Once a Week (magazine)/Series 1/Volume 8/Spectral analysis: a message from the sun
A MESSAGE FROM THE SUN.
"If man has ever been permitted to see otherwise than by the eye, it is when the clairvoyance of reason, piercing through screens of epidermis and walls of bone, grasps, amid the abstractions of number and of quantity, those sublime realities which have eluded the keenest touch and evaded the sharpest eye."
Since the discovery of the planet Neptune—that glorious achievement of the human intellect that called forth this panegyric—there has been no subject of scientific investigation more interesting in its nature, or fruitful in result, than the researches that have recently been made into the physical constitution of the sun by Messrs. Kirchoff and Bunsen, based upon the opto-chemical analysis of the solar spectrum. Apart from the scientific value of these researches, they are so novel and beautiful that a short account of them can scarcely fail to interest the most unscientific reader.
The experiments with which we have to deal are founded upon the phenomena of the dispersion or decomposition of light. With the effects of this we are all so well acquainted—perhaps, in its most familiar form, by the resplendent but never-varying tints of the rainbow, or by the ever-varying forms of the same tints that play about the glass prisms, or drops of a chandelier—that a few words on its causes may not inappropriately preface the subject matter of this paper.
If a ray of sunlight be made to pass through a small hole (A) in a window-shutter, and a screen (B C) be placed at a short distance beyond it; there will be formed upon that screen a small spot of light (D), of the same size and shape as the hole, and in a perfectly straight line with it and the sun; but if the course of the ray of light be intercepted by a prism (E), placed as represented in the diagram, it will no longer pass in a straight line, but will, by refraction through the prism, be thrown upwards upon the screen, and instead of being an image of the hole, it will become considerably elongated in a direction at right angles to the lower edge of the prism; and this elongated band will be painted with all the colours of the rainbow, ranged in the same order, and with the same degrees of relative intensity. This is the solar spectrum, first observed by Newton, for mere amusement, with a prism which he bought at Stourbridge fair. "It was," he says, "a very pleasing divertisement to view the vivid and intense colours produced thereby:" destined, however, to fill a place in the annals of scientific research, not less significant than that other divertisement of his majestic mind which led to the discovery of the universal law "that governs alike the fall of the apple and the precession of the equinoxes." But this philosophy in sport was to Newton's mind science in earnest; and after various theories had failed to explain the production of the brilliant spectrum, he was led to account for it on the supposition that a ray of solar light is composed of seven coloured rays, each possessing a different degree of refrangibility or susceptibility of being turned from its natural course in a straight line by the interposition of a refracting medium. Thus the red ray, being the least refrangible, is found nearest the normal course of the pencil of light, or lowest on the screen, while the violet ray, being most refrangible, is thrown farthest from the normal course, or highest up on the screen; the intermediate colours, in order of increasing refrangibility, being the orange, yellow, green, blue, and indigo. Since this discovery by Newton, it has been contended that instead of seven colours, there are but three—red, yellow, and blue, each extending with varying intensity throughout the whole length of the spectrum, and each displaying its greatest brilliancy at that point where its colour is least confused with its neighbour's: these have therefore been denominated the primary colours, the secondary tints being produced by the blending together of less intense portions of the primaries.
Each of these rays has its own distinct and peculiar properties, and each exercises its individual function of the solar influence. The red ray, called the calorific, is that through which we receive heat. Its superiority in this respect over the other rays was proved by the experiments of Herschel, who placed delicate thermometers in all parts of the spectrum, and found that during ten minutes that placed in the red ray rose 5° above that in the violet ray; but, what will seem more curious, the highest temperature was found to exist at a short distance beyond the red ray, and out of the visible range of the spectrum.
The yellow is called the luminous or light-giving ray. It is the most intensely brilliant of the whole spectrum, and hence has the power of exciting the optic nerves to a greater degree than its companions; but, so far as is known at present, it has no invisible property.
