P H O 819 or citric acid ; but the writer is far from recommending the substi tution. Professor Gamgee has made the very surprising discovery that meta-phosphoric and pyro-phosplioric, although so closely allied to ortho-phosphoric acid, are poisons, as phosphorous acid is. Phosphoric acid readily combines with and neutralizes alkalis, even when these are given as carbonates. The concentrated acid, when heated in porcelain or glass, strongly attacks either material ; hence its concentration ought always to be effected in platinum. In former times, when phosphorus was expensive, the acid, or rather an apology for the same, used to be prepared from bone-ash. Alkaline Phosphates. Of these the di-sodic salt is of the greatest practical importance. It is prepared by somewhat more than neutralizing the hot aqueous acid with carbonate of soda. A cheaper (manufacturing) process is to prepare a solution of "super-phos phate " from bone-ash by the action of vitriol, and, after elimination of the gypsum, to supersaturate the liquid with carbonate of soda and filter off the phosphate of lime produced (see p. 81 5 supra, where the process is explained indirectly). The salt, from sufficiently strong hot solutions, separates out in large transparent crystals of the composition P0 4 HNa + 12H 2 0, which lose their crystal-water on exposure to dry air, even at ordinary temperatures, and very quickly at 100 C. The residue, P0 4 HNa 2 = A(P,0 5 .21S T a,O.H.,0), when heated to redness, loses its remnant of water and becomes pyre-phosphate, which latter retains its specific character on being dissolved in water. A solution of the (original) salt in water has a mild taste (hence its preferential application as a pleasant purga tive) ; it colours red litmus-paper intensely blue, and does not act upon alkaline carbonate. But, when evaporated with the calculated proportion of carbonate of soda (Na.,C0 3 per P B ) to dryness at, ultimately, a red heat, it yields a residue of tri-sodic salt (P0 4 Na 3 ) as a white mass, infusible at the highest temperature producible within a platinum crucible over a glass blowpipe. The solution of this salt in water has all the properties of a mixed solution of P0 4 Na.,H + NaOH ; yet it is capable of depositing crystals of the composition P0 4 Na 3 + 12H..O. The mono-sodic salt (P0 4 H 2 Na), producible by mixing together solutions containing the quantities H 3 P0 4 and Na.,HP0 4 , is of no importance. Of the three potash salts, the mono-metallic salt (P0 4 KH 2 ) is the most readily produced. It forms beautiful anhydrous quadratic crystals which at a red heat lose their H 2 and become meta-phosphate, P0 3 K. Ammonia Salts. A strong solution of the acid, when super saturated with ammonia, deposits on cooling crystals of the di- ammonic salt P0 4 (NH.,)oH, liable to be contaminated with the niono-ammonic salt. The tri-auimonic salt is very unstable, and hardly known. The double salt P0 4 (XH 4 )NaH + 4H 2 was known to the al chemists as "sal microcosmicum urinie" and is interesting his torically as having served Brand as a raw material for the mak ing of phosphorus. It is easily prepared, either by mixing the solution of the two quantities P0 4 Xa 2 HP0 4 and P0 4 (XH 4 ),HP0 4 together and allowing to crystallize, or by dissolving the former along with NH 4 C1 parts of sal-ammoniac in water, and removing the chloride of sodium produced by crystallization in the heat. Microcosmic salt, when heated to redness, leaves a viscid glass of meta-phosphate of soda, which dissolves all basic metallic oxides pretty much as fused borax does, with formation of glasses which often exhibit colours characteristic of the dissolved oxides. Hence its application in blowpipe analysis. Phosphates of Lime. The normal salt P 2 5 .3CaO or P0 4 ca s , where ca = Ca=one equivalent of calcium, or perhaps a compound of it and carbonate of lime, forms the predominating component of bone-ash. A hydrate of the salt is produced by precipitating chloride of calcium solution with exce.js of ordinary phosphate of soda, mixed with enough of ammonia to produce (virtually) tri- alkaline salt, as a gelatinous precipitate similar in appearance and behaviour on filtration to precipitated alumina. A suspension of this precipitate in water, when mixed with a carefully adjusted quantity of hydrochloric acid, gradually passes into a mass of microscopic crystals of di-calcic salt, P0 4 ca. 2 H + xAq, which latter is used medicinally. A solution of the di-calcic or tri-calcic salt, in the proper proportion of hot aqueous hydrochloric acid, deposits on cooling crusts of crystals of the mono-calcic salt P0 4 H 2 ca, which is soluble in about 700 parts of cold water, but is decomposed, by hot water or by prolonged contact with a proportion of cold water insufficient to dissolve it, into free acid and a precipitate of di- calcic salt, 2P0 4 caH 2 = P0 4 H 3 + P0 4 ca 2 H. A very impure form of this salt, known as "superphosphate," enters into the composition of many artificial manures. Such superphosphate is made in dustrially by treating broken-up bones, or powdered bone-ash, or powdered phosphorite, or coprolite, or occasionally apatite with chamber-acid, meaning vitriol of about 60 per cent., as