God's glory in the heavens/The Chemistry Of The Sun
1. Lines of Solar Spectrum. 2. Colours of Solar Spectrum.
3. Spectrum of Sodium. 4. Spectrum of Lithium.
IX.
THE CHEMISTRY OF THE SUN.
Reference has already been made to the results of spectrum observations by Professors Kirchhoff and Bunsen of Heidelberg. The method of research is, however, so important as to demand a separate notice. A new era, both in chemistry and astronomy, has been inaugurated by the discovery of this method. Sufficient time has not yet elapsed for the accumulation of results, but no doubt exists as to the incalculable value of the new analysis.
When a ray of white light is passed through a prism, it is separated into the various colours of the spectrum, viz., red, orange, yellow, green, blue, indigo, and violet. It is thus shewn, that the white ray is compound—and that the prism decomposes this ray into the variously-coloured constituent rays. If we compare the white ray to a closed fan, the opening of the fan will represent the effect of the prism, in spreading out and representing separately the constituent rays. We shall suppose the fan to consist of seven ribs or sectors, representing the seven primitive colours; when the fan is shut, these colours are not visible, but when it is opened it represents the spectrum of white light. A ray of white light may, also, be compared to a rope, made up of strands of the various colours of the spectrum, and the prism serves, by refraction, to open up the rope and present the strands separately. Perhaps a still better illustration is to compare the white ray to a quantity of sand, composed of seven differently-coloured kinds. In this case, the prism separates the different kinds of sand, and assorts them in distinct coloured layers—there being always a regular gradation from the red to the violet. The position of the various colours in the spectrum is perfectly fixed. The fan may be opened more or less, but the relative positions of the ribs are, always the same. In the case of other colours than white, the spectrum of course gives only the primitive colours necessary to make up the given colour. If, for example, yellow is wanting, the spectrum will give a black band instead of a yellow one. If, on the other hand, the colour transmitted through the prism be an elementary colour, there will be only one band, as in the case of the yellow flame produced by sodium; all the other colours are wanting, and only one yellow band is given. The flame in which lithium is diffused, gives only two bands, yellow and red.
The value of the discovery of the researches of the Heidelberg professors lies in this, that these bands are proved to be perfectly characteristic of the substances which produce them; that is, each substance has its own distinctive bands, so that they cannot be confounded with the bands of any other. The distinctive marks are—their number, breadth, tone, and grouping in the spectrum. The characteristic lines are as invariable as the circles of longitude and latitude; and just as these circles fix with certainty the locality of any city, so the bands in the spectrum determine the nature of the substance emitting the colour thus decomposed.
The delicacy of the prism in detecting minute quantities, far surpasses the reagents of the laboratory. One hundred-millionth of a grain can thus be detected. A very minute quantity of sodium diffused in a room, can at once be revealed by its effect on the flame of a burner. The light of the burner has only to be transmitted through the prism in order that it may yield the secret.
The grand discovery, in reference to solar chemistry, is the meaning of the dark lines in the solar spectrum. Besides the bands of the seven primitive colours, there are innumerable black lines which cross the spectrum, and divide the colours into very narrow stripes. In the cut, the diagram of lines must be conceived as superimposed on that of the colours. These lines are seen when the sun's rays are passed through a narrow slit before falling on the prism. If a large beam be taken instead, they cannot be detected, as, in this case, the spectrum is composed of several spectra, overlapping one another and confusing the details. These dark markings are called Frauenhofer's lines. The principal ones are designated by the letters B, C, D, E, F, G, H, and occupy preciselydefined positions in the spectrum. But besides these, there are thousands more; every new refinement in observation detects additional ones. These dark lines have long formed an enigma to physicists. It was plain, that they indicated, that some of the rays were somehow absorbed or obstructed. If we lay a comb on a piece of coloured paper, the teeth will obstruct the view of the paper under them, so that we can see the colour of the ground only in the spaces between the teeth. This is precisely what is seen in the spectrum; the whole coloured spaces have minute dark parallel lines laid upon them, similar to the teeth of the comb. The chief merit of Kirchhoff and Bunsen lies in furnishing a complete explanation of the lines, and, at the same time, putting within our reach a new and universal analysis, applicable alike to celestial and terrestrial objects. A few grains of a gaseous substance can be detected in the remotest orb, as well as in the lamp upon our table.
