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excess of the acid which we wish to retain, since the other, becoming decomposed into gaseous constituents, may be entirely removed by warming the solution.

The nitric acid usually called “fuming” is generally red from the presence of N, O, one of the products of its decomposition : its specific gravity is 1.536; and it solidifies at —49° C. to a very dark red mass.

All nitrates are soluble in water; and therefore this radical cannot be recognized by the formation of any insoluble salts.

It may, however, be identified by other processes :

a. By setting free the hydrogen compound (nitric acid) by the addition of concentrated sulphuric acid to a nitrate, and subjecting it to the immediate action of copper turnings, which must be added simultaneously with the sulphuric acid, decomposition of the nitric acid ensues, with disengagement of nitric oxide gas (N202). The latter is recognized by being itself colourless but capable of acquiring a peculiar brownish red colour by mixture with air ; it absorbs the atmospheric oxygen, and is converted into nitrous and hyponitric anhydrides (N20, and N,0.).

B. When nitric acid is produced from a nitrate by the action of concentrated sulphuric acid, the mixture cooled by immersion of the test-tube containing it in cold water, a crystal of ferrous sulphate added, and the whole allowed to rest, a dark halo is observed to form around the crystal, which, upon the application of heat, disappears with a kind of effervescence. This is due to the formation of a very singular combination of ferrous sulphate and nitric oxide having the formula 4Fe, SOX, N,0,: heat decomposes this combination. The cause of its production is the decomposition of a portion of the nitric acid consequent upon the passage of the ferrous into the ferric salt, thus 10Fe, SO, +4H, SO, +2KNO,=3(Fe,),(802); +K, SO

+4Fe, SO,, N, 0, +4H, O.

brown compound. y. The nitrates do not reduce gold or mercurous salts; but when mixed with hydrochloric acid, they acquire the power of dissolving gold leaf.

8. Upon adding to a nitrate sulphuric acid and sulphindigotic acid, the colour of the latter is destroyed.

Of the acid-radicals containing phosphorus combined with oxygen, those existing in the phosphites and phosphates are of great analytical importance. The hypophosphites are rare.

SALTS OF THE HYPOPHOSPHOROUS RADICAL, OR HYPOPHOSPHITES. This acid-radical is of rare occurrence, being only produced by artificial means. It is obtained in the decomposition of phosphide of barium by water, or by boiling phosphorus with an alcoholic solution of hydrate of potassium, or with the aqueous solutions of hydrate of barium or calcium, until it disappears, and the vapour has no longer the odour of phosphuretted hydrogen. The phosphuretted hydrogen evolved during the process frequently causes dangerous explosions, which may be moderated by employing only a gentle heat. The barium or calcium salts thus obtained (BaH, PO, or CaH, PO,) are very crystallizable.

THE HYDROGEN SALT (H, PO,), or hypophosphorous acid, is a viscid, uncrystallizable, very acid liquid, which when heated decomposes into phosphuretted hydrogen and phosphoric acid, thus-2H, PO,=H,P+H, POZ. Although hypophosphorous acid is thus tribasic, one series of salts only is known, namely, the series of acid salts represented by the general formula MH, PO,; they generally occur with water of crystallization, and are all soluble in water, and many of them also in alcohol.

This radical may nevertheless be recognized by several processes of decomposition :

a. When any dry hypophosphite is rather gently heated in a test-tube, decomposition of its radical occurs ; phosphuretted hydrogen is evolved, which may be recognized by its odour and ready inflammability; and a pyrophosphate of the metal remains behind. The lead salt exhibits this decomposition best :

4MH, PO,=M,P,0,+H, 0+2H, P.

pyrophosphate. B. When mixed with an acid, hypophosphites reduce silver and gold salts, precipitating silver and gold; they also reduce mercuric and cupric salts. The same reactions take place more slowly with concentrated solutions of hypophosphites.

y. When boiled with excess of hydrate of potassium, alkaline hypophosphites evolve pure hydrogen gas :

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SALTS OF THE PHOSPHOROUS RADICAL, OR PHOSPHITES. This acid-radical is obtained in combination with hydrogen by the decomposition of terchloride of phosphorus (PCI) by water, or by the slow oxidation of phosphorus in the air.

THE HYDROGEN SALT (H, PO, ?), or phosphorous acid, is known in the liquid and also in the crystallized state ; when heated, it decomposes, thus

4H, P0,=3H, PO,+H, P. THE ALKALINE PHOSPHITES are soluble in water; most other salts are insoluble or nearly so. The acid salts of the formula MH, PO, are soluble in water; the acid salts having the composition M, HPO, are less so, whilst the neutral phosphites M, PO, are insoluble.

