Prussian and French gunpowder contain the same percentage of nitre as the English; but the proportions of sulphur and charcoal in them are more nearly equal.

CHAPTER XIII.

PHOSPHORUS AND ITS COMPOUNDS WITH THE PRECEDING ELEMENTS. Symbol, P. Equivalent, 31. Density of vapour, 62 (compared with air 4.28).

§ 253. Phosphorus occurs in nature only in combination with other elements-indeed, almost invariably in union with oxygen (with which it forms a powerful acid), in combination with bases. In the primary or lowest, and in recent igneous rocks, as lava, it is found as phosphates of K, Na, Ca, &c. The quantity, however, of P is very small in rocks. Phosphorus occurs in plants, especially in such seeds as are used for food. It forms an essential constituent of bones, the ash of which consists almost wholly of carbonate and phosphate of calcium. Phosphorus is also found in the flesh, blood, brain, and excrements, both liquid and solid, of animals; hence, in part, the use of the latter as manures.

§ 254. Phosphorus, which is very largely employed in the manufacture of lucifer matches, is generally manufactured from the ashes of bones as follows:-The bone-ash, consisting of carbonate and phosphate of calcium, is treated with dilute sulphuric acid in excess, whereby sulphate of calcium is formed, carbonic acid escapes as a gas, and the phosphoric acid remains in solution along with sulphuric acid and lime, forming the so-called superphosphate of lime. The superphosphate is separated by decantation from the insoluble sulphate of calcium, and evaporated nearly to dryness with powdered charcoal. The mass so obtained is placed in earthenware retorts and submitted to a very high temperature. The carbon combines with the oxygen of the phosphoric acid, forming carbonic oxide, which escapes as a gas,

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whereupon the liberated phosphorus distils over and is collected under water, into which the neck of the retort dips. It is then remelted under water and purified by being forced by pressure through the pores of a leathern bag. It is afterwards usually cast into cylindrical sticks, in which form it occurs in commerce. § 255. Freshly made phosphorus is an almost colourless, slightly yellow, transparent solid of waxy consistence. Its density is 1.9; it melts at 44° C., and boils at 290° C. It may be distilled in a current of CO,, H, or N. Exposed to the air, phosphorus gradually oxidizes, gives rise to a faint bluish light (called phosphorescent) and a peculiar smell (ozone, § 83). Heated to 60° C. by friction or otherwise, it enters into rapid oxidation or takes fire, burning with intense light and giving off white clouds of phosphoric acid. Phosphorus is insoluble in water, but soluble in alcohol, ether, &c., and especially in bisulphide of carbon. When kept, as it always is, under water, it gradually gets coated with a white skin of unknown composition.

§ 256. When heated for several hours nearly to its boiling-point in some gas which does not act upon it, phosphorus undergoes a peculiar change in colour, in physical properties, and chemical energy. It becomes red, insoluble in bisulphide of carbon, much less fusible and oxidizable, and altogether less energetic chemically. This red phosphorus is often called amorphous phosphorus, and bears the symbol Ps. It is prepared by heating P for some hours to about 240° C. in a closed vessel, allowing the mass to cool, and washing it with CS2, which removes the unchanged P. So got it forms a vermilion-red powder. If kept for several days at a temperature of about 230°, it is changed into a brittle solid mass. PB is infusible; but when heated to 260° C. it is reconverted into ordinary P, and takes fire if exposed to 0.

$257. Lucifer matches are usually made by first dipping the ends of the wooden splinters into melted sulphur, and then into a paste made of water, gum, nitrate and chlorate of potassium, oxide of lead or of manganese, and phosphorus-the whole being so rubbed up together that the phosphorus is in a fine state of division. When the matches are dry and are rubbed, the small

particles of phosphorus begin to burn, the heat so developed liberates oxygen from the nitrate and chlorate of potassium and the metallic oxides. The increased supply of O so obtained assists the combustion of the rest of the P; and the result is the development of a heat sufficient to ignite the S, and thence the wood.

Combinations of Phosphorus with Oxygen.

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Fig. 66.

