_ "SO REESE LIBRARY OK THK J\LTNIVER.SITY OF CALIFORN Received . . ^-^ /*/?,*', ? 188*? H o> to This we may show graphically by representing these colours by their initials : using open letters to show that it is a case of giving out of light. 10 THE CHEMISTRY OF THE SUN. [CHAP. This experiment will enable us to see the importance of a precaution which we owe to Newton. In his very first experiments he found that when we wish to get the best possible effect out of a prism, we must so arrange it that the particular ray which we wish to observe, whether the yellow, the blue, the green, or any other, leaves that prism at exactly the same angle as the incident compound ray falls on it. This angle is termed the angle of minimum deviation. So much, then, for what we have learnt from Newton touching sunlight. He was far from grappling with the grand problem of solar chemistry ; but he saw at once that it was impossible to imagine the sun to be a cool body, or to assume for it a different origin from our own planet. In his Optics he asks, " Are not the sun and stars great earths vehemently hot ? " When Newton made his classical experiments, lie used, as we have already stated, a beam of light coming through a circular hole in a shutter ; but he was soon able to prove to himself that the circular aperture was not the best thing he could use, because in the spectrum he had a circle of colour representing every ray into which the light could be broken up, and all these circles overlapped and produced a mixed and very impure spectrum. But although he was conscious of the defects of his method, he did not take the best steps to remedy them. We must never forget that in his time the art of making glass had made but a small advance. It was not until the O lapse of 140 years that another step forward was made by Dr. Wollaston, who first employed a very narrow linear slit, an arrangement which gives a very pure spectrum, and is, therefore, adopted in all modern instruments. In consequence of this improvement, Wollaston was enabled to make a discovery of the. highest importance, the first real step, in fact, taken in Solar Chemistry, No sooner had he introduced the fine slit for the examina- tion of the solar spectrum than he observed that the spectrum i.] WOLLASTON'S DISCOVERY. 11 was not absolutely continuous, as Newton had stated it to be, but that it was interrupted by a series of dark spaces. This fundamental observation must here be given in Wol- laston's own words, extracted from his communication to the Royal Society : 1 " The colours into which a beam of white light is separated by refraction appear to me to be neither seven, as they are usually seen in the rainbow, nor reducible by any means (that I can find) to three, as some persons have conceived ; but that, by employing a very narrow pencil of light, four primary divisions of the pris- matic spectrum may be seen with a degree of distinctness that, I believe, has not been described nor observed before. "If a beam of daylight be admitted into a dark room by a crevice ^ of an inch broad, and received by the eye at the distance FIG. 5. Copy of Wollaston's Diagram. of ten or twelve feet, through a prism of flint glass free from veins, held near the eye, the beam is seen to be separated into the four following colours only : Red, yellowish-green, blue, and violet, in the proportions represented in Fig. 5. " The line A that bounds the red side of the spectrum is somewhat confused, which seemed in part owing to want of power in the eye to converge red light. The line B, between red and green, in a certain position of the prism, is perfectly distinct; so also are D and E, the two limits of violet ; but c, the limit of green and blue, is not so clearly marked as the rest ; and there are also, on each side of this limit, other distinct dark spaces, /and g, either of which in an imperfect experiment might be mistaken for the boundary of these colours." 1 Plnl. Trans. 1802, part i. p. 378. 12 THE CHEMISTRY OF THE SUN. [CHAP. Here, then, is an alteration in the solar spectrum with a vengeance. But this is only one part of the story. Wollaston is the true founder of spectrum analysis ; for, entirely unaware of the tremendous importance of the work he was doing, he showed that we may get artificial light sources, the spectra of which are discontinuous, but for a very different reason, as we now know. He writes.: "By candle-light a different set of appearances may be distin- guished. When a very narrow line of the blue light at the lower part of the flame is examined alone, in the same manner, through a prism, the spectrum may be seen divided into five images at a distance from each other. The first is broad red, terminated by a bright line of yellow, the second and third are both green, the fourth and fifth are blue, the last of which appears to correspond with the division of blue and violet in the solar spectrum and the line D of Fig. 5. " When the object viewed is a blue line of electric light, I have found the spectrum to be also separated into several images, but the phenomena are somewhat different from the preceding. It is, however, needless to describe minutely appearances which vary according to the brilliancy of the light, and which I cannot undertake to explain." We shall see the great importance of these observations in the sequel. With Wollaston we quit the introductory stage of our subject. CHAPTER II. FKAUNHOFER, 1814. IN 1814 Fraunhofer extended Wollaston's work in both directions, and he further introduced another improvement by examining the emergent beam from the prisms directly by means of a telescope, instead of allowing it to fall on a screen, and then examining it with the naked eye, as Wol- laston had done. In this way he got a magnified view of the spectrum. In order to insure the utmost purity in some of his experiments the slit was placed at a distance of 92 feet from the prism. As may easily be imagined, by the aid thus afforded, he was enabled to observe a much greater number of lines in the solar spectrum than had been seen by Wollaston.. He con- structed a map, of which Fig. 6 is a much reduced copy, in which he recorded the positions of no less than 576 lines, distinguishing the most prominent ones by letters of the alphabet. From this time these dark lines have been called Fraunhofer lines. He recorded also that the intensity of the light varied greatly in different parts of the spectrum, being greatest in the yellow and falling away rapidly on each side, as shown by the curved line in the figure. Fraunhofer's work, in this direction, the importance of which 14 THE CHEMISTKY OF THE SUN. [CHAP. II.] FRAUNHOFER'S OBSERVATIONS. 15 cannot be over-estimated, will be gathered from the following extract from his communication to the Munich Academy : x " Into a dark room, and through a vertical aperture in the window-shutter, about 15" broad and 36" high, I introduced the rays of the sun upon a prism of flint glass placed upon the theodolite ; this instrument was 24 feet from the window, and the angle of the prism was nearly 60 3 . The prism was placed before the object glass of the telescope, so that the angles of incidence and emergence were equal. In looking at this spectrum for the bright FIG. 7. Fraunhofer's theodolite spectroscope. line which I had found in the spectrum of artificial light, I dis- covered, instead of this line, an infinite number of vertical lines of different thicknesses. These lines are darker than the rest of the spectrum, and some of them appear entirely black. When the prism was turned so that the angle of incidence increased, these lines disappeared, and the same thing happened when the angle was diminished. If the telescope was considerably shortened, these lines reappeared at a greater angle of incidence ; and at a smaller angle 1 Denkschriften de K. Acad. der Wixscnschaften zu Munchen, 1814-15, Band v. pp. -193 -226. Translated in Edinburgh Philosophical Journal, vol. ix. p. 296, 1823, and vol. x. p. 26, 1824. 16 THE CHEMISTRY OF THE SUN, [CHAP. of incidence the eye-glass required to be pulled much further out in order to perceive the lines. If the eye-glass had the position proper for seeing distinctly the lines in the red space, it was necessary to push it in to see the lines in the violet space. If the aperture by which the rays entered was enlarged, the finest lines were not easily seen, and they disappeared entirely when it was about 40". "If it exceeded a minute the largest lines could scarcely be seen. The distances of these lines and their relative proportions suffered no change, either by changing the aperture in the shutter, or varying the distance of the theodolite. The refracting medium of which the prism is made, and the size of its angle did not prevent the lines from being always seen. They only became stronger or weaker, and were consequently more or less easily dis- tinguished in proportion to the size of the spectrum. The propor- tion even of these lines to one another appeared to be the same for all refracting substances ; so that one line is found only in the blue, another only in the red, and hence it is easy to recognise those which we are observing. The spectrum formed by the ordinary and extraordinary pencils of calcareous spar, exhibited the same lines. The strongest lines do not bound the different colours of the spectrum, for the same colour is almost always found on both sides of a line, and the transition from one colour to another is scarcely sensible. "Fig. 6 shows the spectrum with the lines such as they are actually observed. It is, however, impossible to express on this scale all the lines and the modifications of their size. At the point A the red nearly terminates and the violet at i. On either side we cannot define with certainty the limits of these colours, which, however, ap- pear more distinctly in the red than in the violet. If the light of an illuminated cloud falls through the aperture on the prism, the spectrum appears to be bounded on one side between G and H, and on the other at B ; the light of the sun, too, of great intensity, and reflected by a heliostat, lengthens the spectrum almost one-half. In order, however, to observe this great elongation, the light between c and G must not reach the eye, because the impression of that which comes from the extremities of the spectrum is so weak as to be extinguished by that of the middle of the spectrum. At A we observe distinctly a well-defined line. This, however, is not the boundary of the red, which still extends beyond it. At a there is a mass of lines forming together a band darker than the adjacent ii.] HIS NEW MAP. 17 parts. The line at B is very distinct, and of a considerable thickness. From B to c may be reckoned nine very delicate and well-defined lines. The line at c is broad and black like D. Between c and D are found nearly thirty very fine lines, which, however, with the exception of two, cannot be perceived but with a high magnifying power and with prisms of great dispersion ; they are besides well defined. The same is the case with the lines between B and c. The line D consists of two strong lines separated by a bright one. Between D and E we recognise eighty-four lines of different sizes ; that at E consists of several lines, of which the middle one is the strongest. From E to b there are nearly twenty-four lines ; at b there are three very strong ones, two of which are separated by a fine and clear line; they are among the strongest in the spectrum. The space b F contains nearly fifty-two lines, of which F is very strong. Between F and G there are about 185 lines of different sizes ; at G many lines are accumulated, several of which are re- markable for their size. From G to H there are nearly 190 different lines. The bands at H are of a very singular nature; they are both nearly equal, and are formed of several lines, in the middle of which there is one very strong and deep. From H to i they likewise occur in great numbers. Hence it follows that in the space BH there are 574 lines, the strongest of which are shown in the figure. The relative distances of the strongest lines were measured with the theodolite, and placed in the figure from observation. The faintest lines only were inserted from estimation by the eye." Here then we have the first substantial contribution to a map of the newly discovered lines in the spectrum of the sun. Fraunhofer next explained why the lines are not well marked, and why they disappear if the aperture of the slit becomes too large. If the aperture is not such that the light which passes through it cannot be regarded as a single ray, or if the angle of the width of the aperture is greater than that of the width of the line, then the image of the same line will be projected several times parallel to itself, and C 18 THE CHEMISTRY OF THE SUN. [CHAP. will consequently become indistinct, and disappear when the aperture is too great. He was not content with this work. He wanted to know something of the origin of the lines, and he soon came to a conclusion on this point. It occurred to him that they might possibly be attributed to some illusion caused by the narrow aperture through which the light was admitted. We shall see subsequently that the slit has something to do with the forms of these dark spaces, but with their simple existence as spaces it has nothing to do, the mere shape of the lines being quite a trivial matter. To test this he had recourse to a very ingenious method, which is best described in his own words : l " In observing the great quantity of lines in the solar spectrum we might be led to believe that the inflection of light at the narrow aperture in the window-shutter had some connection with them, though the experiments described do not give the least proof of this, and indeed establish the contrary opinion. In order to put this beyond a doubt, and also to make some other observations, I varied the experiments in the following manner : if we make the sun's rays pass through a small round aperture in the window- shutter, nearly 15" in diameter, and cause it to fall on a prism placed before the telescope of the theodolite, it is obvious that the spectrum seen by the telescope can only have a very small width, and consequently will form only a line. In a line, however, almost no breadth it is impossible to see the fine and delicate lines which traverse it ; and, on that account, the fixed lines are not seen in a spectrum of this kind. In order, however, to see all the lines in this spectrum, it is necessary only to widen it by an object-glass, without altering its length. I obtained this effect by placing against the object-glass a glass having one of its faces perfectly plane, and the other ground into the segment of a cylinder of a very great diameter. The axis of the cylinder was exactly parallel to the base of the prism. . . ." 1 E (T* ^7 fr^\ r ^> W U LE; (^\ \( (&) LT.< we get light in the yellow at ' " Next try the circular slit. Instead of a line we get a circle in the same part of the spectrum. What we shall see in passing from the spectrum of the candle to the spectrum of the sodium vapour as seen by a straight or a circular slit is shown in the accompanying woodcut. I I s I I I 3 ' ' :- S -I ,1 ; IS '!:..* FIG. 14. The spectra of continuous and discontinuous light-sources, the latter seen with a line and circular slit. If we treat a salt of lithium in the same manner we shall see with the line slit a bright line in the red, with the circular slit we shall get a circle in the same place. A salt of thallium will, according to the slit we use, give a bright line or circle in the green, and if we examine a very complicated light source we shall arrive at the same result a spectrum characterised by a large number of bright lines or circles, depending upon the shape 32 THE CHEMISTRY OF THE SUN. [CHAP. of the slit, in different parts of the spectrum. We do not get the complete spectrum represented by Wll FE) (rb W /pj\ no) LI LM5 vl^i Li '^J LTA but, in the case of sodium, only Y lithium, only Fo) Lr\l with a substance which gives us a complicated spectrum we see lines, as under, here and there ; the light in each of these cases assuming in the telescope the form of the slit, and occupying different positions along the spectrum according to the light FIG. 15. The spsctrum of a complicated light-source as seen with a circular and a line slit. source. Here then we are in possession of the bright line phenomenon first so called by Fraunhofer. Now why do we see lines ? For this reason The light passing through the lenses forms an image of the slit because in this case the prism deviates the light without dispersing it, and we should see an image of the slit if we used the collimator and telescope alone without the prism. Now apart from the difficulty of constructing fanciful forms of slits there is one very powerful reason that leads to the pre- ference of the line above all other forms. So long as we are using a high dispersion and dealing only with bodies that give out light of very few coloufs, it will not make very much difference in.] A LINE SLIT IS BEST. 33 if we use, say, a circular slit but when we come to substances that give very complex spectra, or when we use only a low dis- persion, it is evident we shall get the result shown in the last figure. The different coloured circles will overlap each other and the whole spectrum will be in a state of confusion. For the sake of comparison the same spectrum is shown as observed with a line slit. When a line slit is used we can make it as narrow as we please, and so reduce the overlapping to a minimum and get a spectrum of the greatest purity. After these statements the meaning of the term " line spectra " should not present any difficulty. We may now then dismiss this part of our subject, in which Wollaston was the pioneer, with the remark that spectra of bright lines are produced by the giving out of light by light- sources which are coloured. In other words, the radiation of coloured light-sources when examined by the spectroscope gives us bright line spectra. .CHAPTEE IV.. BREWSTER, HERSCHEL AND FORBES ; BECQUEREL AND DRAPER. 18221843. THE work of Wollaston and Fraunhofer then, and the im- provements in the methods of observation which they effected, taught us that the quality of the sun's light was very special and that the solar spectrum was a thing per se. Fraunhofer, as we have seen, was firmly of opinion that the special quality was imparted at the siin itself. "We learnt from both of them, moreover, that while sunlight, on the one hand, differed from ordinary white light in having a spectrum of dark lines ; coloured light, on the other hand, differed from the same white light, in having a spectrum in which bright lines are seen. It is in Sir David Brewster's work that we find the next important development. Like his predecessors, he too made a map of the solar spec- trum, and, like them too, he was the first to open out a branch of research on which our present knowledge of solar chemistry is based. In 1822 he laid before the Koyal Society of Edinburgh 1 the results of experiments he had performed, with the view of 1 Edin. Phil. Trans, vol. ix. 1823, p. 433 CHAP. iv.J ACTION OF COLOURED BODIES. 35 constructing a monochromatic lamp for microscopical purposes to prevent the coloured fringes produced by the imperfectly cor- rected lenses then used in the construction of high power object- glasses. He had observed that burning paper, linen, cotton, &c., gave flames in which the yellow rays predominated, and this effect was most marked in the flame of alcohol diluted with water. On examining this flame with the aid of a prism, he found that the great bulk of the light was located in the yellow, and though there were faint traces of green and blue, there was not a trace of red. So far, there is nothing particularly new. But he next proceeded to open up a new field of observation altogether. In the course of his experiments he examined the action of various coloured bodies on the different parts of the spectrum. He found not only that different coloured bodies cut out different rays from the spectrum, but that this action of the same body varied with its thickness and temperature. Shortly after this paper appeared, Mr. J. W. F. Herschel, 1 in a letter to Sir David Brewster, described a similar series of experiments which he had made. In one of these he projected a spectrum obtained by a circular opening and a prism on a sheet of white paper, and examined it through a glass which cut out all but the red rays. He found that the light was perfectly hornogeneous, and produced a well-defined circular image of the aperture. Here then was an instance of light coloured red, not because it was originally tuned to vibrate at that one rate only, for the light was originally complete, and gave the continuous spectrum, w a g Y ^ but because in its passage through the glass the violet, indigo, 1 Edin. Phil. Trans, vol. ix. 1823, p. 445 et seq. D 2 36 THE CHEMISTRY OF THE SUN. [CHAT-. blue, green, yellow, and orange were cut out and stopped, thus, V I B G Y O and by the defect of these rays the light became red. To record his observations Herschel adopted a device which has since come into very general use. He represented the prismatic spectrum by a horizontal line, indicating the various colours by their initial letters, and then at all points where light was transmitted he erected perpendiculars of lengths p n y G :B "V ^B y S B T FIG. 16. Copies of Herschel's Diagrams. proportionate to the intensity of the light observed ; connecting the extremities of all these lines he obtained a curve which he called the type of that particular medium. The accompanying figures which are copies of some of those given by Herschel, show the sort of curves obtained. No. 1 represents the effect of ruby-red glass or port wine on the spectrum. Nearly all the light transmitted is in the red. No. 2 shows the effect of a particular green glass with a tend- ency to redness. 3 shows the action of a solution of ammonio- carbonate of copper in various thicknesses, the absorption IV.] SUNSET COLOURS. 73 commencing in the red, and gradually extending over everything but the violet. 4, 5, and 6 represent the effect produced by various thicknesses of a blue glass with a tinge of purple, 4 showing the action of the least thickness, and 6 that of the greatest. It will be seen from these figures that Herschel observed that this stoppage of light affected not only large reaches of the spectrum, but as shown in Fig. 16 (4), it was sometimes so closely localised as to produce sharp lines. He was the first to point out this fact clearly. These phenomena are produced by the absorption of light by the various bodies interposed in the path of the beam on its way to the eye. We shall have to dwell on this effect at considerable length in the sequel. Such experiments as these explain the phenomena of sunset colours, which deserve notice here because, as it will be seen later on, the work done by our atmosphere on the sunlight requires to be completely studied before we can get any certain knowledge of the light actually emitted by the sun. One form of the aqueous vapour in our atmosphere exerts a powerful obliterating action on the solar light, commencing at the blue end, and gradually encroaching on other parts of the spectrum, thus : {TV No absorption. v a [ v V I [IB Y V I B V I B G VI BGY VI BGYO Showing the ; effect of increasing absorption. 38 THE CHEMISTRY OF THE SUN. [CHAP. so that when the sun is near the horizon, and shines through the greatest thickness of our atmosphere, his light appears red because all the other colours are to a great extent absorbed. The next work of Brewster's carries us much farther in the same direction. He announced to the Eoyal Society of Edin- burgh, 1 in 1833, that he had examined the lines of the solar spectrum with various optical appliances, and had delineated them on a scale four times greater than that employed in the beautiful map of Fraunhofer. Some portions also, which were more particularly studied, had been drawn on a scale twelve times greater. Fraurihofer's spectrum was fifteen and a half FIG. 17. Diagram showing the increasing thickness of air through which the light of the sun has to pass as it descends towards the horizon. When the sun is in the zenith, A, its light will have to pass through a thickness of atmo- sphere represented by a o. When near the horizon, this distance will be increased to y o, and when actually on the horizon to x o. inches long ; a map on the largest scale employed by Brewster would be seventeen feet long. But this was not all. In this same paper Sir Pavid Brewster described " a remarkable series of dark lines and bands, which made their ap- pearance in the spectrum when nitrous acid gas was interposed between the prism and the source of light, whether that were the sun or a burning lamp." In these experiments we get the first adequate glimpse of those wonderful phenomena produced when the light on its way 1 Edin, Phil. Trans, vol. xii. 1834, "p. 519 et scq. iv.] GENERAL AND SELECTIVE ABSORPTION. 