Phosphorus (sym. P; atom. wt. 31; molec. wt., vapour = = 124; do. at white heat, = 62) is one of the metalloids or non-metallic elements, although, in its combining relations, it is closely connected with the metals arsenic and antimony. This substance affords an excellent example of Allotropy (q.v.); that is to say, it may be made to occur under different forms presenting different properties. Ordinary phosphorus and the red variety are the only important forms. We shall speak of them as phosphorus and red phosphorus respectively.
Phosphorus at ordinary temperatures is an almost colourless or faintly yellow solid substance of sp. gr. 1.836, having the glistening appearance and the consistence of wax, and evolving a disagreeable allaceous odour, which, however, is probably due to the action of the oxygen of the air upon it. It fuses at . (.) into a colourless fluid; and, if the air be excluded, it boils at . (.), and is converted into a colourless vapour of sp. gr. 4.35 (air = 1.00). If, however, it be heated to about . (.) in the air it catches fire, burns with a brilliant white flame, and is converted into phosphoric anhydride; and indeed it is so inflammable that it will catch fire at ordinary temperatures by mere friction. As the burns which it occasions are often severe and dangerous, great caution is required in handling it; and in consequence of the readiness with which it catches fire, and of its tendency to oxidise when exposed to the air at a temperature higher than . (.), it is always kept in water, in which it is insoluble. It is slightly soluble in ether, but dissolves freely in benzol, in the fixed and essential oils, and in bisulphide of carbon; and, on allowing its solution in a volatile solvent to fall upon filtering paper, the finely-divided phosphorus absorbs oxygen so rapidly as spontaneously to catch fire as soon as the solvent has evaporated. Phosphorus shines in the dark from the slow combustion which it undergoes; and hence its name, from the Greek words phôs, 'light,' and phoros, 'bearing.' Its power of forming ozone is noticed in the article on that substance. Taken internally, phosphorus is a very powerful irritant poison; and it is the active ingredient of some of the preparations employed for the destruction of vermin. Its fumes give rise to a peculiar form of necrosis of the jaw and to fatty degeneration of the kidney, which used to be common amongst the makers of lucifer matches.
Red phosphorus is prepared from the ordinary variety by heating the latter in a closed iron vessel to a temperature of . (.). It was discovered by Schrötter in 1845, and is a compact solid substance of a dark red colour, and with a metallic lustre. It differs much in its properties from common phosphorus, being devoid of odour, does not shine in the dark, undergoes no change when exposed to the air even for years, and cannot be set on fire by moderate friction or percussion. Moreover, it is insoluble in all the solvents of common phosphorus, and is not poisonous. It bears heating to nearly . (.) without taking fire, and has a specific gravity of 2.16. By using red instead of white phosphorus for lucifer matches there is no risk to the health of the operatives. Safety matches contain chlorate of potash and ignite on a surface containing a mixture of red phosphorus and sulphide of antimony (see MATCHES).
Phosphorus is not met with in nature in an uncombined state; but it occurs in rocks of various kinds and ages, and in some countries abundantly as apatite or phosphorite, both of which are composed of phosphate of lime. It is also found in the form of Coprolites (q.v.), or the dung of extinct animals, and more rarely as wavellite (phosphate of alumina) and vivianite (phosphate of iron). In many volcanic rocks apatite is found in minute crystals or particles, and by the decomposition of these rocks it passes into the soil. From the soil it is extracted by plants, which accumulate it (especially in the seeds of the cereals) in quantity sufficient for the wants of the animals which they supply with food. In the animal system phosphate of lime forms 57 per cent. of the bones; phosphates of the alkalies, especially of soda, occur freely in the animal fluids; and in fibrin, albumen, and nervous matter phosphorus is universally present, although we do not clearly know in what form of combination it occurs.
