Phosphorus tribromide, PBr3, prepared by mixing solutions of its elements in carbon disulphide and distilling, is a transparent, mobile liquid, boiling at 173° and resembling the trichloride chemically. The pentabromide, PBrs, which results from phosphorus and an excess of bromine, is a yellow solid, and closely resembles the pentachloride. The bromochloride, PC13Br2, is an orange-coloured solid formed from bromine and the trichloride, into which components it decomposes at 35°. Phosphoryl tribromide, POBr3, is a solid, melting at 45° and boiling at 195. Thiophosphoryl bromide, PSBr2, obtained after the manner of the corresponding chloride, forms yellow octahedra which melt at 38°, and have a penetrating, aromatic odour. With water it gives sulphur, sulphuretted hydrogen, hydrobromic, phosphorous and phosphoric acids, the sulphur and phosphorous acid being produced by the interaction of the previously formed sulphuretted hydrogen and phosphoric acid. Pyrophosphoryl thiobromide, (PBr¿S)¿S, and metaphosphoryl thiobromide, PS,Br, are also known.

Phosphorus forms three iodides. The subiodide, PI, was obtained by R. Boulough (Comptes rendus, 1905, 141, p. 256), who acted with dry iodine on phosphorus dissolved in carbon disulphide; with alkalis it gives P4(OH). The di-iodide and tri-iodide are formed similarly; the first is deposited as orange-coloured prisms which melt at 110° to a red liquid (see Doughty, Jour. Amer. Chem. Soc., 1905, 27, p. 1444), whilst the second forms dark-red hexagonal plates which melt at 55°.

acids. It does not dissociate on heating as do the pentachloride | sulphuretted hydrogen; alkalis form a thiophosphate, e.g. PS(OK);, and pentabromide, thus indicating the existence of pentavalent and a chloride. 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 2PF5:5NH3, and with nitrogen peroxide at -10° in the proportion PF5:NO2. Phosphorus trifluorodichloride, PF Cl2, prepared from chlorine and the trifluoride, is a pungentsmelling gas, which at 250° gives the pentachloride and fluoride. The trifluorodibromide (see above) is an amber-coloured mobile liquid. Phosphoryl trifluoride, POF3, may be obtained by exploding 2 volumes of phosphorus trifluoride with I volume of oxygen (Moissan, 1886); by heating 2 parts of finely-divided cryolite and 3 parts of phosphorus pentoxide (Thorpe and Hambly, Jour. Chem. Soc., 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 phosphoric acids. The corresponding sulphur compound, thiophosphoryl fluoride, PSF3, obtained by heating lead fluoride and phosphorus pentasulphide to 200°, is a colourless gas, which may be condensed to a clear transparent liquid. It spontaneously inflames in air or oxygen; and when the gas is issuing from a jet into air the flame is greyish green, with a faintly luminous and yellow tip; the flame is probably one of the coldest known. The combustion probably follows the equation PSF+O2 = PF3+SO2, the trifluoride at a higher temperature decomposing according to the equations: 10PF+502=6PFь+2P,О5, 2PF3+O2=2POF3, the complete reaction tending to the equation: 10PSF+1502=6PF6+2PO+ IOSO. The gas dissolves in water on shaking; PSF3+4H2O= ·H2S+H3PO4+3HF, but is more readily taken up by alkaline solutions with the formation of fluoride and thiophosphate: PSF3+ 6NaOH= Na3PSO3+3NaF. Heated in a glass tube it gives silicon fluoride, phosphorus and sulphur, PSF3=PF3+S; 4PF3+3SiO2 = 3SiF+P+302. Electric sparks give at first free sulphur and the trifluoride, the latter at a higher temperature splitting into the pentafluoride and phosphorus. With dry ammonia it gives ammonium fluoride and a compound P(NH2)2SF. Phosphorus trichloride or phosphorous chloride, PCl3, 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 liquid of specific gravity 1.6128 at 0° and boiling-point 76°. With chlorine it gives the pentachloride, PCl5, and with oxygen when heated phosphoryl chloride, POCI. Water gives hydrochloric and phosphorous acids, with separation of red phosphorus if the water be hot. When led with hydrogen into liquid ammonia it gives NH: PNH2, which on elevation of temperature gives P2(NH), (Joannis, Comptes rendus, 1904, 139, p. 364). By submitting a mixture of phosphorous chloride and hydrogen to an electric discharge A. Besson and A. Fournier (Comptes rendus, 1901, 150, p. 102) obtained phosphorus dichloride, PC, as a colourless, oily, strongly fuming liquid, freezing at -28° and boiling at 180° with decomposition. With water it gave phosphorous acid and a yellow indefinite solid. It decomposes slowly at ordinary temperatures. Phosphorus pentachloride, PC15, 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 moist air, giving hydrochloric acid and phosphoryl chloride, POCl3; with water it gives phosphoric and hydrochloric acids.