39 to the eye is made to pass among the molecules of more or less transparent substances. The phenomena were so definite that the experiments did not go on very long before the conviction was forced on Brewster that a new method of chemical analysis was possible by these means, and that such a spectrum analysis was destined to become a chemical agent of great value. In his paper he says, " the first and principal object of my inquiries " was " the discovery of a general principle of chemical analysis, in which simple or compound bodies might be characterised by their action on definite parts of the spectrum ; " and he showed how he had already found it possible, by means of the variety and constancy of the effects thus observed, to distinguish the coloured juices of plants, solutions of salts, minerals, &c., " by merely looking through them at a well-formed spectrum." These investigations of Brewster and Herschel can be easily and cheaply imitated by a few very simple additions to th improvised spectroscope described in Chapter III. To study the absorption of coloured glasses, or other solids, it is only necessary to interpose them between the candle and the slit, and to observe the spectrum as before. The observation of a great many substances, in this manner, has intensified the interesting fact first revealed by Brewster, that while some bodies absorb equally all the prismatic rays, and so give rise to a general darkening of the spectrum, other bodies act locally, their absorption being confined to special regions. So that we have general and selective absorption just as we have general radiation, giving a continuous spectrum, and selective radiation, giving lines here and there. To observe the effect of general absorption it is only neces- sary to employ a piece of smoked, or still better, neutral tinted glass, the only effect of which will be to deaden the spectrum along its whole length. A piece of the ruby-red glass commonly used for photo- graphic purposes will cut off nearly all but the red light, 40 THE CHEMISTRY OF THE SUN. [CHAP. affording an instance of selective absorption, while certain kinds of blue glass will cut off all but the blue. By intro- ducing both the pieces into the beam the spectrum is entirely obliterated. When liquids have to be examined it is necessary to have some sort of transparent cell to hold them. It will not be necessary to buy such an apparatus ; two squares of glass witli a piece of india-rubber tubing between them bent thus U, the glasses being kept in contact with the tubing by two india- Thick. Thin. FIG. 18. Absorption of sunlight by various thicknesses of a solution of the salts of chromium (Gladstone). FIG. 19. Absorption of sunlight by various thicknesses of a solution of potassic permanganate (Gladstone). rubber bands, form a cell which is wonderfully tight, and will serve our purpose excellently. This, with the inclosed liquid, must be placed in front of the slit like the coloured glasses. A little potassic permanganate, or Condy's fluid in water, thus used as a light-filter will produce a deep band in the yellow part of the spectrum and the adjacent regions of the orange and green. Solutions of blood or magenta will give also very definite indi- cations of absorption. In the former case two broad bands of absorption in the green and yellow, and in the latter one in the green, will be seen. IV.] GASEOUS ABSORPTION. 41 To observe the effect of increasing thickness of the medium wedge-shaped cells may be employed, and it will then be found that as the thickness increases the absorption lines broaden out and new regions of absorption appear. 1 But we are not limited to solids and liquids. Gases and vapours can be made to yield absorption spectra. Put a few grains of iodine in a dry flask and place it in front of the slit ; then warm the flask gently with a FIG. 20. Method of studying the absorption of iodine vapour. spirit-lamp. The iodine will soon volatilise and fill the flask with a violet vapour, giving rise to a beautifully regular fluted spectrum as shown in Fig. 21. When the gas or vapour is not distinctly coloured it is neces- sary to pass the light through a great thickness of it. The manner in which this is effected is shown in Fig. 22. The 1 If we have one of those handy little pocket spectroscopes, which now, I am glad to think, are becoming common, the absorption of the light of a candle by the blood in the lobe of a friend's ear, or in the interval between two closed fingers, can be well seen by placing it between the slit and the light. 42 THE CHEMISTRY OF THE SUN. [CHAP. gas is contained in a large glass tube closed at each end by a flat glass plate. At one end of the tube is placed a lamp, and at the other the spectroscope. LL. FIG. 21. Absorption spectrum of iodine (Thalen). We are now, then, in possession of the phenomena first observed by Brewster arid Herschel, and of an easy means of producing them for our own satisfaction. We must now pass on to their applications. FIG. 22. Method of observing the absorption of a great thickness of vapour. Brewster at once applied this new knowledge to the sun. He first of all endorsed Fraunbofer's view that the dark iv.] APPLICATION TO SUN. 43 lines were truly solar in their origin ; and more than this, he could now explain how the lines came about, for if a tube of nitrous oxide gas gave us lines identical in character with the solar lines as a result of its absorptive properties, what more natural than to suppose that the lines were produced by absorption in the atmosphere of the sun? He went further. His observations led him to the conclusion that many of the lines produced by the absorption of the gas in question were identical in their position along the spectrum with several of the Fraunhofer lines themselves ; and he felt himself justified in announcing, on the strength of these coincidences, the discovery that there was nitrous acid gas in the atmosphere of the sun. This is most interesting, for here we have the first chemical touch in solar inquiry. Brewster went on to make another important observation : " When the sun descends towards the horizon and shines through a rapidly increasing depth of air, certain lines which before were little, if at all, visible, become black and well defined, and dark bands appear even in what were formerly the most luminous parts of the spectrum." He states that this effect was observed both at sunrise and sunset, and that it could not be due to any, general obscuration caused by the increasing darkness at sunset, because it was not necessary that the sun should be very low on the horizon in order to produce these effects, but that they were visible while the lines H and K in the violet were still distinct, and therefore before any considerable darkening occurred, which would have obscured this end of the spectrum first. He therefore announced the discovery that the variable bands were "produced by the absorptive effect of the earth's atmosphere," 1 and from 1 Loc. tit. p. 528. 44 THE CHEMISTRY OF THE SUN. [CHAP. the want of variation among the other lines he saw an indication " That the apparent body of the sun is not a flame in the ordinary sense of the word, but a solid body or coating raised by intense heat to a state of brilliant incandescence." 1 It will be seen, then, that the study of the sun was now (1833) in full swing. We had at length, after waiting some centuries, a method of observing a spectrum ; we had, further, the fact that there were dark lines in the solar spectrum ; that coloured flames gave us bright lines ; that certain substances stopped some of the light which passed through them, thus producing dark lines. Hence that the solar lines might be produced in the same way. We next come to Forbes, whose manner of work was interest- ing, though it led him to a conclusion which we now know to be erroneous. We have seen that both Fraunhofer and Brewster were firmly of opinion that the dark lines were produced by some action at the sun. Brewster was more definite, and said some absorption at the sun. The next point investigated was this. It seemed obvious that if Brewster's view were true there should be a difference between the spectrum of the sun's centre and the sun's limb or edge, because the light passing to the eye from the latter would have to traverse a greater thickness of the sun's atmosphere. It did not strike anybody at first to throw an image of the sun on the slit of a spectroscope and see if there were a difference ; the inquiry was, however, made in another way. In 1836, Professor Forbes began it by taking advantage of an annular eclipse of the sun at Edinburgh, during which, of course, only the light from the edge reached us, the centre being cut off by the dark moon. 1 Loc. cit. p. 529. iv.] FORBES'S OBSERVATIONS. 45 In describing the result of his observation in the Philosophical Transactions, 1 mindful of Brewster's work, he first shows that the deficient rays cannot be due to any absorptive action of the glass prisms employed, since whatever material is employed and whatever the length of the path of the light through the prisms the lines remain constant. He then refers to Brewster's discovery of the lines due to the absorption of the earth's atmosphere, but points out that such lines are neither numerous nor important, as compared with the great mass of solar lines, and that if the lines were all or chiefly due either to the absorptive action of the earth's atmosphere or of any matter which may exist in the planetary spaces, we should have the same lines exhibited in the spectra of the fixed stars. Reference is also made to Brewster's idea, suggested by his observations of the absorption spectrum of nitrous acid, that the solar light is originally complete and that the deficient rays have been stopped absorbed in passing through the sun's own atmosphere, which might be supposed to contain nitrous acid or some similar gas as a constituent. On this point the following note is appended to Forbes's paper : " I do not know with whom the idea of the absorptive action of the sun's atmosphere originated. The editors of the London and Edinburgh Philosophical Magazine (December, 1836) have, however, referred me to the mention of it in Sir John Herschel's writings, particularly his Elementary Treatise on Astronomy, from which I extract the following remarkable passage : ' The prismatic analysis of the solar beam exhibits in the spectrum a series of fixed lines totally unlike those of any known terrestrial flame. This may here- after lead us to a clearer insight into its origin. But before we can draw any conclusions from such an indication we must recollect that previous to reaching us it has undergone the whole absorptive action of our atmosphere, as well as of the sun's. Of the latter we know nothing and may conjecture everything. . . . Tt deserves 1 Phil. Trans. 1836. p. 450 et seq. 46 THE CHEMISTRY OF THE SUN. [CHAP. inquiry whether some or all of the fixed lines observed by Wollaston and Fraunhofer, may not have their origin in our own atmosphere. Experiments made on lofty mountains or the cars of balloons on the one hand, and on the other with reflected beams which have been made to traverse several miles of additional air near the surface, would decide this point. The absorptive effect of the sun's atmosphere, and possibly also of the medium surrounding it (whatever it be), which resists the motion of comets, cannot be thus eliminated.' Herschel's Astronomy, p. 212, note. See also his Essay on Light, Encyclopedia Metropolitana, art. 505. The object of the experiment now described is to show a method of elimination which applies, at least, to the sun's atmosphere." To test this view, then, Prof. Forbes determined to take advantage of the opportunity afforded him by the eclipse to examine the spectrum of the edge of the sun without any interference from the central rays, for, he argued, if the absorp- tion is due to the solar atmosphere, it ought to be more marked at the edge of the sun, from which the light has to pass through a thicker stratum of atmosphere than at the centre, from which the rays proceed vertically and through a minimum of atmosphere. That there is a general darkening of the sun's limb is a well-known fact that can be observed at any time by examining the sun through a dark glass or a fog. Prof. Forbes was probably aware of this, and expected to find the darkening accompanied by an increased selective absorption. The result of the observation was that as the eclipse progressed, and the proportion of lateral to central light consequently increased, no change whatever was observed in the number, position, or thickness of the lines, and from this observation Prof. Forbes concluded that " This result proves decisively that the sun's atmosphere has nothing to do with the production of this singular phenomenon." His conclusion, then, was at variance witli that held by his predecessors, and we now know that Prof. Forbes's conclusion SOLAR PHOTOGRAPHY. 47 was wrong. We have, however, not yet finished with the paper. At the close of it he refers to a matter which is of considerable interest, show- ing, as it does, how near one may be to a most important discovery and yet miss it. I give it in the Professor's own words : " Had the weather proved unfavourable for view- ing the eclipse, I intended to have tried the experi- ment by forming an image of the sun by using lens of long focus, stopping alternately by means of a screen the exterior and central moiety of his rays; and restoring the remainder to parallelism by means of a second lens, then suffering these to fall upon a slit as before. The result of my ex- periment during the eclipse seemed, however, so decisive as to no marked change being produced at the sun's edges that I have thought it unnecessary ^. to repeat it." It will be seen that Forbes all but introduced the method of localisation of solar phenomena, which is brought about by throwing an image of the sun on the slit-plate of the spectroscope, whereby we are enabled to examine the spectrum of any particular part of disc or limb at pleasure. Yet, although Forbes came so near applying this principle, which has turned out of so high im- portance, we had to wait until 1866 before it was actually carried out. The year 1842 marked another very great advance in our knowledge of the solar spectrum. While experimenting with photography, l Bec- querel obtained by its means a representation of the whole spectrum with nearly all the lines 1 Bibliotheque Vniverselle de Geneve, t. 39-40, p. 341 (1842). .8 09 'E. 8 J 1 o a 48 THE CHEMISTKY OF THE SUN. [CHAP. iv. mapped by Fraunhofer from the extreme red to the violet, and besides these, certain new lines right beyond the visible spectrum, in the ultra violet. In 1843 Draper 1 accomplished a somewhat similar result in what he called a " tithonographic representation of the solar spectrum." 2 In this he showed certain lines in the extreme visible blue and extreme red, and beyond the visible red he found other lines which even Becquerel had not seen ; but his photograph gave no record of any lines in the yellow, orange, or green. 1 Phil. Mag. vol. xxii. p. 360 (1843). For his earliest work, see Journal Franklin Institute, 1837. 2 Here we had a foretaste of what photography has since done for us in solar studies. CHAPTER V. THE FRAUNHOFER LINES EXPLAINED. WHILE the strange dark lines in the solar spectrum were attracting attention, various attempts were made to account for their existence, and many new facts throwing light on the matter were discovered. We have seen, for instance, that Brewster and Herschel in their work came across substances which seemed to give, so to speak, an artificial solar spectrum, that is, a spectrum crossed by dark lines, I have been favoured by Dr. Gladstone with the following extract from Brewster's note-book, dated St. Andrew's, Oct. 28th, 1841 : " I have this evening discovered the remarkable fact that in the combustion of nitre upon charcoal there are definite bright rays corresponding to the double lines of A and B, and the group of lines a in the space AB. The coincidence of two yellow rays with the two deficient ones at D, with the existence of definite bright rays in the nitre flame, not only at D, but also at A, , and B, is so extraordinary that it indicates some regular connection between the two classes of phenomena." Brewster had already indicated how close the connection might be between absorption phenomena going on here and at the sun. Now he indicates a possible connection between radiation phenomena here and absorption at the sun. E 50 THE CHEMISTRY OF THE SUN. [r:HAP. The bright double yellow line observed by Fraunhofer and Brewster is the bright line of sodium vapour referred to in the previous chapter. In 1849 this work was continued by Foucault, 1 who used a new method of work by getting a new light-source the voltaic arc produced by the passage of an electric current between charcoal poles charged with salts of different substances. With this new engine he also investigated the double yellow line. " The spectrum is marked, as is known, in its whole extent by a multitude of irregularly grouped luminous lines ; but among these may be remarked a double line situated at the boundary of the yellow and orange. As this double line recalled, by its form and situation, the line D of the solar spectrum, / wished to try if it corresponded to it, and in default of instruments for measuring the angles I had recourse to a particular process. " I caused an image of the sun, formed by a converging lens, to fall on the arc itself, which allowed me to observe at the same time the electric and the solar spectrum superposed ; I convinced myself in this way that the double bright line of the arc coincides exactly with the double dark line of the solar spectrum. " This process of investigation furnished me matter for some unexpected observation. It proved to me, in the first instance, the extreme transparency of the arc, which occasions only a faint shadow in the solar light. It showed me that this arc, placed in the path of a beam of so 7 ar light, absorbs the rays D, so that the above- mentioned line D of the solar light is considerably strengthened when the two spectra are exactly superposed. " When, on the contrary, they jut out one beyond the other, the line D appears darker than usual in the solar light, and stands out bright in the electric spectrum, which allows one easily to judge of their perfect coincidence. Thus the arc presents us with a medium which emits the rays D on its own account, and which at the same time absorbs them when they come from another quarter." Thus absorption is produced when we destroy the simplicity of the usual medium through which the light passes. Instead of 1 L'lnstitiit, Feb. 7, 1849. Translated by Professor Stokes in Phil. Mag. vol. xix. p. 194. v..] SODIUM IN THE SUN. 51. ether only, it has to traverse ether plus certain kinds of matter contained in the arc. This association leads to certain vibrations of the ether being used up by the matter, and so the residue only reaches the eye. Foucault then continues : " To make the experiment in a manner still more decisive, I pro- jected on the arc the reflected image of one of the charcoal points, which, like all solid bodies in ignition, gives no lines ; and under these circumstances the line D appeared to me as in the solar spectrum." In 1849, then, we get the statement made that a vapour could absorb the same rays that it radiated, and that the light radiated by sodium vapour occupied the same position in the spectrum as some of the light which was absorbed at the sun. But why ? Was there, then, sodium vapour in the sun ? This conclusion could not be accepted until a valid explanation could be given. If the experiment had been made with cool sodium vapour which might, as we now know, easily have been done the thing might have been at once conceded, but the fact that the same substance could produce a bright line under one condition and a dark one under another was difficult to imagine. An explanation, however, was not long forthcoming. One was first given (though not published) by Prof. Stokes, about the year 1852. The observational and experimental foundation on which Stokes based his explanation has thus been stated by Sir William Thomson : l " 1. The discovery by Fratmhofer of a coincidence between his double dark line, D, of the solar spectrum and a double bright line which he observed in the spectra of ordinary artificial flames. "2. A very rigorous experimental test of this coincidence by Prof. W. H. Miller, which showed it to be accurate to an astonishing degree of minuteness. ] Presidential Address, British Association Meeting, 1871. E 2 52 THE CHEMISTRY OF THE SUN. [CHAP. " 3. The fact that the yellow light given out when salt is thrown into burning spirits consists almost solely of the two nearly dentical qualities which constitute that double bright line. " 4. Observations made by Stokes himself which showed the bright line D to be absent from a candle flame when the wick was snuffed clean, so as not to project into the luminous envelope, and from an alcohol flame when the spirit was burned in a watch-glass ; and, " 5. Foucault's admirable discovery, already referred to, that the voltaic arc between the charcoal points is a medium which emits the rays D on its own account, and at the same time absorbs them when they come from another quarter." The conclusions at which Stokes arrived were : " 1. That the double line D, whether bright or dark, is due to the vapour of sodium. "2. That the ultimate atom of sodium is susceptible of regular elastic vibrations, like those of a tuning-fork or of stringed musical instruments ; that, like an instrument with two strings tuned to ap- proximate unison, or an approximately circular elastic disc, it has two fundamental notes or vibrations of approximately equal pitch ; and that the periods of these vibrations are precisely the periods of the two slightly different yellow lights constituting the double bright line D. "3. That when vapour of sodium is at a high enough temperature to become itself a source of light, each atom executes these two fundamental vibrations simultaneously, and that therefore the light proceeding from it is of the two qualities constituting the double bright line D. " 4. That when vapour of sodium is present in space across which light from another source is propagated, its atoms, according to a well-known principle of dynamics, are set to vibrate in either or both of these fundamental modes, if some of the incident light is of one or other of their periods, or some of one and some of the other : so that the energy of the waves of these particular qualities of light is converted into thermal vibrations of the medium and dispersed in all directions, while light of all other qualities, even though very nearly agreeing with them, is transmitted with comparatively no loss. v.] ANGSTROM'S INVESTIGATIONS. 53 "5. That Fraunhofer 's double dark line, D, of solar and stellar spectra, is due to the presence of vapour of sodium in atmospheres surrounding the sun and those stars in whose spectra it has been observed. "6. That other vapours than sodium are to be found in the at- mosphere of sun and stars by searching for substances producing in the spectra of artificial flames bright lines coinciding with other dark lines of the solar and stellar spectra than the Fraunhofer line D." Although Professor Stokes unfortunately did not publish his theory I say unfortunately because valuable time has been lost the world was not long in ignorance of a matter of such general interest, for in 1853 the idea was published by the celebrated Angstrom. 1 In his memoir, illustrating the absorption of light, he made use of a principle already propounded by v Euler (in his Theoria lucis et caloris), that the particles of a body, in consequence of resonance, absorb principally those ethereal undiilatory motions which have previously been impressed upon them. He also endeavoured to show that a body in a state of glowing heat emits just the same kinds of light and heat which it absorbs under the same circumstances. He further, following in this respect the steps of Foucault, undertook a series of researches on the electric arc just such researches as those Stokes had only suggested and found that in many cases the Fraunhofer lines were an inversion of bright lines which he observed in the spectra of various metals. 2 Of these we shall have much to say in the sequel. Early in 1858 Balfour Stewart independently discovered the law which binds together radiation and absorption, establishing it experimentally and also theoretically as an extension of 1 " Optiska Undersokningar," Trans. Royal Academy of Stockholm, 1853. Translated in Phil. Mag., Fourth Series, vol. ix. p. 327. 2 See Phil. Mag., Fourth Series, vol. xxiv. pp. 2, 3: Monatsbericht, 1859, p. 662. 