226

PREPARATION OF AMORPHOUS PHOSPHORUS.

narrow tube. The steam charged with vapour of phosphorus has all the appearance of a blue flame in a darkened room, but of course combustibles are not inflamed by it, since its temperature is not higher than 212° F. Phosphorus may be distilled with perfect safety in an atmosphere of carbonic acid, the neck of the retort being allowed to dip under water in the receiver.

Although ordinary phosphorus is of a decidedly glassy or vitreous structure, and not at all crystalline, it may be obtained in dodecahedral crystals, by allowing its solution in bisulphide of carbon to evaporate in an atmosphere of carbonic acid.

The conversion of ordinary phosphorus into the red or amorphous phosphorus is one of the most striking instances of allotropic modification. When phosphorus is heated for a considerable length of time to about 450° F. in vacuo, or in an atmosphere in which it cannot burn, it becomes converted into a red infusible mass of amorphous phosphorus. This form of phosphorus differs as widely from the vitreous form as graphite differs from diamond. It is almost unchangeable in the air, evolves no vapour, is not luminous, cannot be inflamed by friction, or even by any heat short of 500° F., when it actually becomes reconverted into ordinary phosphorus. Amorphous phosphorus is insoluble in the solvents for ordinary phosphorus. The two varieties also differ greatly in specific gravity, that of the ordinary phosphorus being 1.83, and of the amorphous variety 2.14.

*

The conversion of vitreous into amorphous phosphorus may be effected by heating it in a flask (A, fig. 204) placed in an oil-bath (B), maintained at a temperature ranging from 450° to 460° F., the flask being furnished with a bent tube (C) dipping

Fig. 204.

into mercury, and with another tube (D) for supplying carbonic acid gas, dried by passing over chloride of calcium. The flask should be thoroughly filled with carbonic acid before applying heat, and the tube delivering it may then be closed with a small clamp (E). After exposure to heat for about forty hours, but little ordinary phosphorus will remain, and this may be removed by allowing the mass to remain in contact with bisulphide of carbon for some hours, and subsequently washing it with fresh bisulphide of carbon till the latter leaves no phosphorus when evaporated.

On the large scale, the red phosphorus is prepared by heating about 200 lbs. of vitreous phosphorus to

450° F. in an iron boiler. After three or four weeks the phosphorus is found to be converted into a hard red brittle mass, which is ground by mill-stones under water, and separated from the ordinary phosphorus either by bisulphide of carbon or caustie soda, in which the latter is soluble. The temperature requires careful regulation, for if it be allowed to rise to 500°, the red phosphorus quickly resumes the vitreous condition, evolving the heat which it had absorbed during its conversion, and thus converting much of the phosphorus into vapour. This reconversion may be shown by heating a little red phosphorus in a narrow test-tube, when drops of vitreous phosphorus condense on the cool part of the tube. The colour of different specimens of amorphous phosphorus varies considerably; that prepared on the large scale is usually of a dark purplish colour, but it may be obtained of a bright scarlet colour. Rhombohedral crystals of phosphorus, resembling crystals of arsenic in form and metallic appearance, have been obtained by fusing phosphorus with lead, and dissolving out the latter with diluted nitric acid (sp. gr. 1·1).

According to Hittorf, the reconversion does not take place till 800° F., the red phosphorus being convertible into vapour below that temperature, without fusion.

PRECIPITATION OF METALS BY PHOSPHORUS.

227

Ordinary phosphorus is very poisonous, whilst amorphous phosphorus appears to be harmless. The former is employed, mixed with fatty substances, for poisoning rats and beetles. Cases are, unhappily, not very rare, of children being poisoned by sucking the phosphorus composition on lucifer matches. The vapour of phosphorus also produces a very injurious effect upon the persons engaged in the manufacture of lucifer matches, resulting in the decay of the lower jaw-bone. This evil is much mitigated by good ventilation, or by diffusing turpentine vapour through the air of the work-room, and attempts have been made to obviate it entirely by substituting amorphous phosphorus for the ordinary variety, but, as might be expected, the matches thus made are not so sensitive to friction as those in which the vitreous phosphorus is used.

The difference between the two varieties of phosphorus, in respect to chemical energy, is seen when they are placed in contact with a little iodine on a plate, when the ordinary phosphorus undergoes combustion, and the red phosphorus remains unaltered.

Black phosphorus has been obtained by heating vitreous phosphorus to a little above its melting point and suddenly cooling it. It is reconverted by fusion and slow cooling. Viscous phosphorus results from the sudden cooling of phosphorus heated nearly to its boiling point.

