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Phosphorescence of dead animals: Fabr. ab Aqua Pendente. De oculo, Cap. 14.- Thom. Bortholinus. De luce animalium, 169.

Rob. Boyle. Works, 3,304.-Beale. Phil. Trans. 11, 299. Gärtner, on the Phosphorescence of Decayed Wood. Scher. J. 3, 3. C. W. Böckmann, on the Phosphorescence of Decaying Wood. Scher. J.

5, 3. Göbel, on the Phosphorescence produced during Vinous Fermentation.

Schw. 40, 257.

On Luminosity produced by Irradiation, Electricity, and Heating. Canton. An easy method of making a Light-magnet or Phosphorus.

Crell. Chem. J. 6, 179.
Grotthuss, on Chlorophane. Schw. 14, 133.

on the Absorption of Light by Phosphorescent Bodies. Schw, 15, 171. Osann, on remarkable Phosphorescent Minerals. Kastn. Archiv. 4, 347; 5, 88. Luminosity produced by Irradiation. Pogg. 33, 405.

. Wach. Preparation of new Phosphorescent Minerals. Schw. 67, 283. Pearsall. Phosphorescence produced by Electricity. J. Roy, Inst. 1, 77

and 267; also Pogg. 20, 202; 22, 566. E. Becquerel. Phosphorescence produced by Irradiation. Bibl. univ. N. S. 20, 344; also Pogg. 48, 540.

Phosphorescence produced by Electricity. Compt. rend. 8, 216; also Pogg. 49, 543. Biot. Phosphorescence produced by Irradiation. Compt. rend. 8, 259;

also Pogg. 49, 557. Biot & E. Becquerel. Phosphorescence by Irradiation. Compt. rend. 8,

315; also Pogg. 49, 563. Wood, on Phosphorescence produced by heating and rubbing. Phil.

Trans. 82, 28. Brewster, on Phosphorescence produced by heating. Ann. Chim. Phys.

14, 288.

On Phosphorescence accompanying Crystallization. Pickel. Taschenb. 1787, 55.-Schönwald. Crell. Ann. 1786, 2, 50. Schiller. Taschenb. 1791, 54.–Giobert. J. Phys. 36, 256; also Gren. J. 2, 437.-Pfaff. Schw. 15, 275.--Herrmann. Schw. 40, 75.-Berzelius. Jahresber. 4, 44; 5, 41.-Schweigger. Schw. 39, 247; 40, 271.Büchner. Repert. 15, 441; also Schw. 41, 221; also Schw. 41, 228.Pleischl. Zeitschr. Ph. Math. 3, 220.-H. Rose. Pogg. 35, 481; 52, 443 and 585.

SYNONYMES. Luminous Substance, Light-producing Matter, Luminous

Matter, Lumière, Photogène. Light is that substance which excites in our eyes the sensation of brightness or vision.

Physical Properties. 1. Light is imponderable. 2. It is in the highest degree expansible: it spreads itself out from its point of origin in straight lines or Rays with extraordinary quickness, passing over 42,100 miles in a second. Its intensity varies inversely as the square

of the distance from the luminous source. 3. It penetrates the air and all other transparent bodies more or less completely; whilst opaque bodies either do not transmit it at all, or only when they are in very thin laminæ.

4. When in passing through a transparent medium it falls on other bodies, transparent or opaque, it is partly thrown back or reflected,and in such a manner that the incident and reflected rays make equal angles with the reflecting surface.

5. When a ray of light travelling through a thin medium passes near a denser body, its course is somewhat altered,-it is inflected—it suffers a bending or Inflexion.

6. When a ray of light passes in an oblique direction from one medium to another of different density and combustibility, its course is likewise altered; the light is broken or Refracted. If the second medium is denser or more combustible than the first, the ray is bent towards the perpendicular,--and conversely.

Inflexion and refraction may perhaps be explained by supposing the attraction or adhesion of bodies for light to increase with their density and combustibility.

