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Ries & Moser (Pogg. 16, 563) and d'Hombres Firmas (Ann. Chim. Phys. 10, 285).- I have myself seen it successfully performed by Morichini.

1 All former researches on the relation between light and maguetism have been completely thrown into the shade by the important discovery of Faraday (Phil. Trans. 1846, 1, 1)—that a ray of polarized light, when made to pass through certain transparent substances placed near a magnetic pole, in such a manner that the lines of magnetic force* shall pass through it in the direction of the ray,—is rotated in a particular direction, depending upon the direction of tlie ray itself and that of the line of magnetic force. -I shall give, nearly in Dr. Faraday's own words, the description of the experiment which first revealed this most remarkable phenomenon.

A ray of light issuing from an Argand lamp was polarized in a horizontal plane by reflection from a surface of glass, and the polarized ray passed through a Nichol's eye-piece revolving on a horizontal axis, so as to be easily examined by the latter. Between the polarizing mirror and the eye-piece, two powerful electro-magnetic poles were arranged, being either the poles of a horse-shoe magnet, or the contrary poles of two cylinder magnets; they were separated from each other about two inches in the directiou of the ray, and so placed that, if on the same side of the polarized ray, it might pass near them; or if on the contrary sides, it might go between then, its direction being always parallel or nearly so to the magnetic lines of force. After that, any transparent substance placed between the two poles would have passing through it, both the polarized ray and the magnetic lines of force, at the same time and in the same direction.

A piece of heavy glass consisting of silico-borate of lead (Phil. Trans. 1830, p. 1) about two inches square and half an inch thick, having flat and polished edges, was placed as a diamagnetict between the poles (not as yet magnetized by the electric current) so that the polarized ray might pass through its length. The glass acted as air, water, or any other indifferent substance would do, and if the eye-piece were previously turned into such a position that the polarized ray was extinguished, or rather the image produced by it rendered invisible, then the introduction of the glass made no alteration in this respect. In this state of circumstances, the force of the electro-magnet was developed by sending an electric current through its coils, and immediately the lamp-flame became visible, and continued so as long as the arrangement continued magnetic. On stopping the electric current, and so causing the magnetic force to cease, the light instantly disappeared. These phenomena could be renewed at pleasure at any instant of time, and upon any occasion, showing a perfect dependence of cause and effect.

The voltaic current was that of five pair of Grove's construction, and the electro-magnets were of such power that the poles would singly sustain a weight of from 28 to 56lbs or more.

By various experiments made in this manner, it was found that the character of the force thus impressed upon the diamagnetic is that of rotation, and that it acts according to the following law:

“If a magnetic line of force be going from a north pole or coming from

* The term Line of magnetic force or Magnetic line of force, or Magnetic curve, denotes that exercise of magnetic force which is exerted in the lines usually called magnetic curves, and which equally exist as passing from or to magnetic poles or forming concentric circles round an electric current.

+ A diamagnetic is a body through which lines of magnetic force are passing, and which does not by their action assume the usual magnetic condition of iron or loadstone,

2.8
2.2

a south pole along the path of a polarized ray coming to the observer, it will rotate that ray to the right-hand; or if such a line of force be coming from a north pole or going from a south pole, it will rotate such a ray to the left hand."

It was likewise found that the degree of rotation is proportional to the extent of the diamagnetic through which the ray and the lines of magnetic force pass. The power of rotating the ray of light increases with the intensity of the magnetic lines of force.

Other bodies besides the heavy glass possess the power of acting on light under the influence of the magnetic force. When those bodies have a rotative power of their own—as is the case with oil of turpentine, sugar, &c.—the effect of the magnetic force is to add to or subtract from their specific force, according as the natural rotation and that induced by the magnetic force are in the same or in opposite directions.

The silico-borate of lead was found to be the best substance for exhibiting the phenomena. Fused borate of lead is nearly as good : flint-glass exhibits the rotation, but in a less degree, and crown-glass still less. Crystallized bodies exhibit little or no influence on the ray when under magnetic influence. All liquids which have been submitted to experiment produce the rotation : air and the other gaseous bodies do not. The following table gives an approximate estimate of the relative amount of the induced rotating force in a few substances, as compared with the natural rotating force of a specimen of oil of turpentine. (Water = 1.) Oil of turpentine..

11.8
Heavy glass..

6.0
Flint-glass
Rock-salt..
Water

1.0
Alcohol.

less than alcohol. The rotating force is also induced by ordinary magnets in the same manner as by electro-magnets, but in a less degree, simply because the magnetic force exerted by the former is less than that exerted by the latter.

By placing various transparent bodies within long helices of wire through which powerful electric currents were passing - it was found that "When an electric current passes round a ray of polarized light in a “ direction perpendicular to the ray, it causes the ray to revolve on its “axis, as long as it is under the influence of the current, in the same direction as that in which the current is passing.” It will be easily seen that this law is identical with that previously stated respecting the rotating power induced by the magnet.

In all cases it is found that the interposition of copper, lead, silver, and other ordinary non-magnetic bodies in the course of the magnetic curves, either between the poles and the diamagnetic, or in other positions, produces no effect on the phenomena, either in kind or in degree. Iron affects the results in a remarkable degree; but it always appears to act either by altering the direction of the magnetic lines or by disposing of their force within itself.

