See also:

• PHOTOCHEMISTRY (Gr. 4&n, light, and " chemistry ")

PHOTOCHEMISTRY (Gr. 4&n, See also:

• LIGHT

light, and " See also:

• CHEMISTRY
• CHEMISTRY (formerly "chymistry"; Gr. xvµela; for derivation see ALCHEMY)

chemistry ")  , in the widest sense, the See also:

• BRANCH (from the Fr. branche, late Lat. branca, an animal's paw)

branch of chemical See also:

• SCIENCE (Lat. scientia, from scire; to learn, know)

science which deals with the See also:

• OPTICAL

optical properties of substances and their relations to chemical constitution and reactions; in the narrower sense it is concerned with the See also:

• ACTION

action of See also:

• LIGHT

light on chemical See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change . The first See also:

• DEFINITION (Lat. definitio, from de-finire, to set limits to, describe)

definition includes such subjects as refractive and dispersive See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power, See also:

• COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour, See also:

• FLUORESCENCE

fluorescence, See also:

• PHOSPHORESCENCE

phosphorescence, optical See also:

• ISOMERISM

isomerism, See also:

• SPECTROSCOPY (from Lat. spectrum, an appearance, and Gr. a-sore y, to see)

spectroscopy, &c.—subjects which are treated under other headings; here we of y discuss the subject See also:

• MATTER

matter of the narrower definition . Probably the earliest photochemical investigations were associated with the darkening of certain See also:

• SILVER

silver salts under the action of light, processes which were subsequently utilized in See also:

• PHOTOGRAPHY (Gr. bias, light, and ypa w, to write)
• PHOTOGRAPHY, CELESTIAL

photography (q.v.) . At the same See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time, however, it had been observed that other chemical changes were regulated by the See also:

• ACCESS (Lat. accessus)

access of light; and the first See also:

• COMPLETE

complete study of such a problem was made by J . W . See also:

• DRAPER
• DRAPER, JOHN WILLIAM (1811-1882)

Draper in 1843, who investigated the See also:

• COMBINATION (Lat. combinare, to combine)

combination of See also:

• HYDROGEN [symbol H, atomic weight x-oo8 (0=16)]

hydrogen and See also:

• CHLORINE (symbol Cl, atomic weight 35`46 (0=16)

chlorine to See also:

• FORM (Lat. forma)

form hydrochloric See also:

• ACID (from the Lat. root ac-, sharp; acere, to be sour)

acid, a reaction which had been previously. studied by See also:

• GAY, JOHN (1685-1732)
• GAY, MARIE FRANCOISE SOPHIE (1776-1852)
• GAY, WALTER (1856– )

Gay-Lussac and See also:

• THENARD, LOUIS JACQUES (1777-1857)

Thenard . 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 See also:

• BUNSEN, CHRISTIAN CHARLES JOSIAS, BARON VON (1791-186o,)
• BUNSEN, ROBERT WILHELM VON (1811-1899)

Bunsen and See also:

• ROSCOE, WILLIAM (1753—1831)

Roscoe See also:

• ILL

ill 1857; and in 1887 See also:

• PRINGSHEIM, NATHANAEL (1823-1894)

Pringsheim suggested that the reaction proceeded in two stages: 1120+ Cl2 = 0120+ 112, 2H2+C120=See also:

• H2O (combined)

H2O+2HC1 . This view demands the presence of See also:

• WATER

water vapour (H . B . See also:

• BAKER, HENRY (1698-1774)
• BAKER, SIR BENJAMIN (1840-1907)
• BAKER, SIR RICHARD (1568-1644/5)
• BAKER, SIR SAMUEL WHITE (1821-1893)
• BAKER, THOMAS (1656-1740)
• BAKER, VALENTINE [BAKER PASHA] (1827-1887)

Baker showed that the perfectly dry gases would not combine), and also explains the See also:

• PERIOD
• PERIOD (Gr. irepioSos, a going or way round, circuit, aepi, round, and &Sis, way, road)

period which elapses before the reaction commenced (the " photochemical See also:

• INDUCTION (from Lat. inducere, to lead into; cf. Gr. ilrayu yli)

induction " of Bunsen and Roscoe) as taken up by the formation of the chlorine monoxide necessary to the second See also:

• PART

part of the reaction . The decomposition of hydriodic acid into hydrogen and See also:

• IODINE (symbol I, atomic weight '26.92)

iodine was studied by Lemoine in 1877, who found that 8o% decomposed after a See also:

• MONTH
• MONTH (a common Teutonic word, cf. Ger. Mond, Du. maand, Dan. maaned, &c., and cognate with Lat. mensis, Gr. µi7v, &c., in other branches of the Indo-Germanic family; all ultimately from the root seen in the word for the moon in nearly all those languages

month's exposure; he also observed that the reaction proceeded quicker in See also:

• BLUE (common in different forms to most European languages)

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 See also:

• VIOLET

violet reduces them, and that with organic compounds the action was entirely oxidizing . These and other reactions suggested the making of actinometers, or See also:

• INSTRUMENTS

instruments for measuring the actinic effect of light waves . The most important employ silver salts; Eder See also:

• DEVELOPED

developed a form based on the reaction between mercuric chloride and ammonium oxalate: 2HgC12+ (NH4)2 C204 = 2HgCl + 2NH4C1 + 2CO2, the extent of the decomposition being determined by the amount: of mercurous chloride or See also:

• CARBON (symbol C, atomic weight 12)

carbon dioxide liberated . The See also:

• ARTICLE (from Lat. articulus, a joint)

article PHOTOGRAPHY (q.v.) deals with See also:

• EARLY
• EARLY, JUBAL ANDERSON (1816-1894)

early investigations on the chemical action of light, and we may proceed here to See also:

• MODERN

modern See also:

• WORK

work on organic compounds . That sunlight accelerates the action of the See also:

• HALOGENS

halogens, chlorine and See also:

• BROMINE (symbol Br, atomic weight 79-96)

bromine, on such compounds, is well known . See also:

• JOHN
• JOHN (1167–1216)
• JOHN (1290-c. 1320)
• JOHN (1296-1346)
• JOHN (1371–1419)
• JOHN (1468-1532)
• JOHN (1801-1873)
• JOHN (Heb. llni')
• JOHN (ZAPOLYA) (1487-1540)
• JOHN, 4TH MARQUESS OF TWEEDDALE (c. 1695-1762)
• JOHN, DON (1545-1578)
• JOHN, DON (1629–1679)
• JOHN, GOSPEL OF ST
• JOHN, or HAYS (1513–1571)
• JOHN, THE APOSTLE
• JOHN, THE EPISTLES OF

John See also:

• DAVY, SIR HUMPHRY

Davy obtained phosgene, COC12, by the See also:

• DIRECT

direct combination of chlorine and carbon monoxide in sunlight (see Weigert, See also:

• ANN

Ann. d . Phys., 1907 (iv.), 24, p . 55);chlorine combines with See also:

• HALF

half its See also:

• VOLUME

volume of methane explosively in sunlight, whilst in diffused light it substitutes; with See also:

• TOLUENE, or METHYLBENZENE

toluene it gives benzyl chloride, C6H5CH2C1, in sunlight, and chlortoluene, See also:

• C5H4

C5H4(CH)3C1, in the dark; with See also:

• BENZENE, C6H6

benzene it gives an addition product, C6H6C16, 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, See also:

• WISLICENUS, JOHANNES (1835-1902)

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 See also:

• OXIDE

oxide lose carbon dioxide; polyhydric See also:

• ALCOHOLS
• ALCOHOLS, ALDEHYDES AND
• ALCOHOLS, BENZYL ALCOHOL, BUTYL ALCOHOLS, METHYL ALCOHOL

alcohols give products identical with those produced by See also:

• FERMENTATION

fermentation; whilst aliphatic See also:

• KETONES

ketones give a See also:

• HYDROCARBON

hydrocarbon and an acid . Among aromatic compounds, See also:

• BENZALDEHYDE (oil of bitter almonds), C6H5CHO

benzaldehyde gives a trimeric and tetrameric benzaldehyde, benzoic acid and hydrobenzoin (G . L . Ciamician and P . Silber, Atli . R . Accad . Lincei, 1909); in alcoholic See also:

• SOLUTION (from Lat. solvere, to loosen, dissolve)

solution it gives hydrobenzoin; whilst with nitro-benzene it is oxidized to benzoic acid, the See also:

• NITROBENZENE, C6H6NO2

nitrobenzene suffering reduction to nitrosobenzene and phenyl-#-See also:

• HYDROXYLAMINE, NH2OH

hydroxylamine; the latter isomerizes to ortho- and See also:

• PARA
• PARA (officially BELEM; sometimes BELEM DO PARA)

para-aminophenol, which, in turn, combine with the previously formed benzoic acid . Similarly See also:

• ACETOPHENONE

acetophenone and See also:

• BENZOPHENONE (DIPIENYL KETONE)

benzophenone in alcoholic solution give dimethylhydrobenzoin and benzopinacone . With nitro compounds Sach and Hilbert concluded that those containing a •CH. See also:

• SIDE
• SIDE (mod. Eski Adalia)

side See also:

• GROUP

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•See also:

• C6H4
• C6H4(CH213r)2

C6H4•CH(OC2H6)2 (E . See also:

• BAMBERGER, LUDWIG (1823-1899)

Bamberger and F .

See also:

• ELGAR, SIR EDWARD (1857– )

Elgar, Ann . 191o, 371, p . 319) . Bamberger also investigated nitrosobenzene, obtaining azoxybenzene as See also:

• CHIEF (from Fr. chef, head, Lat. caput)

chief product, together with various See also:

• AZO (c. 1150-1230)

azo compounds, nitrobenzene, See also:

• ANILINE, PHENYLADIINE, or AMINOBENZENE, (C6H5NH2)

aniline, hydroquinone and a See also:

• RESIN (through O.Fr. resine, modern reline, from Lat. resina, prcbably Latinized from Greek Pirivrt, resin)

resin . For the See also:

• PHOTOCHEMISTRY (Gr. 4&n, light, and " chemistry ")

photochemistry of diazo derivatives see See also:

• RUFF

Ruff and See also:

• STEIN, CHARLOTTE VON (1742-1827)
• STEIN, HEINRICH FRIEDRICH KARL, BARON VOM

Stein, Ber., 19oi, 34, p . 1668, and of the See also:

• TERPENES

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 See also:

• STABLE

stable stereo-isomers from the labile forms, and more rarely in inducing real isomerization or phototropy (Marckwald, 1899) . As examples we may See also:

• NOTICE

notice the observation of Chattaway (Journ .

Chem . See also:

• SOC

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 i3-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 naphthylidene-See also:

• AMINES

amines, which change from yellow to See also:

• ORANGE
• ORANGE (Citrus Aurantium)
• ORANGE, HOUSE OF

orange on exposure to sunlight . Light need not See also:

• ACT (Lat. actus, actum)

act in the same direction as See also:

• HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

heat (changes due to heat may be termed thermotropic) . For example, heat changes the a form of benzyl-0-aminocrotonic ester into the $ form, whereas light reverses this; similarly heat and light have See also:

• REVERSE

reverse actions with as-See also:

• DIPHENYL (phenyl benzene), C6H5

diphenyl See also:

• ETHYLENE, or ETHENE, C2H4

ethylene, See also:

• CH2
• CH2(OH)

CH2: C(See also:

• C6H6

C6H6)2 (R . Stoermer, Ber., Igoo, 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 See also:

• PRODUCTION (Lat. productionem, from producere, to pro-duce)

production of benzene derivatives from See also:

• ACETYLENE

acetylene and its homologues, and of tetramethylenes from the olefines . Theory of Photochemical Action.—Although much work has been done in the qualitative and quantitative study of photo-chemical reactions relatively little See also:

• ATTENTION (from Lat. ad-tendo, await, expect; the condition of being " stretched " or " tense ")

attention has been given to the theoretical explanation of these phenomena . That the solution was to be found in an See also:

• ANALOGY (Gr. avaXo-yLa, proportion)

analogy to See also:

• ELECTROLYSIS (formed from Gr. Xbety, to loosen)

electrolysis was suggested by Grotthuss in 1818, who laid down: (I) only those rays which are absorbed can produce chemical change, (2) the action of the light is analogous to that of a voltaic See also:

• CELL (from Lat. cella, probably from an Indo-European kal —seen in Lat. celare, to hide; another suggestion connects the word with Lat. cera, wax, taking the original meaning to refer to the honeycomb)

cell; and he regarded light as made up of See also:

• POSITIVE (or PORTABLE) ORGAN

positive and negative See also:

• ELECTRICITY

electricity . The first principle received early See also:

• ACCEPTANCE (Lat. acceptare, frequentative form of accipere, to receive)

acceptance; but the development of the second is due to W . D .

See also:

• BANCROFT, GEORGE (1800—1891)
• BANCROFT, HUBERT HOWE (1832– )
• BANCROFT, RICHARD (1544–161o)
• BANCROFT, SIR SQUIRE (1841– )

Bancroft who, in a See also:

• SERIES (a Latin word from serere, to join)

series of papers in the See also:

• JOURNAL
• JOURNAL (through Fr. from late Lat. diurnalis, daily)

Journal of See also:

• PHYSICAL

Physical See also:

• CHEMISTRY
• CHEMISTRY (formerly "chymistry"; Gr. xvµela; for derivation see ALCHEMY)

Chemistry for igo8 and 1909, has applied it generally to the reactions under See also:

• CONSIDERATION (from Lat. considerare, to look at closely, examine, generally taken to be from con-, and the base seen in sidus, sideris, a star, the word being supposed to be originally an astrological or astronomical term)

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 See also:

• LOW, SETH (1850- )
• LOW, WILL HICOK (1853- )

low electromotive force of the light is sufficient to induce decomposition, but in other cases a depolarizer must be See also:

• PRESENT

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 See also:

• ALCOHOL

alcohol . In the same way Bancroft showed that the solvent See also:

• MEDIA

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 See also:

• CASE
• CASE, JOHN (d. 1600)

case they are substances having selective absorption, and hence alter the sensitivity of the See also:

• SYSTEM

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 See also:

• HYDROCARBONS

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 See also:

• NUCLEUS (Lat. for the kernal of a nut, nux, the stone of fruit)

nucleus . See also:

• STANDARD
• STANDARD, BATTLE OF THE

Standard See also:

• TREATISES

treatises are: J . M .

Eder, Handbuch der Photographie, vol. i. pt . 2 (1906); H . W . See also:

• VOGEL, EDUARD (1829-1856)
• VOGEL, SIR JULIUS (1835-1899)

Vogel, Photochemie (1906) . An See also:

• ACCOUNT
• ACCOUNT (through O. Fr. acont, Late Lat. comptum, cornputare, to calculate)

account of the action of light on organic compounds is given in A . W . See also:

• STEWART, ALEXANDER TURNEY (1803-1876)
• STEWART, BALFOUR (1828-1887)
• STEWART, CHARLES (1778–1869)
• STEWART, DUGALD (1753-1828)
• STEWART, J
• STEWART, JOHN (1749—1822)
• STEWART, JULIUS L
• STEWART, SIR DONALD MARTIN (1824–19o0)
• STEWART, SIR HERBERT (1843—1885)
• STEWART, SIR WILLIAM (c. 1540—c. 1605)
• STEWART, STUART
• STEWART, WILLIAM (c. 1480-c. 1550)

Stewart, See also:

• RECENT

Recent Advances in Organic Chemistry (1908) .