10. CHLORIDE OF CALCIUM. Ca Cl.

Uses. It precipitates, as a group, from neutral solutions, phosphoric, hydrofluoric, oxalic, tartaric, and citric acids; also, if the solution is not very dilute, sulphuric acid; better, however, in the presence of alcohol. The different aspects and characteristics of the precipitated lime salts, as to their solubility and insolubility, et cetera, enable the analyst to distinguish certain individual acids-see Lime-Water, page 47.

Tests.-Solution of chloride of calcium must be neutral, and ought not to be rendered turbid or discolored by sulphide of ammonium.

11. NITRATE OF SILVER. Ag 0, N05.

Uses. It is a very important reagent for the classification of acids into groups, and is employed for the detection of certain individual acids, especially hydrochloric, phosphoric, and arsenic acids. With the first, it gives a white precipitate insoluble in nitric acid, but soluble in ammonia; from solutions of tribasic phosphates it precipitates lemon-yellow phosphate of silver, soluble, with difficulty, however, in acetic acid; with arsenic acid it produces a reddish-brown precipitate.

Tests. After adding an excess of pure hydrochloric acid, the filtrate must leave no residue upon evaporation, and should neither be colored nor precipitated by sulphide of hydrogen.

12. SESQUICHLORIDE OF IRON. Fe2 Cl3.

Uses. As the reagent for a group of organic acids, with which, videlicet, succinic, benzoic, hydrosulphocyanic, acetic, and formic acids, it gives very distinct and characteristic reactions. With succinic acid it gives a voluminous brownish-red precipitate, readily soluble in acids, and which is rendered darker on addition of ammonia, owing to the withdrawal of a quantity of succinic acid. The precipitate from benzoic acid is pale-yellow, and comports itself as succinic acid. To solutions of acetates, sulphocyanides, and formiates, it imparts a blood-red color. It is employed also for the detection of hydroferrocyanic acid, in the formation of Prussian blue.

II. SPECIAL REAGENTS.

a. Reagents used for the detection of bases.

1. SULPHATE OF POTASSA. KO, SO3.

Uses. It precipitates from solutions of salts of baryta and strontia the insoluble sulphates of the oxides. It also produces a precipitate in concentrated solutions of lime, but only after some time.

2. CHROMATE OF POTASSA. KO, Cr 03.

Uses. It precipitates, from solutions of the salts of many metallic oxides, chromates; most of them are very difficultly soluble, and possess characteristic colors, by which the particular metal may often be with certainty detected. It is used principally, however, as a test for lead, with which it gives a yellow precipitate.

3. CYANIDE OF POTASSIUM. K Cy.

Uses. It is a valuable reducing agent, and also precipitates many of the heavier metals; with nickel it gives a greenishwhite, with cobalt a brownish-white, with copper a yellowishgreen, and with protoxide and sesquioxide of iron a reddishbrown precipitate. The latter, on addition of an excess of hydrochloric acid, and application of heat, dissolves, giving a beautiful blue liquid.

Tests. The precipitate produced by lead-salts must be perfectly white; the residue left upon evaporating its solution, previously saturated with pure hydrochloric acid, must clearly and perfectly dissolve in distilled water.

4. FERROCYANIDE OF POTASSIUM. K2 Cfy.

Uses.-Principally for the detection of oxide of copper, with which it gives a purple-red precipitate; and for indicating the presence of sesquioxide of iron, from the solutions of which it throws down a beautiful blue precipitate. With salts of the protoxide of iron, absolutely free from sesquioxide, it gives a white precipitate, which, on exposure to the air, becomes blue..

5. FERRICYANIDE OF POTASSIUM. K3 Cfy.

Uses. As a test for protoxide of iron and its salts, with which it immediately gives a blue precipitate of ferricyanide of iron. Fe3 Cfy2.

6. SULPHOCYANIDE OF POTASSIUM. K2 Csy.

Uses.-The same as those of ferrocyanide of potassium for the detection of sesquioxide of iron, the most minute trace of which, in a hydrochloric acid solution, gives with this reagent, a blood-red color.

7. PHOSPHATE OF SODA. 2 Na O, HO, PO5.

Uses.-Phosphate of soda precipitates all the alkaline earths, but is employed, after the separation of baryta, strontia, and lime, for the detection of magnesia, which it precipitates even in the presence of an excess of ammonia or its salts.