54 THE CHEMISTRY OF THE SUN. \CHAF. Prevost's law of exchanges, in the case of the heat-rays, and generalising his conclusion for all rays. 1 In October of the year 1859, Kirchhoff established experi- mentally the same law for light-rays. His first announcement, dated Heidelberg, 20th October, 1859, read before the Berlin Academy on the 27th. 2 must here be given inextenso: " On the occasion of an examination of the spectra of coloured flames not yet published, conducted by Bunsen and myself in com- mon, by which it has become possible for us to recognise the quali- tative composition of complicated mixtures from the appearance of the spectrum of their blowpipe flame, I made some observations which disclose an unexpected explanation of the origin of Fraun- hofer's lines, and authorise conclusions therefrom respecting the material constitution of the atmosphere of the sun, and perhaps also of the brighter fixed stars. " Fraunhofer had remarked that in the spectrum of the flame of a candle there appear two bright lines which coincide with the two dark lines D of the solar spectrum. The same bright lines are obtained of greater intensity from a, flame into which some common salt is put. I formed a solar spectrum by projection, and allowed the solar rays concerned, before they fell on the slit, to pass through a powerful salt flame. If the sunlight were sufficiently reduced, there appeared in place of the two dark lines D, two bright lines ; if, on the other hand, its intensity surpassed a certain limit, the two dark lines D showed themselves in much greater distinctness than without the employment of the salt flame. " The spectrum of the Drummond light contains, as a general rule, the two bright lines of sodium if the luminous spot of the cylinder of lime has not long been exposed to the white heat ; if the cylinder remains unmoved these lines become weaker, and finally vanish altogether. If they have vanished, or only faintly appear, an alcohol flame into which salt has been put, and which is placed between the cylinder of lime and the slit, causes two dark lines of remarkable sharpness and fineness, which in that respect agree with the lines D of the solar spectrum, to show themselves in their stead. 1 Edinburgh Transactions, 1858-9. 2 See translation, by Professor Stokes, in the Phil. Mag , Fourth Series, vol. xix. p. 195. KIRCHHOFFS RESEARCHES. 55 Thus the lines D of the solar spectrum are artificially evoked in a spectrum in which naturally they are not present. . . . " 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. / con- dude 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 t/ie same place. We may assume that the bright lines agreeing FIG. 24. Diagram showing the solar spectrum and th& bright double line D of sodium. 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 " The point marked with italics is the most important part of 56 THE CHEMISTRY OF THE SUN. [CHAP. KirchhofFs work. He insisted that the absorber must be cooler than the radiator. Immediately after the publication of this note of KirchhofFs, Stewart 1 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 heated in the fire. So long as the ruby glass is cooler than 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. In a paper dated Heidelberg, January, 1860, and published in Poggendorffs Annalen? Kirchhoff proves from theoretical con- siderations that the spectrum of an incandescent gas must become reversed (that is, the bright lines become changed into dark ones) when a source of light of sufficient intensity, giving a continuous spectrum, is placed behind the luminous gas. He also describes an extended series of observations similar to those recorded in his first paper, and in a note states that in conjunction with Professor Bunsen he had succeeded in reversing the brighter lines of potassium, calcium, strontium, and barium, by exploding, before the slit, mixtures of their chlorates with milk sugar during the passage of the sun's rays. In discussing the bearing of his observations made up to that time on the constitution of the sun he says : 1 " On the Theory of Exchanges and its Eecent Extension." B.A. Reports, 1861. 2 Translated by Mr. F. Guthrie, in Phil. Mag. vol. xx. July, I860, p. 1. v.J HIS LATER " Imagine a body of very high temperature, in whose spectrum the double line D does not appear, surrounded by a gaseous atmosphere of somewhat lower temperature. If sodium be present in the latter, the spectrum of the whole system so constituted will contain the double line D. From the occurrence of these lines the presence of sodium in the atmosphere may therefore be concluded. Now, the sun is undoubtedly a body of this description ; and therefore, from the occurrence of the lines D in the solar spectrum, the presence of sodium in the sun's atmosphere may be concluded." "What has been stated concerning sodium is equally true of every other substance which, when placed in a flame of any sort, produces bright lines in its spectrum. If these lines coincide with the dark lines of the solar spectrum, the presence in the sun's atmosphere of the substances which produce them must be concluded, provided always that the lines in question cannot have their origin in the atmosphere of the earth. In this way means are afforded of determining the chemical constitution of the sun's atmosphere ; and the same method even promises some information concerning the constitution of the brighter fixed stars." In a subsequent paper * describing the result of the combined work of Kirchhoff and Bunsen, it is stated that, to obtain further confirmation of the observations recorded in Kirehhoffs first paper, the authors produced a bright continuous spectrum by the intense ignition of a platinum wire, and interposed between it and the slit a flame of weak alcohol containing common salt. The dark D line was then seen most distinctly. They also found that the dark D line could be produced by the interposition, between the incandescent wire and the slit, of sodium amalgam heated to boiling, and they pointed out the special importance of this in showing that sodium vapour at a temperature much below that at which it becomes luminous, exerts its absorptive power at exactly the same point of the spectrum as it does at the highest temperatures which we can produce, or at the temperatures existing in the solar atmosphere. 1 Phil, Mag., Fourth Series, vol. xx. p. 108, August, 1860. 58 THE CHEMISTRY OF THE SUN. [CHAP. It has been necessary in dealing with this, so to speak, crucial epoch in the history of our subject to give as far as possible the ipsissima verla of the workers whose genius we have to thank for a series of researches which has not only enabled us to study the chemistry of the sun, but has given us as its first tremendous outcome the conclusion that the sun and earth have the same chemical constituents. Where some, at all events, might have anticipated a new world of matter, we find likeness to the old. With regard to this conclusion it is not too early to make the following remarks on the views concerning the chemical elements held when the spectroscope was introduced. That instrument placed us in presence of phenomena pro- duced by higher temperatures than had ever been employed before. The elements either did or did not bear that tempera- ture without dissociation, and the spectra observed were, therefore, either the spectra of the elements themselves, or of their constituents. Similarly the sun must either give us the spectra of the elements contained in it or the spectra of their constituents. The similarity, therefore, of solar and terrestrial spectra, even if established, cannot be regarded as a final argument in favour of the elementary nature of the bodies which Lavoisier had considered elemental. This point, in fact, was never, so far as I know, raised. On this subject I am permitted to print a memorandum which has been given to me by my friend, Dr. Hodgkinson : The great point in Lavoisier's teaching, affecting the chemistry of his and our own time, was the recognition of the compound nature of some bodies, and the so-called simple nature of others ; and that these compounds can be reduced to the simple state, or elemental condition, of matter. The history of chemistry is one of a development of ideas and theories, more so than in any other science ; a given theory or idea v.] EARLY CHEMICAL VIEWS. 59 opens up a road much too wide to be completely filled by it in its original form, and a more comprehensive one must take its place. The great chemist was careful whilst tabulating the irresolv- able substances in his time into elements, to state that it is probable we do not know the elements themselves, but rather should understand that by analysis we have arrived at a limit at terminal constituents and that for our purpose all such bodies may be looked upon as simple. But this idea of " elements " as the basis of matter took very firm root, and while it has contributed immensely to the growth of chemistry it cannot, latterly at least, be considered as all-sufficient. At the time, however, of Kirchhoff's earlier work the notion of the elements being absolutely elemental appears to have been held very firmly, and no doubt seems to have been felt that the numerous lines forming the spectrum of any one so-called element were other than peculiar to that element alone. In other words, no hesitation was felt at ascribing a most complex spectral appearance to a simple element, regarded as such from its chemical irresolvability under ordinary circumstances. The idea that the " elements " were final substances or forms of matter was first shaken by the discovery and isolation of groups of substances whose chemical behaviour was exactly analogous to that of other substances regarded as elemental. The full meaning and importance of this idea of compound radicals or groups has perhaps only been fully recognised during the past decade, it certainly was not at the time of Kirchhoff's first investigations on spectra. On the con- trary, the " finality of elementary matter" notion was much strength- ened by the coincidence of solar with terrestrial spectral results. With our present knowledge of the behaviour, chemical and physical, of the groups we call radicals, especially those classed as organic ; and our extended notions of allotropism, and isomeric and polymeric forms of bodies ; it is very unlikely, were spectrum analysis to be discovered now, that the notion of an elemental form of matter would be ascribed to substances giving complex spectra, but rather having regard to the analogies above referred to, the phenomena would be ascribed to groups of elements the peculiarity of which is that they act, within certain ranges of condition, as individuals as complexes of atoms of wider range of existence to the limits of which we have not as yet attained. 60 THE CHEMISTRY OF THE SUN. [CHAP. In the case of many compounds of carbon alone, we are able to trace them through their entire range of existence, with others only partially so. In all cases the complex groups formed from simpler ones have related but not identical properties. In other sub- stances this range of condition of existence passes gradually out of our reach, but the phenomena remain the same in kind. Atomic combination or condensation is probably a cause of what is termed the " allotropic state " of some forms of matter. It is strikingly in analogy with polymerization, but its investigation is at present out of our range. Let us now dwell for a little on the principle by the appli- cation of which the great stride we have chronicled has been rendered possible. While in the giving out of light we are dealing with mole- cular vibration taking place so energetically as to give rise to luminous radiation, absorption phenomena afford us evidence of this motion of the molecules when their vibrations are far less violent. The molecules are so apt to vibrate each in its own period, that they will even take up vibrations from light which is passing among them, provided always that the light thus passing among them contains the proper vibrations. Let us try to get a mental image of what goes on. There is an experiment in the world of sound which will help us. If we go into a quiet room where there is a piano, and sing a note and stop suddenly, we find that note echoed back from the piano. If we sing another note we find that it also is re-echoed from the piano. How is this ? When we have sung a particular note we have thrown the air into a particular state of vibration. One wire in the piano was competent to vibrate in harmony with it. It did so, and vibrating after we had finished, kept on the note. 1 1 The following extract will show that this has been long known : " How doth musicke amaze us, when of sound discords she maketh the sweetest Harmony ? And who can show us the reason why two Basons, Bowles, Brasse v.] SYMPATHETIC VIBRATION. 61 This principle may be illustrated in another and very striking manner by means of two large tuning-forks mounted on sound- ing-boxes and tuned to exact unison. One of the forks is set in active vibration by means of a fiddle-bow, and then brought near to the other one, the open mouths of the two sounding- boxes being presented to each other to make the effect as great as possible. After a few moments, if the fork originally sounded is damped to stop its sound, it will be found that the other fork has taken up the vibration and is sounding, not so loudly as the original fork was, but still distinctly. If the two forks are not in unison, no amount of bowing of the one will have the slightest effect in producing sound from the other. Again, suppose we have a long room, and a fiddle at one end of it, and that between it and an observer at the other end of the room there is a screen of fiddles, all tuned like the solitary one, we can imagine that in that case the observer would scarcely hear the note produced upon any one of the open strings of the solitary fiddle. Why ? The reason is that the air-pulses set up by the open strings of this fiddle, in unison with all the other fiddles, would set all the other open strings in vibration, and upon the principle that you cannot eat your cake and save it too, the air-pulses set in motion by the vibration of the fiddle cannot set all those strings vibrating and still pass on to one's ear at the other end of the room as if nothing had happened. The work, in fact, which the air, the medium in this case, pots, or the like of the same bignesse ; the one being full, the other empty, shall, stricken, be a just Diapason in sonnd one to the other : or that there should be such sympathy in sounds, that two Lutes of equal size being laid upon a Table, and tuned unison, or alike in the Gamma, G, sol, re, ut, or any other string ; the one stricken, the other untouched shall answer it?" The Compleat Gentleman Fashioning him dbsolut in the most necessary and commendable Qualities concerning Minde or Body, that may be required in a Noble Gentleman. By Henry Peacham, M.A., Sometime of Trinitic Colledge in Cambridge. Third edition, 1661. 62 THE CHEMISTRY OF THE SUN. [CHAP. would have to do to make the sound-waves passing along it audible to the observer, would be locally done, so to speak, upon the screen of fiddles ; the work done would decrease the amplitude of the sound-waves, and the sound would be weakened. Now apply this to light. Suppose we have at one end of a room a vivid light-source giving us all possible waves of light from red to violet. This we may represent as before by W m V & ':'-'>: ,000i000)0 oo,000, i ouu7 1 A millimetre is '03937 of an inch. 88 THE CHEM1STKY OF THE SUN. [CHAP. that is, 392 billions of waves entering our eye each second in the case of red light, taking the line A in the spectrum, and 29,800,000,000,000,000,000 = 757,000,000,000,000 39,328 that is 757 billions in the case of violet light, taking K in the spectrum. As the velocity of light is the same for all waves, it follows that the number of waves per second varies inversely as the wave-length in each case, and that the number of waves per second multiplied into the wave-length of any particular line must give us a constant quantity, namely, the velocity. In Angstrom's memoir 1 the wave-lengths are given to the second decimal place, the unit being Tsinnnnn^h of a millimetre. In the Atlas which accompanies this memoir the scale is divided so that one division corresponds to TTrcro-Vwo-th of a millimetre of wave-length. In addition to marking the wave- lengths of the solar lines, their relative intensities are shown. The map also gives the origin of each line and its correspond- ence with the lines of metallic spectra so far as these have been determined by Angstrom and Thalen. The first results 2 of Angstrom's comparison of the wave- lengths of metallic vapours with the Fraunhofer lines added the possibility of strontium and aluminium being among the solar elements. He thus allocated the principal Fraunhofer lines : H 1 and H? to Calcium. G ,, Iron F ,, Strontium and Iron (uncertain). b ,, Magnesium and Iron. D Sodium. C Hydrogen. B Potassium. 1 Recherches sur le Spectre Solaire, TJpsala, 1869. 2 Communicated to the Eoyal Academy of Stockholm, Oc tober 8, 1861. Phil. Mag. vol. xxiv. s. 4, p. 1. vn.] SULAK COINCIDENCES. 89 The following is a summary of the coincidences observed : Sodium . . . Iron . . * . 9 (all) 450 Magnesium . . Chromium . . 4(3?) 18 Calcium . V 75 Nickel .... 33 Cobalt ...... Manganese 19 57 Hydrogen . Titanium . 4 (all) 118 Barium. . . 11 (of 26) Zinc .... 2? (of 27) Aluminium . 2 ? (of 14) Copper .... 17 Angstrom remarks that the number of these lines, about 800, might easily be increased by raising the metals to a higher stage of incandescence. Still, he observes, the number already found is quite sufficient to enable him to refer the origin of almost all the stronger lines of the solar spectrum to known elements, thus confirming the opinion he had expressed in a previous memoir, that the substances which constitute the mass of the sun are doubtless the same as those forming that of the earth. But, he writes, the fact must not be lost sight of that there exist nearly midway between F and G strong solar lines, of which the origin is entirely unknown. Angstrom \ gives no list of elements present in the sun such as that given by Kirchhoff, but in its place the table of coin- cidences printed above. Thalen, his associate, in a separate memoir, 2 gives, however, as present in the sun Sodium, Chromium, Hydrogen, Iron, Nickel, Manganese Calcium, Cobalt, Titanium, Magnesium, thus rejecting zinc, barium, and copper from Kirchhoff s list of accepted elements, adding cobalt from the doubtful list, and hydrogen and manganese from Angstrom's, and titanium from his own observations. 1 Recherches sur le Spectre Solairc, par A. J. Angstrom. Spectre Normal du Soldi. Berlin, 1869. 2 Longueurs d'Ondc des Raies Metalliqucs, p. 11. Nova acta. Upsala, 1868. 90 THE CHEMISTRY OF THE SUN. [CHAP. The table of coincidences referred to, and Angstrom's remarks thereon, explain the cause of this. Kirchhoff's evidence for zinc had depended upon the coincidence of two lines only, and these were doubtless thought insufficient, as in the cases of the metals retained in the list the number of the coincidences was much greater. From Angstrom's remarks, which I proceed to give, it is evident that he was not quite satisfied with the brilliancy test relied on by Kirchhoff, and that his doubts concerning zinc arose from this cause. Of aluminium he writes l that although it gives brilliant lines in different parts of the spectrum, yet the two lines situated between Fraunhofer's two H-lines are the only ones which appear to coincide with solar ones. By way of explanation of this phenomenon he points out that the violet rays are much the strongest in the spectrum of this metal. He observes that these two lines often present the same phenomenon of absorp- tion as is shown by the yellow sodium lines, which is a proof of their great intensity. He states finally that the point will be cleared up by ascertaining whether the ultra-violet lines of aluminium coincide or not with faint solar lines in that region. Of zinc he remarks 2 that the two lines he has given of that metal as coincident with solar lines do not correspond with the latter in character, being wide, very strong and nebulous, so that the presence of zinc in the sun remains doubtful. It is note- worthy, however, that there are three lines in the magnesium spectrum which present the same nebulous appearance, and to which there are no corresponding solar lines, and yet magnesium, he thinks, is undoubtedly present in the sun. Angstrom intended to supplement his normal solar spectrum with a series of maps of the bright lines of all the metallic elements on a wave-length scale, but circumstances prevented 1 Loc. cit. p. 36. 2 Idem. vii.] THALEN'S RESEARCHES. 91 his doing so, and at his request Thalen, his associate, undertook their preparation. 1 Thalen's method of work was to compare the metallic lines with the solar lines, and his wave-lengths were then taken from Angstrom's Spectre Normal. He, like Kirchhoff, employed a large Kuhmkorff coil and Ley den jar, and took the spark between electrodes of the metal under observation, or of platinum or aluminium moistened with solutions of salts of the various metals. The salts em- ployed were usually the chlorides, on account of their ready volatility. Occasionally, instead of the spark, an arc produced by fifty cells was used. The dispersion generally employed was afforded by a single prism of bisulphide of carbon, and was magnified by a very powerful telescope. Sometimes a second similar prism was added, and on rare occasions these were replaced by six prisms of dense flint glass. Occasionally also, when low dispersion was required, a single prism of flint glass was used. In some cases when the metallic lines were very faint, the direct method of solar comparison could not be applied, because the glare of the sunlight, even when reduced by a diaphragm, was so great by comparison that the feeble metallic lines could not be discerned. Another method had, therefore, to be re- sorted to. Instead of producing the solar and metallic spectra side by side, they were superposed. The sunlight being then cut off, a pointer was brought into exact coincidence with the line observed, and then the sunlight was again allowed to enter, and the position of the pointer among the Fraunhofer lines gave the position of the metallic line. Thalen gives only the strong lines of the metals in his maps because he deemed it impossible to prove that the feeble lines were not due to some other element present as an impurity, 1 Memoir c sur la Determination des Longueurs cCOndt des Raics Mctalliqucs. Upsala, 1868. 92 THE CHEMISTRY OF THE SUN. [CHAP. but he remarks that the spectroscope itself should be capable of settling this question of impurities an opinion which we shall see subsequently has been in part at least verified. The difficulty Thalen experienced in this question of impuri- ties will be seen from his remarks on titanium. Concerning this element, he states that the faintness and transitory cha- racter of its lines rendered it difficult to determine any solar coincidences, but he nevertheless found some such coinci- dences in the green. Subsequently Angstrom and Thalen observed these lines in the electric arc taken between poles of charcoal soaked in a solution of calcic chloride, but they were not seen in the spark between metallic electrodes moistened with a solution of calcic chloride. Thalen then prepared some chemically pure bichloride of titanium, and satisfied himself spectroscopically, by the absence of the calcium lines, that it contained no trace of calcium, while from the presence of the green lines above mentioned he concluded that those lines are certainly due to titanium and not to calcium ; and, further, that the carbons employed as elec- trodes in the voltaic arc contained an impurity of titanium. From the number of fine lines of titanium concident with solar lines Thalen concluded the existence of titanium in the sun. In the case of iron and other bodies he notes that there is an agreement to some extent between the intensities of the solar and metallic lines, but that in the case of titanium this is less apparent. KirchhofF s and Angstrom's maps are in all our laboratories, nnd there is a very considerable difference between them. This difference arises from the fact that whereas KirchhofF used an induction coil and spark, Angstrom varied his experimental method by placing no longer a spark, but the electric arc in front of the slit of his instrument. In this case, therefore, he was determining the spectrum which was produced at the vii.] SOLAR ELEMENTS. 