The extraordinary influence exerted by the blue ray on all objects of the light-receiving world, and its power of acting upon certain chemical preparations—from which the art of photography took its rise—have earned for it the title of the actinic or chemical ray. It is the most subtle, but most potent of them all, for to its pervading influence we owe most of the benefits and enjoyments we receive, directly or indirectly, from the solar light: the verdure of spring and the sereness of autumn, the odour of the violet, the blush on the rose, the golden glory of the ripening corn, and the copious juices of the luscious fruit, are all developed by its latent power: it is its silent and unseen agency that vivifies the vegetable, and invigorates the animal world; that flushes with the bloom of health the cheek of beauty, and swarths the sweating brow of the toiling husbandman. Light we can produce, and heat we can generate by artificial means, but we cannot counterfeit that "kindest ray" under whose benign aspect
"Reviving sickness lifts her languid head;
Life flows afresh; and young-eyed health exalts
The whole creation round."
To return to our spectrum projected on the screen. We left it a gorgeously-coloured band of light, as Newton found it; in this state it remained without further investigation as to its nature for 130 years, or until Wollaston, in the year 1801, during some experiments on the dispersive powers of different media, which necessitated a close examination of the spectrum, discovered that it was divided into several portions by extremely fine black lines, crossing it at right angles to its length, or apparently separating certain colours; little notice seems to have been taken of this beyond recording the observation, and designating the fine lines, five in number, by the letters A, B, C, D, and E. Fourteen years after, the German optician, Fraunhofer, put forth his observations—made independently and in ignorance of Wollaston's discovery—and published a diagram of the spectrum, on which he had mapped down the positions and relative intensities of 354 lines; this number has since been multiplied tenfold by recent observers, using more delicate and powerful instruments: but they still do honour to their secondary discoverer by the name they bear, as "Fraunhofer's lines." Eight of these, more remarkable for their intensity, and consequent facility of observation, were called by Fraunhofer A, B, C, D, E, F, G, and H; they have since been used as points of reference by later observers, and are shown in their proper positions in our diagram. In the rough experiment we made with the hole in the shutter, the lines could not be seen; but they might be rendered visible by refining the experiment. To do this it would be necessary to limit the aperture through which the light falls upon the prism to an extremely narrow slit, and to view the spectrum with a telescope of small magnifying power; the lines would then make their appearance, and look like the finest spider-threads stretched across the spectrum. The number of lines seen would depend upon the angle and dispersive power of the prism. Messrs. Kirchoff and Bunsen used four prisms, in order to gain as much dispersion, and, consequently, as long a spectrum as possible. In the plates which accompany their memoir, the parts of the spectrum are represented on a scale which would give from sixteen to eighteen feet for its entire length, while the infinite variety of intensity of the lines, and the delicacy required to faithfully reproduce them, necessitated the employment of six different stones and as many shades of ink in the process of lithographing the drawing.
The remarkable feature in these lines is their immutability: they are always seen in the same positions under the same circumstances. Messrs. Brewster and Gladstone, and M. Janseen, have observed certain of them undergo a slight change according to the elevation of the sun; but, as a rule, they may be considered, as they are denominated, "fixed" lines.
The question naturally arises: What do these lines consist of, and how are they produced? To answer this we must leave them for a while, to seek materials for a reply from the labours of Messrs. Kirchoff and Bunsen. It has long been known to the chemist and the pyrotechnist, that the salts of certain metals and alkaline earths possess the property of imparting colours to the flames in which they may be consumed; for instance, if a piece of common salt (chloride of sodium) be held in the flame of a candle, it will cause it to emit a yellow light; if a salt of strontium be used, the colour will be red; and if barium, a green flame will be produced. Now the spectrum formed by the light of a candle, an oil-lamp, or a gas flame, is uniform and continuous, exhibiting the colours of the solar spectrum, but no traces of the dark lines; while the spectra formed by the light emanating from metals and alkaline earths in combustion are strikingly characteristic and peculiar, for instead of being uniform and continuous, they consist of a greater or less number of isolated coloured bands, each occupying a place in the spectrum coincident with the ray of the same colour in the continuous spectrum, by virtue of Newton's law, which fixes the position of each coloured ray by its refrangibility. Every metal yields its own spectrum, completely different from all others, and, however it may be combined or confused with other matters, its presence can be infallibly detected by analysis of the light emitted during its combustion; this peculiar property of the prism, of sifting out a particular element, has led to the discovery of two new metals by Bunsen, who, while engaged in an analysis of the contents of the mineral waters of Baden and Durkheim, detected one by two bright blue lines unknown in any other spectrum, and the other by two equally strange red lines; the first of these he called cæsium, the second rubidium, by virtue of the coloured rays they emitted; a third member of the same family, discovered by Crookes, and yielding a green ray, has been christened thallium.
