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acids. It does not dissociate on heating as do the entachloride and pentabrornide, thus indicating the existence 0 pentavalent phosphorus in a gaseous compound; dissociation, however, into the trifluoride and free fluorine may be brought about by induction sparks of 150 to 200 mm. in length. It combines directly with ammonia in the proportion 2PF5:5NH,, and with nitrogen peroxide at —10° in the pro ortion PF 5:N0=. Phosphorus trifluorodiehloride, PFicb, prepared (Prom chlorine and the trifluoride, is a pungentsmellin gas, which at 250° gives the pentachloride and fluoride. The Irifiuorodibromide (see above) is an amber-coloured mobile liquid. Phosphoryl trifluoride, POFi, may be obtained by exploding 2 volumes of phosphorus trifluoride 'with 1 volume of oxygen (Moissan, 1886); by heating 2 arts of finel -divided cryolite and 3 ~gmrts of phosphorus pentoxide ( horpe and ambly, Jour. Chem. oc., 1889, p. 759); or from phosphoryl chloride and zinc fluoride at 40° to 50°. It is a colourless fuming_gas, which liquefies under ordinary pressure at —50°, and under a pressure of 15 atmospheres at 16°; it may be solidified to a snow-like mass. Water gives hydrofluoric and hosphoric acids. The corresponding sulphur compound, thiophosp oryl fluoride, PSF3, obtained by heating lead fluoride and phosphorus pentasulphide to 200°, is a colourless gas, which ma be condensed to a clear transparent liquid. 1t spontaneously in ames in air or oxygen; and when the as is issuing from a jet into air the flame is greyish reen, with a aintly luminous and yellow tip; the flame is roba ly one of the coldest known. The combustion probably fol owe the equation PSF3+02=PF5+503, the trifluoride at a hi her temperature decom sing according to the equations: 10? i+501=6PF5+2P,Oi, 2 Fi+Oi=2POF;, the complete reaction tending to the equation: 10P5F3+1501=6PF5+2P,05+ 10501. The gas dissolves in water on shaking; PSlii+4H20= 1H,S+H4P04+3HF, but is more readily taken up by alkaline solutions with the formation of fluoride and thiophosphate: PSF3+ 6NaOH=NaiP503+3NaR Heated in a glass tube it gives silicon fluoride, phos horus and sulphur, PSF;=PF,+S; 4PF3+3SiOi= 3SiF¢+Pi+36;. Electric sparks give at first free sulphur and the trifluoride, the latter at a higher temperature splitting into the pentafluoride and phosphorus. With (lry ammonia it gives ammonium fluoride and a compound P(NH2)¢SF.

Phosphorus trichlori'de or phosphorous chloride, PCli, discovered by Gay-Lussac and Thénard in 1808, is obtained by passing a slow current of chlorine over heated red phosphorus or through a solution of ordinary phosphorus in carbon disulphide (purifying in the latter case by fractional distillation). It is a colourless, mobile li uid of specific gravity 1-6128 at 0° and boiling-point 76°. With c lorine it gives the pentachloride, PC15, and with oxygen when heated phosph0ryl chloride, POCli. \Vater gives hydrochloric and phosphorous acids, with separation of red phosphorus if the water be hot. When led with hydrogen into liquid ammonia it ives NHZPNHg, which on elevation of temperature gives P¢(NH)| ( oannis, Campus rendus, 1904, 139, p. 364). By submitting a mixture of phosphorous chloride and hydrogen to an electric discharge A. Besson and A. Fournier (Comples rendus, 1901, 150, p. 102) obtained phosphorus dichloride, P1Cli, as a colourless, oily, strongly fuming liquid, freezing at —28° and boiling at 180° with decomposition. With water it gave phos horous acid and a yellow indefinite solid. It decomposes slow y at ordinary temperatures. Phosphorus pentaohloride, PCli, discovered by Davy in 1810 and analysed by Dulong in 1816, is formed from chlorine and the trichloride. It is a straw-coloured solid, which by fusion under pressure gives prismatic crystals. It sublimes when heated, but under pressure it melts at 148°, giving a normal vapour density, but on further heating it dissociates into the trichloride and chlorine; this dissociation may be retarded by vapourizing in an atmosphere of chlorine. It fumes strongly in moist air, giving hydrochloric acid and phosphoryl chloride, POCli; with water it gives phosphoric and hydrochloric acids.

Phosphoryl trichloride or phosphorus oxychloride, POCli, corresponding to phosphoric acid, (HO);PO, discovered in 1847 by Wurtz, ma be produced by the action of many substances containing hy roxy groups on the pentachloride; from the trichloride and potassium chlorate; by leaving phosphorus pentoxide in contact with hydrochloric acid: 2P105+3HCl=POClg+3HPOfi or by heating the entachloride and pentoxide under pressure: 3PCli+ P,Oi=5POCl:. It is a colourless liquid, boilin at 107-2°, and when solidified it melts at 0-8°. Water gives filydrochloric and phosphoric acids; dilute alcohol gives monoethyl phosphoric acid, C,H5-H=P04, whilst absolute alcohol ives triethyl phosphate, (C,H;);PO.. Pyrophosphoryl chloride, P1030 corresponding to pyrophosphoric acid, was obtained by Geuther and Michaelis ( er., 1871, 4, p. 766) in the oxidation of phosphorus trichloride with

nitro en roxide at low temperature; it is a colourless fuming liqui which boils at about 212° with some decomposition. \IVith Thiophosphoryl

water it ives phosphoric and hydrochloric acids. chloride, l§SCl3, may be obtained by the direct combination of sulphur with the trichloride; from sulphuretted hydrogen and the pentachloride; from antimony trisulphide and the pentachloride; by heating the pentasulphide with the pentachloride; and by dissolvin phosphorus in sulphur chloride and distilling the solution: 2P+3§Clg= 4S+2PSCl_1. It is a colourless mobile liquid, boiling at 125-1° and having a pungent, slightly aromatic odour. It is slowl decomposed by water giving phosphoric and hydrochloric amds, with

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sul huretted hydrogen; alkalis form a thiophosphate, e.g. PS(OK);, an a chloride.

