nitric acid, but are nearly insoluble in oxalic acid and in acetic acid, and practically insoluble in water. The presence of salts of ammonia does not interfere with the formation of these precipitates. Addition of ammonia considerably promotes the precipitation of free oxalic acid by salts of lime. In highly dilute solutions the precipitate is only formed after some time.

6. If hydrated oxalic acid or an oxalate, in the dry state, is heated with an excess of concentrated sulphuric acid, the latter withdraws from the oxalic acid its constitutional water or base, and thus causes its decomposition into CARBONIC ACID and CARBONIC OXIDE (C ̧O = 2CO + 2CO), the two gases escaping with effervescence. If the quantity operated upon is not too minute the carbonic oxide may be kindled; it burns with a blue flame. Should the sulphuric acid acquire a dark color in this reaction, this is a proof that the oxalic acid contained some organic substance in admixture.

7. If oxalic acid or an oxalate is mixed with finely pulverized binoxide of manganese (which must be free from carbonates), a little water added and a few drops of sulphuric acid, a lively effervescence ensues, caused by escaping CARBONIC ACID [2 Mn O2+ C ̧ O ̧ + 2 S 0 ̧ = 2 (Mn O, S 0 ̧)+ 4C0].

2

6

8. If oxalates of alkaline earths are boiled with a concentrated solution of carbonate of soda, and filtered, the oxalic acid is obtained in the filtrate in combination with soda, whilst the precipitate contains the base as carbonate. With oxalates containing for their base oxides of heavy metals, this operation is not always sure to attain the desired object, as many of these oxalates, e.g. oxalate of protoxide of nickel, will partially dissolve in the alkaline fluid, with formation of a double salt. Metals of this kind should therefore be separated as sulphides.

§ 146.

d. HYDROFLUORIC ACID (H F).

1. Anhydrous HYDROFLUORIC ACID is a colorless corrosive gas, which fumes in the air, and is freely absorbed by water. Liquid hydrofluoric acid is distinguished from all other acids by the property of dissolving crystallized silicic acid, and also the silicates which are insoluble in hydrochloric acid. Hydrofluosilicic acid and water are formed in the process of solution (Si Ŏ, + 3 H F = Si F, HF+2 HO). With metallic oxides hydrofluoric acid forms metallic fluorides and water.

2. The FLUORIDES of the alkali metals are soluble in water; the solutions have an alkaline reaction. The fluorides of the metals of the alkaline earths are either insoluble or very difficultly soluble in water. Fluoride of aluminium is readily soluble. Most of the fluorides of the heavy metals are very sparingly soluble in water, as the fluorides of copper, lead, and zinc; many others dissolve in water without difficulty, as the sesquifluoride of iron, protofluoride of tin, and fluoride of mercury. Many of the fluorides insoluble or difficultly soluble in water dissolve in free hydrofluoric acid; others do not. Most of the fluorides bear ignition in a crucible without suffering decomposition.

3. Chloride of barium precipitates aqueous solutions of hydrofluoric acid, but much more completely solutions of fluorides of the alkalies. The bulky white precipitate of FLUORIDE OF BARIUM (Ba F) is almost

absolutely insoluble in water, but dissolves in large quantities of hydrochloric acid or nitric acid, from which solutions ammonia fails to precipitate it, or throws it down only very incompletely, owing to the dissolving action of the neutral ammonia salts.

4. Chloride of calcium produces in aqueous solutions of hydrofluoric acid or of fluorides a gelatinous precipitate of FLUORIDE OF CALCIUM (Ca F), which is so transparent as at first to induce the belief that the fluid has remained perfectly clear. Addition of ammonia promotes the complete separation of the precipitate. The precipitate is practically insoluble in water, and only very slightly soluble in hydrochloric acid and nitric acid in the cold: it dissolves somewhat more largely upon boiling with hydrochloric acid. Ammonia produces no precipitate in the solution, or only a very trifling one, as the salt of ammonia formed retains it in solution. Fluoride of calcium is scarcely more soluble in free hydrofluoric acid than in water. It is insoluble in

alkaline fluids.

