This spectrum soon disappears, and some brilliant lines take its place, particularly a red, a yellow, a green, and a violet one. At this moment the slit is illumined by the famous rose-colored layer, now called the chromosphere, upon which rest the protuberances, formerly so mysterious, seen in total eclipses. We cannot see it in the ordinary way, on account of the atmospheric light; but it comes out in the spectroscope, its light being concentrated in a few bright lines, while that of our atmosphere is spread out in a long spectrum, and consequently much weakened. It has been found that the mean thickness of this gaseous envelope of the sun is more than 5000 kilometres, (3107 miles,) or about four tenths of the earth's diameter, and that its contour is very variable; it is often agitated like the waves of a stormy sea, while in some places it sometimes has a very uniform level. It is now regarded as forming the outer limit or coating of the sun. The only reason which formerly supported the belief in a gaseous atmosphere outside of it, the elective absorption of which gave the dark lines of the solar spectrum, was the phenomenon of the aureola, already mentioned. But the thin layer discovered by F. Secchi will probably account for this; and there are, on the other hand, very strong reasons for rejecting the idea of such a vast exterior envelope. One is the appearance, mentioned above, of the numerous bright lines which Messrs. Frankland and Lockyer attribute to a thin, gaseous coating of the photosphere. The light of these ought seemingly to be absorbed by a thick atmosphere, and the lines reversed to dark ones. Besides, these same observers consider that the change of breadth of the lines shows that the pressure is insignificant at the summit of the chromosphere, and that even at the base it is less than that of our own air. Lastly, no traces have been found of the bright-line spectrum which this envelope ought itself to give in the vicinity of the disc.
To return to the chromosphere: of what gases is it formed? It certainly is principally composed of hydrogen, perhaps in many parts entirely so. When a series of electric sparks is passed through a tube containing pure hydrogen at a very low pressure, the tube is illumined with a light of the same color as that of the protuberances. If this light is examined with the spectroscope, it shows a fine spectrum with a number of brilliant and very fine lines, among which four are conspicuous, broader and brighter than the others. The first is red, the second green, the third and fourth are violet; but this fourth is much the faintest, and even the third is not so bright as the other two. The first is called C, the second F, because their positions exactly correspond to those of the two dark lines thus designated by Fraunhofer in the solar spectrum. The third is very near the dark line G of the sun, which is produced by the vapor of iron. Now, the two first are always found among the lines of the chromosphere; the third also is often visible; and M. Rayet has recently seen the fourth. Hydrogen, then, exists in this layer; for though its other lines are not seen, this may easily be ascribed to their faintness. But there is one line of the chromosphere which is still unexplained, the yellow one between C and F. It would at first seem to be the well-known double line of sodium, called D, which is so frequently met with in spectroscopic experiments; but it is certain that it is somewhat more refrangible than this; and it is not yet known to what substance it is due; it may, perhaps, also belong to hydrogen, under a different pressure or temperature from any under which it has been observed here.
It has been said that the outline of the chromosphere is generally very irregular. Immense columns rise from it, the celebrated protuberances, the height of which is sometimes as much as eleven diameters of the earth, (or 85,000 miles.) It must, therefore, be subject to great agitation, to which the spectroscope bears witness. Mr. Lockyer has observed several times that foreign substances were projected into it; for example, magnesium into one protuberance as far as the sixth part of its height; barium and sodium, and probably other bodies also, were seen, but at smaller elevations. We now understand the breaks in the thin layer detected by F. Secchi; it is probably torn by the upward movement of various substances toward the protuberances. It is, in fact, wanting near the bright spots on the sun, called faculae, and it is now known that these faculae are always covered by protuberances.