Phosphorus tribromi'de, PBri, prflJared by mixing solutions of its elements in carbon disulphide and istilling, is a transparent, mobile liquid, boiling at 173° and resembling the trichloridc chemically. The Pentabromido, PBri, which results from phosphorus and an excess of bromine, is a 'ellow solid, and closely resembles the pentachloride. The bromoc oride, PclgBl'g, is an orange-coloured solid formed from bromine and the trichloride, into which components it decomposes at 35°. Phosphoryl tribromide, POBn, is a solid, melting at 5° and boiling at 1 5°. Thiophosphoryl bromide, PSBri, obtained a ter the manner 0 the corres onding chloride, forms yellow octahedra which melt at 38°, and ave a penetrating, aromatic odour. With water it gives sulphur, sulphuretted hydrogen, h drobromic, phosphorous and phos oric acids, the sulphur and p osphorous acid being produced by t e interaction of the previously ormed sulphuretted hydrogen and phosphoric acid. Pym hosphoryl thiobromide, (PBrZS),S, and metaphosphoryl Ihiobromide, 5381', are also known.

Phosphorus forms three iodides. The subiodide, Pil, was obtained by R. Boulough (Com tes rendus, 1905, 141, p. 256), who acted with dry iodine on phosp orus dissolved in carbon disulphide; with alkalis it gives PAOH). The di-iodide and tri-iodide are formed similarly; the rst is deposited as orange-coloured prisms which melt at 110° to a red li uid (see Dou hty, Jour. Amer. Chem. 800., 1905, 27, p. 1444), whi st the secon forms dark-red hexagonal plates which melt at 55°.

Sulphides and T hio-acids.—-Phos horns and sulphur combine energetically with considerable rise 0 temperature to form sulphides. The researches of A. Stock (Ben, 1908, 41, pp. 558, 657; 1 , 42, p. 2062; 1910, 43, pp. 150, 414) show that three exist, P5,, P2,, 11.5.. The first is prepared by heating red hos horus with finely powdered sulphur in a tube sealed at one en an filled with carbon dioxide. The product is extracted with carbon disulphide and the residue distilled in carbon dioxide. It forms light yellow crystals from benzene, which melt at 1 725° and boil at 407 °—4o8° with slight decomposition. Alkalis give hydrogen and phosphine. The second, P451, is obtained by eating a mixture of red phosphorus and sulphur in the proportions given by PiS-i+5 "A, P.s., an crystallizing from carbon disulphide in which P5; is readily soluble. It forms small, slightly yellow prisms, which melt at 310° and boil at 523°. The third, or entasulphide, P255, was obtained as a substance resembling flowers oPsulphur by A. Stock and K. Thiel (Ber., 1905, 38, p. 2719; 1910, 43, p. 1223), who heated sul but with phosphorus in carbon disulphide solution with a trace 0 iodine to 120 —130°. it exists in two forms, one having the formula. P510, and the other a lower molecular weight. With liquid ammonia it gives PzSi- N Hi, which is a mixture of ammonium iminotrithiophosfihate, P( NHi)s: NH, and ammonium nitrilodithiophosphate, P(S HJZEN. Water converts the former into ammonium thiophosphate, PO(SN H 4),.H,O, whilst the latter heated to 300° in a vacuum gives thiophosphoric nitrile, N i P:S (Stock, ibid., 1906, 39, p. 1967).

Thiophosphates result on dissolving the pentasulphide in alkalis. Sodium monothiophosphate, NaiPSOi-12HIO, is obtained by adding one P185 to six NaOH, adding alcohol, dissolving the precipitate in water and heating to 90°. 11 cooling the salt se arates as white six-sided tablets. Sodium dithiophosphate, Nai SgOfI 111,0, is obtained by heating the above solution only to 50°—55°, cooling and adding alcohol, which precipitates the dithio salt. On heating it ives the monothio salt. Sodium trithiophosphate appears to be ormed when the pentasulphide acts with sodium hydrosulphide at 20°. All thiophosphates are decomposed by acids giving sulphuretted hydrogen and sometimes free su phur. They also act in many cases as reducing agents.