93 temperature of the electric arc instead of the spectrum which was produced at the temperature of the electric spark. The result of the combined attack of KirchhofF, Angstrom, and Thalen is shown in the accompanying table : Elements present in the Sun. Kirchhoff. Angstrom and Thalen. Sodium. Sodium. Iron. Iron. Calcium. Calcium. Magnesium. Magnesium. Nickel. Nickel. Barium. Copper. Zinc. Chromium. Cobalt. Hydrogen. Manganese. Titanium. So far then for that mode of observing the sun which consists in comparing the general light of the light-source with the general light of the sun. This introduces an important consideration. When we have a light-source placed in front of the slit of the spectroscope it is perfectly clear that light from all portions of the light-source must illuminate the slit. Similarly, if we content ourselves by pointing the spectroscope to the sun, or to a cloud illuminated by the sun, it is perfectly obvious that the light from all parts of the sun must enter all parts of the slit, and if there is any localisation of phenomena on the sun this will be lost to a greater or less extent in the spectroscopic record. Such localisations there undoubtedly are, as may be seen with a telescope of very moderate power, and a brief account of what these are, and how they may be effectively got at by the spectro- scope, may be conveniently given in the next chapter. CHAPTEE VIII. A NEW METHOD OF WORK. 1. Details of 'the Solar Surface. WE must now pass to details of another order. Those we have given refer to the light of the sun taken as a whole. But it is in the highest degree necessary for the purposes of our present inquiry, that we should not content ourselves with this general view since when we pass from Kirchhoff' s work we pass to work done on minute portions of the solar surface. This we must now consider, and it is desirable that I should preface it as briefly as may be by a reference to the various differences observed in different parts of the sun. When we look at the sun with a powerful telescope, taking all needful precautions (never look at the sun with a small telescope, for this is a most dangerous proceeding), we find it to be by no means the immaculate body it was thought to be by the schoolmen. Here and there on the disc, but generally limited to those parts of it a little above and below the equator, dark spots may be observed. These generally exhibit three shades of darkness, and float, as it were, on the general bright surface of the sun called the photosphere, the darkness of the spot increasing towards the apparent centre. We have first the penumbra, then the umbra, then the nucleus. But some- times the darker portions are excentric, and very irregular in outline. CHAP. VIII.] SPOTS AND FACUL.E. 95 Changes are going on incessantly in the region of the -spots. Sometimes these are noticed after the lapse of an hour even ; a portion of the penumbra setting sail across the umbra; or a portion of the umbra melting from sight ; or we may have an evident change of position and direction in masses which retain their form. In some spots evidences of cyclonic action are very obvious. We not only get this darkness localised in spots, but it is easy to see that there is a general defect of illumination as we approach the sun's edge. FIG. 35. Sun spot (Secchi). We may now pass to the brighter portions of the general surface. These are most obvious near the edge of the solar disc, and especially about spots approaching the edge ; in these positions bright streaks of diversified form, quite dis- tinct in outline, and either entirely separate or uniting in various ways into ridges and network, are seen. These, which have been termed faculce, are the most brilliant parts of the sun. Where near the edge the spots become invisible, undulated shining ridges still indicate their place, 96 THE CHEMISTRY OF THE SUN. [CHAP. being more remarkable thereabout than elsewhere, though everywhere traceable in good observing weather. Faculse may be of all magnitudes, from hardly visible softly gleaming narrow tracks, 1,000 miles long, to continuous complicated and heapy ridges 40,000 miles and more in length, and 1,000 to 4,000 miles broad. Eidges of this kind often surround a spot, and hence appear the more conspicuous ; but sometimes there appears a very broad white platform round the spot, and from this the white crumpled ridges pass in various directions. So far we have referred only to the phenomena at all times visible to us with ordinary telescopic aid, but those who have been favoured by a sight of a total eclipse, and many more who have read the accounts of total eclipses, know that there is a great deal more of the sun than one generally sees. On these occasions a part of the sun not usually visible its external atmosphere is unveiled for us. The central light of the sun being cut off by the intervening dark moon, surrounding it, on all sides, appears a glorious halo, generally of a silver-white light ; this is called the corona ; now radiated in structure, and again full of strangely- curved markings or long streamers, it extends sometimes beyond the moon to a distance equal to many diameters. Eecent eclipse work has shown that the corona, in part, at all events, reveals to us the sun's outer atmosphere, which is in- visible when the sun's light itself is present, owing to the over- powering light of the latter. But the corona is not all we see at such times. When the totality has commenced, apparently close to the edge of the moon, and therefore within the corona, are observed fantas- tically-shaped masses generally full lake-red, fading into rose- pink in colour, variously called red flames or red prominences. The height of some of these prominences exceeds 70,000 miles. They were first described by Stannyan in 1706. Since his time many have held them to be beautiful effects produced VII I. J PROMINENCES, by the passage of the moon over the sun, or eve*E atmosphere of the moon coloured, by the strange way in which the solar light then fell upon them; but their solar origin has now been conclusively established, chiefly by the observations of the eclipses of 1842, 1851, and 1860. The observations of the eclipse of 1842 afforded evidence that these red prominences or flames these different-coloured phenomena were really, so to speak, upper crests of an almost continuous cloud- sea round the sun. FiG. 36. Total eclipse (Dawes), 1851. Ill the drawings in question, a fine low level, of the same colour as the prominence itself, was shown connecting the pro- minences, while later drawings gave us those prominences alone after the moon had covered all the lower portion. That any doubt should have remained after such an observation as this is as good an indication as I can give of the extreme difficulty of making observations during eclipses, and how important it is that one should have a method which makes us independent of them. The above hasty sketch of localised solar phenomena must suffice for the present ; in the sequel we shall have to return to the various points from the chemical point of view. H 93 THE CHEMISTRY OF THE SUN. [CHAP. 2. How these Details can be Spectroscopically Observed. Now it is evident that if we simply direct a spectroscope to the sun, or reflect sun-light into it, it will receive light coming indiscriminately from spots, faculse, and general surface, and if there are any differences in the spectra of these regions, in other words, if the chemistry of the various regions is different, the differences will be lost, since we shall obtain a mixed spec- trum partly due to the spots, partly to the faculae, and partly to the general surface. But there is another way of observing the spectrum of the sun. We can throw an image of the sun, or of any part of the sun, on the slit of the spectroscope. This kind of work, though, as we have seen, it was suggested by Forbes thirty years earlier, and actually employed by Angstrom to investigate the -darken- ing of the limb, was first applied to the spots and faculai in 1866. If a spot or a facula be visible on the sun, we can throw the image of the sun on the slit plate, and then bring it exactly on the slit. We shall in this way get the spectrum of the sun- spot, or facula, as distinguished from the spectrum of the other portions of the sun. The manner in which this kind of work is carried on is easily grasped. It simply consists in the use of a spectroscope of large dispersion attached at the focal point of a telescope of considerable power. Fig. 37 shows the eyepiece end of Mr. Newall's giant refractor, with a spectroscope with a considerable number of prisms, lixed to the telescope by means of an iron bar. The slit of the spectroscope occupies the focus, so that when the instrument is pointed towards the sun we see an image in the case of this telescope something like four inches in diameter with the spots and brighter portions wonderfully and beautifully clear ; and by means of the different adjustments of the telescope we can THE TELESPECTBOSCOPE. 99 bring now a spot, and now one of the brighter portions of the sun, on to the slit. If there be any difference between the spectrum of the spot and the spectrum of the general surface of the sun, such difference will be thus observed at once, and the differences are in fact very striking Such an instrument, compounded of the telescope and spectro- scope, has been named a telespectroscope. FIG. 37. The eyepiece end of the Newall refractor (of 25 inches aperture) with spectroscope attached. The next point to engage our attention is the change in the phenomena produced by this change in the method of observa- tion. We will first deal with spots. 3. Spot Spectra. When an image of a sun-spot has been thrown on the slit of the spectroscope in the manner described, it shows itself in the H 2 100 THE CHEMISfRY OF THE SUN. [CHAP. spectrum as a dark band, of greater or less width according to the size of the spot, running along the whole length of the spectrum, and crossed, as are the other parts of the spectrum, by the Fraunhofer lines; the nucleus is seen as a narrow band darker than the rest. Besides this continuous absorption, another phenomenon is seen. Some (often many) of the Fraunhofer lines in the parts where they are crossed by the spot spectrum, are considerably thickened, the widening being greatest in the nucleus and gradually fading away in the penumbra. In Fig. 38 this appearance is well depicted. The spectroscope has been so placed that its slit bisects two spots, in each of which many lines are seen widened, notably the two D lines of FIG. 38. Spectrum of sun-spot, showing the widening of the D lines. . sodium. It is not always the same set of lines that are widened. In some spots the sodium lines are most affected, in others the iron lines, and so on. I was induced to start this method of observation in 1866 for the following reason : Two rival theories had been suggested to explain how it was that a sun-spot is dark. 1 One school, led by M. Faye, believed that the interior of the sun is a nebulous gaseous mass of feeble radiating power at a temperature of dissociation, and that surrounding this is a highly radiating photosphere. On this hypothesis, in a sun-spot, the nebulous interior mass is 1 See Proc. Roy. Soc. 1866, vol. xv. p. 256. viii.j THEORIES OF SUN-SPOTS. 101 revealed through an opening in the photosphere, caused by an upward current, and the spot is black by reason of the feeble radiating power of the nebulous mass. The other school, comprising Messrs. De La Rue, Stewart, and Loewy, referred the appearances connected with sun-spots to the effects, cooling and absorptive, of an inrush or descending current of the sun's external atmosphere, which must be colder than the subjacent photosphere. By some observations communicated to the Eoyal Astrono- mical Society l in 1865, 1 was led independently to this latter con- clusion. The observations indicated that instead of a spot being caused by an upward current, it is caused by a downward one, and that the results, or, at all events, the concomitants of the down- ward current, are a dimming and possible vaporisation of the cloud masses carried down. I was led to hold that the current had a downward direction by the fact that one of the cloud- masses observed, passed in succession, in the space of about two hours, through the various orders of brightness exhibited by faculae, general surface, and penumbrse. If we had been dealing with defective radiation, we should still have been dealing with radiation, and should have expected to see bright lines ; but no obvious bright lines were seen in the spectrum of the spot ; what we did see was the thickening and darkening of certain lines and the continuous absorption. In the case of the lines of sodium it was very marked ; so that we were perfectly justified in saying that the sun-spot was really not produced by any defect of radiation, but was truly and really produced by an increased amount of absorption. 4. Spectrum of the Limb. The darkness of the spectrum at the limb is very different from the darkness in a sun-spot. 1 Monthly Notices Roy. Ast. Soe. vol. xxv. p. 237. 102 THE CHEMISTRY OF THE SUN. [CHAP. Angstrom, in extension of Forbes's observation of the spectrum of the limb of the sun during an annular eclipse, repeated the observation under conditions which were more likely to reveal any differences, if such existed, between the spectra of the centre and limb. He projected an image of the sun on the slit of the spectroscope, and then at his leisure was able to see whether the Fraunhofer lines underwent any change when different parts of the image were allowed to fall on the open- ing. The results of this observation are thus described by Angstrom : 1 " Any very remarkable change I could not discover ; all that I fancied I could remark was, that the intensity of the spectrum light is somewhat less when the ray comes from the edge, than when from the centre of the disc ; and this is evidenced by the circumstance that the fainter Fraunhofer lines show themselves in the latter case comparatively stronger, whereas when the light comes from the centre of the solar disc, the fainter lines will sometimes even totally disappear, while the stronger lines, as for example, some of the iron lines, appear with correspondingly increased brilliancy ; as we know by Kirchhoff's experiments that an increased difference of intensity between the source of light and the absorbing gas is favourable to the distinctness of the lines in the gas spectrum, it would seem that this observation, if confirmed, is not repugnant to what we already know concerning the absorbing power of To facilitate the comparison of the spectrum of the limb with that of the centre of the sun, I used a specially-constructed slit plate. One half of the slit is covered by a totally reflecting prism, and another similar prism actuated by a fine screw is fixed in a slide, so that its distance from the former can be varied. The centre of the sun's image is allowed to fall on the uncovered part of the slit, and its edge on the sliding prism, from which it is reflected to the fixed prism, and thence into the 1 Phil. Mag. s. 4, vol. xxiv., July, 1862, p. 3, note. VIII.] METHODS OF COMPARISON. 103 eollimator, which it enters side by side with the direct light from the centre of the disc. FIG. 39. a, right-angled prism on which the image of the limb falls ; b, right- angled prism which receives light from a ; c, slit through which centre of sun is observed ; d, screw to adjust width of slit ; e, screw to adjust distance of a from b, according to the semi-diameter of solar image employed. An arrangement suggested by Mr. Hastings for accomplishing the same result is shown in Fig. 40. The light from the edge of the sun reaches the slit after two total reflections from the faces of the prism, as shown in the figure, while that from the centre enters the uncovered part of the slit directly. FIG. 40. Mr. Hastings's arrangement for comparison of spectra. 1 1', diameter of sun's image ; p, totally reflecting prism. In spite, however, of the new apparatus, it was very difficult to see any special effect at the limb. 104 THE CHEMISTRY OF THE SUN. [CHAP. An increased selective absorption at the limb appeared espe- cially probable for the violet region, because the difference be- tween centre and limb is more marked photographically than visually, a view which received support from Herr Vogel's observations. 1 With minute search however, Mr. Hastings 2 obtained evidence of increased selective absorption not only in the extreme violet, but also in the visible part of the spectrum. He found that the lines \> \, and b B , " become sharper and less hazy near the limb ; " c and F also become sharper in the same region, and D is affected ; while on the other hand a line 7681 (Kirchhoff), which is strongly marked in the centre of the disc, disappears entirely within 16" to 20" from the limb. Two other lines near F (1828-6 and 1830'9 Kirchhoff) which are strong at the limb become much fainter at the centre. "These latter lines also become greatly strengthened over the penumbrse of spots," while "768-1 is not thus affected." Besides these differences which were constantly observed, others were suspected, and Mr. Hastings thus discussed the phenomena : " Since the light from the border of the sun undergoes a general absorption, which reduces its intensity to much less than one-fourth that at the centre, according to Secchi's measurements, and yet the spectroscopic character is changed so slightly, it is impossible for me to escape the conviction that the seat of the selective absorption, which produces the Fraunhofer lines, is below the envelope which exerts the general absorption. But the phenomena of the faculae prove not only that this envelope rests upon the photosphere, but also that it is very thin. The origin of the Fraunhofer lines, then, must be in the photosphere itself, which is in accordance with Lockyer's views. "Any effects which the chromosphere might produce, we would anticipate finding most evident in the lines of those gases which are readily detected there. A reference to the observations shows at 1 Naturforscher, Jahrgang v. p. 321, Oct. 1872. 2 Nature, vol. viii. p. 77, May 22, 1873. vin.] SPECTRUM OF FACULA. 105 once a compliance with this anticipation in the lines of hydrogen, magnesium, and sodium. The line 768'1 is not less strikingly in concordance, if it be regarded as 768'?* (the ? indicates doubt as to the tenths of the scale, and * absence of a corresponding black line) of Young's catalogue of chromospheric lines. The lines 1828'6 and 1830'9, with others of the same class, probably have their origin in the medium which exerts the general absorption, and thus are allied to our telluric lines. It also seems probable that the chromosphere is too transparent to reverse many of its lines. That this is the case in the helium lines is tolerably certain." 5. Spectrum of the Faculce. When a facula is brought on to the slit of the spectroscope, it manifests its presence by a band of greater brightness than elsewhere running . along the whole length of the spectrum. We have in fact an indication of less absorption, general as evidenced by the greater brightness of the continuous spectrum, and selective as shown by the thinning and even disappearance of some of the dark lines. This and other observations have led to the conclusion that the faculse are elevated above the general surface as the spots are depressed, that they are in fact the higher cloud-domes of the photosphere. This will at once explain why the faculse are best seen near the sun's limb. The difference between the general absorption of a facula and that of the general surface of the sun is so small, that it may be scarcely perceptible on the centre of the disc, but becomes very apparent at the limb, where a greater thickness of atmosphere is brought into play. It is seen then that the new method actually does give us important spectroscopic differences, not only between the spectra of spots and faculse, but between the spectra of both spots and faculse as contrasted with that of the general surface. CHAPTEK IX. MOKE RESULTS OF THE NEW METHOD. 1. Artificial Eclipses. ALTHOUGH in 1866 much knowledge had been gathered con- cerning the strange red prominences seen round the edge of the moon in a total eclipse of the sun, they were still a mystery and a source of wonder. After the spectroscope had been used to help us in the study of the spots, it seemed worth while to inquire whether it could also help us in other directions, for if it could it was perfectly clear that we should not be contented with merely observing the chemical nature of the photosphere alone. It did not take long to convince my friend Dr. Balfour Stewart and myself, that by means of the spectroscope, the things which up to that time had only been observed during eclipses, would be more or less felt, if they were not absolutely rendered visible, by this new instrument, if their real nature was anything like what we were justified by our then knowledge in attributing to them ; and for this reason : the things seen round the sun during an eclipse are not there for the instant of the eclipse only : they are always there : why then do we not see them ? The illumi- nation of our own air prevents this. What is our own air illuminated by ? By the sunlight. Now whereas increasing dispersion does considerably dim a continuous spectrum for the reason that it makes it extend over a much larger area, it does not dim to any great extent the brightness of a line ; so CHAP, ix.] THE PRINCIPLE INVOLVED. 107 that by employing a considerable number of prisms we ought to be able to abolish the illumination of our air altogether, and in that way we should no longer be limited to determining merely the chemical nature of the spots, we should be equally able to determine the nature of the surrounding solar atmosphere, supposing the phenomena observed during eclipses were really due to incandescent vapours at the sun, and were not lunar or terrestrial. This principle can be very clearly demonstrated by means of the electric light. The lower pole is charged with some metallic salt, say a salt of lithium, and a single prism is interposed in the beam. There then appears on the screen a mixed spectrum, due partly to the continuous light from the solid poles, and partly to the lithium vapour. The red lithium line stands out on a background v of continuous spectrum. On mounting a second prism, the continuous spectrum from the poles being much more dispersed is enfeebled, while the bright line of lithium retains almost its original brilliancy because it is not dispersed. On the addition of a third prism this effect is enhanced. 'In fact, the brightness of the line relatively to that of the continuous spectrum increases so rapidly with each increase of dispersion, that by employing a sufficient number of prisms, we can practi- cally abolish the latter and see the bright line on a dark ground. That was the principle which it was suggested would enable the spectroscope to be used in making what have been called artificial eclipses.' 1 This method was first applied by Janssen immediately after the eclipse of 1868. Now if we consider what are the conditions presented by eclipses, as well as the sort of thing the spectroscope is called upon to observe, we shall see the very considerable advantage of the introduction of the new method. In the first place, eclipses, which are so full of teachings to be got only at the moment 1 Proc. Roij. Soc. 1866, vol. xv. p. 258. 108 THE CHEMISTRY OF THE SUN. [CHAP. of their rare occurrence, are almost instantaneous, so far as each particular phenomenon is concerned ; and, secondly, when the duration is, say, four or five or six minutes, which happens but very rarely, although a great deal of work may be done, only a very small part of the more interesting regions of the solar atmosphere is uncovered. How then can we avail ourselves of this method ? It should be perfectly clear that if instead of observing a spot we allow the slit to lie on the edge of the sun, and then sweep round it, if the method is competent to abolish the illumination of our atmosphere to make the bright lines visible that here and FIG. 41. Line C (red), with radial slit. there if the slit travels over a prominence it will admit the light of the prominence ; and if we have the image of the sun very accurately focused on the slit, the size of the image of the sun and the length of our slit being known, the length of the slit illuminated by the prominence will enable us readily to deter- mine the exact height of the prominence. Further, if it should happen that there is a sort of vaporous sea round the sun usually invisible, but which this new method will reveal, it would follow that we shall get the depth of the sea sounded for us by the length of the line. Again, if we do sweep round the sun in this way, and if these prominences really do give us lines, we have exactly the same ix.] FIRST RESULTS. 