Near these bright spots are also usually found the dark spots which have been observed for more than two centuries. Some discoveries have just been made regarding these which are perhaps the most interesting of any yet made in the sun. Every one knows that they are composed of two distinct parts—the nucleus, which appears black in a telescope, but which is really quite bright, since it gives a spectrum of its own; and the penumbra, which surrounds this nucleus. The latter consists of portions of the photosphere, drawn out in the form of threads toward the centre of the nucleus; these threads sometimes unite with each other and form bridges, as it were, over the dark space. All the spectral observations confirm the idea previously entertained, that these spots are really cavities in the photosphere; also they indicate that these cavities are filled with absorbing vapors, whose high degree of pressure is manifest by the broadening of their lines. Mr. Lockyer has seen in them sodium, barium, and magnesium; F. Secchi, calcium, iron, and sodium. Above these spots the hydrogen of the chromosphere appears in quantities sufficient for its elective emission to destroy the black lines produced by its absorption upon other parts of the disc, and even sometimes to change them into bright ones. But there are many other peculiarities in the spectra of the spots; and F. Secchi, in examining them, has hit upon an idea which seems to us very suggestive. It was already known by observations of their frequency and size, that the sun is a slightly variable star, with a period of ten and one third years. We now find a new resemblance between it and the other variable stars. It may be remembered that the Roman astronomer has lately divided the stars into four classes, according to the general character of their spectra. He has just compared the different portions of the sun with these four groups, and finds that if its surface was all like the nuclei of the spots, it would have to be put in the class whose type is Betelgeux, all of which are more or less variable; that the penumbras are like Arcturus, and the general surface of the photosphere like Pollux. He has also concluded, from the presence of many of the dark lines in the nuclei, that the vapor of water exists in these regions of the sun; and the appearance of others not yet named has caused him to suspect the presence of many other compound bodies. Up to this time, hardly any thing but the simple substances has been looked for, as the heat of the sun would seem to be so great as to separate all the composite ones; but this temperature probably is not so high in the spots. It became, therefore, of interest to examine the faint red stars which form his fourth group; and in doing so, F. Secchi has obtained the surprising result that the vapor of a compound substance, namely, benzine, gives, when incandescent, a spectrum having bright lines exactly corresponding to the dark ones of one of the stars of this group. This star, then, appears to have an atmosphere of benzine.
Finally, the spectroscope has demonstrated the movement of at least one star. Mr. Huggins has found that the hydrogen lines in the spectrum of Sirius do not exactly coincide with those of this gas when at rest, but are displaced toward the violet; this observation was confirmed at Rome. It would follow from this that Sirius is rapidly approaching us. This is the only observation of this description which seems yet to be well established. But may it not be possible to make others, and even elsewhere than among the stars? The chromosphere is, as we know, the scene of very rapid movements; and may not these be visible by the displacement of the spectral lines? The following remark of Mr. Lockyer, in one of his communications to the Royal Society, would induce us to hope for this: "In the protuberance of which we are speaking, the line F was strangely displaced. It seemed that some disturbing cause altered the refrangibility of this line of hydrogen under certain conditions and pressures." But is it really to pressure that this displacement is due, when we know that rapid movement produces this effect, which has never been known to follow from pressure? But let us hasten to acknowledge that, in a subsequent communication of the same author, we find a sentence much more to the point, and which only needs to be a little more developed to answer our question. Mr. Lockyer is here speaking of movements in the vapors which fill the cavities of the spots. "The changes of refrangibility," says he, "of the rays in question show that the absorbing matter is rising and falling relatively to the luminous matter, and that these movements can be determined with great precision." Let us hope that this will be verified by observation, and that exact measures will show the fertility of such a promising theoretical principle. [Footnote 199]
[Footnote 199: The rapidity of some of these movements has been said to be about one hundred miles a second.]