Nitrogen Com?ounds.—Phosphorus pentachloride combines directly with ammonia, and the com und when heated to redness lmes ammonium chloride and hy rochloric acid and gives phospham, PNil-li, a substance first described by Davy in 1811. It is a white, infusible, very stable solid, which decomposes water on heating, giving ammonia and metaghosphoric acid. whilst alkalis give an analogous reaction. Wit methyl and ethyl alcohols it forms secondary amines (Vidal, Campus rendus, 1891, 112, p. 950; 1892, 115, p. 123). The diamide, PNZH was obtained b Hugot (ibid., 1905, 141, p. 1235) by acting with ammonia gas 011(5) osphorus tribroniide or tri-iodide at —70°; it is very unstable, an decomposes at —25°. Phosphorus combines with nitrogen and chlorine to form several polymeric substances of the general formula (PNClg) x, where 2: may

e 1, 3, 4, 5, 6, 7, or 11; they may be obtained by heatin the pentachloride With ammonium chloride in a sealed tube an separating the mixture by fractional distillation (H. N. Stokes, Amer. Chem. Joan, 1898, 20, 740; also see Besson and Rosset, Comptes rendus, 1906, 37, p. 143). he commonest form is PgNaclq, a crystalline solid. insoluble in water, but soluble in alcohol and ether. Several phosphoamides have been described. The diamide, PO (NH1)(NH), results when the pentachloride is saturated with ammonia gas and the first formed chlorophosphamide, PCl,(NH2),, is decomposed by water. The triamide, PO(NH1),, results from ammonia and p osphorus oxychloride. Both these compounds on heatin give hosphomonamide. PON, of which a polymer (PON), had een escnbed by Oddo (Gazz. chim. 11:11., 1899, 29 (ii.), p. 330). Stokes (Amer. Chem. Joan. i893, 15, p. i 8; i894, 16, pp. 123, i54) has described PO(OH),NH, and PO(OH) NH,)¢, whilst the compound PO(OH)N H was obtained by Schiff (Ama, 1857, l03, . 168) by acting with ammonia on the pentoxide. ,Numerous otlier nitrogen compounds have been obtained.

- The atomic weight of phosphorus was determined by Berzelius, Pélouze, Jacquelin, Dumas, Schrotter, Brodie and van der Plaats. More recent are the investigations of G. Ter Gazarian (Compl. rend., i909, 148, p. 16397) on hydrogen phosphidc, which gave the value 30-906, and of . P. Baxter and G. jones (loam. Amer. Chem Soc., 19 I0, 32, p. 298) on silver phosphate, which gave the value 3i-04.

T herapeutics.—The phosphorus used in the British pharmacopoeia is obtained from calcium phosphate, and is a waxlike non-metallic substance soluble in oils and luminous in the dark. There are various medicinal preparations. In young animals phosphorus has a remarkable influence on the growth of bone, causing a proliferation of the jelly-like masses and finally a deposit in them of true bony material. Owing to this influence it has been used in rickets and osteomalacia. Its most effective use, however, is as a nerve tonic in paralysis agitans, locomotor atain'a, impotence and nerVQus exhaustion. In some .skin diseases such as psoriasis, chronic eczema and acne indurata, phosphorus is very useful, and cases of diabetes niellitus and lymphadenoma have improved under some of its ~compounds. The hypophosphites have been recommended in pulmonary affections, being said to act as free phosphorus without being irritant, and the glycero-phosphates are certainly useful to stimulate metabolism. Dilute phosphoric acid is used as a gastric stimulant. It does not resemble phosphorus in its physiological action and cannot be used to replace it.

T oxicology.—-P0ison0us amounts of phosphorus are frequently taken or administered, criminally or accidentally, it being easily accessible to the public in the form of matches or of vermin pastes. They may have been swallowed several hours before symptoms of acute poisoningsho-wthemselves, with nausea and vomiting, and a burning in the oesophagus, stomach and abdomen. The important thing is to prevent the absorption of the poison, so emetics and purgatives should begiven at once. Sulphate of copper, in doses of 3 to 5gr., freely diluted and repeated every few minutes forms the harmless, black phosphide of copper, which is rapidly eliminated by the kidneys. The stomach may be washed out with warm water and then with a 2 % solutionof permanganate of potash, an enema of the same solution being given. The old French oil of turpentine is the best antidote to use in phosphorus poisoning, delaying the toxic effects; but ordinary oils are not only useless but harmful. When some time has elapsed before treatment and the phosphorus has become absorbed, the organic degenerative changes cannot be easily controlled. For the chronic form of industrial poisoning in the manufacture of lucifer matches—a form of necrosis, known in England as “ phossy jaw ” and in France as “ ma! chimique,” a localized inflammatory infection of the periosteum, ending with the death and exfoliation of part of the bone—see MATCH.

PHOTIUS (0. 820—891), patriarch of Constantinople (858-867 and 878—886). From his early years he displayed an extraordinary talent and appetite for knowledge, and as soon as he had completed his own education he began to teach with distinguished success grammar, rhetoric, divinity and philosophy. The way to public life was probably opened for him by the marriage of his brother Sergius to the princess Irene, sister of 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. The dissensions between the patriarch Ignatius and Bardas, the uncle of the youthful Emperor Michael 111., brought promotion to Photius. Ignatius was arrested and imprisoned (Nov. 858), and upon refusing to 'resign his office was illegally deposed, while Photius, although alayman, received all the necessary sacerdotal orders within six days, and was installed as patriarch in his place. Ignatius, continuing to refuse the abdication which could alone have given Photius's elevation a semblance of legality, was treated with extreme severity. His cause was subsequently espoused by Pope Nicholas in a manner highly offensiVe to the