Tests. It must not become turbid when heated with ammonia. Dilute nitric acid must completely dissolve the precipitates produced in it by nitrate of silver and chloride of barium.

8. OXALATE OF AMMONIA. N H+ 0, Ō.

Uses. It is an invaluable test for lime, with which it gives a white precipitate, insoluble in acetic, but readily soluble in hydrochloric or nitric acids. This reagent produces a similar precipitate with baryta and strontia; the manipulator must, therefore, have positive proof of the absence of those earths before testing for lime.

9. PROTOCHLORIDE OF TIN. Sn Cl.

Uses. It is a very powerful reducing agent, and is employed as a test for mercury, throwing it down as a grey precipitate, which, on rubbing, collects into globules of the metal. It is also used as a test for gold, in the solutions of which it produces a purple color or a similarly colored precipitate.

10. BICHLORIDE OF PLATINUM. Pt Cl2.

Uses. For the detection of ammonia and potassa, from solu

tions of which, especially on addition of a little hydrochloric acid and alcohol, it throws down yellow crystalline precipitates, double chlorides of platinum and ammonium, or potassium as the case may be,―N HCl, Pt Cl2, or KCl, Pt Cl. When testing for potassa, the student must be sure that ammonia is absent, by having previously heated the substance to redness.

11. TERCHLORIDE OF GOLD. Au Cl3.

Uses. As a test for the protosalts of tin, with which it gives a purple color, or a similarly colored precipitate.

12. ZINC. Zn.

Uses.-Metallic zinc reduces many metallic salts and oxides causing their precipitation; this is owing to its affinity for oxygen. It is principally employed, however, for the reduction of antimony and tin, and, when mixed with sulphuric acid, for the evolution of hydrogen gas.

13. COPPER. Cu.

Uses.-Copper turnings are used for the reduction of mercurial salts, and for the detection of arsenious acid; when immersed in a solution of the latter, they immediately become covered with a grey metallic coating, which, on heating, separates in black scales. Copper also indicates the presence of nitric acid, when heated with substances containing it, to which dilute sulphuric acid has been added, by giving off fumes of binoxide of nitrogen; these, on being liberated, absorb oxygen from the atmosphere, and assume a brownish-red color, having the composition N O1.

14. IRON. Fe.

Uses.-Iron, like zinc, reduces and precipitates many metallic salts and oxides. It is employed, in the form of clean wire, for indicating the presence of copper, which falls upon it, when immersed in the solution, in a film, of the usual color of that metal. It is also used in quantitative cupellations, and to reduce phosphates to the state of phosphides. In this process, a white, brittle, metallic bead is formed, which will be noticed under phosphoric acid.

b. Special reagents employed for determining the presence of

acids.

1. ACETATE OF POTASSA. KO, Ā.

Uses. For precipitating phosphate of sesquioxide of iron from hydrochloric acid solutions of phosphates of the alkaline earths, and for precipitating from simple solutions in mineral acids substances insoluble in acetic acid. It is also employed for recognising the presence of tartaric acid, giving a white crystalline precipitate, which, however, from the time occupied in its formation, and its great solubility, is not very characteristic.

2. HYDRATE OF LIME. LIME-WATER. Ca O, HO. Uses.-The former,-hydrate of lime,-is employed for the liberation of ammonia, and the latter,-which should be kept from the air as much as possible, on account of the formation of carbonate of lime-for the detection of carbonic acid, with which it gives a white precipitate, soluble, with effervescence, in hydrochloric acid; oxalic acid, with which a white precipitate is formed, insoluble in acetic acid, but readily dissolved in hydrochloric and nitric acids; tartaric acid, from solutions of which appears, on addition of the reagent, a white precipitate dissolving easily in ammonia and cold potassa; the solution in the latter, on boiling, forms a gelatinous mass, which disappears on cooling; lastly, for indicating the presence of citric acid, with which it gives a white precipitate on boiling; this is soluble in ammonia and its salts, but not in potassa.

3. SULPHATE OF LIME. Ca O, S O3.

Uses. For distinguishing and separating baryta and strontia, producing an immediate precipitate with the former, but only doing so with the latter after a length of time.

4. SULPHATE OF MAGNESIA. Mg O, S O3.

Uses. For the detection of phosphoric acid, which is precipitated by it, even in the presence of ammonia and ammoniacal salts.