by Vincenzo Cascariolo, a cobbler of Bologna, in about 1602. | chlorophane, a variety of fluor-spar, the heat of the hand is sufficient. This was followed by the discovery of a number of other sub- Many yellow diamonds exhibit this form of luminescence. It stances which become luminous either after exposure to light necessary. has been shown, however, that a previous exposure to light is always Sir James Dewar found that if ammonium platinoor on heating, or by attrition, and to which the general name of cyanide, Balmain's paint and some other substances are cooled to "phosphori" (from øŵs and pópos, bringing light) was given. the temperature of liquid air and exposed to light, they do not Among these may be mentioned Homberg's phosphorus (calcium phosphoresce, but as soon as they are allowed to warm up to the chloride), John Canton's phosphorus (calcium sulphide) and hand, some bodies, such as gelatin, celluloid, paraffin and ivory, ordinary temperature they emit a brilliant light. On the other Balduin's phosphorus (calcium nitrate). Of late years it has are phosphorescent at very low temperatures, but lose the property been found convenient to limit the strict meaning of the word at ordinary temperatures. "phosphorescence" to the case of bodies which, after exposure by friction, percussion, cleavage or such mechanical means. Calcium Triboluminescence (from Tpißew, to rub) is luminescence excited to light, become self-luminous (even if only for a fraction of a chloride, prepared at a red heat, exhibits this property. If sugar second). The general term "luminescence" has been proposed is broken in the dark, or two crystals of quartz rubbed together, by E. Wiedemann to include all cases in which bodies give off or a piece of mica cleft, a flash of light is seen, but this is probably light not due to ignition. This general term embraces several of electrical origin. Closely allied to this form of luminescence is subdivisions. Thus, fluorescence (q.v.) and phosphorescence substances crystallize from solution or after fusion. This property crystalloluminescence, a phosphorescent light seen when some are included under the same heading, photoluminescence," is exhibited by arsenious acid when crystallizing from solution in being distinguished from each other only by the fact that hydrochloric acid. fluorescent bodies emit their characteristic light only while chemical action produces light without any great rise of temperature. Chemiluminescence is the name given to those cases in which under the influence of the exciting illumination, while phos- Phosphorus exposed to moist air in a dark room shines with a soft phorescent bodies are luminous for an appreciable time after light due to slow oxidation. Decaying wood and other vegetable the exciting light is cut off. substances often exhibit the same property.

Phosphorescence, in its restricted meaning as above explained, is most strikingly exhibited by the artificial sulphides of calcium, strontium and barium. If any of these substances is exposed for some time to daylight, or, better, to direct sunlight, or to the light of the electric arc, it will shine for hours in the dark with a soft coloured light. The colour depends not only on the nature of the substance, but also on its physical condition, and on its temperature during insolation, that is, exposure to the sun's rays. Thus the phosphorescent light emitted by calcium sulphide may be orangeyellow, yellow, green or violet, according to the method of preparation and the materials used. Balmain's luminous paint, a preparation of calcium sulphide, shines with a white light. The colour also depends on the temperature during exposure to light. Thus A. E. Becquerel found that the light given by a specimen of strontium sulphide changed from violet to blue, green, yellow and orange, as the temperature during the corresponding previous insolation was 20°, 40°, 70°, 100 or 200° C. The duration of phosphorescence varies greatly with different substances. It may last for days or for only a fraction of a second. As in the case of fluorescent bodies, the light produced by phosphorescent substances consists commonly of rays less refrangible than those of the exciting light. Thus the ultra-violet portion of the spectrum is usually the most efficient in exciting rays belonging to the visible part of the spectrum. V. Klatt and Ph. Lenard (Wied. Ann., 1889, xxxviii. 90), have shown that the phosphorescence of calcium sulphide and other phosphori depends on the presence of minute quantities of other substances, such as copper, bismuth and manganese. The maximum intensity of phosphorescent light is obtained when a certain definite proportion of the impurity is present, and the intensity is diminished if this proportion is increased. It appears likely that when a phosphorescent body is exposed to light, the energy of the light is stored up in some kind of strain energy, and that the phosphorescent light is given out during a more or less slow recovery from this state of strain. Klatt and Lenard have shown that the sulphides of the alkaline earths lose the property of phosphorescing when subjected to heavy pressure. Many fluorescent solutions become briefly phosphorescent when rendered solid by gelatin.