109 method of determining the chemical nature of this exterior atmosphere as Kirchhoff employed in determining the compo- sition 'of the general light of the sun. Only we have a great advantage in this case, for whereas Kirchhoff had to suggest an x D. 2 L> 3 FIG. 42. Line D 3 (yellow), with radial slit. hypothesis to explain the possible locus of the region which pro- duced the lines due to the different chemical substances, we have the hard fact beneath our eyes, because if we encounter F FIG. 43. Line F (blue-green), with radial slit. a prominence, and if it be built up, let us say, of iron vapour, then we shall see iron lines ; if it be built up of calcium vapour, then we shall see calcium lines, and so on. Now what are the facts ? The first observation that was recorded with absolute certainty 110 THE CHEMISTRY OF THE SUN. [CHAP. touching the chemical nature of the exterior envelope of the sun had reference to the red line of hydrogen. When therefore we had such an observation as this, showing one of the lines produced by this vaporous sea, coincident with the c line of the solar spectrum, we knew at once, on the assumption made by Kirchhoff, that that line was produced by hydrogen. It was necessary, of course, that the other lines of hydrogen should be investigated. The next obvious line of hydrogen is F in the blue-green, and when the question was put to this line, in that case also it was found that the prominences gave out no uncertain sound that the prominences were really and truly composed to a large extent of what we call hydrogen; that is to say, the spectral lines observed when we render hydrogen incandescent are identical with three of the spectral lines observed when we throw one of the solar prominences on the slit. Here then we see that as in the case of the spots we are in full presence of localised chemical phenomena. All the prominences have not this simple constitution. Some of them exhibit, besides the lines of hydrogen, those of magnesium,, sodium, iron, and other metals ; indeed they may be divided according to their chemical composition into prominences in which among known lines those of only hydrogen are seen, and others in which the lines visible in the spectra of a great number of metals are very brilliant. Extended spectroscopic observation outside the sun's limb on this new method not only revealed the chemical nature of the prominences, but proved that they were merely local heapings up of an envelope chiefly characterised ly the hydrogen lines which entirely surrounded the sun. It was found that outside the photosphere the prominence spectrum was never absent. To the continuous envelope thus revealed I gave the name Chromosphere a term suggested by my late friend, Dr. Sharpey, who was then one of the secretaries of the Royal Society, and who took the keenest interest in the new revelations to distinguish it IX.] THE CHROMOSPHERE. Ill from the fainter outer atmosphere as seen in eclipses on the one hand, and the white photosphere on the other. It was soon found that this continuous ocean, this continuous outer shell of the sun, varied considerably in thickness from time to time, and it was also found that the lines seen in the spectra of other substances be- sides hydrogen, some of which at present we know nothing of, others of which we now think we know a great deal of, also appeared side by side with the lines of hydrogen. Sometimes the spec- trum of the chromosphere is full of lines. In almost all cases, how- ever, we find that these lines are never so long as the hydrogen line, from which we gather that the depth of the layer of the solar atmosphere measured for us by the length of the magnesium lines, to take a case, is much less than that of the hydrogen one. I should further add here that when the sun is moderately active and can be well observed, as in a fine climate like that in Italy, this magnesium layer can be de- tected all round the sun, so that we have in the chromospheric 112 THE CHEMISTRY OF THE SUN. [CHAP. layer, first of all, a layer of hydrogen with its prominences, and then at the bottom of this layer another layer of magnesium, which wells up sometimes where the prominences are most active. The different lines we see in our instruments when we observe the solar prominences are not all alike. Some of the lines vary very much from the appearance of the c line of hydrogen, for instance. In fact, in one line, the F line (see Fig. 43) we get a trumpet-shaped appearance. The line widens as it approaches the sun, so that it resembles an arrow head, resting on the thin absorption line which forms the shaft. This is not only true of the hydrogen lines, but of the lines due to the injection of other substances into the sun's outer atmosphere from below. This artificial eclipse method does more than reveal the heights and chemical nature of the prominences. A slight extension of it enables us to see their actual forms and watch the tremendous changes going on in them from time to time. The first observation of this nature was made in 1868 1 by causing the narrow slit, radial to the edge of the sun, to pass slowly over the prominence. By this means a number of sections of varying length was obtained, which, placed side by side, gave an idea of its shape. It is sufficiently obvious that in this way a perfect view of the prominence can only be obtained by moving the slit with sufficient rapidity to allow of persistent images. At the very outset Janssen and I attempted to accomplish this, Janssen by giving a rotatory motion to a direct- vision spectroscope, I by giving an oscillating motion to the slit, in which plan I was followed by Young, 2 who afterwards ex- panded it. Although these plans were successful, they were subject to the disadvantages of reducing the quantity of light and shaking the instrument. The method now adopted for viewing the forms of the promi- nences is one of extreme simplicity, the principle of which was 1 Proc. Roy. Soc. No. 105, 1868. 2 Nature, Dec. 8, 1870. ix.J VIEWING THE PROMINENCES. 113 first published by Zollner, 1 in February, 1869, and adopted by myself before I had heard of his paper. The method consists simply in the use of a widely opened slit. Zollner, however, fearing that the quantity of light admitted would be so great as to obscure the forms of the prominences, proposed to reduce it if necessary by polarising or absorbing media placed in the eyepiece. On the 16th of the same month in which Zollner announced his idea, Mr. Huggins 2 suggested a similar means of attaining the same end by a combination of an open slit and ruby glass. On the 29th, I heard of this paper of Mr. Huggins', and it at once struck ine that the absorptive media were useless, and that the same result might be accomplished by the use of the open slit only if the field was kept sufficiently small. On putting this to the test I found my expectation fully confirmed. 8 The way this method is applied will be readily under- stood. Having found a prominence and got it carefully focused for the c or F line, the slit is then simply opened widely, when it is found that all the details of the prominence can be seen. The reason of this will be clear on a little consideration. We have already seen that the hydrogen Fraunhofer lines (like all the others) appear dark because the light which would otherwise paint an image of the slit in the place they occupy is absorbed ; but when we have a prominence on the slit, there is light to paint the slit ; and as in the case of any one of the hydrogen lines we are working with light of one refrangibility only, on which the prisms have no dispersive power, we may consider the prisms abolished. Further, as we have the prominence image coincident with the slit, we shall see it as we see the slit, and the wider we open the slit the more of the prominence we shall see. We may use either the red, or green, or blue light of hydrogen for the 1 Aslronomische Nachrichtcn, No. 1772, September 15th, 1869. - Proc. Roy, Soc. vol. xvii. p. 302. 3 Ib. No. 110, 1869. I 114 THE CHEMISTRY OF THE SUN. [CHAP. purpose of thus seeing the shape and details of the promi- nences ; how far the slit may be opened depends upon the purity of the sky at the time. In 1873, Mr. Seabroke and myself communicated a method to the Eoyal Society/ which enabled us to observe the whole of FIG. 45. Diaphragm showing ammlus, the breadth of which may be varied to suit the state of the air. the chromosphere at once by means of a ring slit, which was made coincident with an image of the chromosphere itself, so that the light from the sun itself was stopped out. This arrangement will be readily understood on referring to the diagrams (Figs. 45 and 46). FIG. 46. The anmilus is viewed and brought to focus by looking through aper- tures in the side of the tubes. A, sliding eye-tube of telescope ; B, tube screwing into eye-tube ; c, tube sliding inside B, and carrying lens D and diaphragm E ; r, lenses bringing image of diaphragm to a focus at the place generally occupied by the slit of the spectroscope ; G, collimator of spectro- scope. It afterwards came to our knowledge that Zollner had conceived the same idea, unknown to us, but had rejected it ; and that Professor Winlock had tried a similar arrangement. 2 1 Proc. Roy. Soc. vol. xxi. p. 105. 2 For a more complete account of the methods of viewing the forms of the prominences, see Solar Physics, p. 578 et seq. IX.] POBMS OF PROMINENCES. 115 The different forms which these prominences assume some- times are very striking. Here one is reminded, by the fleecy, infinitely delicate cloud-films, of an English hedge-row with luxuriant elms ; here of a densely intertwined tropical forest, the intimately woven branches threading in all directions, the pro- minences generally expanding as they mount upwards, and changing slowly, indeed almost imperceptibly. It does not at all follow that the largest prominences are those in which the intensest actions, or the most rapid change is going FIG. 47. Solar prominences (Young), showing lateral currents. on. The greatest action is generally confined to the regions just in, or above, the chromosphere. This and subsequent observations led me to propose a division of prominences into two classes : 1 1. Eruptive. Those in which great action is going on, lower vapours being injected ; in the majority of cases these are not high, they last only a short time are throbs, and are oft renewed. They often accompany spots, but are not limited to 1 Proc. Roy. Soc. No. 120, 1870. I 2 116 THE CHEMISTRY OF THE SUN. [CHAP. them, and are not seen so frequently near the sun's poles as near the equator. 2. Nebulous. -Those which are perfectly tranquil, so far as wave-length evidence goes. They are often high, are persistent, and not very bright. These do not as a rule accompany spots. This classification was afterwards accepted by Secchi, Zollner, Sporer, and Young, and also by Kespighi, who, however, divides the eruptive class into a great many sub-classes. Now, not only are these differences in the forms of the promi- nences associated with differences in their chemical constitution, but by observing the form of each prominence, first by means of a line due to one substance and afterwards by means of lines due to others, we can get actual sections of the prominence showing the parts occupied by the substances producing the different lines. The importance of such observations we shall find it difficult to overrate. Detailed observations of the chromosphere showed that its free edge was not only subject to those great local disturbances which form the prominences, but that along its whole length it was subject to the same action in a smaller degree. The edge of the chromosphere is never smooth, but irregular, and the forms of the irregularities are found to conform to one or other of the two types to which we have seen the prominences may be referred. Sometimes it is marked by gentle undulations, at other times thrown into sharp jet-like projections. Since the introduction of the new method, several workers, pre-eminent amongst whom Professors Kespighi, Secchi and Tacchini must be mentioned, have carried on the observation either of the forms and details or the chemical composition of the prominences with indefatigable perseverance, so that as the result of their labours we have an almost daily record of all the prominences visible. It was at first thought impossible that prominences could ix.] PROMINENCES ON DISC. 117 thus be seen on the disc of the sun itself, 1 as this was not in accordance with Kirchhoff s hypothesis. Subsequent observa- tions, however, soon showed that this is quite possible. 2 The first observations of this nature were made by Father Secchi and myself almost simultaneously. My observation was made on the llth of April, 1869, and Secchi's letter to the Paris Academy announcing the same appearance is dated the 13th. FIG. 48. Edge of chromosphere (billowy). This reversal of the hydrogen lines on the sun's disc was also observed by Captain Herschel on June 10th, 1869, and by Pro- fessor Young, who in a communication to the Franklin Institute, dated October 3rd, 1870, 3 thus describes the reversal of the c and F lines in a group of spots then visible : " At 4.05 P.M. the brilliance of the p line increased so greatly that it occurred to me to widen the slit, and to my great delight I FIG. 49. Edge of chromosphere (pointed). saw upon the disc of the sun itself a brilliant cloud, in all its structure and detail identical with the protuberances around the limb. Indeed, there were two of them, and there was no difficulty in tracing out and delineating their form. Fig. 50 represents them 1 Proc. Roy. Soc. No. 110, 1869. See also Secchi, Comptes Rcndus, vol. Ixviii. p. 237. '- Proc, Roy. Soc. vol. xvii. p. 415. 3 Nature, vol. iii. p. 113. 118 THE CHEMISTRY OF THE SUN. [CHAP. as they were from 4.05 to 4.10 ; Fig. 51 gives the form at 4.15 4.20. They were then considerably fainter than at first. During the intervening ten minutes I examined the other lines of the spectrum, and found that the form could be distinctly made out in all the FIG. 50. The prominence at 4.05. hydrogen lines, even in h ; but that the reversal of the other lines, including D 3 , was confined to the region immediately over the spot- nucleus, where the smaller but brighter cloud terminated abruptly, or, I might better say, originated. The larger one faded out at both ends. When the clockwork of the equatorial was stopped, FIG. 51. The prominence at 4.20. the luminous cloud took 16*7 seconds of time to traverse the slit which was placed parallel to the hour-circle. This indicates a length of at least 130,000 miles, without allowing anything for the foreshortening resulting from the nearness to the sun's limb. 2. Contortions of Spectral Lines. In my first observations of the chromospheric lines by the new method, 1 I was greatly puzzled to account for the occasional 1 Phil Tram. 1869, p. 425. ix.] CONTORTIONS OF F. 119 strange behaviour of the F line. Its coincidence with the corresponding dark line could not always be satisfactorily made out it appeared to be more refrangible. Again, a few days later, when observed with a tangential slit, the line assumed the appearance shown in Fig. 52. In the spectrum of the light proceeding from the exact limb of the sun the bright line was seen more refrangible than F, but in the spectrum of the prominence at some distance above the sun the black line F was eclipsed. Hence it appeared that away from the sun's surface the substance which produced the FIG. 52. Non-coincidence of bright and dark F line when tangential slit is used, and when both photospheric and chromospheric light is admitted. line gave out light of less refrangibility than it did at the surface. Again, the bright line was sometimes strangely contorted, as shown in Fig. 53, sometimes being violently deflected to the right or left and sometimes broken as in Fig. 54, part of it being torn away considerably to the more refrangible part of the spectrum. We have in the first figure the F line. The slit the perfectly straight slit has been worked round the limb in search of a prominence, and it has found one. But 120 -w THE CHEMISTRY OF THE SUN. [CHAP. the slit is no longer shown us as a perfectly straight line, it is in fact a very irregular one ; and further than this, it branches at a certain distance from the sun and becomes double. FIG. 53. Contortions of F line in a prominence. The next series of figures (Fig. 55) represents another obser- vation of this bright r line of hydrogen. These various effects (111 111! Ill FIG. 54. Contortions of the hydrogen lines. were produced by varying the position of the slit a very little indeed over a small and very brilliant prominence. The left-hand figure shows the appearance of this line when IX.] BRIGHT LI$TES ON DIS the slit was just on one edge of the prominence, the central figure gives its appearance when it was entirely included in the slit, and the right-hand figure when the slit was on the opposite edge. There was another important fact connected with this : when the phenomena were observed close to the limb it was very often seen that the dark line on the surface of the sun was broken ; in fact we got a doubling of the dark F line in exactly the same way as we got this doubling of the bright line in the prominence itself. FIG. 55. Contortions of the hydrogen lines. The right-hand side of each diagram is the most refrangible. The next point observed was (and this was an observation very difficult indeed to make near the limb) that whenever we got any very considerable activity we got a new order of phenomena altogether, indicated in the two diagrams (Figs- 56 and 57). It was found that the absorption of the hydrogen, or of the magnesium, or of the sodium, as the case might be, was enormously reduced; that for that part of the sun there was practically no absorption; but instead of absorption an ex- cessive brilliance in that part of the spectrum where the dark line would otherwise have been. In the brighter portion between the two small spots (Fig. 57) the absorption was 122 THE CHEMISTRY OF THE SUN. [CHAP. replaced by an exceedingly brilliant radiation, so brilliant, indeed, that it is quite impossible to draw a diagram so as to give any idea of the intense brightness of some of these little spots of light which one sees in the spectroscope ; they fatigue FIG. 56. Contortions of Fline on disc. 1 and 2, rapid downrush and increasing temperature ; 3 and 4, uprush of bright hydrogen and downrush of cool hydro- gen ; 5, local downrushes associated with hydrogen at rest. the eye enormously, although they cover such a very small portion of the field of view, and with these the straight and evenly-bounded image of the slit had given way to an ii regular one. FIG. 57. Contortions of F line on disc, in connection with spots and npmshes of bright hydrogen. Accompanying this intense radiation there was a gradual fading away of the absorption line ; it waned, and faded, and became almost invisible ; while, on the other hand, on the other side or in other places, instead of getting a brilliant patch of IX.] MOTION FORMS. 123 light of the same width as the F line, we got one many times broader. We had also the absorption deflected to the left, or red end of the spectrum, and on that side it was i FIG. 58. Motion forms and lozenges. 1. Prominence much bent. 2. Prominence encroaching over limb bright line crossing black line. 3. Black line (F) curved downwards, sometimes nearly touching iron line below. 4. 5. Prominence nearly divided. 6, 7, 8. My own drawings. gradually fined or eased off, so that it was very difficult to determine exactly where this broadened, deflected F line actu- ally ceased to give us absorption ; whereas at the other side 124 THE CHEMISTRY OF THE SUN. [CHAP. where it changed its refrangibility towards the blue end of the spectrum, we had an enormous patch of light. An observation which I made in April, 1870, 1 pointed un- mistakably to a connection between these bright regions or lozenges, as I called them, and the prominences. On ex- amining the F line, I found that the bright chromospheric line not only intruded very much more than it generally did on to the disc, but that close to it I saw one of those bright lozenges to which reference has just been made, the dark hydrogen absorption appearing over it. Gradually this lozenge travelled very nearly to the edge of the sun, and before it got quite to the edge, I saw a tremendous contortion of the F line, that line being deflected first violently to the violet, and then as violently to the red end. Also, in the same locality, I saw the F line broken into two parts it was doubled. And what was going on while this was happening ? A prominence, obviously with its root some distance from the limb, had gradually travelled beyond the limb ; in appearance it became very much more elevated, and seemed, as it were, in perspective over the limb ; but what I saw first was very rapidly changed. I was not observing with an open slit, so I at once coined the term, " motion forms," because the forms observed did not in any way represent the shape of the prominences. The publication of these observations gave rise to much discussion. Professor Eespighi attributed the phenomena to disturbances in our air, or to the action of the heat of the sun's image on the slit, apparently forgetting that such a cause would affect all the lines in the spectrum, and not a few isolated ones only. In a subsequent memoir he admits having himself seen the phenomena. Father Secchi also for a long time discredited it, but eventually admitted that he had seen the phenomena. Another observer referred to my observations on the changes in question as " les illusions de Mr. Lockyer " ; but Professor 1 Proc. Roy. Soc. No. 120, 1870. ix.] MY "ILLUSIONS.' 1 125 Young from the first observed and recorded exactly similar phenomena. At length, after all this discussion, the new phenomena have been decided to be truly solar in their origin, and not due to any instrumental or other errors, as had been suggested. CHAPTER X. PRELIMINARY SEARCH AFTER EXPLANATIONS. 1. The Widening of Lines. IN the preceding chapter I have attempted to give a rapid and general sketch of the world of wonders in which we had been landed by the new method of solar observation. It became obviously of first importance to attempt to get some explanation of those phenomena, at all events, which .were considered ab- normal because they were new. It was also obvious that if laboratory experiments could not throw light upon such ap- pearances, for instance, as the widening of the lines in spots and at the base of some of the chromospheric lines, we should be in an impasse. With a view to get new facts, and among them some bearing upon the widening of spectral lines, Dr. Frankland and I com- menced, in 1869, a series of researches on gaseous spectra. 1 As the result of this inquiry, which extended over a period of three years, we came to the conclusion that laboratory ex- periments could help us in fact, Dr. Frankland had already succeeded in giving great width to the hydrogen lines, 2 and that 1 Proc. Roy. Soc. vol. xvii. p. 288. 2 These observations were never published, but in a paper communicated to the Royal Society in 1868, Dr. Frankland described a series of experiments on the combustion of jets of hydrogen and carbonic oxide in oxygen under a pressure gradually increasing to twenty atmospheres. In the case of hydrogen he found that under a pressure of two atmospheres " the previously feeble luminosity is CHAP, x.] WIDENING BY PRESSURE. 127 it was possible experimentally to vary the thickness of lines in the way we find them thickened in the sun. The means by which this was effected was an increased quantity or pressure of the vapour producing them. We found, for instance, that by increasing the pressure, say of hydrogen, and rendering the gas incandescent by means of the passage of an electric current, we thickened the lines, and especially the F line, exactly as it is thickened in the lower region of the chromo- sphere, and as some lines are thickened in spots. We carried on the experiments to a pressure of twenty atmospheres, and the spectrum was continuous with maxima indicating the places of the lines. Since by varying the pressure of hydrogen we can thus vary the thickness of the lines, it is possible to observe the spectrum of hydrogen in a tube, and to place the spectrum we get from the hydrogen in the sun, side by side with the spectrum we obtain from the hydrogen in the tube. We can vary the pres- sure of the hydrogen in the tube so that its spectrum exactly fits, so to speak, the spectrum of the hydrogen in the sun ; and hence we are enabled to determine the pressure of the atmosphere at the sun. And this, no doubt, will some day be done, but the thing is not quite so easy as this. There are more chemical substances than hydrogen in that part of the sun which so conveniently gives us these bright lines ; and we have in all questions of pressure not only to take into account the actual pressure of the hydrogen, but the combined pressure, so to speak, of all the vapours which exist in that stratum ; and so we have a very great inquiry before us, before a final estimate of the pressure can be given. very visibly augmented, whilst at ten atmospheres' pressure the light emitted by a jet about one inch long is amply sufficient to enable the. observer to read a newspaper at a distance of two feet from the flame, and this without any reflect- ing surface behind the flame. Examined by the spectroscope, the spectrum of this flame is bright and perfectly continuous from red to violet." Proc. Roy. Hoc. vol. xvi. p. 419. 