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independent feeling of the Eastern Church. Photius felt himself the champion of Eastern Christianity against Latin pretensions; and when in 863 Nicholas finally anathematized and deposed him, he replied by a counter-excommunication. Meanwhile, the situation was suddenly changed by the murder of Photius’s patron, Bardas, by order of the emperor Michael, who was himselfassassinated by his colleague Basil in the following year i867). The fall of Photius immediately ensued; he was removed from his ofiice and banished about the end of September 867, a few days after the accession of Basil, and Ignatius was reinstated on the 23rd of November. About 876 Photius was suddenly recalled to Constantinople and entrusted with the education of Basil’s children. On the death of Ignatius, probably in October 878, Photius, after a decent show of reluctance, again filled the patriarchal throne. He then proceeded to obtain the formal recognition of the Christian world. In November 879 a synod was convened at Constantinople. The legates of Pope John VIII. attended, prepared to acknowledge Photius as legitimate patriarch,_a concession for which John was much censured by Latin opinion. He stood firm, however, on the other two points which had long been contested between the Eastern and Western Churches, the ecclesiastical jurisdiction over Bulgaria and the introduction of the “ filioque ” clause into the creed. He disowned his legates, who had'shown a tendency to yield, again excommunicated Photius, and thus aroused the open hostility which has never been appeased to this day. Strong in the support of the council, Photius simply ignored him. At the height of glory and success he was suddenly precipitated from his dignity by another palace revolution. After the death of Basil (886), his son and successor Leo, who had formerly been devoted to Photius, but in recent years displayed great hatred towards him, deprived him of his office and banished him to the monastery of Bordi in Armenia. From this time Photius disappears from history. -No letters of this period of his life are extant, which leads to the inference that his imprisonment was severe. The precise date of his death is not known, but it is said to have occurred on the 6th of February 891.

For long after Photius’s death his memory was held in no special honour by his countrymen. But when, in the crusading age, the Creek Church and state were alike in danger from Latin encroachments, Photius became a national hero, and is at present regarded as little short of a saint. To this character he has not the least pretension. Few men, it is probable, have been more atrociously calumniated; but, when every specific statement to.his rejudice has been rejected, he still appears on a eneral review of his actions worldly, crafty and unscrupulous. Yet e shows to no little advantage as an ecclesiastical statesman. His firmness was heroic, his sagacity profound and far-seeing; he supported good and evil fortune With equal dignity; and his fall was on both occasions due to revolutions beyond his control. In erudition, literary power, and force and versatility of intellect he far surpassed every contemporary.

The most imjortant of the works of Photius is his'renowned Bibliotheca or yn'obiblon (cd. I. Bekker, 1824—182 ), a collection of extracts from and abridgments of 280 volumes of c assical authors (usually cited as Codices), the originals of which are now to a great extent lost. The Work is special rich in extracts from historical writers. To Photius we are inde ted for almost all we possess of Ctesias. Memnon, Conon, the lost books of Diodorus Siculus, and the lost writings of Arrian. Theology and ecclesiastical history are also very fully represented, but poet and ancient philosophy are almost entirely ignored. It seems that 6 did not think it necessiry to deal with those authors with whom every well-educated man would naturally be familiar. The literary criticisms, generally distinguished by keen and independent judgment, and the excerpts, vary considerably in length. The numerous biogra hical notices are probably taken from the work of Hesychius of ’liletus. TheLemon (A6560! Zwa'yw‘yfi), published later than the Bibliotheca. was probably in the main the Work of some of his pupils. it was intended as a book of reference to facilitate the reading of old classical and sacred authors, whose language and vocabulary were out of date, The only MS. of the Lexicon is the Codex Galeanus, formerly in the possession of Thomas Gale (q.v.), and now in the library of Trinity College. Cambridge (ed. S. A. Naber, i864. with introduction on the authorities, critical commenta , and valuable indexes). His most important theological work is t e Amphilochia, a collection of some 300 questions and answers on diflicult points in Scripture, addressed to Amphilochius, archbishop of Cyzicns (ed. Sophocles Occonomus, Athens, 1858). Other simi ar works are his treatise in four books against the Manichaeans and Paulicians. and his controversy with the Latins on the Procession of the Holy Spirit. His Epistles, political and private, addressed to high church and state dignitaries, are valuable for the light they throw upon the character and versatility of the writer (ed. J. Valettas, London, 1864). A large number of his speeches and homilies have been edited b S. Aristarches (1900). The only complete edition is Bishop Ma ou's in Migne's Palrologia graeca, ci.—cv. R. _Reifzenstein (Der Ara/tan dc: Lexikon: des Photius, 1907) has published a hitherto une it MS. containing numerous ’fragments from various verse and prose authors.

After the allusions in his own writings the chief contemporary authority for the life of Photius is his bitter enemy, Nicetas the Paphlagonian, the biogra her of his rival Ignatius. The standard modern work is that of Cardinal Hergenrother, Photius, Patriarch van Constantinopd (1867—1869). As a dignitar of the Roman Catholic Church, Cardinal Hergenriither is inevitably biased against Photius as an ecclesiastic. but' his natural candour and sympathy with intellectual eminence have made him just to the man.

See also article by F. Kattenbusch in Herzog-Hauck’s Realcnc klopiidie fair Protestantische Theologie (1904), containing full bib iographical details; J. A. Fabricius, Bibliolheca_graeca,_x. 670—~ 776, x1. 1—37; C. Krumbacher, Geschzchle der byzanlzmschcn Litteralur, pp. 73—72, 515—524 (2nd ed., 1897); J. E. Sandys, History of Classical Scholars 11: (2nd ed., 1906).