Electroluminescence is luminescence due to electrical causes. Many gases are phosphorescent for a short time after an electric discharge has been passed through them, and some solid substances, especially diamonds and rubies, are strongly phosphorescent when exposed to kathode rays in a vacuum tube. See generally, Winkelmann, Handbuch der Physik, Bd. vi. (1906); E. Becquerel, La Lumière (1867). (J. R. C.)

Phosphorescence in Zoology.

The emission of light by living substance is a widespread occurrence, and is part of the general metabolism by which the potential energy introduced as food is transformed into kinetic energy and appears in the form of movement, heat, electricity and light. In many cases it is probably an accidental byproduct, and like the heat radiated by living tissues, is not necessarily of use to the organism. But in other cases the capacity to produce light is awakened on stimulation, as when the wind ripples the surface of the sea, or when the water is disturbed by the blade of an oar. It has been suggested that the response to the stimulus may be protective, and that enemies are frightened by the flash of light. In luminous insects and deep-sea fish the power of emitting light appears to have a special significance, and very elaborate mechanisms have been developed. The pale glow of phosphorescence has a certain resemblance to the light emitted by phosphorus, and it was an early suggestion that the phenomenon in living organisms was due to that substance. Phosphorus, however, and its luminous compounds are deadly poisons to all living tissues, and never occur in them in the course of natural metabolism, and the phosphorescence of life cannot therefore be assigned to the oxidation of phosphorus. On the other hand, it is certainly the result of a process of oxidation, as the emission of light continues only in the presence of oxygen. J. H. Fabre showed in 1855 When the duration of phosphorescence is brief, some mechanical that the luminous fungus, Agaricus, discharges more carbonic device becomes necessary to detect it. The earliest and best-acid when it is emitting light, and Max Schultze in 1865 showed known instrument for this purpose is Becquerel's phosphoroscope. It consists essentially of a shallow drum, in whose ends two eccentric holes, exactly opposite one another, are cut. Inside it are fixed two equal metal disks, attached perpendicularly to an axis, and divided into the same number of sectors, the alternate sectors of each being cut out. One of these disks is close to one end of the drum, the other to the opposite end, and the sectors are so arranged that, when the disks are made to rotate, the hole in one end is open while that in the other is closed, and vice versa. If the eye be placed near one hole, and a ray of sunlight be admitted by the other, it is obvious that while the sun shines on an object inside the drum the aperture next the eye is closed, and vice versa. If the disks be made to revolve with great velocity by means of a train of toothed wheels the object will be presented to the eye almost instantly after it has been exposed to sunlight, and these presentations succeed one another so rapidly as to produce a sense of continuous vision. By means of this apparatus we can test with considerable accuracy the duration of the phenomenon after the light has been cut off. For this purpose we require to know merely the number of sectors in the disks and the rate at which they are turned.

Thermoluminescence.-Some bodies which do not emit light at ordinary temperatures in a dark room begin to do so if they are heated to a temperature below a visible red heat. In the case of

that in insects the luminous cells are closely associated with the tracheae, and that during phosphorescence they withdraw oxygen from them. In 1880 B. Radziszewski showed that many fats, ethereal oils and alcohols emit light when slowly combined with oxygen in alkaline fluids at appropriate temperatures. Probably the phosphorescence of organisms is due to a similar process acting on the many fats, oils and similar substances found in living cells. The colour varies much in different organisms; green has been observed in the glow-sworm, fire-flies, brittle-stars, centipedes and annelids; blue in the Italian fire-fly (Luciola italica); blue and light green are the predominant colours in the phosphorescence of marine organisms, but red and lilac have also been observed. The Lantern-Fly (Fulgora pyrorhynchus) is said to have a purple light, and E. H. Giglioli has recorded that an individual Appendicularia appeared first red, and then blue, and then green. P. Panceri, chiefly in the case of Salps, and S. P. Langley and F. W. Very in the case of Pyrophorus, have investigated the light spectroscopically, and

found that it consisted of a continuous band without separate | Halosauridae and Anomalopidae), whilst numerous simple luminous bright lines. The solar spectrum extends farther both towards the violet and the red ends, but is less intense in the green when equal luminosities are compared.