it comes out of the chamber. The phosphate is mixed with the acid in a lead-lined trough by means of machinery, when a rather lively reaction sets in, involving the evolution of vapour of water mixed with hydrofluoric acid, and fluoride of silicon if mineral phosphate is used, possibly also with traces of fluoride or chloride of arsenic, and, in any case, with stinking volatile organic substances. The vapour, therefore, must be removed by means of suitable draught arrangements. The mass passes from the trough into a (ventilated) chamber, where the reaction gradually accomplishes itself with ulti mate formation of a porous friable mass, dry to the touch. This is superphosphate as it goes out into commerce or is used as an ingredi ent in making more complex manures. Its value is determined chiefly by its percentage of "soluble phosphoric acid," meaning the percent age of P 2 5 , extractable as P0 4 H 3 or P0 4 caII 2 by a certain large proportion of cold water. This percentage is liable to decrease on long-continued storing, especially in the case of mineral superphos phate, through a gradual formation of (or regeneration of origin ally present) phosphate of iron and alumina, partly, perhaps, also through the spontaneous decomposition of some of the mono-calcic salt into insoluble di-calcic salt and free acid. The portion of the P 2 5 which has thus become insoluble is designated " reduced " phosphoric acid. In regard to other phosphates than those named reference may be made to the handbooks of chemistry. Analysis. Phosphoric acid, when given in any form, soluble in solution of ammonia, can be detected and determined by "magnesia mixture " (a solution of chloride of magnesium and sal-ammoniac, MgCl., . 2NH 4 C1, strongly alkalinized by addition of aqueous am monia). The phosphoric acid is very gradually, but at last completely, precipitated in microscopic crystals of the salt P0 4 MgNH 4 + 6H 2 6, which, though slightly soluble in water, can be washed pure, with out loss, with dilute ammonia. All other acids except arsenic acid (As 2 5 ) which behaves like phosphoric, and, if present, must be removed by sulphuretted hydrogen remain dissolved. The precipi tate, when kept at a red heat, assumes the composition P 2 5 2MgO, and from the weight of the ignited precipitate that of the phos phoric acid present is easily calculated. Phosphates soluble in acids, and reprecipitated from their solutions as such by ammonia as phosphate of lime or alumina, or ferric oxide used to give great difficulties to the analysts until Sonncnschein founded an excellent quantitative method for their analysis upon a reaction discovered by Swanberg and Struve, which is explained under MOLYBDENUM (vol. xvi. p. 697). The phosphate is dissolved in nitric acid (hydrochloric is less to be recommended) and the solution mixed, and kept for some hours at 40 C., with a large excess of a solution of molybdate of ammonia in excess of nitric acid. The phosphoric acid (along with any arsenic acid that may be present) comes down as yellow crystalline phospho-molybdate of ammonia, soluble in phos phoric acid and slightly in water, but insoluble in dilute nitric acid in the presence of a sufficiency of nitrate of ammonia. The precipitate is soluble in aqueous ammonia, and from the solution its P 2 5 can be precipitated by magnesia mixture as above explained. Neither of the two methods applies directly to meta-phosphates or pyro- phosphates. Regarding these, see the last paragraph of the section " phosphorus " above. (W. D.) PHOTIUS, patriarch of Constantinople from 857 to 867 and again from 877 to 886 A.D., the most eminent literary and ecclesiastical character of his age, was probably born between 820 and 825. If we could credit the asser tions of his adversaries, his father, an official of the im perial court, named Sergius, was of heathen extraction, and his mother, Irene, a faithless nun. It is more certain that he displayed from an early age the most extraordinary talent and appetite for knowledge, and that, having mastered whatever Greek literature could give him (Latin and Hebrew he never acquired), he began to teach with dis tinguished success grammar, rhetoric, divinity, and philo sophy. The way to public life was probably opened for him by the brilliant marriage of his maternal uncle to the princess Irene, sister of the empress Theodora, who, upon the death of her husband Theophilus in 842, had assumed the regency of the empire. Photius became captain of the guard and subsequently first imperial secretary. Some where about 850 he was entrusted with a mission to the " Assyrians," by whom the Saracens must be meant, pos sibly to the court of the caliph of Baghdad. Just previous to his departure on this mission he compiled his Bibliotheca, or Myriobiblion, the noblest monument of his erudition, and, from the number of classical authors whose writings it has partially preserved, by much the most important of his works. Some time after his return from this embassy an un expected path was opened to Photius s ambition by the dissensions between the patriarch Ignatius and Bardas, the uncle of the youthful emperor Michael III., who had suc ceeded to the regency on the disgrace of Theodora. Ignatius,