PHOTOCHEMISTRY (Gr. (#61:, light, and “chemistry ”), in the widest sense, the branch of chemical science which deals with the optical properties of substances and their relations to chemical constitution and reactions; in the narrower sense it is concerned with the action of light on chemical change. The first definition includes such subjects as refractive and dispersive power, colour, fluorescence, phosphorescence, optical isomerism, spectroscopy, &c.-—subjects which are treated under other headings; here we only discuss the subject matter of the narrower definition. ‘

Probably the earliest photochemical investigations were associated with the darkening of certain silver salts under the action of light, processes which were subsequently utilized in photography (q.v.). At the same time, however, it had been observed that other chemical changes were regulated by the access of light; and the first complete study of such a problem was made by I. W. Draper in 1843, who investigated the combination of hydrogen and chlorine to form hydrochloric acid, a reaction which had been previously studied by Gay-Lussac and Thénard. Draper concluded that the first action of sunlight consisted in producing an allotrope of chlorine, which subsequently combined with the hydrogen. This was denied by Bunsen and Roscoe in 1857; and in 1887 Pringsheim suggested that the reaction proceeded in two stages: H10+ Clz= CllO-l- H,, 2H2+C120=H¢O+2HCL This view demands the presence of water vapour (H. B. Baker showed that the perfectly dry gases would not combine), and also explains the period which elapses before the reaction commenced (the “ photochemical induction ” of Bunsen and Roscoe) as taken up by the formation of the chlorine monoxide necessary to the second part of the reaction. The decomposition of hydriodic acid into hydrogen and iodine was studied by Lemoine in 1877, who found that 80% decomposed after a month’s exposure; he also observed that the reaction proceeded quicker in blue vessels than in red. A broader investigation was published by P. L. Chastaing in 1878, who found that the red rays generally oxidized inorganic compounds, whilst the violet reduces them, and that with organic compounds the action was entirely oxidizing. These and other reactions suggested the making of actinometers, or instruments for measuring the actinic effect of light waves. The most important employ silver salts; Eder developed a form based on the reaction between mercuric chloride and ammonium oxalate: 2HgClz-l— (NI-L), C104= 2HgCl + 2NH4C1+ ZCOQ, the extent of the decomposition being determined by the amount: of mercurous chloride or carbon dioxide liberated.

The article PHOTOGRAPHY (q.v.) deals with early investigations on the chemical action of light, and we may proceed here to modern work on organic compounds. That sunlight accelerates the action of the halogens, chlorine and bromine, on such compounds, is well known. ]ohn Davy obtained phosgene, COCli, by the direct combination of chlorine and carbon monoxide in sunlight (see Weigert, Ann. d. Phys., 1907 (iv.), 24, p. 55);

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chlorine combines with half its volume of methane explosively in sunlight, whilst in diffused light it substitutes; with toluene it gives benzyl chloride, C6H5CH¢CL in sunlight, and chlortoluene, CQH‘(CH)3C1, in the dark; with benzene it gives an addition product, CQHGCIQ, in sunlight, and substitutes in the dark. Bromine deports itself similarly, substituting and forming addition products with unsaturated compounds more readily in sunlight. Sometimes isomerization may occur; for instance, Wislicenus found that angelic acid gave dibromangelic acid in the dark, and dibromtiglic acid in sunlight. Many substances decompose when exposed to sunlight; for example, alkyl iodides darken, owing to the liberation of iodine; aliphatic acids (especially dibasic) in the presence of uranic oxide lose carbon dioxide; polyhydric alcohols give products identical with those produced by fermentation; whilst aliphatic ketones give a hydrocarbon and an acid.

Among aromatic compounds, benzaldehyde gives a trimeric and tetrameric benzaldehyde, benzoic acid and hydrobenzoin (G. L. Ciamician and P. Silber, Alli. R. Accad. Lincei, 1909); in alcoholic solution it gives hydrobenzoin; whilst with nitrobenzene it is oxidized to benzoic acid, the nitrobenzene sufl'ering reduction to nitrosobenzene and phenyl-B-hydroxylamine; the latter isomerizes to ortho- and para-aminophenol, which, in turn, combine with the previously formed benzoic acid. Similarly acetophenone and benzophenone in alcoholic solution give dimethylhydrobenzoin and benzopinacone. With nitro compounds Sach and Hilbert concluded that those containing a -CH- side group in the ortho position to the -NO, group were decomposed by light. For example, ortho-nitrobenzaldehyde in alcoholic solution gives nitrosobenzoic ester and 22’ azoxybenzoic acid, with the intermediate formation of nitrobenzaldehydediethylacetal, NOQ‘CQH4'CH(OC2I'IB)2 (E. Bamberger and F. Elgar, Ann. 1910, 371, p. 319). Bamberger also investigated nitrosobenzene, obtaining azoxybenzene as chief product, together with various azo compounds, nitrobenzene, aniline, hydroquinone and a resin.

For the photochemistry of diazo derivatives see Ruff and Stein, Ben, 1901, 34, p. 1668, and of the terpenes see G. L. Ciamician and P. Silber, Ben, 1907 and 1908. !

