with four parts of carbonate of soda and potassa. The fused mass is treated with water, the solution filtered, the filtrate concentrated by evaporation, allowed to cool, transferred to a platinum or silver vessel, hydrochloric acid added to feebly acid reaction, and the fluid allowed to stand until the carbonic acid has escaped. It is then supersaturated with ammonia, heated, filtered into a bottle, chloride of calcium added to the still hot fluid, the bottle closed, and allowed to stand at rest. If a precipitate separates after some time, it is collected on a filter, dried, and examined by the method described in 5. (H. Rose.)

8. Minute quantities of metallic fluorides in minerals, slags, &c., may also be readily detected by means of the blowpipe. To this end, bend a piece of platinum foil, gutter-shape, then insert it in a glass tube as

Fig. 29.

shown in Fig. 29, introduce the finely triturated substance mixed with powdered phosphate of soda and ammonia fused on charcoal, and let the blowpipe flame play upon it in a manner to make the product of combustion pass into the tube. If fluorides of metals are present, hydrofluoric acid gas is evolved, which betrays its presence by its pungent odor, the dimming of the glass tube, and the yellow tint which the acid air issuing from the tube imparts to a moist slip of Brazil-wood paper* (Berzelius, Smithson). When silicates containing metallic fluorides are treated in this manner, gaseous fluoride of silicon is formed, which also colors yellow a moist slip of Brazil-wood paper inserted in the tube, and leads to silicic acid being deposited within the tube. After washing and drying the tube, the latter appears here and there dimmed. In the case of minerals containing water, presence of even a small proportion of metallic fluorides will, upon heating, even without addition of phosphate of soda and ammonia, usually suffice to color yellow a moistened slip of Brazil-wood paper inserted in the tube (Berzelius).

§ 148.

Recapitulation and remarks.-The baryta compounds of the acids of the third division are dissolved by hydrochloric acid, apparently without undergoing decomposition; alkalies therefore reprecipitate them unaltered, by neutralizing the hydrochloric acid. The baryta compounds of arsenious acid, arsenic acid, and chromic acid show, however, the same deportment; these acids must, therefore, if present, be removed before any conclusion regarding the presence of phosphoric acid, boracic acid, oxalic acid, or hydrofluoric acid, can be drawn from the reprecipitation of a salt of baryta by alkalies. But even leaving this point altogether out of the question, no great value is to be placed on this reaction, not even so far as the simple detection of these acids is concerned, and far less still as regards their separation from other acids, since ammonia fails to reprecipitate from hydrochloric acid solutions the salts of baryta in question, and more particularly the borate of baryta and the fluoride of barium, if the solution contains any considerable proportion of free acid or of an ammoniacal salt. Boracic acid may be invariably detected by the characteristic tint which it communicates to the flame of alcohol, provided care be taken to concentrate the solution sufficiently before

* Prepared by moistening slips of fine printing-paper with decoction of Brazil-wood.

adding the alcohol, and in the case of borates, to mix the solution with a sufficient amount of concentrated sulphuric acid. Solutions of free boracic acid must be combined with an alkali before evaporating, otherwise a large portion of the acid will volatilize along with the aqueous vapors. Minute traces of boracic acid are more safely detected by the method given § 145, 6.

The detection of phosphoric acid in compounds soluble in water is not difficult; the reaction with sulphate of magnesia is the best adapted to effect the purpose. The detection of phosphoric acid in insoluble compounds is somewhat more difficult; but even here we have now some excellent tests, in the reaction with sesquichloride of iron and acetate of soda, and more particularly still in that with molybdate of ammonia. With regard to the former of these two tests, I repeat here once more, that a sensitive reaction is not to be expected in a fluid colored red by acetate of sesquioxide of iron, as the latter (as well as acetate of alumina) dissolves phosphate of sesquioxide of iron; the directions given in § 143, 10, must therefore be strictly attended to. As regards the reaction with molybdate of ammonia, I must not omit to remark that, on account of its extraordinary sensitiveness, the greatest possible care and caution must be observed in the application of the test; not but we may almost always or, at all events, in most cases (as arsenic acid shows the same deportment) assume the yellow color of the solution or the formation of the yellow precipitate to indicate the presence of phosphoric acid; but that the reagent used may contain a trace of phosphoric acid, very slight perhaps, yet sufficient to be brought to light by this most delicate test; the purity of the reagents used in the process must therefore always be ascertained with the greatest care, otherwise it may happen that the presence of phosphoric acid is erroneously assumed in a substance simply because one or other of the reagents employed contained a trace of that acid. It must also be borne in mind that the reaction is manifest only in presence of an excess of molybdic acid. If this point is lost sight of, the phosphoric acid may be readily overlooked in the very cases where it is present in the largest proportion. Oxalic acid may always be easily detected in aqueous solutions by solution of sulphate of lime. The formation of a finely pulverulent precipitate, insoluble in acetic acid, leaves hardly a doubt on the point, as racemic acid alone, which occurs so very rarely, gives the same reaction. In case of doubt, the oxalate of lime may be readily distinguished from the paratartrate, or racemate, by simple ignition, with exclusion of the air, as the decomposed paratartrate leaves a considerable proportion of charcoal behind; the paratartrate dissolves moreover in cold solution of potassa or soda, in which oxalate of lime is insoluble. The deportment of the oxalates with sulphuric acid, or with binoxide of manganese and sulphuric acid, affords also sufficient means to confirm the results of other tests. In insoluble salts the oxalic acid is detected most safely by decomposing the insoluble compound by boiling with solution of carbonate of soda. I must finally also call attention here to the fact that there are certain soluble oxalates which are not precipitated by salts of lime; these are more particularly oxalate of sesquioxide of chromium, and oxalate of sesquioxide of iron. Their nonprecipitation is owing to the circumstance that these salts form soluble double salts with oxalate of lime. In salts decomposable by sulphuric acid, the hydrofluoric acid is readily detected; only, it must be borne in mind that the glass cannot be distinctly etched, if, instead of hydrofluoric

