time whether it still continues to precipitate or trouble solution of nitrate of silver. As soon as this ceases to be the case, change the receiver, and distil until a trifling quantity only remains in the retort. Dilute the distillate with water until the specific gravity of the diluted acid is 1.2.

b. Dilute crude nitric acid of commerce of about 1.38 specific gravity, with two-fifths of its weight of water; add solution of nitrate of silver as long as a precipitate of chloride of silver continues to form, [and in considerable excess besides. Mulder.-The solution of a coin in the same acid may be economically employed instead of pure nitrate.] Let the precipitate subside out of access of light, decant the perfectly clear supernatant acid into a tubulated retort; add some nitrate of potassa, free from chlorine, and distil until only a small quantity remains, taking care to attend to the proper cooling of the fumes distilling over. Dilute the distillate, if necessary, with water until it has a specific gravity of 1.2.

Tests.-Pure nitric acid must be colorless, and leave no residue upon evaporation on platinum foil. Addition of solution of nitrateof silver, or of nitrate of baryta, must not cause the slightest turbidity in it. It is advisable to dilute the acid with water before adding the reagents, as otherwise nitrates may precipitate.

Uses.-Nitric acid serves as a chemical solvent for metals, oxides,. sulphides, oxygen salts, &c. With metals, and sulphides of metals, the acid oxidizes the metal present first, at the expense of part of its own oxygen, and then dissolves the oxide, forming a nitrate. Most oxides are dissolved by nitric acid at once as nitrates; and so are also most of the insoluble salts with weaker acids, the latter being expelled in the process by the nitric acid. Nitric acid dissolves also salts with soluble, non-volatile acids, as e. g. phosphateof lime, with which it forms nitrate of lime and acid phosphate of lime. Nitric acid is used also as an oxidizing agent: for instance, to convert protoxide of iron into sesquioxide, protoxide of tin into binoxide, &c.

§ 24.

3. ACETIC ACID (H O, C, H, O =H 0, Ã).

A highly concentrated acetic acid is not required in qualitative analytical processes; the common acetic acid of commerce, which contains 25 per cent. of anhydrous acid, and has a specific gravity of 1.04, answers the purpose.

Tests.-Pure acetic acid must leave no residue upon evapora tion, and-after saturation with carbonate of soda-emit no empy reumatic odor.. Hydrosulphuric acid, solution of nitrate of silver and solution of nitrate of baryta must not impair the transparency

of the dilute acid, nor must sulphide of ammonium, after neutrali. zation of the acid by oxide of ammonium. Solution of indigo must not lose its color, when heated with the acid.

If the acid is not pure, add some acetate of soda and redistil from a glass retort, not quite to dryness; if it contains sulphurous acid (in which case hydrosulphuric acid will produce a white turbidity in it), digest it first with some binoxide of lead or finely pulverized binoxide of manganese, and then distil with acetate of soda.

Uses. Acetic acid possesses a greater solvent power for some substances than for others; it is used, therefore, to distinguish the former from the latter; thus it serves, for instance, to distinguish oxalate of lime from phosphate of lime. Acetic acid is occasionally used also to acidulate fluids where it is wished to avoid the employ. ment of mineral acids.

$25.

4. TARTARIC ACID (2 H O, C, H, O=2 H O, T).

The tartaric acid of commerce is sufficiently pure for the purposes of chemical analysis. It is kept best in powder, as its solution suffers decomposition after a time, a white film forming upon its surface. For use, it is dissolved in a little water with the aid of heat.

Uses. The addition of tartaric acid to solutions of sesquioxide of iron, protoxide of manganese, and various other oxides of metals, prevents the usual precipitation of these metals by an alkali; this non-precipitation is owing to the formation of double tartrates, which are not decomposed by alkalies.

Tartaric acid may therefore be employed to effect the separation of these metals from others, the precipitation of which it does not prevent. Tartaric acid forms a difficultly soluble salt with potassa, but not so with soda; it is, therefore, one of our best reagents to distinguish between the two alkalies.

