Discoveries and Inventions of the Nineteenth Century

bodies, gives a spectrum containing no dark lines. When this light was allowed to fall through a suitable flame, coloured by common salt, dark lines were seen in the spectrum in the position of the sodium lines. The same phenomenon was observed if, instead of the incandescent lime, a platinum wire was used, which, being heated in the flame, was brought to a temperature near its melting point, by passing an electric current through it. The phenomenon in question is easily explained, upon the supposition that the sodium flame absorbs rays of the same degree of refrangibility as those it emits, whilst it is perfectly transparent for all other rays.” (Quoted in Roscoe’s Lectures on “Spectrum Analysis.”) When the light of ignited lime was similarly made to pass through flames containing the incandescent vapours of potassium, barium, strontium, &c., the bright lines which these substances would have produced had the lime-light not been present were found to be in every case changed into dark lines, occupying the very same positions in the spectrum. In such experiments the flames containing the metals in the vapourized state do all the time really give off those rays which are peculiar to each substance; but when a more intense illumination—such as the lime-light, the electric arc, or direct sunlight—passes through them, the rays of the spectrum produced by the intense light overpower those given off by the relatively feebly coloured flames, and hence the portions of the spectrum which are occupied by these, appear black. But as the intense light would give a perfectly continuous spectrum if the incandescent metallic vapour allowed the rays corresponding to its lines to pass through it, the inference is obvious that each vapour absorbs those particular rays which it has itself the power of emitting, but allows all others to pass freely through it. Besides the experimental proofs of this fact which have been already adduced, many others might be named. The flame of a spirit-lamp with a salted wick appears opaque and smoky when we look through it at a large flame of burning hydrogen, also coloured by sodium; for the rays emitted by the latter do not penetrate the former, which, in consequence of its feebler light, appears dark by comparison. Again, if an exhausted tube containing metallic sodium be heated so as to convert the sodium into vapour, the tube viewed by the light of a sodium flame appear to contain a black smoke, and the light from the flame will no more pass through it than through a solid object; yet the tube appears perfectly transparent when viewed by ordinary light, and the light from a lithium or other coloured flame would also pass freely. Kirchhoff was led by purely theoretical reasoning to conclude that all luminous bodies have precisely the same power of absorbing certain rays of light as they have of emitting them at the same temperature, and he thus brought luminous rays under the same general law which had 438previously been established for radiant heat by Prevost, Dessains, Balfour Stewart, and others. Here, then, a law was arrived at, and, abundantly confirmed by direct experiment as regards the more volatile metals, it was ready to supply the most satisfactory explanation of the coincidences which were everywhere discovered to exist between the Fraunhofer lines and those which belong to terrestrial substances. For Kirchhoff also found, when mapping the very numerous lines seen in the spark spectrum of iron, that for each of the 90 bright lines of iron which he then observed, there was a dark line in the solar spectrum exactly corresponding in position. The number of observed bright lines in the iron spectrum has been since extended to 460, and yet each is found to have its exact counterpart in a solar dark line. So many coincidences as these made it certain that these dark lines and the bright lines of iron must have a common cause, for the chances against the supposition that the agreement was merely accidental are enormous. Kirchhoff actually calculated, by the theory of probabilities, the odds against the supposition. He found it represented by 1,000,000,000,000,000,000 to 1. The result arrived at in the case of sodium at once suggested the explanation that these lines were produced by an absorptive effect of the vapour of iron. Now, the existence of such a vapour in our atmosphere could not be admitted, while the temperature of the sun was known to be exceedingly high, far higher, indeed, than any temperature we can produce by electricity, or any other means. Hence, Kirchhoff concluded that his observations proved the presence of the vapour of iron in the sun’s atmosphere with as much certainty as if the iron had been actually submitted to chemical tests. By the same reasoning, Kirchhoff also demonstrated the existence in the solar atmosphere of calcium, chromium, magnesium, nickel, barium, copper, and zinc. To these, other observers have added strontium, cadmium, cobalt, manganese, lead, potassium, aluminium, titanium, uranium, and hydrogen. It has also been demonstrated that a considerable number of the Fraunhofer lines are due to absorption in our atmosphere by its gases and aqueous vapour. This demonstration of the existence of iron and nickel in the sun is an interesting pendent to the known composition of many meteorites which reach us from interplanetary space. Kirchhoff was led to believe that the central part of the sun is formed of an incandescent solid or liquid, giving out rays of all refrangibility, just as white-hot carbon does; that round this there is an immense atmosphere, in which sodium, iron, aluminium, &c., exist in the state of gas, where they have the power of absorbing certain rays; that the solar atmosphere extends far beyond the sun, and forms the corona; and that the dark sunspots, which astronomers have supposed to be cavities, are a kind of cloud, floating in the vaporous