We have now paved the way for an experiment which may be regarded as a key to the researches of the German philosophers. If we take the prism and telescope used for observing Fraunhofer's lines, and substitute for the beam of sunlight the light of a candle or gas-jet, we shall obtain a perfectly uniform or homogeneous spectrum. Now, if we introduce a bead of sodium or a portion of common salt into the flame, we shall perceive two very fine brilliant yellow lines, as it were laid close together upon the gas-flame spectrum; if a colourless flame be employed, such as that obtainable from a Bunsen's gas burner, the yellow lines will be seen without the illuminated background; in this state they will appear as represented in the diagram; and now comes the most interesting and most important feature in the experiment. If a glass tube, containing the heated vapour given off from the volatilisation of a sodium salt be held in front of the incandescent sodium, the brilliant yellow lines will be found to disappear entirely, and, if the luminous spectrum has been used for a background, their places will be exactly filled by two black lines; if, instead of sodium, a salt of lithium be employed, two bright red bands will appear in the places shown in the diagram, which will also be replaced by black ones when the vapour of lithium is interposed between the burning metal and the prisms; if an iron salt be substituted, upwards of sixty bright lines will be seen scattered about the spectrum, all of which will be similarly transmuted when the light is passed through iron vapour: and the same results will be obtained whatever metal be used in the experiment. From observation of these facts, Kirchoff arrived at the important discovery, that the vapour of every substance has the power of absorbing just those rays that its light emits.
Following the course of the German philosophers, we will now revert to the solar spectrum, and see how it can be connected with the above experiment, with the view of tracing the origin of Fraunhofer's lines. By a slight modification of the instrument used in the preceding observations, amounting to the use of the same prism for a double purpose, the solar spectrum may be reduced to half its breadth, and so fill only one half of the field of view of the telescope, leaving the other half to be occupied by the spectrum from any independent source of light; this is done by merely covering half the slit through which the sunlight passes with a small prism or reflector, so placed as to conduct the beam of light under analysis through the apparatus unconfused with the solar beam. Now, with the solar spectrum occupying its half of the field, if we place a gas or other hydrocarbon flame in front of the little prism, we shall see its spectrum in juxtaposition to that of the sunlight, and they will appear as one spectrum, half of which contains the dark lines, the other half being homogeneous; now, if we introduce sodium in any form into the gas-flame, we shall see our yellow line as before produced, and its position will exactly correspond with one of the most prominent of Fraunhofer's lines, namely, that called D; following up our previous experiment, if we interpose the sodium vapour between the burning sodium and the little prism, our bright line will become reversed, and a fine and continuous black line will stretch across both spectra. The relation between the two sections of this line in the different spectra is absolutely identical; the higher the power employed to observe them, the more perfect is their coincidence. If potassium, with its absorbing vapour, be substituted for the sodium, we shall obtain a dark line, corresponding exactly with Fraunhofer's line A; while, if iron with its vapour be employed, about sixty lines will be seen, all in perfect coincidence with some of the more or less distinct of the solar lines.
In this way Kirchoff and Bunsen have compared the spectra of nearly the whole of the known metals and earths with that of the sun, and have thence discovered that its spectrum contains lines identical with those of iron, magnesium, sodium, potassium, calcium, chromium, nickel, and possibly with those of barium, copper, manganese and zinc, while, to the present time, they have discovered no identity between the solar lines and those of gold, silver, lead, tin, antimony, arsenic, mercury, lithium, or strontium.
