Effect of brighter light source. Fraunhofer's lines B and C. If, now, solar rays of mode- CHAP. XII. rate intensity are allowed to fall through the flame on the slit, the line at the place pointed out is seen bright on a darker ground; but with greater strength of sunlight there appears in its place a dark line, which has quite the same character as Fraunhofer's lines. If the flame be taken away, the line disappears, as far as I have been able to see, completely. "I conclude from these observations, that coloured flames in the spectra of which bright sharp lines present themselves, so weaken rays of the colour of these lines, when such rays pass through the flames, that in place of the bright lines dark ones appear as soon as there is brought behind the flame a source of light of sufficient intensity, in the spectrum of which these lines are otherwise wanting. I conclude further, that the dark lines of the solar spectrum which are not evoked by the atmosphere of the earth exist in consequence of the presence, in the incandescent atmosphere of the sun, of those substances which in the spectrum of a flame produce bright lines at the same place. We may assume that the bright lines agreeing with D in the spectrum of a flame always arise from sodium contained in it; the dark line D in the solar spectrum allows us, therefore, to conclude that there exists sodium in the sun's atmosphere. . . . "In the course of the experiments which have at present been instituted by us . . . . . a fact has already shown itself which seems to us to be of great importance. The Drummond light requires, in order that the lines D should come out in it dark, a salt-flame of lower temperature. The flame of alcohol containing water is fitted for this, but the flame of Bunsen's gas-lamp is not. With the latter the smallest mixture of common salt, as soon as it makes itself generally perceptible, causes the bright lines of sodium to show themselves. . . . . . Immediately after the publication of this important Solar lines produced by certain known substances. CHAP. XII. note of Kirchhoff's, Stewart1 explained, in extension of his former work on the theory of exchanges, why it was that a salt flame of lower temperature was required to darken the D lines, pointing out that it was a phenomenon analogous to that presented when a piece of ruby glass is Ruby glass heated in the fire. So long as the ruby glass is cooler than experiment. the coals behind it, the light given out is red because the ruby glass stops the green, the green light is therefore analogous to the line D which is given out by an alcohol flame into which salt has been put. Should however this ruby glass be of a much higher temperature than the coals behind it, the greenish light which it radiates overpowers the red which it transmits, so that the light which reaches the eye is more green than red. This is precisely analogous to what is observed when a Bunsen's gas flame with a little salt is placed in front of the Drummond light, when the line D is no longer dark but bright. Shortly afterwards Kirchhoff independently explained his results on the same theory. In the next chapter we shall show how fruitful of result this experimental verification of Stokes' theory by Kirchhoff and Stewart, really effected by Foucault, has been, and how the new field of investigation thus opened up has been explored. Here we may content ourselves by pointing out how, by the light of modern science, a simple experiment, made by means of sodium vapour and a beam of sunlight, with the powerful aid of a little prism, gave us a tremendous increase of our knowledge about distant worlds-worlds so immeasurably remote that it seemed hopeless for men to try and grapple with them. 1 See "On the Theory of Exchanges and its recent Extension," by Balfour Stewart, B.A., Reports, 1861. I quote here the passage which relates to the connection between the heat and light rays: "We come now to the subject of light; and since radiant light and heat have been shown by Melloni, Forbes, and others, to possess very many properties in common, it was of course only natural to suppose that facts analogous to those mentioned should hold also with regard to light. One instance will at once occur, in which this analogy is perfect. For, as all opaque bodies heated up to the same tempera- CHAP. XII. ture radiate the same description of heat, so also when their common temperature is still further increased, they acquire a red heat, or a yellow heat, or a white heat, simultaneously. "The idea of applying these views to light had occurred independently to Professor Kirchhoff and myself; but, although Kirchhoff slightly preceded me in publication, it will be convenient to defer the mention of his researches till I come to the subject of lines in the spectrum. "In February 1860, I communicated to the Royal Society of London a paper in which certain properties of radiant light were investigated, similar to those already treated of with respect to heat. In this paper it was mentioned that the amount of light radiated by coloured glasses is in proportion to their depth of colour, transparent glass giving out very little light; also that the radiation from red glass has a greenish tint, while that from green glass has a reddish tint. "It was also mentioned that polished metal gives out less light than tarnished metal, and that, when a piece of black and white porcelain is heated in the fire, the black parts give out much more light than the white, thereby producing a curious reversal of the pattern. "All these facts are comprehended in the statement that in a constant temperature the absorption of a particle is equal to its radiation, and that for every description of light. "It was also noticed that all coloured glasses ultimately lose their colour in the fire, as they approach in temperature the coals around them; the explanation being, that while red glass, for instance, gives out a greenish light, it passes red light from the coals behind it, while it absorbs the green, in such a manner that the light which it radiates precisely makes up for that which it absorbs, so that we have virtually a coal radiation coming partly from, and partly through, the glass. "In another paper, communicated to the Royal Society in May of the same year, it is shown that tourmaline, which absorbs in excess the ordinary rays of light, also radiates, when heated, this description of light in excess, but that when the heated tourmaline is viewed against an illuminated background of the same temperature as itself, this peculiarity disappears." Sodium. RESULTS OBTAINED BY THE OLD METHOD. I. KIRCHHOFF'S MAPS AND LIST OF ELEMENTS CHAP. XIII. IN the note of Kirchhoff's which was given almost in extenso in the last chapter, we saw the demonstration of the fact of the existence of sodium in a relatively cool atmosphere surrounding the sun. Now sodium has a very simple spectrum; Kirchhoff was not long before he tested his generalization by the most complicated spectrum he could find. He took for this purpose the spectrum of iron, one of enormous complication, for, as we now know, the spectrum is traversed by lines throughout its whole length, no less than 460 lines having been already mapped, and their positions are now thoroughly well known to us as well known as the position of any star in the heavens. Kirchhoff tried the iron spectrum, and he found, absolutely corresponding in position and in width and darkness to the bright iron lines which he saw in this spectrum, black lines in the solar spectrum. He instantly convinced himself and soon convinced the world, that he had experimentally established not only the fundamental fact, which we have already stated, that gases and vapours have the power of absorbing those very rays which they themselves give out when in a state of incandescence, but that iron as well as sodium was present in the sun. Kirchhoff went on with this magnificent work-which included the construction of the first map and catalogue of the Iron. lines of the solar spectrum approaching completeness, which had ever been given to the world; a monument of industry which cost the illustrious physicist one of his eyes until he had arrived at the conclusion that sodium, iron, calcium, magnesium, nickel, barium, copper, zinc, [potassium?] were in the solar atmosphere; that the existence of cobalt there was doubtful, and that gold, silver, mercury, aluminium, cadmium, tin, lead, antimony, arsenic, strontium, lithium, and silicon, were absent. II. ÅNGSTRÖM'S MAP Kirchhoff, however, was not the only one at work at the problem. I have already alluded to Ångstrom. He, too, like Kirchhoff, was constructing a map of the lines in the solar spectrum, with the important difference that whereas Kirchhoff's scale was an arbitrary one, Ångstrom's scale was based upon the lengths of the light Nor was this all, waves. FIG. 82.-Coincidence of some of the bright lines of iron with some of the Fraunhofer lines. CHAP. XIIT. |