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phite of soda, till the pale yellow tint is removed and the lights remain quite white. The pictures thus finished have a pleasing and peculiar effect. For the other improvements, vid. Hunt's Researches on Light.]

Various other processes have likewise been devised for taking surpictures on paper. Sir John Herschel has obtained some remarkable results with paper washed with solution of ammouio-citrate of iron. Paper thus prepared is very sensitive to light, but the pictures impressed upon it are faint and sometimes scarcely visible: they may however be brought out very strongly and clearly by washing the paper, after exposure to light, with various liquids, e.g. a neutral solution of gold (Chrysotype)—and various compounds of cyanogen, ferrocyanide of potassium, &c. (Cyanotype). These processes will be found fully described in Sir John Herschel's memoirs (Phil. Trans. 1842, II, 181, and 1843, 1, 1), also in Hunt's Researches on Light, pp. 137–149. The latter work likewise contains a detailed account of the action of light on various compounds of silver, also on salts of gold, platinum, mercury, iron, copper, manganese, lead, nickel, tin, cobalt, antimony, and chromium.—The calotype process is however the only one which has been brought into actual use for obtaining sun-pictures on paper, T

Daguerreotype. A silvered copper plate is carefully polished with pumice-stone, dilute nitric acid, and cotton, and placed in a box at the ordinary temperature over iodine, till the vapours of the iodine have covered it with a yellow film of iodide of silver. It is then placed for some minutes in the camera obscura, which allows the illuminated picture of an object to fall upon the place. (The light which falls on the plate probably separates iodine from the iodide of silver and sets silver free, chiefly on those parts where its action is most intense.). The plate, on which no alteration is perceptible, is now placed in a dark covered box at an angle of 45° over a vessel containing mercury heated to 75° C. (167° Fab.) till the desired shading is produced. (The mercury which rises in vapour does not adhere to the portion of the surface covered with undecomposed iodide of silver, but only to the silver which has been set free by the action of light, with which it forms an amalgam in drops about a millimetre in diameter. Lastly, the plate is immersed in a solution of hyposulphite of soda, then washed with hot water and dried. (Daguerre.) --(The hyposulphite of soda dissolves all the iodide of silver; consequently, on those parts of the plate on which the light in the camera obscura has not acted, there remains clean polished silver, which, when the light falls properly upon it, appears black: on the other parts, according to the intensity with which the light has acted on them, there exist various numbers of amalgamated globules, which, by their greater brightness, bring out the contrast of the picture. If a plate of silver polished as above be partly covered with paper and exposed to vapour of mercury, the same contrast is seen on removing the paper. (Fyfe.)—If the iodized plate be covered with different coloured glass plates and then introduced into the camera obscura, a perfect picture will be obtained with blue glass, a tolerably good one with yellow, and none at all with red or green. Ammoniuret of copper acts like blue glass, sesqui-chloride of iron like yellow, acetate of copper like green; on the other hand, the red solution of carmine in ammonia gives a faint picture in which the mercury exhibits a red colour. (Hunt.)— Instead of the tedious polishing of the plate with nitric acid and pumice-stone, it may also be gently rubbed with an aqueous solution of iodide of potassium containing a little excess of iodine,

till all the parts are equally attacked, then exposed to the light for some minutes and polished with dry cotton. (Hunt.) – If the plate be exposed to the vapours of chloride, bromide, &c. of iodine, instead of those of pure iodine, it requires but a few seconds’ illumination in the camera, and thus becomes adapted for taking portraits. (Berres.)—In order to obtain more distinct shading, Fizeau spreads upon the plate prepared by Daguerre's method, a solution of 1 part of chloride of gold, and 3 parts of hyposulphite of soda in 1000 parts of water, and heats it gently for a minute or two. The gold which is precipitated from the silver imparts a deeper black to it; the mercury which combines with it makes the colour darker and more stable. [Vid. also Daguerre, Pogg. 62, 80.]

Thermography. Moser (Pogg. 56, 177) bas shown that: A surface which has been touched by a body in any particular part, acquires the property of precipitating all vapours that may adhere to it or which combine chemically with it on those parts, differently to what it does on the untouched parts.—Thus, if we write on glass with any substance that will not scratch the surface, and then breathe upon it, the writing becomes visible. Again, on placing coins upon a plate of glass or metal, and allowing them to remain in contact for some hours, no change is visible when they are removed: but by breathing on the plate, or exposing it to any vapour (that of mercury or iodine for instance), beautiful images of the coins are produced. Absolute contact is not necessary for the production of these images: mere proximity is sufficient. The general law of the phenomenon may be tbus expressed “When two bodies are sufficiently approximated, they mutually depict each other.” Moser attributes this effect to the action of rays of light which are imperceptible to our eyes, and applies to these rays the somewhat paradoxical appellation of "Invisible Light.—Hunt, who has examined these phenomena with great care, finds that to produce good impressions of coins, &c. on metal plates, it is necessary to nse dissimilar metals. Thus, when a sovereign, a shilling, a large silver medal, and a penny were placed upon a polished copper plate, the plate gently warmed by passing a spirit-lamp under its surface, and when cold exposed to the vapour of mercury, each piece had made its impression,--but those made by the gold and the large medal were the most distinct, the lettering being copied as well as the disc traced out. Impressions of still greater distinctness were obtained when the plate was more strongly heated.

