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Rh 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 by S. Aristarches (1900). The only complete edition is Bishop Malou’s in Migne’s Patrologia graeca, ci.–cv. R. Reifzenstein (Der Anfang des Lexikons des Photius, 1907) has published a hitherto unedited 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 biographer of his rival lgnatius. The standard modern work is that of Cardinal Hergenröther, Photius, Patriarch von Constannnopel (1867–1869). As a dignitary of the Roman Catholic Church, Cardinal Hergenröther 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 Realencyklopädie für protestantische Theologie (1904), containing full bibliographical details; J. A. Fabricius, Bibliotheca graeca, x. 670–776, xi. 1–37; C. Krumbacher, Geschichte der byzantinischen Litteratur, pp. 73–79, 515–524 (2nd ed., 1897); J. E. Sandys, History of Classical Scholarship (2nd ed., 1906).

 PHOTOCHEMISTRY (Gr. , 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 (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 J. 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: H2O + Cl2 = Cl2O + H2, 2H2 + Cl2O=H2O+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: 2HgCl2+ (NH4)2 C2O4=2HgCl + 2NH4Cl + 2CO2, the extent of the decomposition being determined by the amount; of mercurous chloride or carbon dioxide liberated.

The article (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. John Davy obtained phosgene, COCl2, by the direct combination of chlorine and carbon monoxide in sunlight (see Weigert, Ann. d. Phys., 1907 (iv.), 24, p. 55);

chlorine combines with half its volume of methane explosively in sunlight, whilst in diffused light it substitutes; with toluene it gives benzyl chloride, C6H5CH2Cl, in sunlight, and chlortoluene, C6H4(CH)3Cl, in the dark; with benzene it gives an addition product, C6H6Cl6, 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, Atti. R. Accad Lincei, 1909); in alcoholic solution it gives hydrobenzoin; whilst with nitrobenzene it is oxidized to benzoic acid, the nitrobenzene suffering reduction to nitrosobenzene and phenyl--hydroxylamine; the latter isomerizes to ortho- and para-aminophenol, which, in turn, combine with the previously formed benzoic acid. Similarly ace top hen one 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 ⋅NO2 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, NO2⋅C6H4⋅CH(OC2H5)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, hydro quin one and a resin.

For the photochemistry of diazo derivatives see Ruff and Stein, Ber., 1901, 34, p. 1668, and of the terpenes see G. L. Ciamician and P. Silber, Ber., 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 (Atti. R. Accad. Lincei, 1909) on the -naphthylhydrazones (the -compounds are not phototropic), and of A. Senier and F. G. Shepheard (Journ. Chem. Soc., 1909, 95, p. 1943) on the arylidene- and naphthylidene-amines, 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 form of benzyl--aminocrotonic ester into the  form, whereas light reverses this; similarly heat and light have reverse actions with as-diphenyl ethylene, CH2:C(C6H5)2 (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 homologies, and of tetramethylenes from the olefines.

Theory of Photochemical Action.—Although much work has been done in the qualitative and quantitative study of photochemical 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 he 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 