gas, fluosilicic gas alone is evolved; and therefore, in the case of compounds abounding in silica, the safer way is to try, besides the reaction given § 147, 5, also the one given in § 147, 6. In silicates which are not decomposed by sulphuric acid, the presence of fluorine is often overlooked, because the analyst omits to examine the compound carefully by the method given § 147, 7.

Fourth Division of the First Group of the Inorganic Acids.

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a. CARBONIC ACID (CO2).

1. Carbon is a solid, tasteless, and inodorous body. The very highest degrees of heat alone can effect its fusion and volatilization (Despretz). Carbon is combustible, and yields carbonic acid when burnt with a sufficient supply of oxygen or atmospheric air. In the diamond the carbon is crystallized, transparent, exceedingly hard, difficultly combustible; in the form of graphite, it is opaque, blackish-gray, soft, greasy to the touch, difficultly combustible, and stains the fingers; as charcoal, produced by the decomposition (destructive distillation) of organic matters, it is black, opaque, noncrystalline-often dense, shining, difficultly combustibleoften porous, dull, readily combustible.

2. Carbonic acid, at the common temperature and common atmospheric pressure, is a colorless gas of far higher specific gravity than atmospheric air, so that it may be poured from one vessel into another. It is almost inodorous, has a sourish taste, and reddens moist litmus-paper; but the red tint disappears again upon drying. Carbonic acid is readily absorbed by solution of potassa; it dissolves pretty copiously in water.

3. The aqueous solution of carbonic acid has a feebly acid, pungent taste; it transiently imparts a red tint to litmus-paper, and colors solution of litmus wine-red; it loses its carbonic acid upon the application of heat, and also when shaken with air in a half-filled bottle. Part of the carbonates lose their carbonic acid upon ignition; those with colorless oxides are white or colorless. Of the neutral carbonates, only those with alkaline bases are soluble in water. The solutions manifest a very strong alkaline reaction. Besides the carbonates with alkaline bases, those also with an alkaline earth for base, and some of those with a metallic base, dissolve as acid or bicarbonates.

4. The carbonates are decomposed by all free acids soluble in water, with the exception of hydrocyanic acid and hydrosulphuric acid. The decomposition of the carbonates by acids is attended with EFFERVESCENCE, the carbonic acid being disengaged as a colorless and almost inodorous gas, which transiently imparts a reddish tint to litmus-paper. It is necessary to apply the decomposing acid in excess, especially when operating upon carbonates with alkaline bases, since the formation of bicarbonates will frequently prevent effervescence, if too little of the decomposing acid be added. Substances which it is intended to test for carbonic acid by this method, should first be drenched with water, to prevent any mistake which might arise from the escape of air-bubbles upon treating the dry substances with the acid. If it is wished to determine by a direct experiment whether the disengaged gas is really carbonic acid or not, this may be readily accomplished by dipping the end of a glass rod in baryta-water, and inserting the rod into the test-tube,

bringing the moistened end near the surface of the fluid in the tube, when ensuing turbidity of the baryta-water on the glass rod will prove that the evolved gas is really carbonic acid, since

5. Lime-water and baryta-water, when brought into contact with carbonic acid or with soluble carbonates, produce white precipitates of neutral CARBONATE OF LIME (Ca O, CO), or neutral CARBONATE of BARYTA (Ba O, CO,). When testing for free carbonic acid, the reagents ought always to be added in excess, as the acid carbonates of the alkaline earths are soluble in water. The precipitated carbonates of lime and baryta dissolve in acids, with effervescence, and are not reprecipitated from such solutions by ammonia, after the complete expulsion of the carbonic acid by ebullition.

6. Chloride of calcium and chloride of barium immediately produce in solutions of neutral alkaline carbonates, precipitates of CARBONATE OF LIME or of CARBONATE OF BARYTA; in dilute solutions of bicarbonates these precipitates are formed only upon ebullition; with free carbonic acid these reagents give no precipitate.