A

§ 258. Phosphoric acid, anhydrous, P2O..-When phosphorus is heated in dry air or oxygen (§ 80) it takes fire, and burns, giving off white clouds. These clouds consist of anhydrous phosphoric acid, P ̧0. Fig. 66 shows the usual arrangement for making a small quantity of this body. A is a dry glass inverted into a dry dish D, and covering a little cup C, in which a fragment of P is burning. After some time the P2O, is found condensed upon the sides of the glass and on the dish as a snow-white flaky deposit, and may be thence collected. Anhydrous phosphoric

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D

acid has a great affinity for water; it hisses when thrown into that liquid, on account of the heat developed by their union. This property of P2O, is made use of to remove the last traces of water from many liquids and gases-the liquid being shaken with the anhydrous acid, or the gas passed over it. P2O, volatilizes at a temperature below redness.

$259. Hydrated phosphoric acid, HPO,, may be got by dissolving the anhydrous acid in water, evaporating to a syrup, and heating to a red heat in a platinum vessel. On cooling, a brittle glassy solid is obtained, called glacial phosphoric acid, or

and containing one equivalent of water. The same body is more readily got by digesting P in a retort with somewhat diluted HNO,. The HNO, parts with O to the P; when all the P has dissolved, the contents of the retort are distilled until the excess

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of HNO, has been expelled; the excess of water is driven off in a platinum vessel.

§ 260. On exposing HPO, to the action of a quantity of water exactly equal to that already in the acid, crystals of H1PO, are got. Thus :

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If double the quantity of water is used, crystals of HPO are formed. Hence there are three hydrates of phosphoric acid, the mono-,ter-, and tetrahydrate. Of these, the terhydrate is the most stable. On boiling either HPО, or H ̧PO, for some time with water, they are converted into H,PO. Corresponding to each of these acids there are three series of salts-monobasic, tribasic, and tetrabasic. The monobasic phosphates are generally called metaphosphates, the tetrabasic are called pyrophosphates, and the tribasic are the common phosphates. Thus, if M be any metal, we may have

Or, again, either a pyro- or common phosphate may contain different bases, thus :

:

And common phosphate of sodium is tribasic phosphate of sodium and hydrogen.

Na,HPO, phosphate of sodium.

Mg,NH PO.

=

=

phosphate of magnesium and ammonium.

§ 261. Tribasic phosphoric acid and the common phosphates are recognized by giving a yellow phosphate of silver when mixed with soluble silver-salts. Phosphates of calcium, barium, magnesium, lead, &c. are also insoluble in water; but all phosphates are soluble in nitric acid.

$262. Phosphorous acid, anhydrous, P,O,, is got by burning P

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in a limited quantity of dry air. It appears as a white volatile solid, miscible with water.

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$263. Phosphorous acid, hydrated, H,PO,, is obtained by exposing P for a long time to moist air. The P gradually oxidizes, and the H,PO, may be collected in a suitable apparatus. H,PO, may be made more expeditiously and in a state of greater purity by decomposing the terchloride of phosphorus, PC1, (§ 268), by water. When these two bodies are brought into contact they decompose one another.

PC1, 3H,0 = H ̧PO ̧ + 3HC1.

On evaporation the HCl and excess of H2O are expelled.

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§ 264. Hypophosphorous acid, HOH ̧P.—When P is boiled with oxide of calcium (lime) and water, the water is decomposed, a part of the P escapes as a gas in combination with H (§ 265), the rest combines with O to form hypophosphorous acid, which then unites with lime, forming hypophosphite of calcium.

3CaO+P+9H20 = 3(Ca2(H ̧0 ̧P)) + 2PH ̧.

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If baryta be substituted for lime, a corresponding barium-salt is obtained, Ba2H,O,P. From this compound the free acid may be got by treating with just enough sulphuric acid to precipitate all the barium as BaSO,, and filtering from it. Hypophosphorous acid cannot exist either alone or in combination with bases without hydrogen; hence its formula is written HH,OPO or HO HP.

Combinations of Phosphorus with Hydrogen.

Three hydrides of phosphorus are known, PH, PH„, and PH ̧. The first is a solid, the second a liquid, the latter a gas. We shall only consider the latter.

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$265. Phosphoretted hydrogen, PH,.-This gas, which has some similarity to ammonia, may be looked on as ammonia in which the N has been replaced by P. We have (§ 264) seen it formed when H2O, CaO, and P are boiled together. It is made by heating in a flask, filled nearly full, hydrate of potassium, water, and phosphorus.

3KHO + 3H ̧0 + 4P = 3KH ̧OP + PH ̧.