The bitartrate of soda is even better adapted for the detection of potash than the free acid. It is prepared by dividing a solution of tartaric acid into two equal portions, neutralizing one with carbonate of soda (which is best done by adding solution of the latter to the boiling solution of the acid), adding the other portion of acid, and evaporating the solution to crystallization. For use, one part of the salt is dissolved in about 10 parts of water.

b. HYDROGEN ACIDS AND HALOGENS.

$ 26.

1. HYDROCHLORIC ACID (H CI).

Preparation. Pour a cooled mixture of seven parts of concen. trated sulphuric acid and two parts of water over four parts of

chloride of sodium in a retort; expose the retort, with slightly raised neck, to the heat of a sand-bath, until the evolution of gas ceases; conduct the evolved gas, by means of a double limbed tube, into a flask containing six parts of water, and take care to keep this vessel constantly cool. To prevent the gas from receding, the tube ought only to dip about one line into the water of the flask. When the operation is terminated, try the specific gravity of the acid produced, and dilute with water until it marks from 1·11 to 1.12. If you wish to ensure the absolute purity of the acid, and its perfect freedom from every trace of arsenic and chlorine, you must take care to free the sulphuric acid, intended to be used in the process, from arsenic and the oxygen compounds of nitrogen, according to the directions of § 22. A pure acid may also be prepared cheaply from the crude hydrochloric acid of commerce, by diluting the latter to a specific gravity of 1·12, and distilling the fluid, with addition of some chloride of sodium. If the crude acid contains chlorine, this should be removed first by the cautious addition of solution of sulphurous acid, before proceeding to the distillation; if, on the other hand, it contains sulphurous acid, this is removed in the same way by the cautious addition of some chlorine water. Commercial hydrochloric acid not infrequently contains a very slight trace of chloride of arsenic, derived from arsenical oil of vitriol. To remove this, conduct into the acid a stream of hydrosulphuric acid, decant after standing for a day or two, from the deposited sulphur and sulphide of arsenic, and heat until all hydrosulphuric acid is expelled.

Tests.-Hydrochloric acid intended for the purposes of chemical analysis, must be perfectly colorless, and leave no residue upon evaporation. It must not impart a blue tint to a solution of iodide of potassium mixed with starch paste (chlorine), nor discolor a fluid made faintly blue with iodide of starch (sulphurous acid). Chloride of barium ought not to produce a precipitate in the highly diluted acid (sulphuric acid). Hydrosulphuric acid must leave it unaltered. Sulphocyanide of potassium must not impart the least red tint to the diluted acid.

Uses.-Hydrochloric acid serves as a solvent for a great many substances. It dissolves many metals and sulphides of metals, forming chlorides, with evolution of hydrogen or of hydrosulphuric acid. It dissolves lower and higher oxides in the form of chlorides, the solution being, in the case of the higher oxides, mostly attended with liberation of chlorine. Salts with insoluble or volatile acids are also converted by hydrochloric acid into chlorides, with sepa ration of the original acid; thus carbonate of lime is converted into chloride of calcium, with liberation of carbonic acid. Hydrochloric acid dissolves salts with non-volatile and soluble acids, apparently without decomposing them (e. g., phosphate of lime)

but the fact is that, in cases of this kind, a metallic chloride and a soluble acid salt of the acid of the dissolved compound are formed; thus, for instance, in the case of phosphate of lime, chloride of calcium and acid phosphate of lime are formed. With salts of acids forming no soluble acid compound with the base present, hydrochloric acid forms metallic chlorides, the liberated acids remaining free in solution (borate of lime). Hydrochloric acid is also applied as a special reagent for the detection and separation of oxide of silver, suboxide of mercury, and lead (see § 121); and likewise for the detection of free ammonia, with which it produces in the air dense white fumes of chloride of ammonium.

§ 27.