essential oils (oil of cloves) showed the same character, whilst in others (oils of mace and aniseed) there was no phosphorescence. He also noticed a strong garlic-like odour, which we now know to be due to ozone. Frederick Slare noticed that the luminosity increased when the air was rarefied, an observation confirmed by Hawksbee and Homberg, and which was possibly the basis of Berzelius's theory that the luminosity depended on the volatility of the element and not on the presence of oxygen. Lampadius, however, showed that there was no phosphorescence in a Torricellian vacuum; and other experimenters proved that oxygen was essential to the process. It depends on the partial pressure of the oxygen and also on temperature. In compressed air at ordinary temperature there is no glowing, but it may be brought about by heating. Again, in oxygen under ordinary conditions there is no phosphorescence, but if the gas be heated to 25° glowing occurs, as is also the case if the pressure be diminished or the gas diluted. It is also remarkable that many gases and vapours, e.g. Cl, Br, I,NH3, N2O, NO2, H2S, SO2, CS2, CH4, C2H4, inhibit the phosphorescence.

condenser, where it is condensed. It is then cast under water. | minutely studied by Boyle, who found that solutions in some The calcium silicate remains in the furnace in the form of a liquid slag, which may be run off, so that the action is practically continuous. Kaolin may with advantage be used in addition to or in part substitution for sand, because the double silicate thus formed is more fusible than the single silicate of lime. The alternating current is generally used, the action not being electrolytic. One of the special advantages of the electrical over the older process is that the distilling vessels have a longer life, owing to the fact that they are not externally heated, and so subjected to a relatively high temperature when in contact with the corrosive slag formed in the process. The Readman-Parker process (see Jour. Soc. Chem. Ind., 1891, x. 445) appears to be very generally adopted. Readman, experimenting with a Cowles furnace in Staffordshire in 1888, patented his process, and in the same year Parker and Robinson, working independently, patented a similar one. The two inventors then cooperated, an experimental plant was run successfully, and the patents were taken over by the leading manufacturers. With the object of obtaining a valuable by-product in place of the slag produced in this furnace, several patentees (e.g. Hilbert and Frank, Billaudot, Bradley and Jacobs, and others) have sought to combine the manufacture of calcium carbide and phosphorus by using only calcium phosphate and carbon, effecting direct reduction by carbon at a high temperature.

The crude phosphorus is purified by melting under water and then filtering through animal black and afterwards through chamois leather, or by treating it, when molten, with chromic acid or a mixture of potassium bichromate and sulphuric acid; this causes the impurities to rise to the surface as a scum which can be skimmed off. It is usually sent on the market in the form of sticks, which were at one time prepared by sucking the molten material up glass tubes; but the dangers to the workmen and other disadvantages of this method have led to its replacement by a continuous process, in which the phosphorus leaves the melting-pot for a pipe surrounded by water, in which it solidifies and can be removed as a continuous rod.