If a solid body, such as platinum or charcoal, be raised to a white heat, the spectrum given by the prism will be complete, and perfectly continuous,— that is, all the colours will be found following one another in regular order; but they will not be cut up or striated by dark lines. This arises from the solidity of the body. If we saw only the naked incandescent ball of the sun, there would be no dark lines in the spectrum. How, then, are we to account for these lines?—simply by supposing that the sun is surrounded wdth a gaseous envelope, in which the incandescent particles of various substances are diffused. These substances intercept the rays of the incandescent body of the sun, and they intercept in such a way as to betray their nature. The climax of the discovery is, that the dark lines are the negative spectra of the gaseous substances in the sun's atmosphere. What is meant by negative is, that the bright bands in the spectrum of any substance, when diffused in the flame of a lamp, become black in the solar spectrum. Suppose that, when viewmg the solar spectrum, the light of the solid or liquid photosphere of the sun were suddenly extinguished, the gaseous atmosphere still glowing, the colours which we at present see would be at once extinguished, and all the dark lines would become bright bands with their appropriate colours. We would now be viewing a gaseous incandescent atmosphere, and we would have the spectra of all the bodies in this atmosphere represented by these bands. The next total eclipse will present an opportunity of applying this test. The perfect conversion of the positive into the negative spectrum is that which gives its validity to this new analysis. We can read these lines and interpret their meaning, as distinctly as we read the symbols and formulae of the chemist. We have only to ascertain what is the positive symbol of any substance by diffusing it in the flame of a Bunsen's lamp, and then look to the sun for a similar symbol; the only difference is, that, in the former case, the letters are written in coloured inks, whereas, in the latter, they are all black; but the form, grouping, and position are such that their meaning cannot be mistaken.
But how is it that the spectrum of a substance is coloured in terrestrial flame, and black in the gaseous envelope of the sun?—simply from the circumstance, that behind the incandescent atmosphere of the sun there is the brighter incandescent solid or liquid photosphere of the sun. It is this brighter background that makes the lines black, just as the bars of a window appear dark when viewed against the sky, though painted white. A truer idea is afforded by the illustration of the comb. Let us take, for example, the spectrum of sodium. Here there is only one tooth, all the other primitive colours being wanting. This single tooth is yellow, and we shall suppose made of glass. If you look through this yellow glass at another yellow but much brighter object, such as the yellow portion of the sun's spectrum, it becomes a dark line. The yellow tooth, or bar of sodium, when thus seen against the yellow portion of the sun's spectrum, becomes a black line, and we know that it is the same, from its position in the spectrum. Finding that the dark line, called D, in the solar spectrum, corresponds with the bright yellow band of sodium, we conclude that sodium is part of the gaseous solar atmosphere interposed between us and the sun. Other substances have more bands or teeth; and as there are many substances in the sun's atmosphere, the lines are like so many combs laid the one over the other, each having its own character, number, and disposition of teeth, so that, though they appear confused, it is possible to single out the more marked with great ease. The sailor can easily detect, through the forest of masts in a harbour, the rig of his own ship; and the chemist can easily discover, amidst the crowd of lines in the solar spectrum, the pattern corresponding to the various substances. In this way, iron, magnesium, chromium, sodium, and nickel, have been found in the sun's atmosphere. On the other hand, no trace has been found of silver, copper, zinc, aluminium, cobalt, and mercury, though they have very characteristic spectra.