5. If a finely pulverized fluoride, no matter whether soluble or insoluble, is treated in a platinum crucible with just enough concentrated sulphuric acid to make it into a thin paste, the crucible covered with the convex face of a watch-glass of hard glass coated with bees-wax, which has been removed again in some places by tracing lines in it with a pointed piece of wood, the hollow of the glass filled with water, and the crucible gently heated for the space of half an hour or an hour, the exposed lines will, upon the removal of the wax, be found more or less deeply ETCHED into the glass. (The coating with wax may be readily effected by heating the glass cautiously, putting a small piece of wax upon the convex face, and spreading the fused mass equally over it. The removal of the wax coating is effected by heating the glass gently, and wiping with a cloth.) If the quantity of hydrofluoric acid disengaged by the sulphuric acid was very minute, the etching is often invisible upon the removal of the wax; it will, however, in such cases appear when the glass is breathed upon. This appearance of the etched lines is owing to the unequal capacity of condensing water which the etched and the untouched parts of the plate respectively possess. The impressions which thus appear upon breathing on the glass may, however, owe their origin to other causes; therefore, though their nonappearance may be held as a proof of the absence of Huorine, their appearance is not a positive proof of the presence of that element. all events, they ought only to be considered of value where they can be developed again after the glass has been properly washed with water, dried, and wiped.*

At

This reaction fails if there is too much silicic acid present, or if the substance is not decomposed by sulphuric acid. In such cases one of the two following methods is resorted to, according to circumstances.

6. If we have to deal with a fluoride decomposable by sulphuric acid,

J. NICKLES states that etchings on glass may be obtained with all kinds of sulphuric acid, and, in fact, with all acids suited to effect evolution of hydrofluoric acid. I have tried watch-glasses of Bohemian glass with sulphuric and other acids, but could get no etchings in confirmation of this statement. Still, proper caution demands that before using the sulphuric acid, it should first be positively ascertained that its fumes will not etch glass. Should the sulphuric acid contain hydrofluoric acid, the latter may be easily removed by diluting with an equal volume of water and evaporating in a platinum dish to the original strength.

but mixed with a large proportion of silicic acid, the fluorine in it may be detected by heating the mixture in a test-tube with concentrated sulphuric acid, as FLUOSILICIC GAS is evolved in this process, which forms dense white fumes in moist air. If the gas is conducted into water through a bent tube moistened inside, the latter has its transparency more or less impaired, owing to the separation of silicic acid. If the quantity operated upon is rather considerable, hydrate of silicic acid separates in the water, and the fluid is rendered acid by hydrofluosilicic acid.

The following process answers best for the detection of small quantities of fluorine. Heat the substance with concentrated sulphuric acid in a flask closed with a cork with double perforation, bearing two tubes, one of which reaches to the bottom of the flask, whilst the other terminates immediately under the cork. Conduct through the longer tube a slow stream of dry air into the flask, and conduct this, upon its reissuing through the other tube, into a U tube containing a little dilute ammonia, and connected at the other end with an aspirator. The silicofluoric gas which escapes with the air, decomposes with the ammonia, more particularly upon the application of a gentle heat towards the end of the process, fluoride of ammonia and hydrated silicic acid being formed. Filter, evaporate in a platinum crucible to dryness, and examine the residue by 5. For more difficultly decomposable substances bisulphate of potassa is used instead of sulphuric acid, and the mixture, to which some marble is added (to ensure a continuous slight evolution of gas), heated to fusion, and kept in that state for some time.

7. Silicates not decomposable by sulphuric acid must first be fused 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 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, and insert it in a glass tube as shown in fig. 43, introduce the finely triturated sub

Fig. 43.

stance mixed with powdered phosphate of soda and ammonia fused on charcoal, and let the blowpipe flame play upon it so that the products of combustion may pass into the tube. A metallic fluoride treated in this way yields hydrofluoric acid gas, which betrays its presence by its pungent odor, the dimming of the glass tube (which becomes perceptible only after cleaning and drying), 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 Hluorides are treated in this manner gaseous fluoride of silicon is formed, which also colors yellow a moist slip of Brazil-wood paper inserted

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

in the tube, and leads to silicic acid being deposited within the tube. After washing and drying the tube, it appears here and there dimmed. A small quantity of a fluoride present in a mineral containing water may generally be detected by heating the substance by itself in a glass tube sealed at one end and inserting a slip of Brazil-wood paper in the tube; under the circumstance the paper will usually turn yellow (BERZELIUS).