Properties. When perfectly pure phosphorus is a white, transparent, waxy solid, but as usually prepared it is yellowish owing to the presence of the allotropic "red phosphorus," J. Böeseken (Abs. Jour. Chem. Soc., 1907, ii. 343, 760) prepares perfectly pure phosphorus by heating the crude product with chromic acid solution, washing and drying in a vacuum, first at 40°, then at 80°. It remains colourless in vacuum tubes in the dark, but on exposure it rapidly turns yellow. At 25° to 30° C. it is soft and flexible, but it hardens when strongly cooled, and can then only be cut with difficulty. The fracture is distinctly crystalline; large crystals, either regular dodecahedra or octahedra, may be obtained by crystallization from carbon bisulphide, sulphur chloride, &c., or by sublimation. It is a ductor of electricity. Its density at o° is 1-836; this regularly diminishes up to the melting-point, 44.3°, when a sudden drop occurs. Molten phosphorus is a viscid, oily, highly refractive liquid, which may be supercooled to 32° before solidification. It boils at 290°, forming a colourless vapour which just about the boiling-point corresponds in density to tetratomic molecules, P1; at 1500° to 1700°, however, Biltz and Meyer detected dissociation into P2 molecules. Beckmann obtained P, molecules from the boiling-point of carbon bisulphide solutions, and Hertz arrived at the same conclusion from the lowering of the freezing-point in benzene solution; E. Paternò and Nasini, however, detected dissociation. Phosphorus is nearly insoluble in water, but dissolves in carbon bisulphide, sulphur chloride, benzene and oil of turpentine.

The theory of this action is not settled. It is certain that the formation of hydrogen peroxide and ozone accompany the glowing, and in 1848 Schonbein tried to demonstrate that it depended on the ozone. E. Jungfleisch (Comptes rendus, 1905, 140, p. 444) suggested that it is due to the combustion of an oxide more volatile than phosphorus, a view which appears to be supported by the observations of Scharff (Zeit. physik. Chem., 1908, 62, p. 178) and of L. and E. Bloch (Comptes rendus, 1908, 147, p. 842).

The element combines directly with the halogens, sulphur and selenium, and most of the metals burn in its vapour forming phosphides. When finely divided it decomposes water giving hydrogen phosphide; it also reduces sulphurous and sulphuric acids, and when boiled with water gives phosphine and hypophosphorous acid; when slowly oxidized under water it yields. hypophosphoric acid.

Allotropic Phosphorus.-Several allotropic forms of phosphorus have been described, and in recent years much work has been done towards settling their identities. When the ordinary form immersed in water is exposed to light, it gradually loses its transparency and becomes coated with a thin film. This substance was regarded as an allotrope, but since it is not produced in non-aerated water it is probably an oxide. More important is the so-called "red phosphorus," which is produced by heating yellow phosphorus to about 230° for 24 hours in an inert atmosphere, or in closed vessels to 300°, when the change is effected in a few minutes. E. Kopp in 1844 and B. C. Brodie in 1853 showed that a trace of iodine also expedited the change. The same form is also produced by submitting ordinary phosnon-con-phorus to the silent electric discharge, to sunlight or the ultraviolet light. Since this form does not inflame until heated to above 350°, it is manufactured in large quantities for consumption in the match industry. The process consists in heating yellow phosphorus in iron pots provided with air-tight lids, which, however, bear a long pipe open to the air. A small quantity of the phosphorus combines with the oxygen in the vessel, and after this the operation is practically conducted in an atmosphere of nitrogen with the additional safety from any risk of explosion. The product is ground under water, and any unchanged yellow form is eliminated by boiling with caustic soda, the product being then washed and dried and finally packed in tin boxes. The red variety is remarkably different from the yellow. It is a dark red microcrystalline powder, insoluble in carbon bisulphide, oil of turpentine, &c., and having a density of 2.2. It is stable to air and light, and does not combine with oxygen until heated, to above 350° in air or 260° in oxygen, forming the pentoxide. It is also non-poisonous. When heated in a vacuum to 530° it sublimes, and on condensation forms microscopic needles.