No rotating power is induced by lines of electro-static tension.

The results here described, important as they are in themselves, become still more so when viewed in connection with the magnetic condition of all matter, as developed in the 20th and 21st series of Faraday's Experimental Researches. A brief abstract of these researches will be found at the end of Chap. III,

less than water

Ether

II. RELATIONS OF LIGHT TOWARDS PONDERABLE BODIES.

1. Changes produced in Ponderable Bodies by the Action of Light.

Chemical effects of Light.

A. Combinations produced by the agency of Light. a. Chlorine gas does not combine with hydrogen to form hydrochloric acid gus at ordinary temperatures and in the dark, but only under the influence of light (Gay-Lussac & Thénard): according to Seebeck, the combination takes place under white or blue, but not under red glass. Neither does sunlight transmitted through bichromate of potash effect the combination. (Draper: vid. Formation of Hydrochloric Acid.)-. Chlorine gas combines with carbonic oxide gas only under the influence of light. (J. Davy.)-c. Iodine and olefiant gas combine only in sunshine. (Faraday.)-d. Many kinds of plate-glass, which have only a faint violet tint, become purple after exposure to light for a year, while the same glass kept in the dark retains its original pale tint. (Faraday, Qu. J. of Sc. 15, 164; also Pogg. 24, 387.) This may arise from a higher oxidation of the manganese contained in the glass.-e. Hyacinths exposed to light lose their reddish tint and become browner. (G. F. Richter, Pogg. 24, 386.) This case belongs perhaps to the head of decompositions.

B. Combinations accompanied by Decompositions. a. Phosphorus kept in various gases or in water, is changed in the sunshine under colourless or blue glass (not however, or but very slowly, under red) into red oxide of phosphorus. (Böckmann, A. Vogel.) Hence light brings about the combination of the phosphorus with the oxygen of the air or of the water.-6. Chlorine combines at ordinary temperatures with the hydrogen of water and liberates oxygen gas, but only under the influence of light.—Aqueous solution of chloride of platinúm mixed with lime water gives a precipitate in colourless or violet, but not in red or yellow light. (Herschel.)—d. Chlorine decomposes light carburetted hydrogen gas when moist, forming hydrochloric acid and carbonic acid, but only when exposed to light. (W. Henry.)-e. Chlorine converts the oil of olefiant gas at under temperatures into chloride of carbon and hydrochloric acid, but only under the influence of light. (Faraday.)—f. The oil of olefiant gas covered with water and placed in the sunshine is resolved into hydrochloric acid and acetic ether. (Mitscherlich.)-Chlorine converts anhydrous hydrocyanic acid under the influence of the sun's rays into hydrochloric acid and solid chloride of cyanogen; it also decomposes moist cyanide of mercury in different ways accordingly as light is concerned in the action or not. (Serullas.)—h. The brown selution of iodine in absolute alcohol saturated with sulphurous acid gas deposits crystalline sulphur when exposed to sunshine. (Dobereiner, Pneumat. Chem. 5, 72.)

i. Many metallic oxides combined with acids and dissolved in alcohol or ether give up oxygen, only under the influence of light, to these organic liquids, and are thus either brought to a lower degree of oxidation or reduced to the metallic state. Yellow hydrochlorate of peroxide of uranium dissolved in ether is converted by light into precipitated dark

hydrochlorate of protoxide ef uranium. (Gehlen.)—Hydrochlorate of peroxide of iron dissolved in ether is resolved under white and blue, not under red glass, into hydrochlorate of protoxide of iron. (A. Vogel.) Red sulph-hydrocyanate of peroxide of iron is converted into colourless sulph-bydrocyanate of protoxide of iron (in the focus of a concave mirror in a few minutes,-more quickly by the bluish green ray of the spectrum than by any other-and even by the light of an Argand lamp placed at the distance of an inch. (Grotthuss.)-Hydrochlorate of protoxide of copper dissolved in alcohol or ether is reduced to dichloride of copper. (Gehlen, Neumann, Schw. 13, 358.)—Hydrochlorate of protoxide of mercury (corrosive sublimate) is converted under white or blue, but not under red glass, into calomel

