to rise to the surface as a scum, leaving the phosphorus as a clear liquid beneath.

Since the advent of the electric furnace, phosphorus is now being manufactured direct from calcium phosphate by a process which threatens to entirely supersede the method of distillation already described. The calcium phosphate is mixed with carbon and simply heated in the electric furnace. At the high temperature of the electric arc the calcium phosphate is decomposed, the calcium uniting with the carbon to form calcium carbide, while the phosphorus in the state of vapour escapes along with carbon monoxide by the pipe P (Fig.119), and is condensed in suitable condensers

Ca1(PO4)2+14C=3CaC2+2P+8CO.

The molten calcium carbide is tapped off from the furnace from

D

FIG. 120.

time to time as fresh charges of phosphate and carbon are introduced.

Phosphorus usually comes into commerce either in the form of wedges or as sticks. The operation of casting the phosphorus into sticks is performed beneath water. A quantity of phosphorus beneath a shallow layer of water is placed in the vessel C (Fig. 120), which is contained in a tank of water through which a steam-coil passes. Connected to the phosphorus reservoir is a glass tube, G, which passes into a second shallow tank of cold water. On open

ing the cock D, the liquid phosphorus flows into the cold glass tube where it congeals, and it may then be drawn through as a continuous rod of phosphorus if care be taken not to draw it out faster than it solidifies. It is the custom to adopt a uniform length and thickness of stick, namely, 7 inches long and inch diameter. Nine such sticks weigh 1 lb.

Properties. When freshly prepared and kept in the dark, phosphorus is a translucent, almost colourless, wax-like solid. Even in the dark it soon loses its transparency and becomes

coated with an opaque white film; while if exposed to the light the film that forms becomes first yellow, then brown, and in time the phosphorus assumes a red and even a black colour throughout its entire mass. Its specific gravity at 16° is 1.82. At o phosphorus becomes moderately brittle, and a stick of it may be readily snapped, when its crystalline character will be seen. At 15° it becomes soft, and may be cut with a knife like wax. Phosphorus melts under water at 43.3°, and the liquid exhibits the property of suspended solidification. If the melted material, which has been cooled below its solidifying point, be touched with a fragment of phosphorus upon the end of a capillary glass tube, the mass instantly congeals with rise of temperature.*

Phosphorus contained in a closed vessel without water melts at as low a temperature as 30°,† and when heated in air to 34° it takes fire. At a temperature of 269° phosphorus boils and forms a colourless vapour.

Phosphorus is volatile at ordinary temperatures: if a small quantity of phosphorus be sealed in a vacuous tube, and the tube be placed in the dark, the phosphorus will slowly vaporise; and if one end of the tube be kept slightly cooler than the rest, the phosphorus will sublime upon that part in the form of brilliant, colourless, and highly refracting rhombic crystals, which retain their beauty as long as they are kept in the dark. The density of the vapour of phosphorus is 62.0, giving a molecular weight of 124.0, which is four times the atomic weight, showing that the molecule of phosphorus contains four atoms. Even at temperatures as high as 1040° these tetratomic molecules are stable, but it has been shown that at high temperatures dissociation begins to take place.

On account of its ready inflammability, phosphorus is always preserved under water, which exerts practically no solvent action upon it. It is extremely soluble in carbon disulphide, I part of this liquid dissolving 9.26 parts of phosphorus. On evaporation, the element is deposited in the form of colourless crystals. Phosphorus is also soluble, but to a less extent, in chloroform, benzene, turpentine, alcohol, olive oil, and many other solvents. A solution of phosphorus in carbon disulphide, when allowed to evaporate upon a piece of blotting-paper, leaves the element in so finely divided a condition, that its rapid oxidation almost immediately raises the * See "Chemical Lecture Experiments," new ed., Nos. 528, 529. + Readman.

temperature to the ignition point of the phosphorus, when it takes fire.

On exposure to moist air in the dark, phosphorus appears faintly luminous, emitting a pale greenish-white light, and at the same time evolving white fumes which possess an unpleasant, garlic-like smell, and are poisonous. These fumes consist mainly of phosphorus oxide, P406, and the glowing of the phosphorus is the result of its oxidation; phosphorus does not glow when placed in an inert gas which is perfectly free from admixed oxygen, although the presence of very small traces of free oxygen in such a gas is sufficient to cause the phosphorescence.