7. Dispersion of Colour.–At every refraction, a colourless ray of light is separated into seven coloured rays. These are, beginning with the most refrangible: violet, indigo, blue, green, yellow, orange, red. [Luminous and Coloured Spectra.] The yellow and green rays are the most luminous. Far beyond the boundary of the violet there exists, according to Seebeck, a faint violet light which gradually becomes colourless* : similarly, red light exists beyond the assigned limit of the red. None of the coloured rays experience any further change of colour by a second refraction.

8. When light falls at a particular angle (35° for glass) on the surface of a body, the reflected light is found to possess peculiar properties; it is Polarized. For, if the ray thus reflected fall on a second flat surface of the same body at the same angle, it is completely reflected therefrom, when the second surface is parallel to the first or makes a right angle with it; but no reflection takes place when the second surface is turned round through an angle of 90°, so that the ray which falls on the first surface would make a right angle with the ray reflected from the secondt.

9. In a great number of media, viz., in crystals not belonging to the regular system, and in certain uncrystallized animal substances, a ray of light is split into two distinct rays oppositely polarized: Double Refraction. Comp. Brewster (Edinburgh J. of Sc. 5, 1; also Schw. 33, 340).

* The existence of luminous rays having a faint violet or rather a lavender grey colour has likewise been proved by Sir John Herschel (Phil. Trans. 1840, Pt. I.)

of The ray is completely reflected when the planes of incidence on the first and second surfaces coincide: no reflection takes place when they are perpendicular to each other. [W.]

CHEMICAL RELATIONS OF LIGHT.

1. RELATIONS OF LIGHT TO THE OTHER IMPONDERABLES.

1. Relation of Light to Heat.

A. Development of Heat by Light. All ponderable bodies absorb a portion of the light with which they come in contact. The quantity thus absorbed is greater in proportion to their opacity, and the darkness and roughness of their surfaces. Transparent bodies, with white shining surfaces, absorb the least, inasmuch as the greater part of the light which falls on them is either transmitted or reflected. The more light a body absorbs, the hotter does it become when exposed to the sun's rays.

In sunshine which raises a mercurial thermometer to 38° C. (100° Fah.) a ball of pure bismuth, one inch in diameter, rises to 50°; the same ball, covered with indian ink, to 56°; with lamp-black, to 59°; and with white paint to 43°: when covered with a blue colour, the ball is more strongly heated than when clean; but less strongly when painted red. (Böckmann.)—If a number of pieces of copper of equal sizes are covered with various colours, the black becomes most heated in the sun, then the blue, then the red and green, then the yellow, and lastly the white. (H. Davy.)--Pieces of cloth laid upon snow in the sunshine sink deeper the darker they are in colour. (Franklin.)

(Heat-collector of Saussure and Ducarchat.]

The concentration of the sun's rays by means of burning glasses or mirrors is one of the most powerful means of producing a high temperature.

Flaugergues has shown, by experiments made during a solar eclipse, that the light of the sun has the same heating power, whether it proceeds from the edge of the disc or from the centre.

Daniell's supposition, that the solar rays have less heat-producing power at the equator than in the temperate regions, has been shown by Gay-Lussac (Ann. Chim. Phys. 26, 375) and Foggo (Edinb. Phil. J. 14, 63) to be incorrect.

The light of the sun loses but very little of its heating power by passing through a plate of glass.

Solar light refracted through a prism shows the greatest heating power: according to Landriani, in the yellow; according to Rochon, between the yellow and the red; according to Herschel and Englefield, beyond the utmost limit of the red; according to Bérard, at the furthest edge of the red, whilst the heating power of the almost invisible rays situated beyond the red is only as great; according to Leslie, in the red, whilst beyond the red scarcely any heating effect was produced. Seebeck however has shown that the heating power of the coloured rays varies with the nature of the prism. According to that philosopher, the beating power of the coloured rays gradually increases from the extreme edge of the violet or about an inch beyond it (where it is weakest) through the blue and green, and attains its maximum :-with a prism filled with water, in the yellow; with a prism filled with oil of vitriol, or with a solution of sal-ammoniac and corrosive sublimate together, between the yellow and the red; with a prism of common white glass and crown glass, in the full red; and with a prism of flint-glass, beyond the red.

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 beat 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 1) from the burning glass than the focus of light.

I 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. T

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 higber 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.)-(6). 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 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?—(6). 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),

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