cooled mixture of sulphuric acid with a little nitrate solution, a deep greenish-brown halo is produced at the junction of the two liquids, as shown in fig. 36. This test is also extremely delicate.

ε. Solution of gold. Gold leaf when boiled in hydrochloric acid remains unchanged, but upon the addition of a little nitrate becomes dissolved wholly or in part. To demonstrate the solution of the gold in the latter case, protochloride of tin may be added, which will give rise to a purplish precipitate or turbidity.

(116.) IN ORGANIC MIXTURES.

a. Solid matters should be digested for some time in cold water, and the liquid filtered off. This method is applicable to stains on clothing, when not of too long standing. Viscid and turbid liquids are simply mixed with water and filtered. The suspected filtrate is next tested with blue litmus paper, and if found to be acid, neutralised carefully with a solution of carbonate or hydrate of sodium, evaporated down to the crystallising point, and set aside. If practicable, the resulting deposit may be collected, dried by pressure between folds of bibulous paper, dissolved in a little warm water, and the solution, filtered if necessary, evaporated, and recrystallised. Lastly, the crystalline residue is to be examined microscopically, and by the several chemical tests mentioned in the preceding section. The acid reaction may be wanting in organic mixtures, through a neutralisation of the nitric acid originally administered.

§ III.-HYDROCHLORIC ACID.

(117.) CONCENTRATED.

a. Appearance. The pure solution is colourless, or of a scarcely perceptible greenish tinge; but the commercial acid has frequently a bright yellow colour, from the presence of perchloride of iron.

B. Volatility. At ordinary temperatures the strong acid evolves colourless, almost transparent fumes, which have a marked

acid reaction, a characteristic smell, and, in common with other acid fumes, become opaque upon admixture with ammoniacal vapour. The liquid acid heated on a watch-glass evaporates without leaving any residue.

y. Action on organic matter. Most organic tissues are gradually corroded and tinged of a yellow colour by immersion in the strong acid; but the stains produced on black cloth are at first distinctly red, and after some days reddish-brown.

d. Want of action on metals. Hydrochloric acid boiled with a little copper or mercury simply evaporates, leaving the metal unchanged, or very nearly so.

ε. Evolution of chlorine. Peroxide of manganese warmed with hydrochloric acid in a test tube produces an abundant evolution of chlorine gas, recognisable by its greenish-yellow colour and irritating smell. It quickly bleaches litmus paper, and produces a purple colouration on starch paper moistened with iodide of potassium.

(118.) DILUTED.

a. Neutralisation by carbonates, &c. Dilute hydrochloric acid is completely volatile, has a marked acid reaction, and dissolves most carbonates with effervescence, forming chlorides, which may be obtained in the solid state on evaporation; the alkaline chlorides, and particularly chloride of sodium, crystallising in cubical or stauroid forms. Chlorine may be liberated from the evaporated residue by the action of sulphuric acid and peroxide of manganese.

B. Precipitation of chloride of silver. Solution of nitrate of silver added to hydrochloric acid throws down a white clotty precipitate of chloride of silver, which subsides readily after brisk agitation, and by exposure to light acquires a grey or purplish colour. One portion of the precipitate may be treated with ammonia, in which it will dissolve, and another portion boiled with nitric acid, by which it will be unaffected, while the remainder may be washed, dried, and ignited in a porcelain capsule, when it will fuse into a horny mass.

L

(119.) IN ORGANIC LIQUIDS OR SOLIDS.

a. Distillation, &c. The liquid having shown an acid reaction to test paper, may be strained or filtered if necessary, and then distilled nearly to dryness from a retort, or a flask to which a cork and delivery tube have been adapted, as in fig. 40.

The earlier portions of the distillate are usually little else than water, but the later portions should manifest all the properties of dilute hydrochloric acid. Solid substances may be digested in distilled water, and the resulting solution examined with test paper and nitrate of silver. If possible, a portion of the liquid. should be evaporated to thorough dryness, and the dissolved residue again tested with nitrate of silver to ascertain the absence or comparative absence of metallic chlorides.

a. Appearance, &c.

(120.) SOLID.

Oxalic acid generally occurs in colourless, more or less well-defined four-sided prisms, which dissolve readily in boiling water to form a solution having a marked acid reaction.

B. Volatility without charring. A few crystals of the acid, when heated upon platinum foil, melt, evolve fumes, and disappear without leaving any carbonaceous residue.

7. Effervescence with manganese. A little peroxide

of manganese, free from carbonates, when added to oxalic acid moistened with water, sets up an active effervescence of carbonic acid gas.

(121.) DISSOLVED.

a. Crystalline form. One or two drops of the strongly acid liquid evaporated cautiously upon a glass plate until a solid white margin appears, yield on spontaneous cooling a crystalline residue of delicate, long flat prisms, as shown in fig 41.

B. Precipitation of oxalate of silver. Nitrate of silver produces in solution of oxalic acid, more abundantly after its neutralisation or partial neutralisation with ammonia, an opaque white precipitate of oxalate of silver, which is not discoloured by ebul

Fig. 41.

lition, but may dissolve if the excess of oxalic acid be large, and is readily soluble in dilute nitric acid. The precipitate collected on a filter, washed, dried, and then ignited upon platinum foil, is dissipated with a slight explosion, leaving an inconsiderable pulverulent residue of metallic silver.

y. Precipitation of oxalate of calcium. Solution of sulphate of calcium added freely to aqueous oxalic acid, throws down a white precipitate of oxalate of calcium, insoluble in acetic, but readily soluble in dilute nitric acid. By ignition, the precipitate is converted into carbonate of calcium, which dissolves in acetic acid with effervescence.

(122.) IN ORGANIC LIQUIDS.

a. Reaction to test paper, &c. The acid reaction is very

decided even when the poison exists but in very small proportion. It is often necessary to filter the liquid, after previous dilution with water or proof spirit, or at any rate to strain it through muslin, before applying any reagent.

B. Precipitation of oxalate of lead. Solution of acetate of lead added to the strained or filtered liquid, throws down a precipitate of oxalate of lead, either white or discoloured by the organic matter present. The addition of the reagent is to be continued until it no longer produces a fresh precipitate. This point is easily ascertained by repeatedly testing the supernatant liquid, which separates readily after briskly agitating the mixture. y. Production of oxalic acid. The above precipitate of oxalate of lead having been collected on a filter and thoroughly

Fig. 42.

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washed, is made into a thin magma with water, and treated with a current of washed sulphuretted hydrogen evolved from some