Light is also powerful in producing isomerization and polymerization. Isomerization chiefly appears in the formation of stable stereo-isomers from the labile forms, and more rarely in inducing real isomerization or phototropy (Marckwald, 1899). As examples we may notice the observation of Chattaway (Journ. Chem. Soc. 1906, 89, p. 462) that many phenylhydrazones (yellow) change into azo compounds (red), of M. Padoa and F. Graziani (Am'. R. Accad. Lincei, 1909) on the B-naphthylhydrazones (the a-compounds are not phototropic), and of A. Senier and F. G. Shepheard (Journ. Chem. Soc., 1909, 95, p. 1943) on the arylidene- and naphthylideneamines, which change from yellow to orange on exposure to sunlight. Light need not act in the same direction as heat (changes due to heat may be termed thermotropic). For example, heat changes the a form of benzyl-B-aminocrotonic ester into the 6 form, whereas light reverses this; similar-1y heat and light have reverse actions with as-diphenyl ethylene, CH1: C(C6H5)¢ (R. Stoermer, Ber., 1909, 42, p. 4865); the change, however, is in the same direction with Senier and Shepheard’s compounds. With regard to polymerization we may notice the production of benzene derivatives from acetylene and its homologues, and of tetramethylenes from the olefines.

Theory of Photochemical {lation.—Although much work has been done in the qualitative and quantitative study of photo~ chemical reactions relatively little attention has been given to the theoretical explanation of these phenomena. That the solution was to be found in an analogy to electrolysis was suggested by Grotthuss in 1818, who laid down: (1) only those rays which are absorbed can produce chemical change, (2) the action of the light is analogous to that of a voltaic cell; and be regarded light as made up of positive and negative electricity. The first principle received early acceptance; but the development of the second is due to W. D. Bancroft who, in a series of papers in the Journal of Physical Chemistry for 1908 and 1909, has applied it generally to the reactions under consideration. Any electrolytic action demands a certain minimum electromotive force; this, however, can be diminished by suitable depolarizers, which generally act by combining with a product of the decomposition. Similarly, in some photochemical reactions the low electromotive force of the light is sufficient to induce decomposition, but in other cases a depolarizer must be present. For example, ferric chloride in aqueous solution is unchanged by light, but in alcoholic solution reduction to ferrous chloride occurs, the liberated chlorine combining with the alcohol. In the same way Bancroft showed that the solvent media employed in photographic plates act as depolarizers. The same theory explains the action of sensitizers, which may act optically or chemically. In the first case they are substances having selective absorption, and hence alter the sensitivity of the system to certain rays. In the second case there are no strong absorption bands, and the substances act by combining with the decomposition products. Bancroft applied his theory to the explanation of photochemical oxidation, and also to the chlorination and bromination of hydrocarbons. In the latter case it is supposed that the halogen produces ions; if the positive ions are in excess side chains are substituted, if the negative the nucleus. '

Standard treatises are: I. M. Eder, Handbuch der Photographic, vol. i. pt. 2 (1906); H. W. Vogel, Photochemic (1906). An account of the action of light on organic compounds is given in A. W. Stewart, Recent Advances in Organic Chemistry (1998).

PHOTOGRAPHY (Gr. are, light, and 7ph¢ew, to write), the science and art of producing pictures by the action of light on chemically prepared (sensitized) plates or films.

History.

It would be somewhat difficult to fix a date when what we now know as “ photographic action ",was first recorded. No doubt the tanning of the skin by the sun’s rays was what was first noticed, and this is as truly the effect of solar radiation as is the darkening of the sensitive paper which is now in use in photographic printing operations. We may take it that K. W. Scheele was the first to investigate the darkening action of sunlight on silver chloride. He found that when silver chloride was exposed to the action of light beneath water there was dissolved in the fluid a substance which, on the addition of lunar caustic (silver nitrate), caused the precipitation of new silver chloride, and that on applying a solution of ammonia to the blackened chloride an insoluble residue of metallic silver was left behind. He also noticed that of the rays of the spectrum the violet most readily blackened the silver chloride. In Scheele, then, we have the first who applied combined chemical and spectrum analysis to the science of photography. In 1782 J. Senebier repeated Scheele’s experiments, and found that in fifteen seconds the violet rays blackened silver chloride as much as the red rays did in twenty minutes.l In 1798 Count Rumford contributed a paper to the Philosophical Transactions entitled “ An inquiry concerning the chemical properties that have been attributed to light," in which he tried to demonstrate that all effects produced on metallic solution could be brought about by a temperature somewhat less than that of boiling water. Robert Harrup in 1802, however, conclusively showed in N ichalson’s Journal that, at all events, salts of mercury were reduced by visible radiation and not by change of temperature.

In 180! we come to the next decided step in the study of photographic action, when Johann Wilhelm Ritter 0776—1810) proved the existence of rays lying beyond the violet, and found that they had the power of blackening silver chloride. Such a discovery naturally gave a direction to the investigations of others, and Thomas Johann Seebeck (1770—1831) (between 1802 and i808) and, in ISI 2, Jacques Etienne Bérard (1789—1869) turned their attention to this particular subject, eliciting valuable information. We need only mention two or three other cases

1 It may here be remarked that had he used a pure s trum he would have found that the red rays did not blacken t e material in the slightest degree. '

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where the influence of light was noticed at the beginning of the

- 19th century. William Hyde Wollaston observed the conversion

of yellow gum guaiacum into a green tint by the violet rays, and the restoration of the colour by the red rays—both of which are the effect of absorption of light, the original yellow colour of the gum absorbing the violet rays, whilst the green colour to which it is changed absorbs the red rays. Sir Humphry Davy found that puce-coloured lead oxide, when damp, became red in the red rays, whilst it blackened in the violet rays, and that the green mercury oxide became red in the red rays—again an example of the necessity of absorption to eflect a molecular or chemical change in a substance. U. R. T. Le Bouvier Desmorties in 1801 observed the change eflected in Prussian blue, and Carl Wilhelm Bockman noted the action of the two ends of the spectrum on phosphorus, a research which John William Draper extended farther in America at a later date.