At a few degrees below o°, phosphorus ceases to glow in the air. Although the glowing is due to oxidation, phosphorus does not appear luminous in pure oxygen at temperatures below about 15°. If, therefore, a stick of phosphorus which is glowing in the air, be immersed in a jar of oxygen, its phosphorescence is at once stopped. If, however, the oxygen be slightly rarefied, the phosphorus again becomes luminous. Similarly, the phosphorescence that is exhibited in air is stopped if the air be compressed.* The glow of phosphorus is believed to be associated with the formation of ozone, for the presence in the air of traces of such gases and vapours as ethylene, turpentine, or ether, which are known to possess the power of destroying ozone, at once stops the glowing of a stick of phosphorus.

Phosphorus is incapable of uniting with oxygen if the gas be perfectly pure and free from aqueous vapour. It has been shown that in oxygen which has been dried by prolonged exposure to the desiccating action of phosphorus pentoxide, phosphorus may not only be melted, but even distilled, without any combination with the oxygen taking place.

If water, beneath which is a small quantity of melted phosphorus, be boiled, the phosphorus vaporises with the steam, and renders the steam luminous use is made of this property, as a means of detecting free phosphorus, in toxicological analysis.

Phosphorus is a powerfully poisonous substance; in large doses it causes death in a few hours, in smaller quantities it produces stomachic pains and sickness, usually ending in convulsion. Persons constantly exposed to the vapours arising from the handling of phosphorus, either in its manufacture or in the manufacture

* Chemical Lecture Experiments," new ed., Nos. 530 to 534.

of matches, are very liable to suffer from caries of the bones of the jaw and nose; it is believed that this injurious effect is caused by the white fumes which are the product of oxidation, and not by the actual vapour of phosphorus.

Red Phosphorus.-When phosphorus is heated to a temperature between 240° and 250°, out of contact with air, it passes into an allotropic modification. The same transformation takes place when phosphorus is heated to 200° with an extremely small proportion of iodine.

Red phosphorus is manufactured by heating ordinary phosphorus

in a cast-iron pot, provided with a cover, through which passes a short open pipe, D (Fig. 121). The pot is carefully and uniformly heated to between 240° and 250°, as indicated by the thermometers C C', which are encased in metal tubes, to prevent the phosphorus from attacking the glass. A small quantity of the phosphorus becomes oxidised by the air within the vessel, but after this atmospheric oxygen is used up, no further oxidation takes place. If the temperature be allowed to rise above 260°, the red phosphorus is

reconverted into the ordinary modification, and with the evolution of so much heat, that unless the open tube be provided, as a safetyvalve, the iron vessel is liable to burst. The material that is obtained at the end of the operation is in the form of hard, solid lumps, which still contain a certain amount of the unchanged phosphorus mixed with them. It is first ground to powder beneath water, and then boiled with a solution of sodium hydroxide (caustic soda), to remove the ordinary phosphorus, and finally washed and dried.

It

Properties.-Red phosphorus, as usually sent into commerce, is a chocolate-red powder, having a specific gravity of 2.25. is not luminous in the dark, and has no taste or smell. It is not poisonous, and when taken into the system is excreted unchanged. It is not soluble in carbon disulphide, or in any of the solvents which dissolve ordinary phosphorus. Red phosphorus is unaffected by exposure to dry air or oxygen, but in the presence of moisture it is very slowly oxidised. If red phosphorus which has been perfectly freed from ordinary phosphorus, and carefully washed and dried, be exposed to air and moisture, it is found after the lapse of some time to have become acid, owing to slight oxidation into phosphoric acid. When heated in contact with air, red phosphorus does not ignite below a temperature of 240°. Red phosphorus may be obtained in the form of rhombohedral crystals by heating the substance under pressure to a temperature of 580°.

The chief use of phosphorus is in the manufacture of matches. When ordinary phosphorus is employed, the bundles of wooden splints are first tipped with melted paraffin wax, and afterwards dipped into a paste, made of an emulsion of phosphorus, chlorate of potash, and glue. Matches so made ignite when rubbed upon any rough surface; the paraffin (which is sometimes replaced by sulphur) serving to transmit the combustion from the phosphorus to the wood. Since the discovery of red phosphorus, and its noninjurious properties, the old phosphorus match has been largely superseded by the so-called safety matches. In these matches the splints are tipped with a mixture of potassium chlorate, potassium dichromate, red lead, and antimony sulphide, and they are ignited by being rubbed upon a prepared surface consisting of a mixture of antimony sulphide and red phosphorus. Although these matches will not ignite by ordinary friction upon any but the specially prepared surface, they may be inflamed by being swiftly drawn. along a sheet of ground glass or strip of linoleum.