Phosphorus was originally discovered in 1669 by Brandt, a Hamburg chemist, who obtained it from urine. Gahn and Scheele were, however, the first to discover its presence in bone, and to employ that material for its preparation. The following are the leading steps of the method now usually employed in obtaining it on the large scale. Bones are burned to whiteness, and powdered; and this bone-ash is then mixed with sulphuric acid in such quantity as to decompose the phosphate of lime occurring in the ash, , partly into insoluble sulphate of lime, partly into a soluble superphosphate of lime, whose composition is represented by the formula . The solution of the superphosphate is evaporated to a syrup, mixed with charcoal, and submitted to distillation in an earthen retort exposed to a red heat. Phosphorus rises in vapour, and is conveyed by means of a bent tube into water, in which it condenses in yellow drops. Two distinct processes take place within the retort. The first consists in the decomposition of the superphosphate of lime into bone-earth and hydrated phosphoric acid; while the second consists in the deoxidation, by means of the carbon, of the liberated phosphoric acid into phosphorus—a process accompanied by the evolution of carbonic oxide gas. After being pressed in a fused state through wash-leather, and further purified, it is forced into tubes, in which it is allowed to solidify, giving it the usual form of sticks. Sombrerite (see APATITE) is now largely substituted for bones in the manufacture of phosphorus.
Phosphorus forms two known oxides—phosphorous anhydride, , obtained by the slow oxidation of phosphorus in dry air; phosphoric anhydride, , obtained by the combustion of phosphorus in an excess of dry air or oxygen. The latter is a snow-like substance which has a great avidity for water, and is therefore very useful in the laboratory as a desiccating agent.
Phosphorus forms five acids—hypophosphorous acid, , monobasic (the anhydride, , is not known); phosphorous acid, (), dibasic, obtained by slow oxidation of phosphorus in moist air or by passing chlorine gas through phosphorus under hot water; metaphosphoric acid, (), monobasic, obtained by dissolving in water or by heating ordinary phosphoric acid to redness; pyrophos- phoric acid, , tetrabasic, obtained by acting upon lead pyrophosphate with sulphuretted hydrogen, the lead salt being prepared from soda pyrophosphate, which is obtained by heating such a phosphate as , of which two molecules give off one of water and form ; phosphoric or orthophosphoric acid, , tribasic, prepared by boiling in water, or from bone-ash and sulphuric acid; the last-named acid has three sets of salts, the hydrogen in the acid being more or less completely replaced—e.g. , , and , which are obtained by more or less completely neutralising phosphoric acid with soda.
The discovery of phosphoric acid was made in 1740 by Marggraf; the discovery of its true chemical nature is, however, due to Lavoisier, and that of its various modifications and its polybasicity to the investigations of Graham. The salts of phosphoric acid are phosphates; and they are of great value, when in a soluble form or when they can become soluble through weathering, as manures (see MANURE). Of late years mineral phosphates have been largely worked up; and a new source of phosphatic manure has become available in the alkaline slag of the Thomas-Gilchrist process, whereby the phosphorus is removed from iron by means of lime.
Phosphorus combines with hydrogen in three proportions to form phosphuretted hydrogen gas, ; liquid phosphide of hydrogen, ; and solid phosphide of hydrogen, . Of these the first alone requires notice in these pages. There are various processes for obtaining the gas, one of the simplest being by boiling fragments of phosphorus in a concentrated solution of hydrated potash, in which case hypophosphate of potash is formed, while phosphuretted hydrogen gas is extricated. The reaction is explained by the equation . The gas thus evolved is colourless, possesses a characteristic fetid odour, and has the remarkable property of taking fire spontaneously in atmospheric air or in oxygen gas, with the production of anhydrous metaphosphoric acid and water. There is reason to believe that perfectly pure phosphuretted hydrogen gas does not possess the power of igniting spontaneously, and that the self-lighting gas always contains a minute quantity of the vapour of the liquid phosphide, . The luminous phenomenon known as Ignis Fatuus (q.v.) has been referred to the natural evolution of the gas. The compounds of phosphorus with sulphur, chlorine, iodine, bromine, &c. are not of practical importance.
Phosphorus is rarely employed in medicine as a nervous stimulant, in consequence of its poisonous properties. The symptoms induced by this poison are those of acute inflammation of the stomach and bowels; the treatment is the administration of large quantities of mild demulcent fluids, and of magnesia. Dilute phosphoric acid is included in the British Pharmacopœia, but is not very much employed. It may be prescribed in much the same cases as those in which sulphuric and nitric acids are employed, and is less likely to disturb the digestive functions, if employed for a long period, than the other mineral acids. It has also been recommended, when properly diluted, as a serviceable acidulated drink for assuaging the thirst in diabetes. It may be prescribed in half-drachm doses. For medicinal phosphates, see SYRUP.