To England belongs the honour of first producing a photograph by utilizing Scheele’s observations on silver chloride. In June 1802 Thomas Wedgwood (1771-1805) published in the Journal of the Royal Institution the paper—“ An account of a method of copying paintings upon glass and of making profiles by the agency of light upon nitrate of silver, with observations by H. Davy.” He remarks that white paper or white leather moistened with a solution of silver nitrate undergoes no change when kept in a dark place, but on being exposed to the daylight it speedily changes colour, and, after passing through various shades of grey and brown, becomes at length nearly black. The alteration of colour takes place more speedin in proportion as the light is more intense.

“ In the direct beam of the sun two or three minutes are sufficient to <produce the full effect, in the_ shade several hours are required, an .hfihttransmitted through different-coloured glasses acts upon it wrt different degrees of intensity. Thus it is found that red rays, or the common sunbeams passed through red glass, have very little action upon it; yellow and green are more efficacious. but blue and violet light produce the mast decided and powerful effects."

Wedgwood goes on to describe the method of using this prepared paper by throwing shadows on it, and inferentially by what we now call “ contact printing."- He states that he has been unable to fix his prints, no washing being suflicient to eliminate the traces of the silver salt which occupied the unexposed or shaded portions. Davy in a note states that he has found that, though the images formed by an ordinary camera obscura were too faint to print out in the solar microscope, the images of small objects could easily be copied on such paper.

“ In comparing the effects produced by light upon muriate of silver (silver chloride) with those upon the nitrate it seemed evident that the muriatc was the more susceptible, and both were more readily acted upon when moist than when dry—a fact long ago known. Even in the twili ht the colour of the moist muriate of silver, spread

‘ upon paper, slow y changed from white to faint violet; though under

similar circumstances no intermediate alteration was produced upon the nitrate. . . Nothing but a method of preventing the unshaded parts of the delineations from being coloured by exposure to the day is wanting to render this process as useful as it is elegant."

In this method of preparing the paper lies the germ of the silver-printing processes of modern times, and it was only by the spread of chemical knowledge that the hiatus which was to render the “ process as useful as it is elegant” was filled up—when sodium thiosulphate (hyposulphite of soda), discovered by Francois Chaussier in 1799, or three years before Wedgwood published his paper, was used for making the print permanent. Here we must call attention to an important observation by Seebeck of Jena in 1810. In the Farbenlehro of Goethe he says:—

“ \Vhen a spectrum produced by a properly constructed prism is thrown upon moist chloride of silver paper, if the printing be continued for from fifteen to twenty minutes, whilst a. constant sition for the spectrum is maintained by any means, I observe the fo lowing. In the Violet the chloride is a reddish brown (sometimes more violet, sometimes more blue), and this coloration extends Well beyond the limit of the violet; in the blue the chloride takes a clear blue tint, which fades away, becoming lighter in the green. In the yellow l_ usuall found the chloride unaltered; sometimes, however, it had a light ye low tint; in the red and beyond the red it took a rose or lilac not. This image of the spectrum shows beyond the_re_d and the violet a region more or less light and uncoloured. This is how the decomposition of the silver chloride is seen in this region. Beyond the brown band, . . . which was produced in the violet, the silver chloride was coloured a grey-violet for a distance of several inches. In proportion as the distance from the violet increased, the tint became lighter. Beyond the red, on the contrary, the chloride took a feeble red tint for a considerable distance. When moist chloride of silver, having received the action of light for a time, is ex osed to the spectrum, the blue and violet behave as above. In tfiie yellow and red regions, on the other hand, it is found that the silver chloride becomes paler; . . . the parts acted upon by the red rays and by those beyond take a light coloration."

This has been brought forward by J. M. Eder as being the first record we have of photographic action lending itself to production of natural colours. This observation of Seebeck was allowed to lie fallow for many years, until it was again taken up and published as a novelty.