These experiments seem to show that the calorific relations of the metals materially influence the result; and this is more strikingly shown by the following arrangements.—Pieces of blue, red, and orange-coloured glass, also of crown and flint glass, mica, and a square of tracing paper, being laid for half an hour on a plate of copper, the space occupied by the red glass was found to be well marked, that covered by the orange glass was less distinct, but the blue glass left no impression. The shapes of the flint and crown glass were well made out, and a remarkably strong impression left where the crown glass rested on the tracing paper, but the mica had not left any impression. The same glasses, together with a piece of well smoked glass, were placed for half an hour, t of an inch below a polished plate of copper." Vapour of mercury brought out the image of the smoked glass only.-All these glasses were placed on the copper and slightly warmed. The red and smoked glasses gave, after vaporization, equally distinct images: the orange the next: the others left but faint marks of their forms. Polishing with Tripoli and putty powder would not remove the images of the smoked and red glasses.

The same coloured glasses, &c., were placed, together with a thick piece of charcoal, upon a plate of copper and exposed to fervent sunshine. Mercurial vapour brought out the images in the following order: Smoked glass, crown glass, red glass, mica, orange glass, paper, charcoal, the coin, blue glass,—thus distinctly proving that the only rays which had any influence on the metal were the calorific rays. For this reason, Mr. Hunt applies the term Thermography to the production of images in this manner.

The thermographic process is applicable to the copying of engravings. An account of the method will be found in Hunt's Researches on Light. p. 233. The same work, pp. 219—242, also contains a variety of other interesting details relating to this curious mode of action. [Vid. also Ann. Chem. Pharm. 48, 164.] I

Solar light may be supposed to consist of three kinds of rays,—the heating, the luminous and coloured, and the chemical rays,—the first of which are the least and the last the most refrangible. According to this view, the light of the sun refracted by a prism produces three spectra: in the middle the light and colour spectrum; on the one side, the heat spectrum with its maximum in the neighbourhood of the red; and on the other, the chemical spectrum with its maximum in the neighbourhood of the violet. The rays from the green to the red likewise exhibit chemical action, inasmuch as they impart a somewhat lighter colour to chloride of silver: but on the other hand, they seem to exhibit an action contrary to that of the chemical rays, since they remove the blackening from chloride of silver which has been acted upon by white light, and instantly destroy the power of magnets formed by the action of light. The assertion of Grotthuss, that blue iodide of starch is most quickly decolorized by yellow and green light, and the blood-red alcoholic solution of sulphocyanide of iron by blue and green light (from which he concludes that a ray of coloured light most easily decolorizes substances of opposite hue, and endeavours to impart its own colour to theni) requires further examination, inasmuch as it is opposed to the experience of other observers. The same may be said of Sir H. Davy's assertion (Elem. 1, 187) that the red ray acts on a mixture of chlorine and hydrogen gases and on wet peroxide of lead, more powerfully than the other coloured rays.

Since the chemical rays of light often cause the separation of oxygen from metals, they have been called de oxidizing, and those towards the red and of the spectrum, oxidizing rays; incorrectly however, since the chemical rays also bring about the combination of oxygen with guiacum and colonring matters, of chlorine with hydrogen, &c.

Many of the changes produced by light may also be brought about by slight elevation of temperature; e.g. the efflorescence of salts; many by a boiling heat, as the decomposition of most metallic salts dissolved in alcohol or ether (sulpho-cyanide of iron dissolved in alcohol is, according to Grotthuss, but little discoloured by boiling); others again by a temperature of from 150° to 200°, as the combination of chlorine with hydrogen, and the bleaching of coloured fabrics exposed to the air; others also at a red heat, at which light may likewise assist,---as the production of oxide of phosphorus. But in many cases the action of light cannot be replaced ly that of heat.

Chloride of silver does not blacken even at a red heat,

at which it melts and sublimes. The green parts of plants do not separate oxygen from carbonic acid at any temperature in the absence of light. It is also to be remarked that the rays by wbich chemical action is produced are not the hottest but the coldest rays in the spectrum. Hence it is only in a few cases that we can adopt the views of Rumford and of Gay-Lussac & Thénard, and attribute the chemical effects of light to the rise of temperature which it produces on coming in contact with ponderable bodies.

2. Development of Light by Ponderable Substances. The development of light is either vivid and attended with considerable rise of temperature-Fire-or it is faint and accompanied by little or no development of heat-Phosphorescence, Luminosity.

4. Development of Light by the mutual chemical action of Ponderable

Bodies. d. Development of Light as a consequence of actual Chemical Combination.