It may be objected, that it is a mere assumption, that the brighter opaque photosphere of the sun is shining through a gaseous atmosphere. This, however, is not the case. These precise conditions can be represented artificially, and the same results obtained. The charcoal points in the electric light, represent the opaque incandescent photosphere of the sun. A Bunsen's lamp, which gives a great heat without much light, represents, with substances diffused in its flame, the gaseous envelope of the sun. When the flame of the lamp is viewed with the brighter electric light as a background, phenomena precisely similar to those of the solar spectra are observed. The only difference is, that in the lamp we can diffuse any one substance at a time and get its spectrum isolated, while, in the sun's atmosphere, numerous substances are diffused, and we have thus various spectra mixed up together.
But this spectrum analysis is not confined to the sun. We can now analyse the remotest stars that burn in space. As far as observations have already gone, each star has its own individuality as to chemical composition. The line D, characteristic of sodium, has been detected in the spectra of Pollux, Capella, Beltigeux, and Procyon; and it is probable that the colour of yellow stars is due to this widely- diffused element. It is probable that there are systems where metals chiefly prevail, and others where the characteristic elements are alkalies.
This new instrument of research is by far the most comprehensive that has yet been put into man's hand. It is destined alike to deal with worlds and with atoms. It can decompose the distant star, and the invisible molecule. All science tends to bind, under the same mechanical laws, the motions of orbs and atoms. Light and heat are regarded merely as modes of motion. The elementary atoms are in a constant state of vibration; that vibration produces a vibration in the ether, and the vibration of the ether, in reaching us, is light or heat—a light vibration being merely a heat vibration, with the rate so intensified that it becomes luminous. It is exceedingly probable that the complexity of the spectrum of any elementary body is related to the complexity of the atom itself. If, for example, the spectrum of an atom, hitherto supposed to be elementary, give several colours, it is probable that there are several component atoms vibrating at different rates—corresponding to the colours—each atom having its own chemical characteristics. How simple the means by which results so momentous are achieved! Strange that an angular bit of glass, turning aside a ray of light, should be the means of revealing to us the secrets of the two extremes of the material universe—worlds and atoms! The unity of the Creator is shewn, not merely in the unity of plan pervading the universe, but in the existence of instruments by which the discovery of that unity might be made known. We might conceive intellectual beings so circumstanced, that they would have no means of discovering the unity of plan. They might dwell in a world of marvellous order, unity, and beauty, while they might be shut out for ever, on account of the want of proper instruments, from the discovery of these elements of God's glory. When, then, we have the means of discovery so marvellously provided, the existence of one Creator and Governor is more forcibly impressed on our minds.
The new light thrown upon the constitution of the sun will greatly unhinge former speculations on this subject. The body of the sun, if it is to be taken as the source of light, can no longer be regarded as an abode cool enough for inhabitants. It is now to be held as hotter and brighter far than the gaseous envelope. But how, on this supposition, are we to explain the dark body of the sun seen down through the perforations of the outer envelopes? Or may we still regard the body as comparatively dark, while the photosphere underlying the red stratum is to be regarded as an opaque, liquid substance, comporting itself, as to radiation, like a solid body? Is the red stratum, with its rose-coloured prominences, or the corona above it, the region of the gaseous atmosphere in which the various substances are diffused? The luminous envelope can no longer be regarded as a gaseous body, but then the phenomena which it presents are altogether inconsistent with the supposition that it is solid. Neither does the idea of continuous liquidity furnish a satisfactory solution. But does not the analogy of other parts of the solar system furnish a clue? If we suppose it to consist of discrete meteoric matter, as in the case of Saturn's rings, the principal phenomena will receive an adequate explanation. The condition of solidity is satisfied, and the porous structure is accounted for. The zones of meteoric matter round the sun, the limits of the mass of which have been determined by Levenier, indicate the sources from which it is fed. The visible envelope of Saturn has probably a similar constitution, and if incandescent, as it probably once was, would, with his rings, present a complete analogue to the photosphere of the sun and his encircling zones. These unsettled questions shew, that our theories regarding the constitution of the sun have been merely provisional. We have now, however, got our foot on solid ground, and there is every probability that the great outstanding questions will soon be, to a large extent, settled, though the scaling of every new mountain top will only bring into view other loftier and more distant summits.