We are thus put in possession of the facts necessary to answer the question relative to the nature of these fixed lines, and with it to partially solve the problem of the physical constitution of the sun; we cannot do this otherwise so well as by quoting the words of Kirchoff himself: "The sun consists of a glowing gaseous atmosphere surrounding a solid nucleus possessing a still higher temperature. If we could see the spectrum of the solar atmosphere without that of the solid nucleus, we should observe in it the bright lines which are characteristic of the metals it contains. The more intense luminosity of the internal nucleus does not, however, permit the spectrum of the solar atmosphere to become apparent; it is reversed (as in our experiment we reversed the soda line), so that instead of the bright lines which the luminous atmosphere itself would have shown, dark ones appear. We do not see the spectrum of the solar atmosphere, but a negative image of it. This can, however, with an equal degree of certainty, serve to detect the metals present in the sun's atmosphere; all that we require for the purpose is a very accurate knowledge of the solar spectrum, and the spectra of the individual metals."
Thus have the solar rays revealed the history of their birth, and thus has the minute sunbeam admitted through the chink in the window-shutter, borne on its silent course and in occult language telegraphed its secret message: by successive steps the philosopher, first creating an alphabet, next a lexicon, then a grammar of the mystic language, has at last deciphered the cunning telegram by which the solar beam betrays the secrets of its prison-house.
Much as we have already seen accomplished, the work is yet far from complete; for a vast number of solar lines remain unmatched and unclaimed by terrestrial partners. For a time, however, we must rest content, at least so far as the German philosophers are concerned; for Kirchoff tells us his eyes have become so weakened by continual observation, that he is compelled to suspend his labours till they shall have repaired their exhausted power. Meanwhile, stimulated by the results of these researches, astronomers are actively engaged in mapping the spectra of the heavenly bodies. Fraunhofer, in his time, observed the lines given by the planets and a few of the fixed stars: in our time, Donati (of comet celebrity) and Padre Secchi, of Rome, have each observed and published a few stellar spectra, while the magnificent equatorial of our national Observatory, equipped with a spectrometric apparatus, has already furnished accurate micrometric comparisons between the solar spectrum and the spectra of about forty stars.
The spectra of the moon and planets show lines coinciding with those of the sun. This, though curious, is only natural, when we consider that these bodies are merely reflectors, giving off the light they, in common with the earth, receive from the sun. The spectra of stars, however, differ widely, not only from the sun, but from each other. In our diagram we have shown two of quite different characters, the first of Betelgeux, the bright star in Orion, the second of Sirius, or the Dog star. In the spectrum of Betelgeux, the most remarkable coincidence is that of a line with the sodium line D of the solar spectrum, and it is curious that several other stars show this line. This is primâ facie evidence of the existence of sodium in the atmosphere of these stars. But the most remarkable circumstance is connected with the solar line F, for out of forty stars, twenty-eight exhibit a line perfectly coincident with it: Sirius is one of them. This line has not yet been found to correspond with any metallic line, but the group of which it forms the nucleus contains several strongly-marked iron lines. At first sight therefore, it would seem probable that the stellar line owes its source to the presence of iron in those stars that show it; but, with our present knowledge it would be premature to adopt this supposition without more conclusive evidence. Although twenty-eight stars possess this line in common, their other lines are not coincident, for out of the twelve not more than two or three give spectra whose lines all agree. This proves that the source of the F line is independent of that of the other lines, and hence that, although these bodies possess one common component, they are not all of exactly similar nature. Our present ignorance as to the nature of this and the less diffused components of the stellar suns detracts nothing from the importance and little from the interest of these investigations; for in these days of insatiable research, to establish the existence of one element in one star—as has been done in the case of sodium in Betelgeux—is to tread the lowest step of a ladder whose topmost round will not be left unattained till the constitution of every spectrum-yielding star is more or less certainly known.