§ 147.

Recapitulation and remarks.-The baryta compounds of the acids of the third division are dissolved by hydrochloric acid, apparently without decomposition; alkalies therefore reprecipitate them unaltered, by neutralizing the hydrochloric acid. The baryta compounds of the acids of the first division 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 is well characterized by the coloration which it imparts to the flame of alcohol, and also by its action on turmeric-paper. The latter reaction is more particularly suited for the detection of very minute traces. Oxides of the heavy metals, if present, are most conveniently removed first by hydrosulphuric acid or sulphide of ammonium. Before proceeding to concentrate dilute solutions of boracic acid the acid must be combined with an alkali, otherwise a large portion of it will volatilize with the aqueous vapors. Small quantities of boracic acid may also be safely and easily detected by the spectroscope. The detection of phosphoric acid in compounds soluble in water is not difficult; the reaction with sulphate of magnesia, &c., is the best adapted for the purpose. The detection of phosphoric acid in insoluble compounds cannot be effected by means of magnesia solution. Sesquichloride of iron (§ 142, 9) is well suited for the detection of phosphoric acid in its salts with the alkaline earths, and more particularly for the separation of the acid from the alkaline earths; the nitric acid solution of molybdate of ammonia is more especially adapted to effect the detection of phosphoric acid in presence of alumina and sesquioxide of iron. I must repeat again that both these reactions demand the strictest attention to the directions given. If present in combination with oxides of the fourth, fifth, or sixth group, it may be separated by the method given § 142, 11, or by precipitating the bases with hydrosulphuric acid or sulphide of ammonium.

Oxalic acid may always be easily detected in aqueous solutions of oxalates of the alkalies, 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 racemate, by simple ignition, with exclusion of air, as the decomposed racemate leaves a considerable proportion of

I must

charcoal behind; the racemate 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 them by boiling with solution of carbonate of soda, or by hydrosulphuric acid or sulphide of ammonium (§ 145, 8). 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 non-precipitation is owing to the circumstance that these salts form soluble double salts with oxalate of lime. Hydrofluoric acid is readily detected in salts decomposable by sulphuric acid; only it must be borne in mind that an over large proportion of sulphuric acid impedes the free evolution of hydrofluoric gas, and thus impairs the delicacy of the reaction; also 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 § 146, 5, also the one given in 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 in 7.

§ 148.

PHOSPHOROUS ACID (PO).

Anhydrous phosphorous acid is a white powder, which admits of sublimation, and burns when heated in the air. It forms with a small proportion of water a thickish fluid, which crystallizes by long standing. Heat decomposes it into hydrated phos phoric acid, and phosphuretted hydrogen gas, which does not spontaneously take fire. It freely dissolves in water. Of the salts those with alkaline base are readily soluble in water, all the others sparingly soluble; the latter dissolve in dilute acids. All the salts are decomposed by ignition into phosphates, which are left behind, and hydrogen, or a mixture of hydrogen and phosphuretted hydrogen, which escapes. With nitrate of silver separation of metallic silver takes place, more especially upon addition of ammonia and application of heat; with nitrate of suboxide of mercury, under the same circumstances, separation of metallic mercury. From chloride of mercury in excess phosphorous acid throws down subchloride of mercury after some time, more rapidly upon heating. Chloride of barium and chloride of calcium produce in not over-dilute solutions of phosphorous acid, upon addition of ammonia, white precipitates, soluble in acetic acid. A mixture of sulphate of magnesia, chloride of ammonium, and ammonia will precipitate only rather concentrated solutions. Acetate of lead throws down white phosphite of lead, insoluble in acetic acid. By heating to boiling with sulphurous acid in excess phosphoric acid is formed, attended by separation of sulphur. In contact with zinc and dilute sulphuric acid phosphorous acid gives a mixture of hydrogen with phosphuretted hydrogen, which accordingly fumes in the air, burns with an emerald-green color, and precipitates silver and phosphide of silver from solution of nitrate of silver.

Fourth Division of the First Group of the Inorganic Acids.

§ 149.

a. CARBONIC ACID (CO).

1. CARBON is a solid tasteless and inodorous body. The very highest degrees of heat alone can effect its fusion and volatilization (DESPRETZ). All carbon is combustible, and yields carbonic acid when