. (A. Vogel.)-Hydrochlorate of oxide of gold dissolved in ether or alcohol is discoloured on exposure to light, and according to Grotthuss, most quickly in the blue ray, metallic gold being separated ;-also hydrochlorate of oxide of platinum dissolved in ether deposits a small quantity of platinum when exposed to light, and is at the same time considerably decolorized. (Gehlen.)-k. The red aqueous solution of oxalate of sesqui-oxide of manganese loses its colour when exposed to sunshine (slowly in blue and violet light) and is converted into carbonic acid and oxalate of protoxide of manganese. (Döbereiner.)— Aqueous oxalate of peroxide of iron is resolved, when exposed to the direct rays of the sun, into carbonic acid and oxalate of protoxide of iron. The change also takes place in violet and blue, but not in yellow or red light, neither is it produced by boiling. (Döbereiner.) - Oxalic acid added to aqueous solutions of hydrochlorate of oxide of gold, hydrochlorate of oxide of platinum, and ammonio-chloride of iridium, precipitates the metal and generates carbonic acid. In the case of iridium, the action cannot be replaced by that of a boiling heat. (Döbereiner, Schw. 62, 90.)-An aqueous solution of tartrate of peroxide of iron and potash, containing copper, deposits metallic copper on the side exposed to light. (Ehrmann, Repert. 49, 112.)-1. Many other organic substances also take oxygen from metallic oxides dissolved in acids, only when their action is assisted by light. Thus, charcoal and oils precipitate the pure metals from gold and silver solutions only when exposed to light or at a boiling heat. (Rumford.) Linen marked with nitrate of oxide of silver blackens only when exposed to light. Paper moistened with solution of gold reddens in presence of light,—and likewise in the dark, when it has been exposed to light till the reddening has commenced. (Seebeck.)

m. Light favours the combination of the oxygen of the atmosphere with the carbon and hydrogen of organic substances, by which they are more or less decomposed and altered. In this manner may be explained the decolorization and rotting of fabrics coloured with safflower, logwood, Brazil wood, turmeric, and woad when exposed to the sun-an effect which, according to Gay-Lussac & Thénard, is likewise produced in the dark by heating them to 160°...200° C. with access of air (Chevreul's investigations on the behaviour of tissues or fabrics dyed with various colouring matters and placed in different media: vid. Ann. Chim. Phys. 66, 71]; —the decoloration of the petals of Papaver Rheas, which according to A. Vogel takes place more quickly through blue than through white glass;— the blanching of the blue flowers of Cichorium sylvestre, which takes place more rapidly in fine than in dull weather (Pajot Descharmes, J. Phys. 95, 112);--the decoloration of the green tincture obtained by digesting the green leaves of plants in alcohol (this tincture, according to Grotthuss retains its colour longest in the blue-green ray of the spectrum);—the decoloration of the yellow oils of linseed, savin, and peppermint, and the yellow colouring of blue oil of camomile and colourless oil of turpentine. În a similar manner may also be explained the green colouring of yellow guiacum-either pulverized or spread upon paper by means of an alcoholic solution—when exposed either to colourless or violet light in a glass containing air or oxygen gas, or when strongly heated, whereas concentrated red light restores the yellow colour. (Wollaston, Gilb. 39, 291.) [See also Herschel, Phil. Mag. J. 22, 5.)

n. The blue solution of iodide of starch in water is completely decolorized, with development of hydriodic acid, by colourless light and by the yellow and green ray of the spectrum (in the focus of a concave mirror the effect takes place in a few minutes),-slightly by the red and blue, and not at all by the violet, the last on the contrary counteracting the decolorizing action of daylight. (Grotthuss.)

0. The green parts of living plants, under the influence of light decompose carbonic acid, liberating oxygen gas from it and retaining the carbon, which enters into organic combinations. In this case also the violet appears to be the most active of the coloured rays, whilst under yellow glass plants become paler. (Senebier, Tessier.) Moonlight and candle-light appear also, according to Tessier and Vasalli, to have some effect.

C. Decompositions produced by Light. a. Salts containing water of crystallization lose it when exposed to the sun, and much more quickly behind blue than behind red glass. (A. Vogel.)

b. Concentrated nitric acid is decomposed by exposure to light (as also by a red heat) into oxygen gas and hyponitric acid, N 0+ (Scheele); and according to Seebeck, this effect is produced behind wbite and blue, but not behind orange-coloured glass.

c. Many metallic oxides, when exposed to light, give up the whole or a part of their oxygen, and are resolved into pure metal and a higher oxide. Thus, brown peroxide of lead is resolved into oxygen gas and red lead (the latter becomes of a darker colour by the long continued action of light: the cause of this phenomenon remains to be discovered);grey oxide of mercury is resolved into metallic mercury and red oxide;the red oxide, under water, is decomposed, according to Seebeck, into mercury, grey oxide, and oxygen gas, provided that the light comes to it through colourless or blue glass, not through red;—(dry red oxide of mercury loses in four months under colourless glass 0:9, under violet 0:5, under green

0.2 and under red 0.1 per cent. of oxygen acquiring at the same time a grey colour, which is strongest under colourless, and scarcely observable under red glass. Dulk.)–Oxide of silver is resolved into silver and oxygen gas ;-carbonate of silver into silver, oxygen and carbonic acid gas;—(pyrophosphate of silver blackens on exposure to the

light);-oxide of gold into gold and oxygen gas. (Scheele.)—Manganic • acid dissolved in water and exposed to sunshine slowly deposits peroxide of manganese, while oxygen gas is set free. (Fronherz.)

d. Proto-chloride of mercury dissolved in water is resolved in sunshine into dichloride of mercury, hydrochloric acid, and oxygen gas. (Boullay.) - Terchloride of gold dissolved in water deposits flakes of gold when exposed to the sun. (Scheele.)

e. White chloride of silver, as obtained by precipitating nitrate of silver with hydrochloric acid or a soluble chloride, changes when exposed

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