The element is highly inflammable, taking fire in air at 34° and burning with a bright white flame and forming dense white clouds of the pentoxide; in perfectly dry air or oxygen, however, it may be distilled unchanged, H. B. Baker showing that a trace of water vapour was necessary for combination to occur. When exposed to the air a stick of phosphorus undergoes slow combustion, which is revealed by a greenish-white phosphorescence when the stick is viewed in the dark. This phenomenon was

Hittorf's phosphorus is another crystalline allotrope formed by heating phosphorus with lead in a sealed tube to redness, and removing the lead by boiling the product with nitric and

200° C. Oxidizing agents decompose it with great violence. When warmed with alcoholic potash it yields gaseous phosphine, hydrogen and a hypophosphite. It reduces silver salts.

hydrochloric acid. It is also obtained by heating red phosphorus | solid, which is insoluble in water. It burns when heated to about under pressure to 580°. It forms a lustrous, nearly black crystalline mass, composed of minute rhombohedra. G. E. Linck and P. Möller (Ber., 1908, 41, p. 1404) have affirmed that the product of the first process always contains lead. E. Cohen and J. Olie, Jun. (Abs. Jour. Chem. Soc., 1909, ii. 998) regard red phosphorus as a solid solution of the white in Hittorf's, but this is contradicted by A. Stock (Ber., 1909, 42, p. 4510), who points out that ordinary red phosphorus melts at 605°-610°, whilst Hittorf's melts at 620°; moreover, the latter is less reactive than the former at high temperatures. Another form was obtained by R. Schenck (Zeit. Elektrochem, 1905, ii. 117) as a scarlet amorphous powder by deposition of solutions of phosphorus in the tri-iodide, tribromide or sulphide (PS3). It phosphoresces in ozone, but not in air, and is nonpoisonous; from its solution in alcoholic potash acids precipitate the hydride P12H6, and when heated it is transformed into the red modification. It has been used in combination with potassium chlorate as a composition for matches to strike on any surface. Finally a black phosphorus was described by Thénard as formed by rapidly-cooling melted phosphorus.

Phosphine (phosphoretted hydrogen), PH,, a gas formed in the putrefaction of organic matter containing phosphorus, was obtained by Gengembre (Crell's Ann., 1789, i. 450) by the action of potash upon phosphorus, the gas so prepared being spontaneously inflammable. Some time later Davy, by heating phosphorous acid, obtained a phosphoretted hydrogen which was not spontaneously inflammable. These gases were considered to be distinct until Le Verrier (Ann chim. phys., 1835 [2], 60, p. 174) showed that the inflammability of Gengembre's phosphine was due to small quantities of liquid phosphoretted hydrogen, P2H4. Phosphine may be prepared by the decomposition of calcium phosphide with water (PH, being formed simultaneously); by the decomposition of phosphorous and hypophosphorous acids when strongly heated; and by the action of solutions of the caustic alkalis on phosphorus: P1+3NaOH+3H2O = PH:+3NaH2PO2; hydrogen and PH, are produced at the same time, and the gas may be freed from the latter substance by passing into a hydrochloric acid solution of cuprous chloride, and heating the solution, when pure phosphine is liberated (Riban, Comptes rendus, 58, p. 581). The pure gas may also be obtained by heating phosphonium iodide with caustic potash (A. W. Hofmann, Ber., 1871, 4, p. 200); by the decomposition of crystalline calcium phosphide or of aluminium phosphide with water (H. Moissan, Bull. soc. chim., 1899 (3), 21, p. 926; Matignon, Comptes rendus, 1900, 130, p. 1391); and by the reduction of phosphorous acid with nascent hydrogen.

It is a colourless, extremely poisonous gas, possessing a characteristic offensive smell, resembling that of rotting fish. It becomes liquid at 90° C., and solid at -133° C. (K. Olszewski, Monats., 1866, 7, p. 371). It is only slightly soluble in water, but is readily soluble in solutions of copper sulphate, hypochlorous acid, and acid solutions of cuprous chloride. It burns with a brightly luminous flame, and is spontaneously inflammable at about 100° C. When mixed with oxygen it combines explosively if the mixture be under diminished pressure, and is violently decomposed by the halogens. It is also decomposed when heated with sulphur or with most metals, in the latter case with the liberation of hydrogen and formation of phosphide of the metal. It combines with the halide derivatives of boron and silicon to form, e.g. PH3.2BF3, 2PH-SiCl4 (Besson, Comptes rendus, 1890, 110, 80, pp. 240, 516; 1891, 113, p. 78), with the halogen acids to form phosphonium salts, PHX (X=Cl, Br,I), and with sodammonium and potassammonium to form PH,Na, PH,K (Joannis, Comptes rendus, 1894, 119, p. 557). It oxidizes slowly in air, and is a reducing agent. It decomposes when heated, hydrogen and red phosphorus being formed.