Many substances whose affinity for each other is considerable,-the elementary bodies therefore most of all (according to Law 2, a. p. 144),-develop light and heat at the moment of combination. The element which most frequently develops light and heat in combining with others is oxygen; and the act of its combination with other bodies is preeminently denominated Combustion. The body which, next to oxygen, most commonly produces light and heat in combining with others, is chlorine; next follow bromine and iodine; then selenium, sulphur, and phosphorus.--But few compound bodies develop light in combining with others.—Hydrate of potash or soda produces light in combining with sulphuric, nitric, or concentrated acetic acid dropt upon it; baryta or lime with water or one of the acids just mentioned; magnesia with sulphuric or nitric acid. (Heinrich.)

The light must either have existed ready formed in one or both of the combining bodies, and be merely separated by the act of combination, or it must be evolved during the combination of the ponderable bodies out of imponderable elements contained in them; on the latter hypothesis, it is most probable that one kind of electricity is supplied by oxygen, chlorine, &c. and the other by the metals and bodies like them. | Vid. Development of Heat in the chapter on Heat, and Combustion in the chapter on Oxygen.]

b. Development of Light as a consequence of probable Chemical

Combination.

(a.) Phosphorescence of living Organized Bodies.

1. Phosphorescence of Living Animals. The phosphorescence of these animals appears to arise from this circumstance, that they eliminate a peculiar and in most cases liquid substance, containing phosphorus or some other element, which combines, at common temperatures, with the oxygen of the air or of water containing air, producing a faint luminous appearance. Not only does the separation of this fluid appear to depend upon the life of the animal, but its will seems likewise to determine whether the fluid shall—partly by means of the respiratory process—come in contact with the oxygen of the air, and thus produce a development of light, or not. This view is maintained by Spallanzani, Heinrich, Treviranus, and Tilesius, --whilst Carradori and Macartney regard this luminosity as a consequence of the vital process, and suppose that the increased luminosity in oxygen gas proceeds from augmented vital activity, and the diminution of light in other media from diminished vital energy. But since the liquid retains its luminosity even when separated from the living animals, and the animals often continue luminous even after death, the vital process cannot be the immediate cause of the phosphorescence.

The animals which exhibit phosphorescence during life, all belong to the lower classes, principally insects and worms.

Amphibia: The fresh eggs of Lacerta agilis (Heinrich), and of some serpents (Langrebe).

Fishes: A peculiar kind of Leptocephalus. The bodies of dead fish also appear phosphorescent at times. (Langrebe.)

Insects. 1. Coleoptera: Elater noctilucus, ignitus, Lampadion, retrospiciens, lucidulus, nictitans, Lucernula, Speculator, Janus, pyrophanus, luminosus, lucens, extinctus, Cucujus, Lucifer, and phosphoreus, Lampyris noctiluca, splendidula, Italica, and hemiptera (this according to Illiger is not phosphorescent); Pausus sphaerocerus, Scarabeus phosphoreus (?) (Luce, J. Phys. 44, 300); Buprestis occellata.

2. Orthoptera : Acheta Gryllotalpa.

3. Hemiptera. Fulgora laternaria (the luminosity of which is doubted by Martius and Spix) and candelaria.

4. Diptera: Calex pipiens (Hablitzl, Neue nordische Beiträge, 4, 396).

5. Myriapoda: Scolopendra electrica, phosphorea and morsitans; Julus.

6. Arachnidee: Phalangium.

7. Crustacea: Many Squille; Cancer fulgens, macrourus and others; Gammarus Pulex (sometimes only), caudisetus, longicornis, truncatus, circinnatus, heteroclitus and crassimanus; Oniscus fulgens; Cyclops exiliens; Amymone and Nauplius (the young of Cyclops) Monoculus.

8. Annulata: Nereis noctiluca, phosphorans, cirrigera, mucronata, radiata, and others; Spirographis Spallanzanii, Lumbricus terrestris, simplicissimus and Hirticauda; Planaria retusa; Branchiurus quadripes.

9. Mollusca; Pholas Dactylus; all kinds of Salpa (or Biphora), as pinnata, affinis, zonaria, vaginata, bicornis, cornuta, venosa; and of Pyrosoma, viz., Pyrosoma Atlanticum or Telephorus Australis.

Zoophytes. 1. Radiata: Asterias noctiluca; Ophiura phosphorea.

2. A calepha: Very many species of Medusa, as pelagica, pellucens (these two species perhaps identical), scintillans, simplex, lucida, hemisphoerica, ovata, tuberculata, panopyra, noctiluca, aurita, the several species of Aurelia: many species of Beroë, as fulgens, Pileus, globosa, Brasiliensis, micans, flava; Physalia Arethusa and glauca; several species of Physsophora, together with Rhizophysa and Stephanomia.

3. Polypi: Sertularia neritina and volubilis (Qr. J. of Sc. N. S. 4, 383); Pennatula phosphorea, grisea, and all others; Veretillum Cynomorium, according to Leuckart; Isis; Gorgonia; Alcyonium exos, according to Leuckart; Spongia.

4. Infusoria; Leucophoa echinoides; Trichoda triangularis, granulosa, Clava and echinoides; Gleba Pseudohippus, crispa, crystallina, deformis, Conus and spiralis, Vorticella; Cercaria: Vibrio, Volvox.

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