Sulphides and Thio-acids.-Phosphorus and sulphur combine energetically with considerable rise of temperature to form sulphides. The researches of A. Stock (Ber., 1908, 41, pp. 558, 657; 1909, 42, p. 2062; 1910, 43, pp. 150, 414) show that three exist, PS3, PAST, PS5. The first is prepared by heating red phosphorus with finely powdered sulphur in a tube sealed at one end and 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 1725° and boil at 407-408° with slight decomposition. Alkalis give hydrogen and phosphine. The second, P4S7, is obtained by heating a mixture of red phosphorus and sulphur in the proportions given by P4S7 +5% PS, and crystallizing from carbon disulphide in which PS3 is readily soluble. It forms small, slightly yellow prisms, which melt at 310° and boil at 523°. The third, or pentasulphide, PS, was obtained as a substance resembling flowers of sulphur by A. Stock and K. Thiel (Ber., 1905, 38, p. 2719; 1910, 43, p. 1223), who heated sulphur with phosphorus in carbon disulphide solution with a trace of iodine to 120°-130°. It exists in two forms, one having the formula P.Sio, and the other a lower molecular weight. With liquid ammonia it gives P2S5.7NH,, which is a mixture of ammonium iminotrithiophosphate, P(SNH4)3: NH, and ammonium nitrilodithiophosphate, P(SNH4)2N. Water converts the former into ammonium thiophosphate, PO(SNH1)s.H2O, whilst the latter heated to 300° in a vacuum gives thiophosphoric nitrile, NIP:S (Stock, ibid., 1906, 39, p. 1967).

Thiophosphates result on dissolving the pentasulphide in alkalis. Sodium monothiophosphate, Na,PSO, 12H2O, is obtained by adding one P2S to six NaOH, adding alcohol, dissolving the precipitate in water and heating to 90°. On cooling the salt separates as white six-sided tablets. Sodium dithiophosphate, Na PS2O2-11H2O, is obtained by heating the above solution only to 50°-55°, cooling and adding alcohol, which precipitates the dithio salt. On heating it gives the monothio salt. Sodium trithiophosphate appears to be formed when the pentasulphide acts with sodium hydrosulphide at 20°. All thiophosphates are decomposed by acids giving sulphurinetted hydrogen and sometimes free sulphur. They also act in many cases as reducing agents.