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b. SILICIC ACID (Si O).

1. Silicic acid is colorless or white, even in the hottest blowpipe flame unalterable and infusible. It fuses in the flame of the oxyhydrogen blowpipe. It is met with in two modifications (more correctly speaking, in the crystalline and in the amorphous state). It is insoluble in water and acids, with the exception of hydrofluoric acid; whilst its hydrate is soluble in acids, but only at the moment of its separation. The amorphous silicic acid and the hydrate dissolve in hot aqueous solutions of caustic alkalies and of fixed alkaline carbonates; but the crystallized acid is insoluble or nearly so in these fluids. If either of the two is fused with pure alkalies or alkaline carbonates, a basic silicate of the alkali is obtained, which is soluble in water, and from which acids again separate hydrated silicic acid. The silicates with alkaline bases alone are soluble in water.

2. The solutions of the alkaline silicates are decomposed by all acids. If a large proportion of hydrochloric acid is added at once to even concentrated solutions of alkaline silicates, the separated silicic acid remains in solution; but if the hydrochloric acid is added gradually drop by drop, whilst stirring the fluid, the greater part of the silicic acid separates as gelatinous hydrate. The more dilute the fluid, the more silicic acid remains in solution, and in highly dilute solutions no precipitate is formed. But if the solution of an alkaline silicate, mixed with hydrochloric or nitric acid in excess, is evaporated to dryness, silicic acid separates in proportion as the acid escapes; upon treating the residue with hydrochloric acid and water, the silicic acid remains in the free state (or, if the temperature in the process of drying was restricted to 212°, as hydrate, H O, 4 Si O,), as an insoluble white powder. Chloride of ammonium produces in rather concentrated solutions of alkaline silicates precipitates of hydrate of silicic acid.

3. Part of the silicates insoluble in water are decomposed by hydrochloric acid or nitric acid, part of them are not affected by these acids, not even upon boiling. In the decomposition of the former, the greater

portion of the silicic acid separates usually as gelatinous, more rarely as pulverulent hydrate. To effect the complete separation of the silicic acid, the hydrochloric acid solution, with the precipitated hydrate of silicic acid suspended in it, is evaporated to dryness, the residue heated at a temperature above the boiling point of water until no more acid fumes escape, then moistened with hydrochloric acid, heated with water, and the fluid containing the bases filtered from the residuary insoluble silicic acid.

4. Of the silicates not decomposed by hydrochloric acid, many, e. g., kaolin, are completely decomposed by heating with hydrated sulphuric acid, the decomposition being attended with separation of silicic acid in the pulverulent form; many others are acted upon to some extent by this reagent.

5. If any silicate, reduced to a fine powder, is fused with 4 parts of carbonate of potassa and soda until the evolution of carbonic acid has ceased, and the fused mass is then boiled with water, the greater portion of the silicic acid dissolves as alkaline silicate, whilst the alkaline earths, the earths proper, and the heavy metallic oxides are left undissolved. If the fused mass is treated with water, then, without previous filtration, hydrochloric or nitric acid added to strongly acid reaction, and the fluid treated as directed in 3, the silicic acid is left undissolved, whilst the bases are dissolved. If the powdered silicate is fused with 4 parts of hydrate of baryta, the fused mass digested with water, with addition of hydrochloric or nitric acid, and the acid solution treated as directed in 3, the silicic acid separates, and the bases, especially the alkalies, are found in the filtrate.

6. If hydrofluoric acid, in aqueous solution or in the gaseous state, is made to act upon finely pulverized silicates, fluosilicic gas escapes, and the bases are converted into silicofluorides, which upon heating with hydrated sulphuric acid change to sulphates, with evolution of fluosilicic gas. If the powdered silicate is mixed with 5 parts of fluoride of calcium in powder, the mixture made into a paste with hydrated sulphuric acid, and heat applied (best in the open air), until no more fumes escape, the whole of the silicic acid present volatilizes as fluosilicic gas. The bases present are found in the residue as sulphates, mixed with sulphate of lime.

7. If silicic acid or a silicate is fused with carbonate of soda on the loop of a platinum wire, a frothing is observed in the fusing bead, owing to the disengagement of carbonic acid. If the proper proportion of carbonate of soda is not exceeded, the bead of silicate of soda formed in the process will remain transparent on cooling.

8. Phosphate of soda and ammonia, in a state of fusion, fails nearly altogether to dissolve silicic acid. If therefore silicic acid or a silicate is fused, in small fragments, with phosphate of soda and ammonia on a platinum wire, the bases are dissolved, whilst the silicic acid separates and floats about in the clear bead, as a more or less transparent mass, exhibiting the shape of the fragment used in the experiment.

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Recapitulation and remarks.-Free carbonic acid is readily known by its deportment with lime-water; the carbonates are easily detected by the evolution of a nearly inodorous gas, which takes place when they