Liquid Phosphoretted Hydrogen, PH4, first obtained by P. Thénard (Comptes rendus, 1844, 18, p. 652) by decomposing calcium phosphide with warm water, the products of reaction being then passed through a U tube surrounded by a freezing mixture (see also L. Gattermann, Ber., 1890, 23, p. 1174). It is a colourless liquid which boils at 57°-58° C. It is insoluble in water, but soluble in alcohol and ether. It is very unstable, being readily decomposed by heat or light. By passing the products of the decomposition of calcium phosphide with water over granular calcium chloride, the PH gives a new hydride, P12He and phosphine, the former being an odourless, canary-yellow, amorphous powder. When heated in a vacuum it evolves phosphine, and leaves an orange-red residue of a second new hydride, PH2 (A. Stock, W. Böttcher, and W. Lenger, Ber., 1909, 42, pp. 2839, 2847, 2853).

Solid Phosphoretted Hydrogen, P4H2, first obtained by Le Verrier (loc. cit.), is formed by the action of phosphorus trichloride on gaseous phosphine (Besson, Comptes rendus, III, p. 972); by the action of water on phosphorus di-iodide and by the decomposition of calcium phosphide with hot concentrated hydrochloric acid. It is a yellow

Phosphonium Salts.-The chloride, PH,Cl, was obtained as a crystalline solid by Ogier (Comptes rendus, 1879, 89, p. 705) by comfrom 14-20 atmospheres; it can also be obtained at --30° to -35° C. bining phosphine and hydrochloric acid gas under a pressure of under ordinary atmospheric pressure. It crystallizes in large transparent cubes, but rapidly dissociates into its constituents on exposure. The bromide, PH,Br, was first obtained by H. Rose (Pogg. Ann., 1832, 24, p. 151) from phosphine and hydrobromic acid; it also results when phosphorus is heated with hydrobromic acid to 100120° C. in sealed tubes (Damoiseau, Bull. soc. chim., 1881, 35, p. 49). It crystallizes in colourless cubes, is deliquescent, and often inflames spontaneously on exposure to air. It is readily decomposed by water and also by carbonyl chloride (Besson, Comptes rendus, 1896, 122, p. 140): 6PH4Br+5COCl2 10HCl + 5CO + 6HBr + 2PH; + PH2. The iodide, PHI, first prepared by J. Gay-Lussac (Ann. chim. phys., 1814, 91, p. 14), is usually obtained by the action of water on a mixture of phosphorus and iodine (A. W. Hofmann, Ber., 1873, 6, p. 286). It is also prepared by the action of iodine on gaseous phosphine, or by heating amorphous phosphorus with concentrated hydriodic acid solution to 160° C. It crystallizes in large cubes and sublimes readily. It is a strong reducing agent. Water and the caustic alkalis readily decompose it with liberation of phosphine and the formation of iodides or hydriodic acid. It is also decomposed by carbonyl chloride (Besson, loc. cit.).

4PH1I+8COC12=16HCl+8CO+P2I1+2P.

