Whatever may be the nature of the prism, heat is always manifested beyond the red, but gradually diminishes as the distance from the extreme limit of the red increases. With a prism of rock-salt the maximum of heat is situated far beyond the red. The solar light contains heating rays of various degrees of refrangibility; rock-salt transmits them all, even the least refrangible; glass and water only the more refrangible: for this reason the maximum of heat, when prisms of glass or water are used, is found within the coloured spectrum. (Melloni.) According to Powell, the heating power of the coloured rays depends also upon the colour of the body to be heated; according to his observations, a thermometer painted with vermilion is more strongly heated in the orange rays than in the red. According to the same philosopher, the heating rays of the prism pass like the solar rays through glass without perceptible loss of heating power.-There exists therefore a Heat-spectrum in connection with the coloured spectrum. According to Herschel, the coloured spectrum takes up only about of the space occupied by the heat-spectrum; and in consequence of the smaller refrangibility of the heat-rays, the focus of heat is somewhat farther (according to Wollaston about) from the burning glass than the focus of light. Sir John Herschel (Phil. Mag. J. 22, 505) has obtained some remarkable results by exposing thin writing paper, blackened on one side by holding it over a smoky flame, and afterwards thoroughly wetted with alcohol applied to the unsmoked side, to the action of the solar spectrum. The influence of the calorific rays was shown by a whitening of the paper, marking by a clear and sharp outline the lateral extent of these rays, and by due gradations of intensity in a longitudinal direction, their law or scale of distribution, both within and without the luminous spectrum. The thermic spectrum thus impressed extended from about the middle of the violet to a distance considerably beyond the red; moreover, it was found to consist of a number of distinct patches, the brightest of which were situated in and just beyond the visible red rays. Three other spots subsequently came into view at continually greater distances from the luminous spectrum and successively diminishing in brightness. This want of continuity in the thermic spectrum may arise from an absorbent effect in the atmosphere of the sun, or of the earth, or of both; if such absorbent action be exerted by the earth's atmosphere, it will follow that a large portion of the solar heat never reaches the earth's surface at all, and that the heat incident on the summits of lofty mountains differs, not only in quantity but also in quality, from that which the plains receive. The two spectra formed by a prism of double refracting spar have equal heating powers. (Bérard.) Moonlight, the intensity of which, according to Bouguer, is to that of sunlight as 1 from 250000 to 30000, produces, when concentrated by a burning mirror, only a very slight degree of heat barely perceptible by a delicate thermometer (Howard, Sillim. Amer. J. 2, 327); according to most observers is has no effect on the thermometer; and Forbes (Phil. Mag. J. 6, 138) observed no trace of heating, when he caused moonlight concentrated 3000 times by a glass lens to fall on a thermo-multiplier. B. Development of Light by Heat. All bodies when heated to a certain temperature become incandescent. Iron becomes hot when hammered; by long-continued hammering it may be made red-hot.-All bodies become red-hot at the same temperature, excepting that air requires, according to Wedgewood's experiments, a higher temperature to render it luminous. According to Newton, iron becomes dull red in the dark at 335° C. (635° Fah.), bright red at 400° C. (752° Fah.), luminous in the twilight at 474° C. (903° Fah.), and luminous in bright daylight at about 538° C. (1000° Fah.) Modes of explaining the facts stated in A and B. 1. Light and heat are the same substance. Light arrested in its motion by the adhesion of ponderable bodies shows itself as heat. When too much heat becomes accumulated in a body, part of it escapes again with great velocity in the form of light: the body becomes incandescent. Against this very simple theory-to which Berthollet also (Stat. Chim. 1, 191) gives the preference the following objections may be urged: (a). Moonlight, ever so much concentrated gives no heat. (This may perhaps be explained by its very small intensity.)-(b). The brightest, most luminous rays of the coloured spectrum, the yellow and green, give very little heat, and the heating power likewise shows itself where neither light nor colour can be perceived.-Light produces chemical alterations of ponderable bodies, which heat alone is unable to effect. (This may perhaps be explained by the more rapid motion of light.)-Phosphorescence by Irradiation, and more particularly that produced by heating, is difficult to reconcile with this hypothesis. 2. The solar rays consist of rays of light and rays of heat distinct from one another; the former are more refrangible than the latter; hence two spectra of different kinds. The solar rays give heat therefore only in consequence of the heat which they contain. The solar light reflected to the earth from the moon has left its heat-rays on the moon and therefore cannot give heat. (Herschel.) Objections: (a). What becomes of the rays of light which bodies absorb together with the rays of heat, seeing that the bodies suffer no change from the absorption, excepting change of temperature?-(b). Why cannot a body become very hot without emitting light? 3. All ponderable substances contain the hypothetical Principle of Fire, which, when united with the light which falls on them, produces heat. (Deluc.) 2. Relation of Light to Electricity. Light often appears as an attendant of electrical phenomena:-the electrical spark, lightning. Is it an element of electricity, or on the other hand is light composed of the two electricities or is it merely separated by electricity from the surrounding medium? 3. Relation of Light to Magnetism. If the violet ray of the spectrum concentrated by a lens be made to pass uniformly for about half an hour over one half of a steel needle, proceeding from the middle towards one of the extremities, that extremity being directed to the north, and the temperature being between 0° and 27° C. the needle will become perfectly magnetic. (Morichini, Schw. 20, 16; further in Kastn. Arch. 8, 105.-This experiment was also successfully made by Ridolfi (Schw. 20, 10), Mary Somerville (Ann. Phil. 27, 224; abstr. Pogg. 6, 493), Müller (Kastn. Archiv. 13, 397), Baumgartner (Zeitschr. Ph. Math. 1, 263), Zantedeschi (Bibl. univ. 41, 64; also Schw. 56, 109; also Pogg. 16, 187), and Barlocci (Bibl. univ. 42, 11; also Schw. 58, 69). It did not succeed in the hands of Configliachi (Gilb. 46, 335), Ries & Moser (Pogg. 16, 563) and d'Hombres Firmas (Ann. Chim. Phys. 10, 285).—I have myself seen it successfully performed by Morichini. ¶ All former researches on the relation between light and magnetism have been completely thrown into the shade by the important discovery of Faraday (Phil. Trans. 1846, I, 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 the 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 direction 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 them, 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. 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.) 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 bo found at the end of Chap. III, 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 gas 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.)-b. 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. b. 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 platinum 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. (Dóbereiner, 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 |