2. All the OXALATES undergo decomposition at a red heat, the oxalic acid being converted into carbonic acid and carbonic oxide. Those with an alkali or an alkaline earth for base are in this process converted into carbonates (if pure, almost without separation of charcoal); oxalate of magnesia yields pure magnesia on gentle ignition; those with a metallic base leave either the pure metal or the oxide behind, according to the greater or less degree of reducibility of the metallic oxide. The alkaline oxalates, and also some of the oxalates with metallic bases, are soluble in water. 3. Chloride of barium produces in neutral solutions of oxalates a white precipitate of OXALATE OF BARYTA (2 Ba O, C, O。 + 2 Aq.), which is very slightly soluble in water, more largely in acetic or oxalic acid, and in solution of chloride of ammonium, easily in nitric acid and in hydrochloric acid. From solution in the last named acids it is reprecipitated by ammonia unaltered.

4. Nitrate of silver produces in aqueous solutions of oxalic acid and alkaline oxalates a white precipitate of OXALATE OF SILVER (2 Ag O, C, O.), which is almost insoluble in water, difficultly soluble in dilute nitric acid, easily soluble in hot strong nitric acid and in ammonia.

5. Lime-water and all the soluble salts of lime, and consequently also solution of sulphate of lime, produce in even highly dilute solutions of free oxalic acid or of oxalates, white, finely pulverulent precipitates of OXALATE OF LIME (2 Ca O, C, O. + 2 aq. and sometimes 2 Ca O, C, O. + 6 aq.) which dissolve readily in hydrochloric acid and in nitric acid, but are nearly insoluble in oxalic acid and in acetic acid, and almost totally 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 the 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, and thus causes its decomposition into CARBONIC ACID and CARBONIC OXIDE (C, O = 2 CO + 2 CO2), the two gases escaping with effervescence. If the quantity operated upon is not too minute, the escaping car bonic oxide gas 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 some 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 the escaping carbonic acid: 2 Mn O2+ C, O + 2 SO = 2 (Mn O SO ) + 4 CO.

8 If oxalates of the alkaline earths are boiled with a concentra

ted solution of carbonate of soda, and the fluid filtered, the oxalic acid is obtained in the filtrate in combination with soda, whilst the precipitate contains the base as carbonate. Some oxalates of the heavy metals, e. g., oxalate of nickel, are not entirely decomposed in this manner, soluble double oxalates being formed. Sulphide of ammonium or hydrosulphuric acid must, therefore, be employed to separate these bases.

$149.

d. HYDROFLUORIC ACID (HF).

1. Anhydrous HYDROFLUORIC ACID is a colorless, corrosive gas, which fumes when exposed to the air, and is freely absorbed by water. Aqueous hydrofluoric acid is distinguished from all other acids by the exclusive property it possesses of dissolving crystallized silicic acid, and also the silicates which are insoluble in hydrochloric acid. Fluoride of silicon and water are formed in the process of solution (2 H F + Si O2 = Si F2 + 2 H O). Hydrofluoric acid decomposes with metallic oxides in the same manner, metallic fluorides and water being formed.

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 altogether insoluble in water, or they dissolve in that menstruum only with very great difficulty. Fluoride of aluminum is readily soluble. Most of the fluorides corresponding to the oxides of the heavy metals are very difficultly soluble in water, for instance, fluorides of copper, lead, and zinc; many other of the fluorides of the heavy metals dissolve in water without difficulty, as, for instance, the sesquifluoride of iron, protofluoride of tin, fluoride of mercury, &c. 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 free hydrofluoric acid imperfectly, more completely the solutions of alkaline fluorides; the volu minous white precipitate of FLUORIDE OF BARIUM (Ba F) is quite insoluble in water, but dissolves in a large excess of hydrochloric or nitric acid. From these solutions it is thrown down incompletely or not at all by ammonia, being somewhat soluble in 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 precipitated fluoride of calcium is insoluble in water, is 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. It 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 i soluble, is treated in a platinum crucible with just enough con centrated sulphuric acid to make 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 bit 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 ETCHED into the glass. 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 reappear when the plate is breathed upon. This reappearance 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 appearance of lines, after breathing on the glass, is not necessarily evidence of the presence of hydrofluoric acid in the substance under examination, unless they can be reproduced after the glass has been rinsed, dried and wiped, though their non-appearance is proof of its absence.

This reaction (5) fails if there is too much silicic acid present, or if the body under examination is not decomposed by sulphuric acid. In such cases the one or the other of the two following methods is resorted to, according to circumstances.

6. If we have to deal with a fluoride decomposable by sulphuric acid, 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 ig conducted into water, through a bent tube moistened inside, the latter has its transparency more or less impaired, by the separation of silicic acid. If the quantity operated upon is rather considerable, hydrate of si icic acid separates in the water, and the fluid is rendered acid by hydrofluosilicic acid.

* 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 the wax off with a cloth.

The statement of Nickles, that sulphuric acid and all acids adapted to decom pose quorides, etch glass, I have not been able to confirin when employing Bohemian glass. It is, however, advisable to prove by trial that the sulphuric acid one use really does not attack the glass.

The following process answers best for the detection of smaller quantities of fluorine. The substance is heated with concentrated sulphuric acid in a flask closed by a perforated cork bearing two tubes. Through one tube which should reach nearly to the bottom of the flask, is passed a slow current of dried air which finds exit through the other short tube, and is made then to stream through some ammonia contained in a U tube by attaching the latter to an aspirator. The fluoride of silicon carried over by the stream of air, is decomposed on reaching the ammonia, especially when the latter is heated, fluoride of ammonium and hydrated silica being formed. The liquid is filtered, the filtrate is evaporated to dryness in a platinum crucible and the residue tested according to 5.

In the case of more difficultly decomposable substances bisulphate of potassa is used instead of sulphuric acid, and the mixture, to which some marble is likewise added, heated to fusion, and kept in that state for some time.

7. Compounds 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 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

Fig. 29.

glass tube as 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 products 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, gaseo ̈s fluoride of silicon is formed, which also colors yellow a moist slip

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

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 yel low a moistened slip of Brazil-wood paper inserted in the tube (Berzelius).

$ 150.

Recapitulation and remarks.-The baryta compounds of the acids of the third division are dissolved by hydrochloric acid, apparently without undergoing decomposition; alkalies therefore veprecipitate them unaltered, by neutralizing the hydrochloric acid. The baryta compounds of chromic, sulphurous, hyposulphurous, and iodic acids 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, as well as by its reaction with turmeric paper. The latter reaction is particularly adapted to detect minute quantities. Care should be taken to concentrate the solution sufficiently before testing. 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. Metallic oxides, if present, should be removed, which may be done by hydrosulphuric acid or sulphide of ammonium.

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. To compounds which are insoluble in water, sulphate of magnesia cannot be applied. In phosphates of the alkaline earths, phosphoric acid may be detected and sepa rated by means of sesquichloride of iron (§ 145, 9). In presence of alumina and sesquioxide of iron, solution of molybdate of ammonia in nitric acid is best employed. Both reagents must be used strictly according to the directions already given (§ 145, 9 and 10),