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Heat, Light, and Chemical Rays.

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Speaking generally, we may say that the rays which affect the optic nerve lie midway between the heating and the chemical rays. The more hidden and minute changes, which we call chemical, are thus affected by the more rapid and minute ethereal vibrations; while the long, slow rays of heat appeal to our grosser senses, That there is only a difference in degree, and not in kind, among

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the different rays, is proved by the following two facts :-(i.) By concentration with a lens the heat-rays may be caused to act upon a solid body and make it white-hot, i.e., visible, the slow heat rays being thereby quickened in rapidity, and correspondingly shortened. (ii.) By allowing the invisible chemical rays to fall on certain substances known as fluorescent, they are reduced in length and rapidity, and so brought within the pale of the visible.

899. Fluorescence.—As calorescence applies to the red end of the spectrum, so fluorescence is applied to certain phenomena at the violet end. This term has been applied to the internal dispersion of the rays of light on certain solids and fluids. Sulphate of quinine forms with water a perfectly colourless solution, but under certain aspects, it presents at the surface a splendid light-blue colour. Sir J. Herschel noticed this in 1845, and it has been observed in a solution of the bark of the horse-chestnut in water, as well as of the green colouring matter of leaves (chlorophyll) in alcohol. It is called fluorescence or epipolization. This property serves to detect the presence of quinine even when the solution is much diluted. Stokes found that the rays causing dispersion, are the invisible rays beyond the violet, those which can be seen in the spectrum by the aid of solution of quinine on paper, or by the use of uranium glass. As these chemical rays are not found in the light emitted from a candle, the fluorescent appearance cannot be well seen by artificial light.

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900. Colour of bodies.—Coloured bodies, as we term them, possess the power of extinguishing or absorbing certain rays or waves of the solar beams, and reflecting others. A body which reflects all the rays in the same proportion as it receives them is white; a body which absorbs all the rays and reflects none is black. A red powder, such as the red iodide of mercury, absorbs all the blue and other rays, and reflects the red only; and a red glass stops all the rays excep*ing the red, sifting them out, and allowing them to pass, as being of a period or temperament conformable with itself. The petal of a scarlet geranium is red, because by its molecular structure it absorbs and fixes all the rays excepting the red. When held in the green rays of the spectrum, it cannot be distinguished from black velvet. That a coloured body exercises this passive or secondary sort of action on the luminous rays, and does not impart to them any new property, is proved by the following experiment : If we place a red wafer at the confines of the green and blue part of the spectrum, a yellow wafer in the indigo blue part, and a blue one in the red of the spectrum, all three wafers will seem equally black, showing that thev are unable to reflect the lights which fall on them, or have an atomic constitution which accepts the rate of ethereal pulsation corresponding to the respective colours named, and does not return the motion. The facts seem analogous to the case of the mechanical impacts of ivory balls (see Art. 170). If the impinging ball be of the same weight as the ball it strikes, the motion of the former is accepted and passed on, none being returned; whereas, if the two be of unequal masses, corresponding degrees of the motion will be accepted and returned.

901. Dark Lines of the Solar Spectrum.-When the light of the sun, passing through a fine slit, is carefully examined by means of a good prism, it is seen that the coloured spectrum or extended band of light is crossed at frequent intervals by dark lines parallel to the line of the slit. It thus appears that the sun does not transmit to us luminous waves of all varieties of length between the short violet and the long red. There are interruptions corresponding to definite wave-lengths in the solar spectrum, examined by any number and variety of transparent prisms, which we do not find when we examine the light of a candle, or of a gas jet, or of an incandescent wire, by the same means. The dark lines in the solar spectrum were first detected by the English philosopher, Wollaston, about the year 1802; but as they were first accurately mapped by the German optician, Fraunhofer, abou! the year 1814, they are often designated

Red.

Solar Spectrum. Fraunhofer's Lines.