128 THE CHEMISTRY OF THE SUN. [CHAP. It happened, however, that in the Geissler tubes employed when the molecules of the gas were most agitated so as to give the phenomena of great pressure, the current might be held to give us the highest temperature. It was therefore of importance to eliminate electrical effects altogether. This was accomplished by volatilising a piece of sodium in a glass tube in an atmo- sphere of hydrogen and observing its absorption spectrum. The experiments were conducted as follows : l (1.) Into a piece of hard glass combustion-tube, thoroughly cleaned and closed at one end, a few pieces of metallic sodium, clean and as free as possible from naphtha, were introduced. The end of the tube was then drawn out and connected with a Sprengel pump and exhausted as rapidly as possible. Hydrogen was then admitted, and the tube re-exhausted, and when the pressure was again reduced to a few millimetres, carefully sealed up. The tube thus prepared was placed between the slit plate of a spectroscope and a source of light giving a continuous spectrum. Generally, unless the atmosphere of the laboratory was very still and free from dust, the two bright D lines could be distinctly seen on the background of the bright continuous spectrum. The tube containing the sodium was then heated with a Bunsen flame and the spectrum carefully watched. Soon after the application of the heat, a dark line, thin and delicate as a spider's thread, was observed to be slowly creeping down each of the bright sodium lines and exactly occupying the centre of each. Next, this thin black line was observed to thicken at the top, where the spectrum of the lower denser vapours was ob- served, and to advance downwards along the D line, until, arriving at the bottom, they both became black throughout ; and if now the heat was still applied, thus increasing the density of the various layers of the sodium vapour, the lines began to broaden 1 Phil. Trans. 1872, p. 253. x.] WIDENING BY QUANTITY. 129 until, in spite of considerable dispersion, the two lines blended into one. The source of heat being now removed, the same changes occurred in inverse order; the broad band split into two lines, gradually the black thread alone was left, and finally that vanished, and the two bright lines were restored. (2.) This experiment was then varied in the following way. Some pieces of metallic sodium were introduced into a test- tube, and a long glass tube conveying coal-gas passed to the bottom, an exit for the gas being also provided at the top. The sodium was now heated and the flow of coal-gas stopped. In a short time the reversal of the D lines was complete. The gas was now admitted, and a small quantity only had passed when the black lines were reduced to threads. In this manner we were able to artificially reproduce the thickness observed in the D lines when they are thinnest in a high prominence and thickest in a deep spot, and this in a way which eliminated the effect of temperature. When we consider the widening of the lines seen in spots we are in presence of another cause than pressure to which that widening may be ascribed. There are many reasons for believ- ing that a spot is the seat of a downrush of comparatively cool vapours, so that in it we have an accumulation of those vapours, and the increased quantity of any particular substance leads to a widening of its spectral lines independently of the change in the same direction that would be produced by the pressure which also is doubtless increased. Experimentally we can vary the thickness of the D line of sodium so as to exhibit the effects to a large audience, and if we then examine the conditions under which this is effected, we get an additional strengthening of the above explanation of the thickening of the lines. 1 1 It is the more necessary to insist upon this, as quite recently the thickening has been attributed by M. Fievez to temperature. This would make the tempera- ture of the sun-spots lower than that of the sodium vapour in the experiment referred to above ! K 130 THE CHEMISTRY OF THE SUN. [CHAP. The experiment is managed as follows. The condenser of a Duboscq lantern being removed, a combination of a slit and lens is put in its place, the slit being placed inside the lantern close to the carbons at the principal focus of the lens. The carbons, about f- of an inch in diameter, have their ends filed perfectly flat, and a very thin layer of metallic sodium is spread on the lower pole. The rays of light proceeding through the slit fall upon the lens, which renders them parallel, and then enter two hollow prisms filled with carbon disulphide. They are next focused by means of a lens of long focus upon a screen. When the two poles are brought together the sodium line is seen to be very strongly reversed upon the screen, the line being about 15 inches wide in a spectrum of between 3 and 4 yards long, and it remains so, as long as the carbons are nearly in contact, till all the sodium is volatilised. By separating the poles the absorption line can be thinned down until it becomes a fine line, and then, by opening them a little more, the line can be obtained as a bright one. The cause of this is perfectly obvious. The temperature is practically the same all the time, but we have a very considerable quantity of sodium vapour surrounding the incandescent poles in the first instance. On the continued application of the heat this sodium vapour goes away by degrees, and we gradually deal with a smaller quantity, and as we deal with a smaller quantity the line thins. We therefore are justified in saying that when in a sun-spot we get the line of sodium considerably thickened, that is due to the fact that there is a greater quantity of sodium vapour present in that spot. These principles afford a satisfactory explanation not only of the widening of F and other lines near the limb, but of those bright patches or lozenges which are sometimes seen interrupting or accompanying the dark hydrogen lines on the surface of the sun itself, especially in the locality of spots. The fact that the x.] HYDROGEN EJECTIONS. 131 lines are bright indicates that the hydrogen which produces them is more intensely heated than usual, and we may assume, therefore, that it comes from below the region of higher tem- perature. We can therefore regard them as enormous ejections, or uprushes of hydrogen, so intensely hot that it radiates much more light than it absorbs. This gradually replaces the absorbing hydrogen, which is driven down again with such a considerable velocity that it commonly suffers a displacement towards the red, while the hot ascending hydrogen undergoes a similar alteration in the direction of the violet. In the great width of some of these bright patches we find a parallel phenomenon to the widening of the bases of the chro- mospheric lines and of the lines in spots, and this resemblance naturally leads to the suspicion that all these phenomena may be due to some common cause, that cause, as we have seen in the case of spots and chromospheric lines, being increased pressure or quantity of any particular vapour. 2. The Simplification of Spectra. We have already seen that the spectrum of the chromosphere is far simpler than the general solar spectrum, and that the lower regions of the chromosphere are more complex than the upper reaches. It was next observed that the spectrum of the same substance varied at different distances from the sun. Of the hydrogen lines, F was found to be the longest, 1 so that at a certain elevation all the other lines were left behind, and we had a stratum in which hydrogen was repre- sented by a single line only. Further observation disclosed the same fact in the case of magnesium. Its lines & 1 , & 2 , were of nearly equal height, while 5 4 was much shorter. To explain these facts Dr. Frankland and I continued our researches on gaseous spectra, and found that under certain 1 See letter to Mr. Warren De La Rue, Oct. 23rd, 1868, printed in Proc. Roy. Soc. No. 105, 1868. K 2 132 THE CHEMISTRY OF THE SUN. [CHAP. conditions of temperature and pressure, the very complicated spectrum of hydrogen could be reduced in the laboratory to one line in the Hue-green corresponding to F in the solar spectrum ; and also that the equally complicated spectrum of nitrogen is similarly reducible to one bright line in the green, with traces of other more refrangible faint lines. From a mixture of the two gases we obtained a combination of the spectra in question, the relative brilliancy of the two bright green lines varying with the amount of each gas present in the mixture ; and by remov- ing the experimental tube a little further away from the slit of the spectroscope, the combined spectra were reduced to the two bright lines. By reducing the temperature all spectroscopic evidence of the nitrogen vanished ; and by increasing it, many new nitrogen lines made their appearance, the hydrogen line always remaining visible. 1 These experiments enabled us at once to connect the two series of observations. It was only necessary, in fact, to assume that, as in the case of hydrogen and nitrogen, the spectrum became simpler when the density and temperature were less, to account at once for the reduction in the number of the lines visible in those regions where the pressure and temperature of the absorbing vapours of the sun would be reduced. The results of the continuation of this line of inquiry will be stated further on. 3. The Contortion of Lines. Those strange contortions which as we have seen are commonly observed in the bright chromospheric lines, and in the dark lines on the disc of the sun, were at first a very great puzzle, not only to me, but to two eminent physicists whom I consulted at the time. The accompanying phenomena, however, soon suggested the explanation which, unknown to me then, had been applied 1 Proc, Roy. Soc. No, 112, 1869, x.] DOPPLER'S PRINCIPLE. 133 by Secchi and Huggins, a few months earlier in the same year, to somewhat similar effects observed in the spectra of stars. The explanation depends upon a view first advanced by Doppler in 1842, that the light from a moving light-source is not the same in all its qualities as light from a fixed one. Doppler 's idea will be gathered from the following analogies from sound : The colours which we see in the spectrum are exactly analo- gous to the notes which we hear in a piano when we go from one end of the scale to the other. Doppler imagined the equi- valent of a piano going away from or coming towards the listener with considerable velocity a velocity comparable, in fact, to the velocity of sound in air. It is clear that under these circumstances we should no longer get true concert pitch for each note, since the note which gives us a certain tone, because it produces in the air so many waves per second, will change its tone if the source of the note is coming to us. Take, for instance, a tuning-fork giving concert C, and imagine it rapidly coming to us : the waves of sound will be crushed to- gether, we shall have more waves in a second falling on the ear, and we shall get a higher note. If we imagine, on the other hand, the tuning-fork to be going away from us, the notes will be paid out at longer intervals, so to speak, and we shall get a lower note. In neither case shall we continue to have concert C. A very familiar instance in which we do get this change of pitch due to change of motion, is produced in these days of very rapid railway travelling. Any of us who have been at a country railway station when an express is coming by will know that as the train approaches us the note of its whistle is at one pitch, and as it goes from us after passing, it changes and gives a lower note according to the velocity of the train. An experimental illustration of this principle is to attach a whistle to the end of a long india-rubber tube. If then a person sounds the whistle 134 THE CHEMISTRY OF THE SUN. [CHAP. by blowing through the open end of the tube, and while still blowing whirls it round rapidly in a vertical plane in which an observer is standing, that observer will note that when the whistle is approaching him in one part of the curve, and the waves are therefore being crushed together, the note will appear higher than when it is receding from him in the opposite part of the curve, where the waves are being, as it were, pulled asunder. Now to apply this to the light from the hydrogen promi- nences in which the effects are most pronounced. The long notes of light are red, and the short notes are blue, and if we sharpen or shorten any light note in any part of the spectrum we shall give that light a tendency to go towards the blue, and if we lengthen or flatten it we shall give it a tendency to go towards the red ; so that, for instance, if a mass of hydrogen gas giving the line or note in the green indicated by F, is approach- ing us with a velocity comparable to the velocity of light, the line will change its position in the spectrum towards the blue ; and if we are careful to note the exact amount of change of refrangibility, as it is called, we shall have then an absolute method of determining the rate of relative motion of that mass of gas. This will help us in more ways than one. Suppose we observe the gas at the limb of the sun, we shall then, if we get any change of refrangibility, be justified in calling it a solar wind, because the motion thus indicated would be very nearly parallel to the surface of the sun ; but if on the disk of the sun itself take a spot, for instance, in the very middle of the disk we get any change of wave-length such as I have referred to, it is perfectly clear that we shall no longer be dealing with what we can justly call a wind, it will really be an upward or down- ward current. So that this principle enables us at the limb of the sun to determine the velocity of solar winds, and at the centre of the sun to determine the velocity of uprushes or downrushes. If the hydrogen lines were invariably observed to broaden out x.] FIZEAU'S RESEARCHES 135 on both sides, the idea of movement would require to be received with great caution ; we might be in presence of phenomena due to greater pressure, both when the lines observed are bright or black upon the sun; but when they widen out sometimes on one side, sometimes on the other, and sometimes on both, this explanation was soon seen to be untenable, as Dr. Frankland and myself in our researches at the College of Chemistry had never failed to observe a widening out on both sides of the F line when the pressure of the gas had been increased. In the explanation, I have been compelled to refer to a light- note, and as a matter of fact, on the sun the phenomena are limited to certain lines. It was on this point that Doppler was in error ; he dealt with the total light, which could never be changed in consequence of the change of motion he assumed. The true criterion was first pointed out by Fizeau, who showed that the change must take place at a particular wave- length to be noticeable at all. M. Fizeau's researches in this direction are so little known in this country that I give the account of them at length in a foot-note. 1 1 "Siun corps sonore emettaut un son continu et toujours identique se meut avec tme vitesse comparable a celle du son, les ondes sonores ne seront pas symctriquement disposees autour du corps sonore, comme cela a lieu lorsqu'il est au repos ; mais elles seront plus rapprochees les unes des autres dans la region vers laquelle aura lieu le mouvement, et plus eloignees dans la region opposee, pour un observateur place en avant ou en arriere du corps sonore ; le son sera done different, plus aigu dans la premiere position, plus grave dans la seconde. "Si 1'observateur a son tour est suppose en mouvement, le corps sonore restant immobile/ le resultat sera semblable, mais la loi du phenomene est differente. "En calculant les vitesses qui correspondant aux intervalles de la gamme on trouve les nombres suivants : pour produire une elevation d'un dcmi-ton, le corps sonore doit avoir une vitesse par seconde de 21'25, pour un ton majeur 37 '8, pour la tierce 68, pour 1'octave 170. Dans le cas du corps sonore immobile etpour obtenir les niemes notes 1'observateur doit avoir les vitesses 22'6, 42'5, 85, et340. Lea sons emis ou recus dans des directions differentes de celles du mouvement se calculent en projetant la vitesse sur la nouvelle direction. "L'auteur donne la description d'un appareil qu'il a employe et au moyen duquel on peut verifier et demonstrercommodemont cos curieuses proprietes du son, 136 THE CHEMISTRY OF THE SUN. [CHAP. To see the bearing of this, let us suppose that one of the constituents of the solar atmosphere, let us say hydrogen, while it is giving out its light is moving towards us at the rate of say fifty miles (= 80,466 metres) a second. Now the wave-length of one of the lines due to hydrogen, F, is, as we have seen, 0-00048606 at the normal velocity. With a higher velocity, therefore, the number of crests per second reaching the eye must be greater, and therefore the effective wave-length must be shorter. In the spectrum this shortening of the wave is indi- cated by the position of the line F changing towards the violet the region of shorter waves. If we supposed the hydrogen receding from the eye, then the dans le cas du mouvement du corps sonore. Cette appareil est fonde sur le principe des'roues dentees ;de M. Savart, mais la disposition est inverse. An lieu de dents mobiles recontrant dans leur mouvement un corps elastique fixe, c'est le corps elastique qui est place sur la circonference d'une roue et qui recontre dans son mouvement des dents fixes placees sur la concavite d'un arc exterieur immo- bile, 1'on a ainsi un appareil fixe qui jouit de la propriete d'emettre des sons differents dans chaque direction particuliere. Pour une certaine vitesse de rotation par example on aura en avant le son fondamental, en arriere 1'octave, et toutes les notes de la gamme dans des directions intermediares. "En appliquant ces considerations a la lumiere on arrive a des consequences curieuses et qui pourraient acquerir de I'importance si 1'experience venait a les confirmer. Un mouvement tres-rapide et comparable a la vitesse de la lumiere, attribue au corps lumineux ou a 1'observateur, aura pour eftet d'alterer la longueur d'ondulation de tous les rayons simples qui composent la lumiere re?ue dans la direction du mouvement. Cette longueur sera augmentee ou diminuee suivant le sens du mouvement. Considere dans le spectre, cet effet se traduira par un emplacement de raies correspondant au changement de la longueur d'ondu- lation. ' ' En calculant la value du deplacement angulaire de la raie D dans la cas ou le corps lumineux aurait la vitesse de la planete Venus, le spectre etant forme au moyen d'un prisme de flint de 60 on trouve 2"'65. " Pour le cas ou 1'observateur seul serait en mouvement et anime d'une vitesse egale a celle de la terre on trouve 2" '2 5. "En supposant que Ton mesure les deviations doubles et que Ton se place successivement dans des conditions ou les mouvements en question seraient de signe contraire, ces quantites peuvent etre quadruplets, et Ton a 10" '6 et 9 pour les valeurs precedentes. "L'auteur termine en examinant si ces consequences pourront etre soumises a 1'observation, et il pense que les difficultes ne sont pas telles qu'on ne puisse esperer de les surmounter." Societe Philomathique de Paris. Extraitsdes Proces- verbaux des Sciences, 23 Dec. 1848, p. 81. x.] CONTORTIONS EXPLAINED. 137 position of the line would be changed towards the region of longer waves i.e. towards the red. Let us suppose such a change to be observed, say a change of the F line, the normal wave-length of which is 0'00048606 from that position to 0*00048716. Obviously the wave has been lengthened by the recession of the source of light from the eye, and the amount of recession, about thirty-nine miles a second, is measured by the increased length of wave, the difference in the wave-length bearing the same ratio to the total wave-length as the difference in the velocity bears to the velocity of light. 1 We can now interpret the meaning of the strange doubling of the F line shown in Fig. 54. We there get, according to the principle just laid down, an indication of the fact that the hydrogen up to a certain height was very nearly at rest, and that beyond, part of it was torn away, the line being deflected towards the blue, indicating that it was approaching us. Now the Fraunhofer lines in the diagram may be looked upon as so many milestones which enable us to measure by the deflection the number of miles traversed by the gas in one second; for these deflections are nothing more nor less than alterations of wave-length, and, thanks to Angstrom's map, we can measure distances along the spectrum in TwoWo-zr mm., and we know that an alteration of Tiruwoinj- mm - * n tne wave-length of the F line towards the violet means a velocity of thirty-eight miles a second towards the eye ; and that a similar alteration towards the red means a similar velocity from the eye ; so that carrying the part of the line which has the greatest deflection from the normal down to the dots, we find that the velocity of the solar wind under observation at that time was something like 114 miles per second. In the second figure this same prominence is seen a short time afterwards. The tremendous rush of hydrogen has descended somewhat nearer the sun, and bringing that in the 1 See Clerk-Maxwell, Phil. Trans. 1868, p. 532. 138 THE CHEMISTRY OF THE SUN. [CHAP. x. same way down to our milestones, we can give that velocity at something like fifty miles per second. The wind velocities measured in this way have amounted to 140 miles a second, while the convection currents give us velocities which very often amount to forty or sixty miles a second. This method enables us to determine a matter which a few years ago we could not have determined in any other way. I refer to the fact that the motions of the solar winds are to a very great extent cyclonic. Eef erring again to the series of drawings (Fig. 55), it will be seen that in the first of these drawings the hydrogen line indi- cates by its change of refrangibility towards the red that the gas is receding from us. In the third diagram we see that in that part of the prominence the rays were being deflected to- wards the violet ; that is to say, they were approaching us. In the middle drawing, which represents what was seen when the entire cyclone was included in the slit, we get indications both of recess and approach. Now if anybody in the moon had as good a method as this of measuring an earthly cyclone, he would see exactly this sort of thing the part of the cyclone receding from him would give a deflection in one direction, a complete view of the cyclone would give him loth deflections, because he would get currents going in both directions, and on the other part of the cyclone he would get a deflection in the other direction. CHAPTEE XL THE NEW METHOD APPLIED TO LABORATORY WORK. ATTENTION must next be drawn to another method of obser- vation of spectra, or rather to the extension of the method we have seen applied to the localization of solar phenomena, to a different line of work. Kirchhoff, as we have seen, examined the sun as a whole, and compared it with the light of a light-source as a whole. We have stated the difference in the results obtained when we pass from the method of observing the sun as a whole to that other more detailed one of observing a small portion at a time. Now is it worth while to do this with the light-source ? that is the question. Let us deal with some simple considerations which should enable us to give an answer to this question. The spectroscope, however simple or complex it may be, is an instrument which allows us to observe the image of the slit through which the light enters it, in the most perfect manner. If the light contains rays of every wave-length, then the images formed by each will be so close together that the spectrum will be continuous, that is, without break. If the light contains only certain wave-lengths, then we get certain, and not all, of the possible images of the slit, and the spectrum will be discontinuous. Again, if we have an extremely complex light-source, let us 140 THE CHEMISTRY OF THE SUN. [CHAP. say a solid and a mixture of gases giving out light, and we allow the light to enter, so to speak, indiscriminately into the spec- troscope, then in each part of the spectrum we shall get a summation a complex record of the light of the same wave- length proceeding from all. the different light- waves. But if by means of a lens we form an image of the light-source, so that each particular part shall be impressed in its proper place on the slit-plate, then in the spectrum the different kinds of light will be sorted out. FIG. 59. Arrangement for projecting an image of a candle flame on^the slit-plate of a spectroscope. There is a simple experiment which shows clearly the dif- ferent results obtained. If we observe the light of a candle or a lamp with the spectroscope in the ordinary manner, that is by placing the candle in front of the slit at some little distance from it, we see a band of colour a continuous spec- trum ; and in one particular part of the band we see a yellow line, and occasionally in the green and in the blue parts of the band other lines are observable. Now, if we throw an image of the candle or lamp on to the slit the slit being horizontal and the image of the candle vertical we then get three perfectly XI.] USE OF THE LENS. 141 distinct spectra. We find that the interior of the flame, that is the blue part (best observed at the bottom), gives us one spec- trum, the white part gives us another, while on the outside, so faint as to be almost invisible to the eye, there is a region which gives us a perfectly distinct spectrum consisting of a line in the FIG. 60. Showing arrangement of lens with large spectroscope. A, collimator ; B, observing- telescope ; c, spark ; D, lens. yellow. In this way there is no difficulty whatever in deter- mining the co-existence of three light- sources, each with its proper spectrum, in the light of a common candle or lamp. What a pity that Wollaston did not use such a lens in 1804 ! We see in a moment that much the same condition of affairs will be brought about if, instead of using a candle, we use an 142 THE CHEMISTRY OF THE SUN. [CHAP. electric arc or spark in which the pure vapour of the substance which is being rendered incandescent fills the whole interval between the poles, the number of particles being smaller and the degree of incandescence being less intense at the sides of the arc. Take, for instance, the case of the volatilisation of iron in an electric arc. Since light from every part of the arc placed in front of a slit must enter every part of it, the differences between the light proceeding from the upper pole or the lower pole, or from the globule of iron which is being melted and exists in a liquid form, or from the vapours of iron which surround that liquid globule if there are any such differences are absolutely lost in observations made in the ordinary way. But if we introduce the lens between the light- source and the slit of the spectroscope ; if, as we throw an image of the sun on the slit, we throw an image of the light-source on the slit, we ought to see any difference that may exist. We readily do see that there are very considerable optical differences in the various parts of the image of the light-source. We have the upper and lower pole, the globule of iron vola- tilising, and the vapour, both in the arc, properly so-called, and the accompanying flame, each with its own special spectrum. By an easily understood artifice we can throw an image of a horizontal arc on a vertical slit : the slit will give then the spec- trum of a section of the arc at right angles to its length. The vapour which exists furthest from the core of the arc has a much more simple spectrum than that of the core of the arc itself, the spectrum of the core consisting of a large number of lines, all of which die out until the part of it furthest from the centre gives but one line. It is obvious that if we throw the image of the electric arc on the slit in this manner we can examine the vapour without being inconvenienced by the bright continuous spectrum of XL] LOCALIZATION OF PHENOMENA. 