THE BARIUM and CALCIUM SALTs are slowly precipitated by the addition of alkaline phosphites to soluble barium or calcium salts, as white precipitates having the formula M, HPO,: by boiling, the precipitation is accelerated; but the precipitate which then falls is M, PO,,—MH, PO, (the acid salt) remaining in solution. They are soluble in most acids, and by nitric acid are converted into phosphates.

THE MAGNESIUM Salt is not precipitated when a dilute solution of a magnesium salt is added to a soluble phosphite in the presence of the chloride and hydrate of ammonium.

THE FERROUS Salt is white, changing to red basic ferric phosphate. THE FERRIC Salt is white.

THE Zinc Salt is soluble.
THE CUPRIC Salt is blue.
THE MERCUROUS, MERCURIC, and SILVER Salts do not exist.
THE LEAD Salt is white.

This radical may also be recognized as follows :· a. The majority of the phosphites do not evolve phosphuretted hydrogen, but pure hydrogen slightly contaminated with it, when the dry salts are heated. The lead salt is an exception.

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B. Soluble phosphites reduce solutions of cupric, silver, mercuric, or gold salts, and precipitate the metal. This takes place especially on boiling.

y. Phosphites, when boiled with excess of hydrate of potassium, are not altered, nor does any evolution of hydrogen occur.

SALTS OF THE PHOSPHORIC RADICAL, OR PHOSPHATES. The phosphates occur somewhat abundantly in nature; they are found distributed in small quantities through the mineral and vegetable kingdoms, whence they pass into the bodies of animals, to which they are absolutely essential for the formation and renewal of their skeleton.

THE HYDROGEN SALT (H, PO.), or phosphoric acid, is produced by the decomposition of these, or by the solution of phosphoric anhydride (the body P,0s, which is the white crystalline solid obtained when phosphorus is burnt in oxygen or air) in water. The substance P,00, when combining with water, produces three distinct acids, thus

P,0,+H,0 =2HPO,, meta- or monobasic phosphoric acid. P,0,+2H,0=H, P,0, pyro- or bibasic phosphoric acid.

P,03+3H,0=2H, PO,, ordinary or tribasic phosphoric acid. These are each the representative of three distinct classes of salts, called respectively the metaphosphates, pyrophosphates, and phosphates, or the mono-, bi-, and ordinary phosphates : whether in these the three allotropic varieties of phosphorus exist, we do not know; but they singularly correspond to the two classes of phosphides previously mentioned (p. 366). The phosphates which ordinarily occur in nature are the tribasic phosphates. When the anhydride P,0, is dissolved in abundance of water, and the solution heated, the latter of these acids is produced ; but if the phosphoric anhydride be allowed to dissolve in cold water, the second is obtained,—whilst, again, if the aqueous solution of either acid be evaporated in a platinum dish until water no longer is expelled, the residue solidifies on cooling into a glassy substance, which is called “glacial” phosphoric acid, and which is in reality HPO3, or the monobasic acid. This will

volatilize altogether at a red heat. The solution of the ordinary acid (H, PO.) may be evaporated without decomposition up to a temperature of 149° C., when it becomes of a syrupy consistence; between that temperature and 213° C. it loses the elements of water, and is converted into pyrophosphoric acid; and if cooled from that point, it solidifies in the form of a soft glass, or in small granular crystals; if heated further, it again loses the elements of water, and is converted into the monobasic acid, as has before been stated.

Many phosphates behave in a characteristic manner when heated, owing to their peculiar constitution : thus with the tribasic radical PO,, which forms the three series of salts,

acid salt.

acid salt.

neutral salt. If heat is applied to the second of these acid salts, it decomposes into water and a neutral pyro- or bibasic phosphate, thus

2M, HPO =M, P,0,+H,0: and again, if the first acid salt be heated, water is again obtained and a meta- or monobasic phosphate; for

MH, PO,=MPO, +1,0. The pyrophosphates exhibit similar features. Notwithstanding, however, that the acid salts of the various phosphoric radicals are thus unstable, all neutral salts are but little affected by heat: the generality fuse to a glassy mass, and yield to few decomposing agents except carbon, which, at a high temperature, removes the oxygen from most phosphates except those of potassium and sodium, and eliminates the phosphorus, which then distils and is suitably collected. (See p. 33.)

Of neutral phosphates, those of the first subdivision are almost the only soluble ones; many of the acid phosphates, especially those of the formula MH, PO,, are soluble in water; most are soluble in acids.

THE BARIUM, STRONTIUM, and CALCIUM Salts are white precipitates of the formula M, HPO, when produced by the action of ordinary phosphate of sodium (Na, HPO.). They are soluble in

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