=

Just as the amines are derived from ammonia, so from phosphine are derived the primary, secondary and tertiary organic phosphines by the exchange of hydrogen for alkyl groups, and corresponding to the phosphonium salts there exists a series of organic phosphonium bases. The primary and secondary phosphines are produced when the alkyl iodides are heated with phosphonium iodide and zinc oxide to 150° C. (A. W. Hofmann, Ber., 1871, 4, pp. 430, thus: 605), 2 RI + 2 PHI + ZnO = 2 R·PH2 HI + Żnĺ2‍+ H2O, mixture on treatment with water yields the primary phosphine, 2RI + PH1I + ZnO R2 PH HI+ ZnI2 + H2O. The reaction the secondary phosphine being then liberated from its hydriodide by caustic soda. The tertiary phosphines, discovered by L. Thénard Comptes rendus, 1845, 21, p. 144; 1847, 25, p. 892), are formed (together with the quaternary phosphonium salts) by heating alkyl iodides with phosphonium iodide to 150-180° C.: PH1I+3CH,I= P(CH3)3HI+3HÍ; P(CH3)2HI + CH2Ĩ = P(CH3) 4I+HI (see also Fireman, Ber., 1897, 30, p. 1088). They are also formed by the interaction of phosphorus trichloride and zinc alkyls (Cahours and Hofmann, Ann., 1857, 104, p. 1): 2PCl3+3 Zn (Č2H6)2=3ZnCl2+ 2P(C2H6) 3.

The primary and secondary phosphines are colourless compounds, and with the exception of methyl phosphine are liquid at ordinary temperature. They possess an unpleasant odour, fume on exposure to air, show a neutral reaction, but combine with acids to form salts. They oxidize very rapidly on exposure, in many cases being sponOn oxidation with nitric acid the primary taneously inflammable. compounds give monoalkyl phosphinic acids, R.PO(OH)2, the secondary yielding dialkyl phosphinic acids, R,PO(OH). The primary phosphines are very weak bases, their salts with acids being readily decomposed by water. The tertiary phosphines are characterized by their readiness to pass into derivatives containing pentavalent phosphorus, and consequently they form addition compounds with sulphur, carbon bisulphide, chlorine, bromine, the halogen acids and the alkyl halides with great readiness. On oxidation they yield phosphine oxides, R.P·Ŏ. The quaternary phosphonium salts resemble the corresponding nitrogen compounds. They are stable towards aqueous alkalis, but on digestion with moist silver oxide yield the phosphonium hydroxides, which are stronger bases than the caustic alkalis. They differ from the organic ammonium hydroxides in their behaviour when heated, yielding phosphine oxides and paraffin hydrocarbons: R4P·OH=R ̧PÓ+RH. The boiling-points of some members of the series are shown in the table:

The alkyl phosphinic acids are colourless crystalline compounds
which are easily soluble in water and alcohol. They yield two
series of salts, viz. RHM PO, and RM,PO, (M = metal). The
dialkyl phosphinic acids are also colourless compounds, the majority
of which are insoluble in water. They yield only one series of salts.
Oxides.-Phosphorus forms three well-defined oxides, P40%, P2O1
and P2Os; two others, P40 and P2O, have been described.
Phosphorus suboxide, P4O, is said to be formed, mixed with the

Phosphorus tetroxide, PO4, was obtained by Thorpe and Tutton by heating the product of the limited combustion of phosphorus in vacuo as a sublimate of transparent, highly lustrous, orthorhombic crystals. They are highly deliquescent, and form with water a mix-phorus pentoxide or meta- or pyrophosphoric acids at low temperature of phosphorous and phosphoric acids: P2O.+3H2O=H2PO2+ H3PO4. The vapour density at about 1400° is 230, i.e. slightly less than that required by PO16 (West, Jour. Chem. Soc., 1902, p. 923). Phosphoric oxide, or phosphorus pentoxide, PO10, formed when phosphorus is burned in an excess of air or oxygen, or from dry phosphorus and oxygen at atmospheric pressure (Jungfleisch, loc. cit.), was examined by Boyle and named " flowers of phosphorus by Marggraf in 1740. It is a soft, flocculent powder, which on sublimation forms transparent, monoclinic crystals. It is extremely deliquescent, hissing when thrown into water, with which it combines to form phosphoric acid. It is reduced when heated with carbon to phosphorus, carbon monoxide being formed simultaneously. Its vapour density at 1400° points to the double formula (West, Jour. Chem. Soc., 1896, p. 154).