Ordinary phosphorus is capable of direct union with oxygen, chlorine, bromine, iodine, sulphur, and most of the metals, with which it forms phosphides or phosphurets. Even gold and platinum unite with this element when heated, so that crucibles of these metals are liable to corrosion when heated in contact with a phosphate in the presence of a reducing agent, such as carbon. Thus the inside of a platinum dish or crucible is roughened when vegetable or animal substances containing phosphates are incinerated in it. The presence even of small quantities of phosphorus in metallic iron or copper produces considerable effect upon their physical qualities.

If a

Phosphorus has the property, a very remarkable one in a non-metal, of precipitating some metals from their solutions in the metallic state. stick of phosphorus be placed in a solution of sulphate of copper, it becomes coated with metallic copper, the phosphorus appropriating the oxygen. This has been turned to advantage in copying very delicate objects by the electrotype process, for by exposing them to the action of a solution of phosphorus in ether or bisulphide of carbon, and afterwards to that of a solution of copper, they acquire the requisite conducting metallic film, even on their finest filaments. Solutions of silver and gold are reduced in a similar manner by phosphorus.

By floating very minute scales of ordinary phosphorus upon a dilute solution of chloride of gold, the metal will be reduced in the form of an extremely thin film, which may be raised upon a glass plate, and will be found to have various shades of green and violet by transmitted light, dependent upon its thickness, whilst its thickest part exhibits the ordinary colour of the metal to reflected light. By heating the films on the plate, various shades of amethyst and ruby are developed. If a very dilute solution of chloride of gold in distilled water be placed in a perfectly clean bottle, and a few drops of ether, in which phosphorus has been dissolved, poured into it, a beautiful ruby-coloured liquid is obtained, the colour of which is due to metallic gold in an extremely finely divided state, and on allowing it to stand for some months, the metal subsides as a purple powder, leaving the liquid colourless. If any saline impurity be present in the gold solution, the colour of the reduced gold will be amethyst or blue. These experiments (Faraday) illustrate very strikingly the use of gold for imparting ruby and purple tints to glass and the glaze of porcelain.

228

MANUFACTURE OF LUCIFER MATCHES.

162. Lucifer matches are made by tipping the wood with sulphur or wax or paraffine to convey the flame, and afterwards with the match composition, which is generally composed of saltpetre or chlorate of potash, phosphorus, red lead, and glue, and depends for its action on the easy inflammation, by friction, of phosphorus when mixed with oxidising agents like saltpetre (KO. NO,), chlorate of potash (KO. ClO), or red lead (Pb,O), the glue only serving to bind the composition together and attach it to the wood. The composition used by different makers varies much in the nature and proportions of the ingredients. In this country, chlorate of potash is most commonly employed as the oxidising agent, such matches usually kindling with a slight detonation; but the German manufacturers prefer either nitrate of potash or nitrate of lead, together with binoxide of lead or with red lead, which produce silent matches.

Sulphide of antimony (which is inflamed by friction with chlorate of potash, see p. 157) is also used in those compositions in which a part of the phosphorus is employed in the amorphous form, and fine sand or powdered glass is very commonly added to increase the susceptibility of the mixture to inflammation by friction.

The match composition is coloured either with ultramarine blue, Prussian blue, or vermilion. In preparing the composition, the glue and the nitre or chlorate of potash are dissolved in hot water, the phosphorus then added and carefully stirred in until intimately mixed, the whole being kept at a temperature of about 90° F. The fine sand and colouring matter are then added, and when the mixture is complete, it is spread out upon a stone slab heated by steam, and the sulphured ends of the matches are dipped into it.

The safety matches, which refuse to ignite unless rubbed upon the bottom of the box, are tipped with a mixture of sulphide of antimony, chlorate of potash, and powdered glass, which is not sufficiently sensitive to be ignited by any ordinary friction, but inflames at once when rubbed upon the amorphous phosphorus mixed with glass which coats the rubber beneath the box. On this principle some French matches have been made which can be ignited only by breaking the match and rubbing the two ends together.

It would be very desirable to dispense entirely with the use of phosphorus in lucifer matches, not only because of the danger from accident and disease in the manufacture, but because a very large quantity of phosphate of lime which ought to be employed for agricultural purposes is now devoted to the preparation of phosphorus, of which six tons are said to be consumed annually in this country for the manufacture of matches. The most successful attempt in this direction appears to be the employment of a mixture of chlorate of potash and hyposulphite of lead, in place of the ordinary phosphorus composition.

