Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/527

Rh PHOSPHORUS.] CHEMISTRY 515 PHOSPHORUS. Symbol, P; At. wt., 30 96 ; Mol. wt. of gas, 123 84. This element never occurs in the free state, but is al ways found in combination as a salt of phosphoric acid. Considerable deposits of more or less pure calcium phos- phats occur in a few places, and phosphates are found in minute proportions in most rocks and in soils, and in river and spring waters. Phosphates are necessary to the life of all plants and animals. In plants they accumulate chiefly in the seed; in animals they accumulate in the bones, of which calcium phosphate is the chief earthy con stituent, but they are also an important element of blood and nervous tissue. Phosphorus was discovered in 1669 by Brandt of Ham burg, who obtained it by distilling the residue of evapo rated urine with charcoal. Scheele in 1775, however, was the first who devised a process for its extraction from bones, and it has always since been prepared from this source. The bones are burnt to a white ash, which is finely powdered and mixed with a sufficient quantity of diluted sulphuric acid to displace by hydrogen two-thirds of the calcium in the tricalcium phosphate, which is the main constituent of bone ash, in the manner represented by the equation Ca ? (P0 4 ) 2 + 2H 2 SO 4 = H 4 Ca(PO 4 ) 2 + 2CaS0 4. Tricalcium Sulphuric Monocalcium Calcium phosphate. acid. phosphate. sulphate. The solution of monocalcium phosphate, or superphos phate of lime as it is ordinarily termed, which is obtained, is separated from the insoluble calcium sulphate, and evaporated to a syrup ; this is mixed with about one- fourth its weight of charcoal powder, and heated gradually to dull redness in an iron pot with constant stirring. By this means the elements of two molecules of water are re moved, the monocalcium phosphate being converted into calcium metaphosphate CaH 4 (P0 4 ) 2 = Ca(P0 3 ) 2 + 2H 2 O. The porous mixture of calcium metaphosphate and char coal thus obtained is introduced into earthen retorts, and distilled at a bright red heat. The calcium metaphos phate is then converted into tricalcium phosphate and phosphoric anhydride, and the latter is decomposed by the charcoal into carbonic oxide and phosphorus, which passes over in vapour and is condensed in water The crude phosphorus which is produced is usually puri fied by adding a mixture of potassium chromate and sul phuric acid to it when in a melted state. The impurities are thus oxidized, and rise to the surface in the form of a scum, and the pure phosphorus remains colourless and trans parent at the bottom of the vessel. Freshly-prepared phosphorus is almost perfectly trans parent and colourless, or has, at most, a faint yellow tinge. It melts at 44 C., forming a viscid oily liquid, and boils at 290 C. Its vapour at a temperature of 1040 C. is about 62 times as heavy as hydrogen gas, and hence, as the atomic weight of phosphorus is about 31, it appears that the molecule of gaseous phosphorus is tetra- tomic. Phosphorus has a specific gravity of about 1 82. It is a non-conductor of heat and electricity. It is in soluble in water, but freely soluble in carbon disulphide, phosphorus trichloride, and sulphur chloride, S 2 C1 2. It crystallizes in forms of the regular system. Phosphorus is extremely inflammable, taking fire in the open air at a temperature very little above its melting point ; if it contains impurities, it inflames still more easily. It gradually absorbs oxygen when exposed to the air at ordinary temperatures, giving off a white vapour, which has a peculiar garlic odour ; in presence of moisture phosphorous acid, H 3 PO 3, and phosphoric acid, H 3 P0 4 , are produced. Phosphorus, like sulphur, can exist in several allotropic modifications, the most remarkable being that produced by exposing ordinary phosphorus to light, or by heating it for some hours to a temperature of about 240 C. in an atmo sphere free from oxygen. It is thus converted into a red amorphous substance, which is insoluble in carbon disul phide, and may be heated to 250 C. without alteration, but at 200 C., under the ordinary pressure, it is reconverted into ordinary phosphorus. The red modification has a much higher specific gravity (2 14) than ordinary phosphorus, and it is also distinguished by its inertness as compared with the latter ; thus, it is not oxidized in the air at com mon temperatures, and it emits no odour. The transfor mation of ordinary phosphorus into this variety is attended with development of heat. Troost and Hautefeuille have shown that the formation of the red modification is governed by different laws according as the phosphorus is in the state of gas or liquid, and that it takes place much more rapidly in the latter case. When the tension of the vapour becomes diminished to a certain minimum value, varying for each temperature, the transformation ceases ; this tension of transformation is established only after some time. Liquid phosphorus at 280 C., for instance, becomes wholly transformed into red phosphorus. The vapour given off at 260 C. is stable, but that formed at higher temperatures becomes slowly and partially converted into red phosphorus, the production of which ceases when the tension attains a given minimum ; the rapidity with which this change takes place is greater the higher the tempera ture. At temperatures up to 520 C., however, the maxi mum tensions of phosphorus vapour are much higher than the tensions of transformation ; thus, at 360 C. the tension of transformation is 6 atmosphere, but to prevent phos phorus boiling at this temperature, a pressure of 3 2 atmo spheres must be exerted. Above 520 C. the maximum tension is not established on account of the rapidity with which the transformation takes place ; and at temperatures above 550 C. the tension observed is no greater than the tension of transformation, because the transformation of the liquid phosphorus takes place more rapidly than its evaporation. The observed maximum tension and tension of transformation at different temperatures are given in the following table : Temperature. 360 440 487 494 503 510 511 531 550 577 Maximum tension. 3 - 2 atmospheres 7-5 18-0 21-9 26-2 Tension of transformation. 6 atmospheres 175 16-0 31-0 56-0 The red phosphorus produced by heating ordinary phos phorus in closed vessels differs, however, in appearance according to the temperature at which it is prepared. That obtained at 265 C. has a bright red colour and glassy fracture ; that produced at 440 C. is orange-yellow in colour, and exhibits a dull fracture ; at 500 C. it is denser, and has a violet grey colour ; and, lastly, that prepared at 580 C. has a conchoidal fracture, exhibits signs of commencing fusion, and often encloses ruby red crystals. These modifications differ also in specific gravity, and when oxidized to phosphoric acid by iodic acid solution they develop different amounts of heat. Thus