of nitre with an alkaline carbonate or hydrate (which, of course, must be free from sulphuric acid); and the resulting acid solution tested for sulphuric acid with chloride of barium. Many organic compounds containing sulphur, are decomposed by merely boiling with a solution of caustic potash, sulphide of potassium being formed, which may be detected either by acetate of lead, or nitroprusside of sodium.

5. Chlorine. Cl. Atomic weight, 35.5.

Found principally in combination with sodium, as chloride of sodium, or common salt, both in solution in sea-water, and in the

solid state as rock-salt. At the ordinary temperature chlorine is a heavy gas of a yellowish green colour, and a most irritating smell: in presence of water it bleaches litmus, indigo, and other vegetable colours. It is readily soluble in water, forming a solution possessing the characteristic colour and smell of the gas, which is commonly called chlorine-water. This solution gives with hydrosulphuric acid a precipitate of sulphur; with nitrate of silver, chloride and chlorate of silver; with acetate of lead, chloride and peroxide of lead. Chlorine-water always contains hydrochloric and hypochlorous acids, owing to spontaneous decomposition. Cl2+ OH2=CIH+CIOH. Hence all the chlorine cannot be removed from chlorine-water by nitrate of silver; for, after the separation of chloride of silver, hypochlorous acid remains in solution, and may be detected by its bleaching action on vegetable colours.

ACIDS AND OXIDES OF CHLORINE. a. Hydrochloric Acid, CIH. Metallic Chlorides. - Hydrochloric acid is a colourless gas, that fumes in contact with the air: it possesses a strong acid reaction and a suffocating smell. It is very readily absorbed by water: a saturated solution of it in water constitutes concentrated hydrochloric acid. This is a colourless liquid, fuming strongly on contact with air. When the concentrated acid is heated, only a portion of the dissolved gas can be driven off, dilute acid remaining. With all basic metallic oxides, hydrochloric acid forms metallic chlorides and water: with peroxides (as well as with chromates, chlorates, and hypochlorites), it evolves chlorine, especially on heating (Mn2 O2+4CIH=2MnCl+2OH2+Cl2): with nitric acid it evolves chlorine (Aqua-regia, p. 8). Pure hydrochloric acid does not bleach vegetable colours.

Metallic chlorides differ considerably in their physical properties. Some are liquid, or of the consistency of butter, and volatile without decomposition (chlorides of antimony, arsenic, tin): others are solid, fusible, and non-volatile (chlorides of silver, lead, potassium, sodium, barium): others are decomposed by heat into chlorine and the metal (chlorides of gold and platinum), or if heated in contact with air, into chlorine and the metallic oxide (chloride of iron). Most chlorides are soluble in water: chloride of silver, ClAg, and subchloride of mercury, ClHg2, are insoluble; subchloride of copper,