For illustration, very excellent matches may be made upon the small scale in the following manner. The slips of wood are dipped in melted sulphur so as to acquire a slight coating. 30 grains of gelatine or isinglass are dissolved in 2 drachms of water in a porcelain dish placed upon a steam-bath; 20 grains of ordinary phosphorus are then added, and well mixed in with a piece of stick; to this mixture are added, in succession, 15 grains of red lead and 50 grains of powdered chlorate of potash. The sulphured matches are dipped into this paste, and left to dry in the air. To make the safety matches: 10 grains of powdered chlorate of potash and 10 grains of sulphide of antimony are made into a paste with a few drops of a warm solution of 20 grains gelatine in 2 drachms water, the sulphured matches being tipped with this composition. The rubber is prepared with 20 grains of amorphous

COMPOUNDS OF PHOSPHORUS AND OXYGEN.

229

phosphorus, and 10 grains of finely-powdered glass, mixed with the solution of gelatine, and painted on paper or card-board with a brush.

163. Phosphorus-fuze composition.-To ignite the Armstrong percussion shells, a very sensitive detonating composition is employed, which is composed of amorphous phosphorus, chlorate of potash, shellac, and powdered glass made into a paste with spirit of wine. This is placed in the little cap designed for it, and when dry is waterproofed with a little shellac dissolved in spirit.

Such a composition may be prepared with care in the following manner:-4 grains of powdered chlorate of potash are moistened on a plate with 6 drops of spirit of wine, 4 grains of powdered amorphous phosphorus are added, and the whole mixed at arm's-length with a bone knife, avoiding great pressure. The mixture, which should still be quite moist, is spread in small portions upon ten or twelve pieces of filtering paper, and left in a safe place to dry. If one of these be gently pressed with a stick, it explodes with great violence. It is dangerous to press it with the blade of a knife, as the latter is commonly broken, and the pieces projected with considerable force. A stick dipped in oil of vitriol of course explodes it immediately. If a bullet be placed very lightly upon one of the pellets, and the paper tenderly wrapped round it, a percussion shell may be extemporised, which explodes with a loud report when dropped upon the floor.

OXIDES OF PHOSPHORUS.

164. There are only two compounds of phosphorus with oxygen which have been obtained and satisfactorily examined in the separate state, viz., phosphorous acid (PO), and phosphoric acid (PO). The sub-oxide of phosphorus (PO) is said to have been obtained, but very little is known of it, and hypophosphorous acid (PO) has only been obtained in combination with water.

165. Phosphoric acid is by far the most important of the compounds of phosphorus. It has been already noticed as almost the only form of combination in which that element is met with in nature, and as an indispensable ingredient in the food of plants and animals. No other mineral substance can bear comparison with it as a measure of the capability of a country to support animal life. The acid itself is very useful in calicoprinting and some other arts.

The mineral sources of this acid appear to be phosphorite, coprolite, and apatite, all consisting essentially of phosphate of lime (3CaO. PO), but associated in each case with fluoride of calcium, which is also contained, with phosphate of lime, in bones, and would appear to indicate an organic origin for these minerals. Phosphorite is an earthy-looking substance,

230

PREPARATION OF PHOSPHORIC ACID.

forming large deposits in Estremadura. Apatite (from áraráw, to cheat, in allusion to mistakes in its early analysis) occurs in prismatic crystals, and is met with in the Cornish tin-veins. Both these minerals are largely imported from Spain, Norway, and America, for use in this country as a manure.

Coprolites (kómpos, dung, Xíbos, a stone, from the idea that they were petrified dung) are rounded nodules of phosphate of lime, which are found abundantly in this country.

Large quantities of phosphoric acid, combined with lime and magnesia, are imported in the form of guano, the partially decomposed excrement of sea-fowl, which sometimes contains one-fourth of its weight of phosphoric acid.

Bones, however, must be regarded as the chief immediate source whence the phosphate of lime for agricultural purposes is derived.

Hydrated phosphoric acid is obtained from bone-ash by decomposing it with sulphuric acid, so as to remove as much of the lime as possible in the form of sulphate, which is strained off, and the acid liquid neutralised with carbonate of ammonia, which precipitates any unchanged phosphate of lime, and converts the phosphoric acid into phosphate of ammonia, consisting of phosphoric acid, water, and ammonia. On evaporating the solution, and heating the phosphate of ammonia, the ammonia is expelled, and hydrated phosphoric acid (HO. PO) is left in a fused state, solidifying to a glass on cooling. Thus prepared, however, it always retains some ammonia, and is contaminated with soda derived from the bones.

The pure hydrated acid is prepared by oxidising phosphorus with diluted nitric acid (sp. gr. 1-2), and evaporating the solution in a platinum dish, until the hydrated phosphoric acid begins to volatilise in white fumes

Some phosphorous acid is formed at an intermediate stage. A transparent glass (glacial phosphoric acid) is thus obtained, which eagerly absorbs moisture from the air, and becomes liquid.