Nitrogen Compounds. Phosphorus pentachloride combines directly Phosphoryl trichloride or phosphorus oxychloride, POCl3, correspond- with ammonia, and the compound when heated to redness loses ing to phosphoric acid, (HO),PO, discovered in 1847 by Wurtz, ammonium chloride and hydrochloric acid and gives phospham, may be produced by the action of many substances containing | PN2H4, a substance first described by Davy in 1811. It is a white, hydroxy groups on the pentachloride; from the trichloride and infusible, very stable solid, which decomposes water on heating, potassium chlorate; by leaving phosphorus pentoxide in contact giving ammonia and metaphosphoric acid, whilst alkalis give with hydrochloric acid: 2P2O5+3HC1=POCl3+3HPO3; or by an analogous reaction. With methyl and ethyl alcohols it forms heating the pentachloride and pentoxide under pressure: 3PCl5+ secondary amines (Vidal, Comptes rendus, 1891, 112, p. 950; 1892, 115, P2O6=5POCÎ3. It is a colourless liquid, boiling at 107.2°, and p. 123). The diamide, PN2H1, was obtained by Hugot (ibid., 1905, when solidified it melts at 0-8°. Water gives hydrochloric and 141, p. 1235) by acting with ammonia gas on phosphorus tribromide phosphoric acids; dilute alcohol gives monoethyl phosphoric acid, or tri-iodide at -70°; it is very unstable, and decomposes at −25°. C2H H2PO4, whilst absolute alcohol gives triethyl phosphate, Phosphorus combines with nitrogen and chlorine to form several (C2H5)3PO4. Pyrophosphoryl chloride, P2O3Cl4, corresponding to polymeric substances of the general formula (PNCl2) x, where x may pyrophosphoric acid, was obtained by Geuther and Michaelis be 1, 3, 4, 5, 6, 7, or 11; they may be obtained by heating the penta(Ber., 1871, 4, p. 766) in the oxidation of phosphorus trichloride with chloride with ammonium chloride in a sealed tube and separating nitrogen peroxide at low temperature; it is a colourless fuming the mixture by fractional distillation (H. N. Stokes, Amer. Chem. Jour., liquid which boils at about 212° with some decomposition. With 1898, 20, p. 740; also see Besson and Rosset, Comptes rendus, 1906, 37, water it gives phosphoric and hydrochloric acids. Thiophosphoryl p. 143). The commonest form is P3N3C16, a crystalline solid, insoluble chloride, PSCl3, may be obtained by the direct combination of sulphur in water, but soluble in alcohol and ether. Several phosphoamides with the trichloride; from sulphuretted hydrogen and the penta- have been described. The diamide, PO (NH2) (NH), results when the chloride; from antimony trisulphide and the pentachloride; by heat-pentachloride is saturated with ammonia gas and the first formed ing the pentasulphide with the pentachloride; and by dissolving phos- chlorophosphamide, PCI (NH2)2, is decomposed by water. The phorus in sulphur chloride and distilling the solution: 2P+3Š2Cl2 = triamide, PO(NH2), results from ammonia and phosphorus oxy4S+2PSC13. It is a colourless mobile liquid, boiling at 125-1° chloride. Both these compounds on heating give phosphomonamide, and having a pungent, slightly aromatic odour. It is slowly decom- PON, of which a polymer (PON), had been described by Oddo posed by water giving phosphoric and hydrochloric acids, with (Gazz. chim. Ital., 1899, 29 (ii.), p. 330). Stokes (Amer. Chem. Jour..

1893, 15, p. 198; 1894, 16, pp. 123, 154) has described PO(OH)2NH2 | independent feeling of the Eastern Church. Photius felt himself and PO(OH) (NH2)2, whilst the compound PO(OH)NH was obtained by Schiff (Ann., 1857, 103, p. 168) by acting with ammonia on the pentoxide. Numerous other nitrogen compounds have been

authors.