The first to found a process of photography-which gave pictures that were subsequently unaffected by light was Nicéphore de Niepce. His process, which he called provisionally “héliographie, dessins, et gravures,” consists in coating the surface of a metallic plate with a solution of asphaltum in oil of lavender and exposing it to a camera image. He recommends that the asphaltum be powdered and the oil of lavender dropped upon it in a wine-glass, and that it be then gently heated. A polished plate is covered with this varnish, and, when dried, is ready for employment in the camera. After requisite exposure, which is very long indeed, a very faint image, requiring development, is seen. Development is effected by diluting oil of lavender with ten parts by volume of white petroleum. After this mixture has been allowed to stand two or three days it becomes clear and is ready to be used. The plate is placed in a dish and covered with the solvent. By degrees the parts unaffected by light dissolve away, and the picture, formed of modified asphaltum', is developed. The plate is then lifted from the dish, aIIQWed to drain, and finally freed from the remaining solvents by washing in water. Subsequently, instead of using oil of lavender as the asphaltum solvent, Niepce employed an animal oil, which gave a deeper colour and more tenacity to the surface-film. I _ Later, Louis Jacques Mandé Daguerre (1789—1851) and Niepce used as a solvent the brittle residue obtained from evaporating the 'oil of lavender dissolved in ether or alcohol—a transparent solution of a lemon-yellow colour being formed. This solution was used for covering glass or silver plates, which, when dried, could be used in the camera. The time of exposure varied somewhat iri length. Daguerre- remarked that “the time required to procure a photographic copy of a landscape is from seven to eight hours, but single monuments, when strongly lighted by the sun, or which are themselves very bright, can be taken in about three hours.” Perhaps there is no sentence that illustrates more forcibly the advance made in photography from the days when this process was described. The ratio of three hours to Thth of a second is a fair estimate of the progress made since Niepce. The development was conducted by means of petroleum-vapour, which dissolved the parts not acted upon by light. As a rule silver plates seem to have been used, and occasionally glass; but it does not appear whether the latter material was chosen because an image would be projected through it or whether simply for the sake of effect. Viewed in the light of present knowledge, a more perfectly developablc image in half-tone would be obtained by exposing the film through the back of the glass. The action of light on most organic matter is apparently one of oxidation. In the case of asphaltum or bitumen of Judaea the oxidation causes a hardening of the material and an insolubility in the usual solvents. Hence that surface of the film is generally hardened first which first feels the influence of light. Where half~tones exist, as in a landscape picture, the film remote from the surface first receiving the image is not acted upon at all, and remains soluble in the solvent. It is thus readily seen that, in the case of half-tone pictures, or even in copying engravings, if the action were not continued sufficiently long when the surface of the film farthest from the glass was first acted upon, the layer next the glass would in some places remain soluble, and on development would be dissolved away, carrying the top layer of hardened resinous

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matter with it, and thus give rise to imperfect pictures. In carbon-printing development from the back of the exposed film is absolutely‘essential, since it depends on the same principles as does heliography, and in this the same mbde of procedureis advisable.

It would appear that Niepce began his researches as early as 1814, but it was not till 1827 thathe had any success worth recount' ing. At that date he communicated a aper to Dr Bauer of Kew, the secretary of the Royal Society of ondon, with a view to its presentation to that society. Its publication, however, was prevented because the process, of which exam les were shown, was a secret one. In an authentic MS. co y o Niepce’s “ Mémoire," dated “ Kew, le 8 Décembre 1827,',’ e says that “in his framed drawings made on tin the tone is too feeble, but that by the use of chemical agents the tone may be darkened." This shows that Niepce was familiar with the idea of using some darkening medium even with his photographs taken on tin plates.

Daguerreotypc.—We have noticed in the joint process of Daguerre and Niepce that polished silver plates were used, and we know from the latter that amongst the chemical agents tried iodine‘suggested itself. Iodine vapour or solution applied to a silvered plate would cause the formation of silver iodide on those parts not acted upon by light. The removal of the resinous picture would leave-an image formed of metallic silver, whiLst the black parts of the original would be represented by the darker silver iodide. This was probably the origin of the daguerreotype process. Such observers as Niepce and Daguerre, who had formed a partnership for prosecuting their researches, would not have thus formed silver iodide iwithout noticing that it changed in colour when exposed to the light. What parts respectively Daguerre and Niepce played in the development of the daguerreotype will. probably never be known with absolute accuracy, but in a letter from Dr Bauer to Dr J. J. Bennett, F.R.S., dated the 7th of May 1839, the former says:—

“I received a very interesting letter from Mons. Isidore Niepce, dated 12th March [about a month after the publication of the daguerreotype process], and that letter fully confirms what I sus

ectedof Daguerre’s manoeuvres with r Nicéphore, but Mr sidore observes that for the present that etter might be considered confidential."

Dr Bauer evidently knew more of “ poor Nicéphore’s ’ work than most people, and at that early period he clearly thought that an injustice v,had been, done to vNiepce at the hands of Daguerre. It should be remarked that Nicéphore de N iepce died in 1833, and a new agreement was entered into between his son Isidore de Niepce and Daguerre to continue the prosecution of their researches. It appears further that Niepce communicated his process to Daguerre on the 5th of December 1829. At his death some letters from Daguerre and others were left by him in which iodine, sulphur, phosphorus, &c., are mentioned as having been used on the metal plates, and their sensitiveness to light, when thus treated, commented upon. We are thus led to believe that a great part of the success in producing the daguerreoty-pe is due to the elder Niepce; and indeed it must have been thought so at the time, since, on the publication of the process, life-pensions of 6000 francs and 4000 francs were given to Daguerre and to Isidore Niepce respectively. In point of chronology the publication of the discovery of the daguerreotype process was made subsequently to the Talbottype process. It will, however, be convenient to continue the history of the daguerreotype, premising that it was published on the 6th of February 1839, whilst Talbot’s process was given to the world on the 25th of January of the same year.

Daguerreotype pictures were originally taken on silver-plated copper, and. even now the silvered surface thus prepared serves better than electro-deposited silver of any thickness. An outline of the operations is as follows. A brightly- olishcd silver late is cleaned by finely-powdered umice and oive oil, and t en by dilute nitric acid, and a soft ufi is employed to give it a brilliant polish, the slightest trace of foreign. matter or stain being fatal to the production of a perfect picture. The late, thus repared, is ready for the iodizing operation. Small ragmcnts of) iodine are scattered over a saucer, covered with gauze. Over this the plate is placed, face downwards, resting on supports, and the vapour from the iodine is allowed to form upon it a surface of silver iodide. It is essential to note the colour of the surface-formed iodide at its several stages, the varying colours being due to interference colours

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