atmosphere. During total eclipses of the sun, certain red-coloured prominences have been noticed projecting from the sun’s limb, and visible only when the glare of its disc is entirely intercepted by the moon. Fig. 228 represents a total eclipse, and will give a rude notion of the appearance of the red prominences seen against the fainter light of the corona, which extends to a considerable distance beyond the sun’s disc. Now, two distinguished men of science simultaneously and independently made the discovery of a mode of seeing these red prominences, even when the sun was unobscured. M. Janssen was observing a total eclipse of the sun in India, and the examination by the spectroscope of the light emitted from the red prominences showed him that they were due to immense columns of incandescent hydrogen, for 439he recognised the red line and blue lines which belong to the spectrum of this gas (see No. 12, Plate XVII.). Mr. Norman Lockyer at the same time also succeeded in viewing the solar prominences in London without an eclipse. He found a red line perfectly coinciding in position with Fraunhofer’s C line and that of hydrogen, another nearly coinciding with F, and a third yellow line near D. Soon after this, Dr. Huggins discovered a mode of observing the shape of the red prominences at any time, by using a powerful train of prisms and a wide slit, so that the changes in the forms of the red flames can be followed. Now, since the red prominences give off only a few rays of particular refrangibility, it is not difficult to understand that the light of the sun might be, as it were, so diluted by stretching out the spectrum, by means of a train of many prisms, that almost only the red rays, C, should enter the telescope, and occupy the field with sufficient intensity to overpower all others, and produce an image of the object from which they originated. The nature of this action may be illustrated thus: If we hold vertically a prism, and look through it at a candle-flame, we may perceive a lengthened-out image of the flame, showing the succession of prismatic colours, and formed, as it were, of a red image of the flame close to a yellow one, and so on, but presenting no defined form. If, still viewing this spectrum, we introduce into the flame on a platinum wire a piece of common salt, we shall perceive a well-defined yellow image of the candle start out, because the rays which are emitted by the incandescent sodium, being all of one refrangibility, the prism simply refracts without dispersing them. The dispersion which weakens the light of the continuous spectrum by lengthening it out, does not sensibly detract from the brilliancy of the bright lines, as their breadth is scarcely increased—they are refracted but not dispersed. Hence, when a sufficient number of prisms is employed, the bright lines of the solar chromosphere may be seen in full sunshine, in spite of the greater intensity of the light emanating from the photosphere, which produces the continuous spectrum. The bright C line is, of course, a virtual image of the slit produced by rays of that particular refrangibility; but by using a very high dispersive power, the slit may be opened so wide that the C rays form in the telescope a red image of the prominence from which they issue, since their light will predominate over that of any rays belonging to the continuous spectrum. Fig. 228.—Solar Eclipse, 1869. In the hands of Mr. Norman Lockyer the science of the physical and chemical constitution of the sun has made rapid progress, and new facts are continually being observed, which serve to furnish more and more definite views. Mr. Lockyer considers that, extending to a great distance around the sun is an atmosphere of comparatively cooler hydrogen, or perhaps of some still lighter substance which is unknown to us. It is this which forms what is termed the corona, or circle of light which is seen surrounding the 440sun in a total eclipse. Immersed in this, and extending to a much smaller distance from the nucleus of the sun, is another envelope, termed the chromosphere, consisting of incandescent hydrogen and some glowing vapours of magnesium and calcium. The brightest part of this envelope, which lies nearest the sun, is that which gives off the red rays by which the prominences may be observed without an eclipse. These prominences have been shown to be tremendous outbursts of glowing hydrogen, belched up with sometimes an enormous velocity from below, since they have been observed to spring up 90,000 miles in a few minutes. Beneath the chromosphere, and nearer to the body of the sun, are enormous quantities of the vapours of the different elements—sodium, iron, &c.—to which the dark lines of the solar spectrum are due. This stratum Mr. Lockyer calls the reversing layer, because it reverses (turns to dark) the lines which would otherwise have appeared bright, just as Kirchhoff’s sodium vapour did in the experiment described on page 437. Beneath the reversing layer is the photosphere, from which emanates the light that is absorbed in part by the reversing layer, and which there is good reason to believe is either intensely heated solid or liquid matter. In 1861 Dr. Huggins devoted himself, with an ardour which has since known no remission, to the extension of prismatic analysis to the other heavenly bodies. The difficulties of the investigations were great. There was first the small quantity of light which a star sends to the spectator; this was obviated by the use of a telescope of large aperture, which admitted and brought to a focus many more rays from the star, and therefore the brightness of the image was proportionately increased. Not so the size of the image: the case of the fixed stars for this always remains a mere point. It was, of course, necessary to drive the telescope by clockwork, so that the light of the star might be stationary on the field of the spectroscope. As the spectrum of the image of the star formed