143 the light from either of the carbon poles. It is also obvious that if we arrange the slit horizontally while the current is passing in a vertical direction from one pole to the other, we shall be able, by moving the slit upwards, to see if there are any differences observable in the vapour, first in the region where we have intense boiling and volatilisation going on, and in the necessarily cooler region where the arc is in contact with the outer air. In this way the spectrum of each substance furnishes us with long and short lines, the long lines being common to the more and the less intensely heated parts of the arc, because we are FIG. 61. Arrangement for obtaining long and short lines. Image of the horizontal arc on slit-plate of spectroscope. not dealing with a simple section, and the short lines being confined to the more heated ones only. Whether we use the artifice of a horizontal arc with a vertical slit, or a vertical arc with a horizontal slit, does not matter, pro- vided we keep the slit immersed in the light of the arc, and thus get rid of the light from the poles, and at the same time arrange the slit so that we can compare the light in the interior portion of the light-source with the light nearer its boundaries if we take all these precautions we shall then get in the case of every substance such a result as shown in Fig. 62. We have in the centre a complete spectrum, its intensity being gradually toned 144 THE CHEMISTRY OF THE SUN. [CHAP down, and some of the lines being left behind as we look up and down towards the boundary where we have the spectrum of that portion of the arc which was the last to retain its lumi- nosity in consequence of cooling. XI.] LONG AND SHORT LINES. 145 If we take horizons from the central portion of the diagram to the point most distant from that central axis, we find, in the case of every substance, the light at last becomes prac- tically monochromatic. The vapour at this distance from the central axis radiates to us light spectroscopically represented by one line. As we get nearer and nearer the centre of agitation the spectrum becomes more complex, until at length, when very near the central axis, we get a great many short lines introduced, so that the spectrum at that point is most complex. This I am anxious to draw attention to with FIG. 63. Spectrum of sodium, showing the long and short lines. some insistence, because we shall understand at once the terms long and short lines, about which there will be a great deal in the sequel. Figure 63 shows the much more simple spectrum of sodium. The longest line in the middle is D, that to the left, the line in the green, and we find that one set of the double lines excels all the others, and reaches a greater distance from the central axis. An electric lamp can be arranged to show the long and short L 146 THE CHEMISTRY OF THE SUN. CHAP. line? of sodium on a screen to a large audience. The arrange- ment is rather a delicate one, but the point is that we have not, as in the case of the electric lamp as ordinarily used, vertical FIG. 64. Another arrangement of apparatus for projecting the long and short lines on a screen, using a horizontal slit in the lamp close to the poles. poles, but horizontal ones, and we have a vertical slit close to the poles in the very middle of the lamp, so that if the experiment is carried far enough we can then prove the accuracy of the state- ment that the line is an image of the slit, because the slit XL] FIRST GLIMPSES. 147 generally melts, and we see the shape of the lines varying on the screen as the melting of the slit goes on. I began work on this method in 1869, but long before this other observers had obtained a glimpse of some of the pheno- mena, so obvious when it is employed. Thus Dr. Gladstone, in I860, 1 in the course of some experi- ments with the light produced by the passage of an electric current through an interrupted stream of mercury, remarked that "the more intense lines appeared broad on account of irradiation, and of greater extent because the slightly luminous environment of the spark gives a perceptible amount of those rays." This observation was followed up in 1862 by Professor Stokes, 2 who used the spark itself instead of a slit, and remarked that the metallic lines are "distinguished from air lines by being FIG. 65. Copy of Dr. Miller's spectrum of cadmium, showing the " dots." formed only at an almost insensible distance from the tips of the electrodes, whereas air lines would extend right across." Miller, 3 who used a slit and a spark close to it, referring to his photographs of electric spectra, remarked, ' The marginal extremities of the metallic lines leave a stronger image than their central portions," and the extremities of these interrupted lines he terms "dots." The accompanying woodcut of the spectrum of cadmium, as photographed by him, will show the expressiveness of the word, On the same subject Eobinson 4 writes, "At that boundary of the spectrum which corresponds to the negative electrode (and 1 Phil. Mag. s. 4. vol. xx p. 250. ' 2 Philosophical Transactions, vol. clii. 1862, p. 603. a Op. cU. p. 877. Op. cit. p. 947. I, 2 148 THE CHEMISTRY OF THE SUN. [CHAP. in a much less degree at the positive) extremely intense lines are seen, . . . which, however, are short." Thalen observed this localisation to a certain extent, doubt- less on account of the long collimator which he employed. 1 In 1869, I observed that in a solar storm on the edge of the sun, when -the magnesium lines were visible, the lines of that metal did not all attain the same height. Thus of the ~b lines, b l and lr were of nearly equal height, but 6 4 was much shorter. Next I found that of the many iron lines observed by Angstrom, only a very few were indicated in the spectrum of the chromosphere when iron vapour was injected into it from below. Dr. Frankland and myself were enabled at once to connect these phenomena by reasoning that, as in the case of hydrogen, the spectrum became simpler and the lines thinner where the density was less. To test the truth of this assumption by some laboratory experiments, we took the spark in air between two magnesium poles, so separated that the magnesium spectrum did not extend from pole to pole, but was visible only for a little distance, indicated by the atmosphere of magnesium vapour round each pole. We then threw an image of the magnesium pole and surrounding vapour on the slit of the spectroscope exactly as the sun and its surrounding vapours were thrown on the slit in the observatory. 2 1 He remarks : " II y a aussi des raies brillantes qu'on n' observe que dans des cas exceptionnels, comme, par exemple, quaud la quantite de la substance soumise a 1' experience est tres-abondante ou quand 1'incandescence devient tres-vive, Ces raies qui se presentent ordinairement aux bords du spectre sous la forme de points d'aiguille, meme quand les autres raies du metal forment des lignes continues a travers du spectre, out ete representees surlaplanche par des lignes tres-courtes. " Memoire sur la determination des longueurs d'onde des raies metalliques, p. 1 2, printed in the Nova Ada Regioe Societatis Scientiarum Upsaliensis, ser. iii. vol. vi. Upsala, 1868. 2 Proc. Roy. Soc. No. 115, 1869. ; xi.] SPECTRUM OF MAGNESIUM. 149 We then carefully examined the disappearance of the b lines and found that they had behaved exactly as they do on the sun. Of the three lines the most refrangible was the shortest. In fact, had the experiment been made in hydrogen instead of in air, the phenomena presented in the telescope would have been almost perfectly reproduced ; for each increase in the temperature of the spark caused the magnesium vapour to extend further from the pole, and increased the disparity in the relative lengths of the lines. I have given the above as the first experiment which indi- cated that this method of observation seemed likely to bring us much new knowledge, and indeed it will not be difficult to show that the results obtained by this method have a very important bearing upon every question connected with solar chemistry. When these spectra were observed the spectra of the longs and shorts of course we had a perfectly new set of phenomena to deal with. In all spectra observed on the old method all the lines had been practically seen of the same length, or else lengths shown in maps had represented intensities. But now we soon found that we had, in the case of each chemical substance, to deal with the remarkable fact that when the spectrum of that chemical substance was examined in this way, some of the lines were long and some of them short ; and the question naturally arose, How is it that some of the lines are long and some of them short ? That question was an exceedingly difficult one to answer. I do not know that it has been thoroughly answered yet. More than this, we wanted to know ,how the long and short lines behaved when the spectrum of any particular chemical substance was seen in the chromosphere. We shall see that while researches were being made for the answer to these questions, certain general statements soon became possible which are of very considerable importance to us in our inquiry. CHAPTER XIT. SOME RESULTS OF THE " LONG AND SHORT " METHOD. 1. The Short Lines die out ~by mixing. IN order to apply this new knowledge to the investigation of the chemistry of the sun, the first thing to be done was to prepare maps showing the long and short lines of those sub- stances stated by Angstrom, Kirchhoff and others, to exist in the solar atmosphere, and then to see if it was possible to get a lead by comparing these with the lines observed either in the general spectrum of the sun, or in the spectrum of any particular por- tion be it spot or prominence. This work was begun in 1870. The following facts forced themselves at last upon the attention : x 1. When a metallic vapour was subjected to admixture with another gas or vapour, or to reduced pressure, I found that its spectrum became simplified by the abstraction of the shortest lines and by the thinning of many of the remaining ones. To observe the effect of reduction of pressure, the metals were inclosed in tubes in which a partial vacuum was produced. In all these experiments it was found that the longest lines invariably remained most persistently. 2. When we used metals chemically combined with a metal- loid in other words, when we passed from a metal to one of it? salts (I used the chlorides) only the longest lines of the metal 1 Phil. Trans. 1873, p. 253 et scq. CHAP, xii.] SPECTRUM OF A MIXTURE. 151 remained. Their number was large in the case of elements of low atomic weight, and small in the case of elements of high atomic weight. 3. When we used metals mechanically mixed, of the smallest constituent, only the longest lines remained. On this point I must enlarge somewhat by referring to a series of experiments recorded in the Philosophical Transactions (1873). 1 A quantity of the larger constituent, generally from live to ten grammes, was weighed out, the weighing being accurate to the fraction of a milligramme ; and the requisite quantity of the smaller constituent was calculated to give, when combined, a mixture of a definite percentage composition by weight (this being more easily obtainable than a percentage composition by volume). The quantities generally chosen were 10, 5, 1, and O'l per cent. In a few cases, with metals known to have very delicate spectral reactions, a mixture of O'Ol per cent, was prepared. Observations were then made of the spectrum of each speci- men, and the result was recorded in maps in the following manner : First, the pure spectrum of the smallest constituent was observed, and the lines laid down from Thalen's map. The series thus mapped was as follows : Tin + Cadmium percentages of Cadmium 10, 5, 1, 0'15 Lead + Zinc Zinc 10, 5, 1, O'l Lead + Magnesium Magnesium 10, 1, O'l, O'Ol The observations showed that the lines of the smallest constituent disappeared as the quantity got less. These ob- servations then rendered possible such a general statement as this, for instance, that if we take, say, some iron, observe its spectrum, and then mix some manganese with it, and observe the spectrum of the mixture : if the quantity of manganese 1 Part ii. p. 479 et scq. 152 THE CHEMISTRY OF THE SUN. [CHAP. is very small, we shall only get the longest line of man- ganese ; if the quantity of manganese is increased, the next longest line will come in ; and so on. So that if the spectrum of any specimen of iron was observed, it was at once easy to see whether there was an impurity of man- ganese in that iron, if we made the admission that the spectra of iron and manganese, and so on, were the spectra of bodies not decomposable at the temperature which we were employing. If, for instance, there was a great quantity of manganese exist- ing as an impurity in the iron, we got a great many lines, and of course with the quantity of admixture the number of lines would go on increasing until we might have fifty per cent, of each, when we should practically have the greatest number of lines of iron and the greatest number of lines of manganese we could ever get together. This was the general statement, but certain exceptions were noted. Although we had here the germs of a quantitative spectrum analysis, the germs only were present, because, from the exist- ence of several " critical points," and great variations due to other causes, the results obtained were not constant. In a subsequent research on the gold-copper alloys used in the coinage, 1 Mr. Roberts, the Chemist of the Mint, and myself were able to show that the shortening in the length of the lines by reduced quantity was such a definite physical effect that a difference of yo-Juir part of copper in gold could be detected. In the case of the metals, the electric spark was passed between metallic poles. For gases of course this method could not be employed. In practice there are very great objections to the use of Geissler tubes, one very valid objection being that the gas becomes much, less luminous as its pressure is reduced ; but in fact, the work connected with gaseous spectra can be done at atmospheric pressure ; we can get the lines down, 1 Phil. Trans. 1873, Part ii. p. 495 xn.j COINCIDENT LINES. 153 not by reducing the pressure, but by reducing the quantity of any particular gas in a mixture. If we take, for instance, a spark in air and observe its spec- trum, we find the lines of the constituents of atmospheric air considerably thick ; but if we wish to observe accurately the lines of one of the constituents, say oxygen, these should be fine, in order to enable us to determine their absolute position. To accomplish this, the spark is taken in a glass vessel with two adits and one exit tube. If we wish to observe the oxygen lines fine, the vessel is flooded with nitrogen so that there is only a small quantity of oxygen present, and pass the current between the inclosed electrodes. If we wish to observe nitrogen lines fine, it" is flooded with oxygen, so that there is only a small quantity of nitrogen present. In this way, by merely making a mixture in which the gas to be observed is quantitatively reduced, so that the lines which we have to investigate are just visible in their thinnest state, we have a perfect means of doing it without any appa- ratus depending on the use of low pressures. A very great simplicity of work is thus introduced. 2. The Long Lines are in some cases common to many Spectra. While on the one hand, as we have seen, the short lines are reduced in number with reduced quantity of the substance producing them, it soon became apparent on the other, that the long lines had a trick of making their appearance in many spectra. Any one who will consult Thalen's tables, or Kirch- hoff's or Angstrom's map, will note the many coincidences. This point, then, deserves careful study. The following general statements were soon hazarded, with the proviso that it was possible that further inquiry might modify them. 1st. If the coincident lines of the metals are considered, those cases are rare in which the lines are of the first order of 154 THE CHEMISTRY OF THE SUN. [CHAP. length in all the spectra to which they are common, especially if the volatility of the metals in question is about the same : those cases are much more common in which they are long in one spectrum and shorter in the others. 2nd. As a rule, in the instances of those lines of iron, cobalt, nickel, chromium, and manganese (substances volatilised with difficulty), which are coincident with lines of calcium (which volatilises easily), the calcium lines are long. Hence we are justified in assuming that lines seen in the spectra of iron, cobalt, nickel, chromium, and manganese, coincident with long and strong lines of calcium, are really due to traces of the latter metal occurring in the former as an impurity. The question then naturally arose whether, in cases of coin- cidences of lines found between the lines of various spectra, the line may be fairly assumed to belong really to that one in which it is longest and brightest, and make its appearance merely as an impurity in all the others. Indeed, a pro- longed examination of various spectra was not required to afford evidence not only of the great impurity of most of the metals used, but of the fact that many of the coincidences observed by Thalen and others might be explained without having recourse to the idea of physical coincidences. A study of Angstrom's map of the solar spectrum, to take an instance, shows many cases in which a line has been observed to be common to two or more spectra ; and this is especially the case with the lines of iron, titanium, and calcium, nearly every other solar metallic spectrum exhibiting one or more cases of coincidence with the latter. In those cases which were exa- mined in the light of the " long and short lines," it was frequently found that a line coincident in different spectra was long and bright in only one of them, and that in others it was short, or faint, or both ; or even, in certain specimens of the substances, altogether absent from the spectrum. As an instance of this difference of behaviour, the following xii.] IMPURITIES. 155 cases in the spectra of calcium and strontium may be given. 1 The longest line in the visible portion of the calcium spectrum (Thalen), wave-length 4226 '3, is found in the strontium spectrum as a line of medium length. 4607 '5, one of the longest lines of strontium, appears in the calcium spectrum as a short line. Another very long line of strontium occurs at 42 15 '3, in close proximity to the longest calcium line, and, according to Thalen, occurs also in the spectrum of that metal. We have here, then, two metals with two lines common to their spectra ; and it is found that the line which is long and bright in one spectrum, is faint in the other ; and with regard to a third line, one observer finds it in both spectra, the other in one only, and after many attempts succeeds in observing it in the second, but only in a specimen known to be contaminated with the first. The simplest explanation of the case, bearing in mind the facts already dwelt upon, is that the calcium used to produce the spectrum was contaminated to a certain extent with stron- tium, the strontium in turn containing calcium a state of things which a moment's consideration will show to be not only possible but most probable, the close chemical relation of the two metals and the extreme difficulty of making even an approximate separation when mixed, being well known. The long lines of calcium at wave-lengths 4226, 3968, and 3933, occur also in the spectrum of iron, cobalt, nickel, barium, and other metals, as observed in the arc using carbon poles, and assume very considerable proportions, equalling or surpassing in length many undoubted lines of those elements which are less easily volatilised by the action of the current; on the other hand, the iron lines at wave-lengths 4071, 4063, and 4045, occur in calcium, strontium, and barium, and in other metals under like conditions. 1 Proc. Roy. floe. No. 147, 1873, p. 511. 156 THE CHEMISTRY OF THE SUN. [CHAP. Again, the longest lines of aluminium (wave-lengths 3961 and 3943) occur usually in the spectrum of iron as longish lines, and are to be found in the arc spectra of cobalt, nickel, calcium, strontium, barium and other metals, where they are even longer than some of the true lines of the metals in which they occur owing to their lower volatility. It will be seen, then, that amongst the first questions raised was that of impurities, and it became clear that if this view were well founded, the revision of all cases of coincidences re- corded by previous observers became absolutely necessary, since at last we had a method of eliminating impurities with certainty. 3. Light thrown on the Fraurihofer Lines. There was another point in which our knowledge was at once increased in the most definite manner. Only those who knew least about solar matters took KirchhofFs statement as to the identity of solar and terrestrial spectra au pied de la lettre ; and Angstrom had already pointed out discrepancies in the case of aluminium and zinc. Hence I attempted to see whether there was anything to help us in the simplification of spectra brought about by passing from the metal to the chloride. It was soon found that when we compared the spectra of metallic vapours reversed in the solar spectrum, such spectra being mapped by the new method, i.e., showing the long and short lines, the reversed lines in the spectrum were invariably those which are longest. Here we had, in fact, a new test to apply to the reversal of solar lines, and a guide of the highest value in spectrum observations of the chromosphere and photo- sphere. It was seen at once that to the last published table of solar elements (that of Thalen) must be added zinc, alu- minium, and possibly strontium, as a result of the application of the new test which it seemed worth while to pursue further. In order to continue the inquiry under the best conditions, complete maps of the long and short lines of all the elements xii.] A PRELIMINARY SEARCH. 