When papers of this kind have been blackened by colourless sunlight, they acquire, when exposed to it for several days under coloured glasses, the following tints:

On further exposure other colours are also developed. (Hunt.) Bromide and iodide of silver are affected by light much in the same way as the chloride.

Iodide of silver precipitated by iodide of potassium from nitrate of silver, washed and dried, blackens slowly when exposed to light, remains colourless in the dark when dry, but if wetted resumes its yellow colour in the dark. (Hunt.)

Daguerre's Bromide of Silver Paper. The paper is first spread over with solution of silver, then with solution of bromide of potassium, then again with solution of silver, and dried after each operation. This paper is insensible to heat, but very sensitive to light, in which it becomes coloured first bluish green, then olive green, and finally almost black.

Hunt's Iodide of Silver Paper. The paper is first moistened with a solution of iodide of potassium in 48 parts of water, then with a solution of 1 part of nitrate of silver in 58 parts of water. This paper is very delicate. In white light it becomes brown, under solution of ammoniuret of copper, a rich light blue, under solution of acetate of copper, brown, under solution of sesqui-chloride of iron, green, and under solution of carmine in aqueous ammonia, brown-red. (Hunt.)

Talbot's Calotype Process.-A sheet of the best writing paper of smooth surface and close, even texture, is washed by means of a soft brush with a solution of 50 gr. of crystallized nitrate of silver in one ounce of distilled water, and then dried-either by holding it at some distance before a fire, or by spontaneous evaporation in a dark room. When dry or nearly so, it is immersed for two or three minutes in a solution of iodide of potassium, containing 500 grains of that salt in a pint of water. It is then immersed in water for a quarter of an hour, lightly dried with blotting paper, and the drying completed by holding it near a fire. All these operations are best performed by candle light. The paper thus prepared is called iodized paper; it is not sensitive to light, and may be preserved for any length of time without change. When required for use, it is to be washed with a liquid prepared by dissolving 100 gr. of crystallized nitrate of silver in 2 oz. of distilled water, adding to the solution its own volume of strong acetic acid, and mixing the liquid thus formed with from one to twenty volumes of a saturated solution of crystallized gallic acid in cold distilled water. It is best to mix these liquids in small quantities at a time, as the mixture does not keep long without spoiling. This solution (which Mr. Talbot calls the gallo-nitrate of silver) is to be washed over the paper on the side previously iodized, and after the liquid has

been allowed to remain on it for half a minute, it must be dried lightly with blotting paper. This operation requires the total exclusion of daylight. The paper prepared in this manner is exquisitely sensitive, exposure of less than a second to diffused daylight being quite sufficient to produce a decided effect upon it.

The calotype paper may be used for taking portraits from the life by means of the camera obscura. It is the only paper yet discovered sufficiently sensitive for this purpose. When properly prepared, it yields pictures of exquisite beauty and fidelity. It likewise serves for taking representations of buildings, scenery, &c. The picture, as first produced, is generally latent and invisible, but may be brought out immediately by again washing the paper with a mixture of 1 part of the silver solution with 3 parts of the saturated solution of gallic acid. In cold weather, the development of the picture may be accelerated by holding it over a basin of hot water. The picture being thus obtained, it is to be fixed by washing in clean water, lightly drying between blotting paper, and then washing it over with a solution of bromide of potassium, containing 100 gr. of that salt in 8 or 10 ounces of water: after a minute or two, it is to be again well washed with water and then finally dried. Where bromide of potassium is not at hand, a strong solution of common salt may be used in its stead.

The calotype picture is a negative one,-but positive copies may be obtained from it by placing it with its face against the sensitive side of a piece of ordinary photographic paper (Talbot's chloride of silver paper already described), pressing it into close contact by a board below and a glass above, and exposing it for a short time to good sunshine.-A more sensitive copying paper may be prepared by soaking letter paper for a minute in a solution of salt and water-one oz. of salt to a gallon of water-drying it, and then washing it over with a mixture made by adding to a solution of 70 grains of nitrate of silver in an ounce of water sufficient caustic ammonia to nearly redissolve the precipitate at first produced. The positive pictures are fixed by washing in water and immersion for ten minutes in a solution of hyposulphite of soda (1 oz. to a pint of water)—then finally washing and drying.

Since the calotype was first patented, some further improvements have been made by Mr. Talbot, among which may be mentioned: 1. The removal of the yellowish tint of the iodide of silver from the paper, by immersing the picture for about ten minutes in a bath consisting of a solution of hyposulphite of soda in ten times its weight of water and heated nearly to the boiling point, then well washing and drying it. [A stronger solution of the hyposulphite may also be used cold, the paper being immersed in it for a longer time: this will be found the more convenient method of the two.] After this, the picture is placed upon a hot iron and wax melted into its pores to increase its transparency. The excess of wax is removed by pressing between blotting paper with a hot iron.-2. Rendering the calotype paper more sensitive by placing a warm iron behind it in the camera while the light is acting upon it.-3. The improvement of photographic drawings by exposing them twice the usual time to the action of sunlight. The shadows are thus rendered too dark and the lights are not sufficiently white. The drawing is then washed, and plunged into a bath of iodide of potassium of the strength of 500 grains to a pint of water, and allowed to remain in it for a minute or two, which makes the picture brighter and causes its lights to assume a pale yellow tint. It is then washed, and immersed in a hot bath of hyposul

VOL. I.

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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 sunpictures 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, I, 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° Fah.) 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 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,

pure

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 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) has 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 thus 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 use 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, 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.] ¶

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 them) 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 colouring 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 by that of heat. Chloride of silver does not blacken even at a red heat,