against the Manichaeans and Paulicians, and his controversy with | chlorine combines with half its volume of methane explosively the Latins on the Procession of the Holy Spirit. His Epistles, in sunlight, whilst in diffused light it substitutes; with toluene political and private, addressed to high church and state dignitaries, it gives benzyl chloride, CH,CH2Cl, in sunlight, and chlortoluene, are valuable for the light they throw upon the character and versatility of the writer (ed. J. Valettas, London, 1864). A large C6H4(CH)3Cl, in the dark; with benzene it gives an addition number of his speeches and homilies have been edited by S. product, C6H6Cle, in sunlight, and substitutes in the dark. Aristarches (1900). The only complete edition is Bishop Malou's Bromine deports itself similarly, substituting and forming in Migne's Patrologia graeca, ci.-cv. R. Reifzenstein (Der Anfang des Lexikons des Photius, 1907) has published a hitherto unedited addition products with unsaturated compounds more readily MS. containing numerous fragments from various verse and prose in sunlight. Sometimes isomerization may occur; for instance, Wislicenus found that angelic acid gave dibromangelic acid in After the allusions in his own writings the chief contemporary the dark, and dibromtiglic acid in sunlight. Many substances authority for the life of Photius is his bitter enemy, Nicetas the Paphlagonian, the biographer of his rival Ignatius. The standard decompose when exposed to sunlight; for example, alkyl iodides modern work is that of Cardinal Hergenröther, Photius, Patriarch darken, owing to the liberation of iodine; aliphatic acids (especivon Constantinopel (1867–1869). As a dignitary of the Roman ally dibasic) in the presence of uranic oxide lose carbon dioxide; Catholic Church, Cardinal Hergenrother is inevitably biased against polyhydric alcohols give products identical with those produced Photius as an ecclesiastic, but his natural candour and sympathy by fermentation; whilst aliphatic ketones give a hydrocarbon with intellectual eminence have made him just to the man. See also article by F. Kattenbusch in Herzog-Hauck's Real- and an acid. encyklopädie für protestantische Theologie (1904), containing full bibliographical details; J. A. Fabricius, Bibliotheca graeca, x. 670776, 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. pws, 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 or y 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 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 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. 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);

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-ẞ-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 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, NO 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, hydroquinone 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 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 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 a form of benzyl-ß-aminocrotonic ester into the ẞ 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 homologues, 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

papers in the Journal of Physical Chemistry for 1908 and 1909, | where the influence of light was noticed at the beginning of the 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: J. M. Eder, Handbuch der Photographie, vol. i. pt. 2 (1906); H. W. Vogel, Photochemie (1906). An account of the action of light on organic compounds is given in A. W. Stewart, Recent Advances in Organic Chemistry (1908),

PHOTOGRAPHY (Gr. pws, light, and ypάpew, 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. 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 Nicholson's Journal that, at all events, salts of mercury were reduced by visible radiation and not by change of temperature.

In 1801 we come to the next decided step in the study of photographic action, when Johann Wilhelm Ritter (1776-1810) 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 1808) and, in 1812, Jacques Étienne 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 spectrum he would have found that the red rays did not blacken the material in the slightest degree.

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 effect a molecular
or chemical change in a substance. U. R. T. Le Bouvier
Desmorties in 1801 observed the change effected in Prussian
blue, and Carl Wilhelm Böckman 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 photo-
graph 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 speedily 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, and light transmitted through different-coloured glasses acts upon it with 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 most 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 sufficient 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. silver (silver chloride) with those upon the nitrate it seemed evident In comparing the effects produced by light upon muriate of that the muriate was the more susceptible, and both were more readily acted upon when moist than when dry-a fact long ago known. Even in the twilight the colour of the moist muriate of silver, spread upon paper, slowly 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 François 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 Farbenlehre of Goethe he says:

When 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 position In the violet the chloride is a reddish brown (sometimes more violet, for the spectrum is maintained by any means, I observe the following. 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 I usually found the chloride unaltered; sometimes, however, it had a light yellow tint; in the red and beyond the red it took a rose or lilac tint. This image of the spectrum shows beyond the red 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 | matter with it, and thus give rise to imperfect pictures. In chloride was coloured a grey-violet for a distance of several inches. In proportion as the distance from the violet increased, the tint carbon-printing development from the back of the exposed film became lighter. Beyond the red, on the contrary, the chloride is absolutely essential, since it depends on the same principles as does heliography, and in this the same mode of procedure is advisable.