The length of this bulletin is beginning to alarm us; but since it should include all the last scientific developments concerning the subject of ethereal vibrations, a word must be added on some curious experiments of Mr. Tyndall. The chemical action of these vibrations had hardly been examined hitherto, except in the nutrition of plants, in the formation of chlorhydric acid, and in the transformation of various substances, principally used in photography. The successor of Faraday has recently studied their effects upon vapors, and has applied the curious results of his investigations to some as yet unexplained facts of meteorology and astronomy. Passing a cylindrical beam of light down a long glass tube full of the vapor which he wished to examine, he found that the vapor soon ceased to be completely transparent. An incipient cloud, as he calls it, soon appeared, so thin that it could only be seen by the light of the beam producing it, but became invisible in the full light of day. Some vapors undoubtedly will not produce it; but the experiment succeeds perfectly with many different ones, especially with nitrite of amyle, bisulphide of carbon, benzine, etc. The following explanation of this phenomenon seems quite probable. The vibrations of the ethereal medium, or at least some of them, are communicated to the atoms of which the composite molecules of the vapor are formed. Owing to isochronism, the movement becomes strong enough to break up the molecule, the atoms of which are formed into new combinations, which are better able to resist the action of light. If the new substance cannot remain under the given pressure and temperature in the gaseous state, it will be precipitated in liquid particles, which are at first extremely small, but gradually increase in size, so as to intercept the light and become visible. If the vapor employed satisfies these conditions, the experiment ought to succeed. The chemical analysis of the products has, we believe, in some cases confirmed this explanation; we will now confirm it by some facts of another kind.
In Mr. Tyndall's experiments, the vapor examined was never unmixed; when it was put into the tube, some other gas was also introduced, usually atmospheric air; but other gases were also employed. With hydrogen, a remarkable effect was produced. On account of its small density, it failed to sustain the liquid particles, and they slowly settled in the bottom of the tube. By a suitable diminution of the pressure of these mixtures of gas and vapor, the chemical action of the rays could be retarded at pleasure. The "incipient cloud" could then be seen to form gradually; and whatever was the character of the vapor used, the cloud had always at first a magnificent blue color. Continuing the experiment, the brilliancy of the cloud increased, but its blue tinge diminished, until it became as white as those usually formed. The natural explanation of this change is found in the gradual growth of the liquid particles.
The cloud was not usually formed all along the course of the rays. After having traversed a certain thickness of vapor, the rays, though seeming as bright as ever, lost their chemical power. This result might easily be predicted by the theory. Only a few of these rays had the proper length of wave to act by isochronism upon the atoms of the vapor. These would be absorbed shortly after entering; and the others, though vastly more numerous and escaping absorption, would produce no chemical effect. It was even probable that, by passing the light at the outset through a small thickness of the liquid, the vapor of which was contained in the tube, all its active rays could be taken out; and experiment confirmed this conclusion. It is to be regretted that the light was not examined with the prism before being employed; the wave-length of the active rays would then have been known. It is no doubt very probable that they are toward the violet extremity, either among the visible rays or beyond. But the colored glasses, which the English physicist interposed, only partially resolve the question. The prism would undoubtedly have shown that the wave-length of the active rays varies with the substance exposed to them.
Some vapors taken alone are almost insensible, while their mixture is immediately affected by the passage of the rays. Such is the case of that of nitrite of butyle with chlorhydric acid. This is very easily explained theoretically. The disturbance communicated to the atoms by the ethereal vibrations, though very decided, may be insufficient to break up the molecules. But if another cause, though itself insufficient alone, comes to its assistance, the atoms may be separated. Such another cause is that which chemists have long known as affinity, the manifestations of which are very numerous; but which has not yet been submitted to a precise analysis. In the case just mentioned, the affinity of the elements of the nitrite of butyle for those of the chlorhydric acid conspires with the vibrations to destroy the molecules of the two substances and form a new one, which is precipitated. The phenomenon is like that observed in the growth of plants. Light alone is not sufficient to decompose the carbonic acid of the air; neither are the leaves when in the dark. But when the sun's rays fall upon them, the carbonic acid is decomposed, its oxygen uniting with the atmosphere and its carbon with the plant. It is now easy to justify what was said in the beginning as to the formation of chlorhydric acid by the action of the rays on a mixture of chlorine and hydrogen. It is only necessary that the molecules of these gases, or, at least, of one of them, should be composed of several atoms. Affinity alone could only break the union of these very slowly; but the light would shake them apart, and enable the affinity to act immediately.