2. CHLORINE (Cl) AND CHLORINE Water.

Preparation.-Mix 18 parts of common salt in coarse powder or crystals with 15 parts of finely pulverized good binoxide of manganese; put the mixture in a flask, pour a completely cooled mixture of 45 parts of concentrated sulphuric acid and 21 parts of water upon it, and shake the flask: a uniform and continuous evolution of chlorine gas will soon begin, which, when slackening, may be easily increased again by the application of a gentle heat. This method of Wiggers is excellent, and can be highly recommended. Conduct the chlorine gas evolved, first through a flask containing a little water, then into a bottle filled with cold water, and continue the process until the fluid is saturated. To procure a solution of chlorine entirely free from bromine, the wash-flask and receiving bottle must be changed when about half the gas has passed over. The last portion is pure. The chlorine water must be kept in a cellar, and carefully protected from the action of light; since, if this precaution is neglected, it speedily suffers complete decomposition, being converted into dilute hydrochloric acid, with evolution of oxygen (resulting from the decomposition of water). Smaller quantities, intended for use in the laboratory, are best kept in a stoppered bottle, protected from the influence of light by a case of pasteboard. Chlorine water which has lost its strong peculiar odor is unfit for use.

Uses.-Chlorine has a greater affinity than iodine and bromine for metals and for hydrogen. Chlorine water is therefore an efficient agent to effect the expulsion of iodine and bromine from their compounds. Chlorine serves, moreover, to convert sulphurous acid into sulphuric acid, protoxide of iron into sesquoxide, &c.; and also to effect the destruction of organic substances, in presence of water, as it withdraws hydrogen from the latter, enabling thus the liberated oxygen to combine with the vegetable matters and to effect their decomposition. For this latter purpose it is most advisable to evolve the chlorine in the fluid which contaias the

organic substances; this is effected by adding hydrochloric acid to the fluid, heating the mixture to boiling, and then adding chlorate of potassa. This gives rise to the formation of chloride of potas sium, water, free chlorine, and bichlorate of chlorous acid, which acts in a similar manner to chlorine.

$ 28.

3. NITRO-HYDROCHLORIC ACID. Aqua regia. Preparation-Mix one part of pure nitric acid with from three to four parts of pure hydrochloric acid.

Nitric acid and hydrochloric acid decompose each other, the decomposition resulting, in most cases, as Gay-Lussac has shown, in the formation of two compounds which are gaseous at the ordinary temperature, N O, Cl2 and N O, Cl, and of free chlorine and water. If one equivalent of N O, is used to three equivalents of Cl H, it may be assumed that only chloro-hyponitric acid (N O, Cl), chlorine and water, are formed (N O; + 3 H Cl = N O., Cl2 + Cl + 3 H O).

2

This decomposition occurs as soon as the fluid is saturated with the gas; but it recommences the instant this state of saturation is disturbed by the application of heat or by decomposition of the acid. The presence of the free chlorine, and also, but in a very subordinate degree, that of the acids named, makes aqua regia our most powerful solvent for metals (with the exception of those which form insoluble compounds with chlorine). Nitro-hydrochloric acid serves principally to effect the solution of gold and platinum, which metals are insoluble both in hydrochloric and in nitric acid; and also to decompose various metallic sulphides, e. g., cinnabar, pyrites, &c.

§ 29.

4. HYDROFLUOSILICIC ACID (H Fl, Si Fl). Preparation.-Take quartz sand, wash off every particle of dust, and dry thoroughly. Mix one part of the dry sand intimately with one part of perfectly dry fluor spar in powder; pour six parts of concentrated sulphuric acid over the mixture in a strong glass flask or in a non-tubulated retort, dry inside, and mix carefully by shaking the vessel. As the mixture swells up, when getting warm, it must at first fill the vessel only to one-third. The mouth of the flask or retort must be closed with a perforated cork, or a cap of india-rubber, into which one end of a somewhat wide glass tube, suitably bent at right angles, is fitted air-tight; the other limb reaching to the bottom of a tall flat-bottomed glass jar with just sufficient mercury to allow the end of the tube to dip into it to the extent of several lines; the mercury in the jar is covered with four