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Fraunhofer's Lines. Wollaston had observed only the principal lines, such as are given in our diagram (fig. 233); but Fraunhofer, by examining the prismatic band through a telescope, counted and mapped accurately down on paper no less than 576, naming the most conspicuous lines by the letters of the alphabet, by which

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they are still recognized. Thus A is at or near the extreme red end of the spectrum; B is between the red and the orange; C is situated in the orange; D is a double line in the yellow; is in the green; F, a group of fine lines in the blue; G is just at the confines of the indigo; and H is a host of fine lines in the violet.

Fraunhofer observed, also, that these lines are always present in the same relative positions in sunlight, whether direct or reflected. Thus the light of the moon and of the planets presents exactly the same set of dark lines as the light of the sun, as we should expect, from their being but reflectors of the sun's light. On the other hand the fixed stars, which we know to be independent sources of light, show different groups

of lines, with different relative
arrangements. Fraunhofer E
concluded from this that the
dark lines, however they may
be caused, are not due to any
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sphere with these ultra-mun-
dane lights.

902. Fraunhofer's application of the telescope to the examination of the solar spectrum resulted in the construction of the spectroscope, an

Fig. 234.

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instrument indispensable to the chemical analyst of the present day. The figure (234) will give an idea of its leading features. It

Violet.

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Spectra of Heated Solids.

consists essentially of a tube, T 0, which is provided with a slit at the further end, o, capable of being made larger or smaller by means of a screw. At the other end of the same tube is a collimator or lens which throws the light passing through the slit into parallel rays. A secord tube, E T', is a telescope whose focus is adjusted till it gives a distinct image of the slit. On introducing a prism, P, between the two tubes, we must of course move the latter tube round till its axis coincide with the direction of the bent or refracted beam from the slit. We have thus the means of inspecting the prismatic band more minutely, and of distinguishing the dark lines with facility. This instrument also enables us to study the spectra of artificial lights; and by its means we can see in the spectrum of an incandescent gas or vapour, characteristic features which serve for their identification in the case of a compound incandescence.

903. An incandescent solid or liquid placed in front of the slit of the spectroscope, always gives a spectrum unmarked by dark lines. The light of a common gas flame is almost entirely due to the incandescence of the solid carbon particles suspended in the flame ; and this is the reason that the spectrum of such a flame is continuous, it being, in fact, not the spectrum of the hydrogen gas at all, but of the carbon particles.

When, however, we examine the flame of a body really in the gaseous or vaporous state, placed in front of the slit of the spectroscope, we find its spectrum is discontinuous, consisting usually of a characteristic set of luminous bands of one or more colours. Thus, for instance, if we burn a salt of sodium, such as common salt, which is the chloride of sodium, or common soda, or Glauber's salt (sulphate of sodium), in the flame of a spirit lamp, or of a common Bunsen burner, we volatilize and decompose the salt and obtain, as the spectrum of the burning metallic vapour, a bright yellow line corresponding to the place of the dark D line in the solar spectrum (fig. 233). No other simple substance is found to give the same line; it is consequently assumed to be indicative of the presence of the metal sodium, in some form or other, wherever it appears in the spectrum. The study of spectra has revealed the important fact that there is no substance more universally diffused than sodium; the air is impregnated with it; the very particles of dust seem to be crusted with its salt, for if we strike two books together near a common gas flame at the slit of our spectroscope, we find that the inflamed dust makes the well-known D line flash forth with more or less distinctness. This is doubtless due to tiny

Spectra of different Metals.

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particles of salt evaporated from the large surface of salt-water on the face of our globe.

Again, if we examine with the spectroscope the light of the vapour of the metal potassium, or the light obtained from any salt of potassium burning in a Bunsen or other smokeless flame, we see two red bands (occupying the same relative position in the spectrum as the dark A and B lines in the solar spectrum); and also near the farther end of the spectrum we find flashing forth a violet line between the G and H dark lines of the solar spectrum (fig. 235).

904. The adjoining figure (235) will serve to convey, more clearly and briefly than words can do, an idea of the peculiar sets of lines

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corresponding to the vapours of the two metals we have mentioned, as well as a few others.

Calcium, the metallic base of common lime, gives a number of bands in the orange, yellow, yellowish green, and one bright band in the blue.

Strontium, salts of which are used for giving the beautiful crimson red in fireworks and illuminations, exhibits some very bright lines

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