157 would have been of course necessary, but it was not absolutely essential for the purposes of a preliminary inquiry to wait for such a complete set of maps, since we had already learned that the lists of lines given by the various observers may be made to serve as a means of differentiating between the longest and shortest lines, because, as we have seen, the lines given at a low temperature, by a feeble percentage composition, or by a chemical combination of the vapour to be observed, are pre- cisely those lines which appear longest when the complete spectrum of the pure dense vapour is studied. Now with regard to the various lists and maps published by various observers, it was known (1) that very different tem- peratures were employed to produce the spectra, some investi- gators using the electric arc with great battery-power, others the induction-spark with and without the jar ; (2) that some observers employed in certain cases the chlorides of the metals the spectra of which they were investigating, while others used the metals themselves. It is obvious, then, that these differences of method could not fail to produce differences of result ; and accordingly, in refer- ring to various maps and tables of spectra, we find that some include large numbers of lines omitted by others. A reference to these tables, in connection with the methods em- ployed, shows at once that the large lists are those of observers using great battery-power or metallic electrodes, the small ones those of observers using small battery-power or the chlorides. If the lists of the latter class of observers be taken, we shall have only the longest lines, while those omitted by them and given by the former class will be the shortest ones. In cases, therefore, in which I had not then already (1873) mapped the spectrum in the new way, I in the first instance took the longest lines as thus approximately determined. A preliminary search having been determined on, I endea- voured to get some guidance by seeing if there was any quality 158 THE CHEMISTRY OF THE SUN. [CHAP. which differentiated the elements already traced in the sun from those not traced ; lists were prepared showing broadly the chief chemical characteristics of the elements traced and not traced. This was done by taking a number of the best known com- pounds of each element (such, for instance, as those formed with oxygen, sulphur, chlorine, bromine, or hydrogen), stating after each whether the compounds in question were unstable or stable. Where any compound was known not to exist, that fact was indicated. Two tables were thus obtained, one containing the solar, the other the more important non-solar elements (according to our knowledge at the time). These tables gave me, as the differentiation sought, the fact that in the main the known solar elements formed stable oxygen- compounds. I have said in the main, because the differen- tiation was not absolute ; but it was sufficiently strong to make me commence operations by searching among the Fraunhofer lines for the longest lines of the outstanding strong oxide- forrning elements. The immediate result l was that strontium, cadmium, lead, copper, cerium, and uranium were shown with considerable probability to exist in the solar atmosphere, in addition to the elements in Thalen's last list. Certain of those elements which form unstable compounds with oxygen were also sought for; gold, silver, mercury, being examples. None of these were found, however. This was in 1874. The total result then was as follows : Metals present in the Sun. Sodium Cobalt Cadmium Iron Hydrogen Lead Calcium Manganese Copper Magnesium Titanium Cerium Chromium Strontium Uranium. Nickel Proc. Roy. Soc. No. 147, 1873, p. 512. xii.] SPECTRA OF COMPOUNDS. 159 4. The Spectroscopic Effects produced ~by the Dissociation of knoivn Compounds. There was another way in which the new method of work proved itself very valuable, though at first it did not seem to be connected in any way with solar chemistry. With regard to this, I should commence by stating that from a beautiful series of researches carried on by several methods, Mitscherlich concluded, in 1864, 1 that every compound of the first order heated to a temperature adequate for the production of light, which is not decomposed, exhibits a spectrum peculiar to this compound. In some experiments of my own, made in 1873, I observed : 2 First. That whether the spectra of iodides, bromides, &c., be observed in the flame or a weak spark, only the longest lines of the metals are visible, showing that only a small quantity of the simple metal is present as a result of partial dissociation, and that by increasing the temperature, and consequently the amount of dissociation, the other lines of the metal appear in the order of their length with each rise of temperature. Secondly. I convinced myself that while in air, after the first application of heat, the spectra and metallic lines are in the main the same, in hydrogen the spectra are different for each compound, and true metallic lines are represented according to the volatility of the compound, only the very longest lines "being visible in the spectrum of the least volatile compound. Thirdly. I found that with a considerable elevation of temperature, the spectrum of the compound faded almost into invisibility. These results enabled us to make the following statement : A compound body, such as a salt of calcium, has as definite a spectrum as that given by the so-called elements ; but while 1 Phil. Mag. 1864, vol. 28, p. 176. - Phil. Trails. 1873, pp. 650, 651. 160 THE CHEMISTRY OF THE SUN. [CHAP. xn. the spectrum of the metallic element itself consists of lines, the number and thickness of some of which increase with increased quantity, the spectrum of the compound consists in the main of flutings and bands, which increase in like manner. In short, the molecules of a simple body and a compound one are affected in the same manner by an increase in their quantity in so far as their spectra are concerned ; in other words, both spectra have their long and short lines, the lines in the spectrum of the element being represented in the spectrum of the com- pound by bands or fluted lines ; and in each case the greatest simplicity of the spectrum depends upon the smallest quantity, and the greatest complexity upon the greatest. The heat required to act upon such a compound as a salt of calcium, so as to render its spectrum visible, dissociates the com- pound according to its volatility ; the number of true metallic lines which thus appear is a measure of the quantity of the metal resulting from the dissociation, and as the metal lines increase in number, the compound bands thin out. CHAPTER XIII. DIFFICULTIES. AT this point we must pause. The work referred to in the immediately preceding chapters, which brought us down to the end of the year 1873, was chiefly laboratory work. We must now return to the sun, and study not only the application of this new work to solar chemistry, but refer to some purely solar observations as well. In this way we shall be able to note how the solar theories then in vogue stood the strain of the new tests we could apply to them ; in short, we can inquire if the line was all clear for future progress. We were working on two hypotheses or inferences more or less clearly enunciated by KirchhofF. I. The absorption which produced the Fraunhofer lines took place at some distance above the photosphere, the spots being solar clouds. II. The chemical elements present in the solar atmosphere were identical with some of those existing on the earth, and their spectra were identical. Was the way then perfectly clear, taking the work as it stood in 1873 ? Did work on the sun's atmosphere both during eclipses and independently of them, confirm Kirchhoff's views ? And, moreover, did our chemical theories which Kirchhoff had taken as a base for his second hypothesis explain all the facts which M 162 THE CHEMISTRY OF THE SUN. [CHAP. had been gathered by many men in many lands touching solar chemistry ? The inquiry had depended on using existing maps, whether tainted with impurities or not, and observing the lines in all prominences and spots. I shall have to show that things were by no means clear; that both eclipse observations and others had not endorsed KirchhofFs views, and that any one who took the trouble to bring together all the results which had been obtained up to that time would have found not only that there was a rift in the lute, but that there was a very big one ; and that the discord which grew upon one as one went into details either with regard to the spectrum of the spots, to the spectrum of the prominences, or to the localisation of the solar atmosphere, was very much more remarkable than the accord. Of course, an immense amount had been done towards elaborating a view of solar chemistry a great part of which would stand ; still there was a great deal which required a considerable amount of attention, and a great deal more which suggested that there was still a higher light to be got before we could really face the magnificent problem with which we. were attempting to grapple. In 1873 we had not only results garnered by the new method in six years to deal with, but we had the observations made in the eclipses of 1868, 1869, 1870, and 1871 to co-ordinate with them, and after such co-ordination to compare with the hypo- theses in vogue. With regard to eclipses it is difficult to overrate the value of the observations which they enable us to collect, for they in- crease our knowledge of the higher reaches of the sun's atmo- sphere which are inaccessible at any other times, and the lower layers can at the same time be seen under better conditions because there is less atmospheric light to contend with. So much, indeed, has of late years been discovered by the new method touching both spots and prominences, that these are now regarded as but ordinary phenomena we are familiar with xni.] LOCUS OF SOLAR ABSORPTION. 163 them ; and we are apt to forget the scale on which the changes rendered visible to us by our telescopes take place. This is not the case with the different class of phenomena which is revealed to us only during eclipses, when the moon shields the place of observation from ordinary sunlight and, by interposing herself exactly between us and the sun, allows us to inspect the sun's atmosphere with perfect ease. Then new glories are rendered visible which make the moments of the totality as precious to scientific men as they are terrible and awe-inspiring to ordinary beholders. 1. The Solar Atmosphere. Let us, then, attempt to compare the totality of the know- ledge thus acquired with that part of KirchhofFs hypothesis FIG. 66. The sun and its atmosphere on KirchhofFs hypothesis. A, luminous haze resting on liquid photosphere ; B, locus of absorption ; c, outer atmosphere. which deals with the locus of absorption. The first year's work with the new method showed that the facts did not fit the theory, for the greatest widening of lines was always seen close to the photosphere; as a rule though they were generally thinner than the Fraunhofer lines they were thicker at the bottom than at the top. A diagram, Fig. 66, will show the importance of the last remark. If we imagine a slit of a spectroscope normal to the sun cutting the three layers, then the lines should be thickest in the spectrum of B according to Kirchhoffs view. But they were actually thickest at A. M 2 164 THE CHEMISTRY OF THE SUN. [CHAP. It must never be forgotten (but it often is) that in our obser- vations we are dealing with a sphere, and not with a section. So that if we imagine a layer to exist at any height above the sun, its spectral lines will be visible down to the spectrum of the sun itself. Whether, therefore, there is such a layer is very difficult 1234 H Su n's ec t rum FIG. 67. Diagram showing that if the sun's atmosphere consists of layers the lines will still extend down to the solar spectrum. For the layer A is also represented at B and B', so that we shall see (2) in the spectrum and not (1). If thickness of stratum increased the width of the line, we should see (3) if the layer A did not extend to the sun, and (4) if it did. of determination if we neglect to notice the various thicknesses of the lines. In fact the thickening towards the sun seems the only criterion ; if we suppose it not to exist, then it is Sun's Spectrum FIG. 68. Spectral phenomena on the assumption of three layers, c, outermost layer, being represented at c', will give us a long line as at c, as it would do if the stratum extended from c to A along the line x Y, and so with B. easy to see that there is no spectroscopic method of determining the existence of the layers if we neglect temperature. If we t^ke temperature into consideration, then, as the lines will be less bright as the distance from the sun is increased, xiii.] MODIFICATIONS SUGGESTED. 165 and therefore the temperature is reduced, the longest lines, will be the dimmest if they are produced by layers. The lines of A the hottest layer will be "brightest and shortest. The lines of B the next cooler layerwill be less bright and longer, and will also appear to rest on the spectrum of the sun, as before explained, on account of the part of the layer at B', although it is unrepresented at A, along the section X Y. And so on with c. Now, chiefly because the lines increased both in width and number as we neared the photosphere, and also on account of other considerations, Dr. Frankland and myself soon came to the conclusion that in many particulars KirchhofFs theory required modification. In 1869 we wrote as follows 1 : " We believe that the determination of the above-mentioned facts leads us necessarily to several important modifications of the received theory of the physical constitution of our central luminary, the theory we owe to Kirchhoff, who based it upon his examination of the solar spectrum. According to this hypothesis, the photosphere itself is either solid or liquid, and it is surrounded by an atmo- sphere composed of gases and the vapours of the substances incan- descent in the photosphere. " We find, however, instead of this compound atmosphere, one which gives us merely, or at all events mainly, the spectrum of hydrogen (it is not, however, composed necessarily of hydrogen alone, and this point is engaging our special attention) ; and the tenuity of this incandescent atmosphere is such that it is extremely improbable that any considerable atmosphere such as the corona has been imagined to indicate, lies outside it ; a view strengthened by the fact that the chromospheric bright lines present no appear- ance of absorption, and that its physical conditions are not statical. " With regard to the photosphere itself, so far from being either a solid surface or a liquid ocean, that it is cloudy or gaseous, or 1 Proc. Roy. Soc. vol. xvii. p. 288. 166 THE CHEMISTRY OF THE SUN. [CHAP. both, follows, both from our observations and experiments. The separate prior observations of both of us have shown : " 1. That a gaseous condition of the photosphere is quite con- sistent with its continuous spectrum. The possibility of this con- dition has also been suggested by Messrs. De la Rue, Stewart, and Loewy. "2. That the spectrum of the photosphere contains bright lines when the limb is observed, these bright lines indicating probably an outer shell of the photosphere of a gaseous nature. "3. That a sun-spot is a region of greater absorption. " 4. That occasionally photospheric matter appears to be in- jected into the chromosphere. May not these facts indicate that the absorption to which the reversal of the spectrum and the Fraun- hofer lines are due takes place in the photosphere itself, or ex- tremely near to it, instead of in an extensive outer absorbing atmosphere ? And is not this conclusion strengthened by the con- sideration that otherwise the newly discovered bright lines in the solar spectrum itself should be themselves reversed on KirchhofFs theory ? This, however, is not the case. We do not forget that the selective radiation of the chromosphere does not necessarily indicate the whole of its possible selective absorption ; but our experiments lead us to believe that were any considerable quantity of metallic vapours present, their bright spectra would not be entirely invisible in all strata of the chromosphere." Our view received, apparently, very strong confirmation after- wards. In my own observations occasionally in passing over a metallic prominence, the spectrum would be " full of lines " ; and during the eclipse of 1870, at the moment of disappear- ance of the sun, the same effect was noticed ; we had, to quote Prof. Young, " A sudden reversal into brightness and colour of the countless dark lines of the spectrum at the commencement of totality." The instrument used was an integrating spectro- scope directed to the sun. In 1871 Captain Maclear and myself watched these lines on the retreating cusps. On these observations was based the view that there was a region some 2" high above the photosphere which reversed for us all the xiii.] COMPOSITE ATMOSPHERE. 167 lines visible in the solar spectrum ; and on this ground the name " reversing layer " was given to it. Still, undoubtedly some absorption was produced in the upper region of the solar atmosphere, for with each eclipse, against authority, against chemical arguments based upon the low atomic weight of the substances in the chromosphere, the corona grew and grew. In 1870 we had hydrogen 8' from the sun, " far above .any possible atmosphere ;" l and in 1871 this was carried higher still ; while towering above the hydrogen lines was another line, the famous 1474, which indicated something else existing in the exterior reaches of the atmosphere. What, then, was the totality of the knowledge which had been acquired a few years ago with regard to the chemical nature of the sun's atmosphere taken as a whole the sun's atmosphere from the upper reaches of the coronal atmosphere down to the region where, doubtless, the spot phenomena are located ? The view of the sun's atmosphere, in 1873, was one something like this : We had, let us say, first of all an enormous shell of some gas, probably lighter than hydrogen, about which we know absolutely nothing, because so far none of it has been found here. Inside this we had a shell of hydrogen ; inside this one of calcium, another of magnesium, another of sodium, and then a complex shell which has been called the reversing layer, in which we got all the metals of the iron group plus such other metals as cadmium, titanium, barium, and so on. The solar atmosphere then, from top to bottom, consisted, it was imagined, of a series of shells, the shells being due not to the outside substance existing only outside, but to the outside sub- stance extending to the bottom of the sun's atmosphere, and encountering in it, at a certain height, another shell which again found another shell inside it, and so on; so that the 1 Young. 168 ..-HrM-.*' 3 ^ CHEMISTRY OF THE SUN. [CHAP. composition of the solar atmosphere as one went down into it got more and more complex ; nothing was left behind, but a great many things were added. We had, dealing with known elements, Highest . . Hydrogen. Medium . . Magnesium, calcium, sodium. Lowest / ' Y . Iron, nickel, manganese, chromium, cobalt, barium, copper, zinc, tita- nium, and aluminium. One word about the hydrogen. We may have hydrogen, in a large globe, at such a pressure that we can make it luminous with a feeble current. If we exa- mine it with the spectroscope we find it gives the F line alone, there is nothing red about it. Now there is a region around the sun which gives us something very like that in colour, and something absolutely like it, so far as the result of spectro- scopic observation is concerned. Again, we may have hydrogen in a narrow tube in a condition to be considerably agitated ; instead of allowing the current to act throughout a globe, it has to pass through a fine capillary space in which the gas is confined. That is a condition which is supposed to give us the effect of high temperature. It really does give us something like what we see in the next lower solar region. As we pass from few encounters of molecules to many, the gas is very much more luminous, and it is red. The level which gives such a spectrum as is got from the capillary tube is considerably lower than the one which gives us the F line alone (Fig. 69). We may go more into detail with regard to the lower reaches. Further down, as has been already pointed out, we got all round the sun at certain periods of the solar activity some lines seen in the spectrum of magnesium. Underneath this again we got a layer in which lines seen in the case of sodium and calcium are almost as constantly seen. Still a lower depth XIII.] LAYERS. [69 practically there is no end of them in which we got the lines of iron and other substances. There are many lower variable layers depending upon local disturbance. We have by these observations a means of determining the fact that the solar atmosphere consists of what may be very conveniently and justly called a very considerable number of layers ; arid what happens with these layers is this : If the sun is quiet, or if we observe any particular part of it at any time at which it is not agitated, the layers visible at that time, few in number, are nearly concentric (Fig. 69), but the moment there is any agitation in the subjacent photosphere H.W/w FIG. 69. Imagined stratification of the solar atmosphere (1873). (H = Hydrogen ; Mg = Magnesium ; Na = Sodium ; Fe = Iron.) the lower layer shoots up into the next layer above it ; the next shoots up into the one next above that ; and so on (Fig. 70). How far into the very confines of the solar atmo- sphere this sort of action goes we do not know, because it wants more time to observe than is afforded by an eclipse, but it is certainly known that from the very lowest layer to the upper hydrogen one the layers are made to obey this same impulse, and bulge up like so many domes on that part of the sun which is being violently agitated. The following extract from a lecture I gave at Cambridge in 1871 refers to an experiment of some interest in this connection : 170 THE CHEMISTRY OF THE SUN. [CHAP. "It is easy to show also that with increased pressure comes such an increased brilliancy as we get in the sun, where, going inwards, we have first the faint corona, then the hydrogen layers, visible only in an eclipse, then those observed every day by the new method, and last of all, the photosphere itself. " There is a simple experiment bearing upon this point. If we take a tube containing hydrogen at a very low pressure, and at the bottom of it some mercury, so long as the mercury is cool the spark passes through nearly pure hydrogen, and the tube is lighted FIG. 70. Stratification of the solar atmosphere disturbed by the upheaval of a prominence. up with only a faint glimmer, the equivalent of the auroral discharge. " But if instead of having our mercury vapour almost absent from the tube, in consequence of the low temperature of the liquid supply at the bottom, we drive this liquid mercury at the bottom of the tube into a state of vapour, we find that we not only change the colour of the discharge in the lower regions of the tube, but imitate what we get on the sun itself. As the discharge passes through the denser layers, and renders them incandescent as they are at the sun, the brilliancy increases enormously as the source of the vapour is approached, and in each stratum of different density of the mixed gas and vapour in the tube we have the same increase xiii.] BRIGHT LINES. 171 of brightness as we observe in the similar strata of the sun's atmosphere." l So much for the first results obtained in localising the solar chemistry. We pass from a general theory, that the absorption is above the sun, and that the sun consists of such and such chemical substances, to a very much more complete picture, in which we see that the solar absorption is built up by vapours extending to certain definite heights in the sun's atmosphere, and that they are ever changing their heights and distribution at different times. It should be seen clearly, from the foregoing statements, how wide, after the accumulation of these facts, was the chasm which separated KirchhofFs view of the construction of the solar atmosphere from the facts as we imagined them in 1873. There is, indeed, another point to which a slight reference may be made. It is impossible on KirchhofF s hypothesis to explain the existence of bright lines in the ordinary solar spectrum, and yet that such bright lines do exist is undoubted. They were first noticed in 1869, when a large dispersion was employed, and they have been re-discovered two or three times since. The bright lines seen on the disc of the sun also from time to time, which have been previously referred to while discussing the phenomena of the spots, also militate against this hypothesis. 2. The Chemical Nature of the Sun unlike that of the Earth. Let us now leave the distribution of the chemical substances, and deal with the atmosphere of the sun taken as a whole. This is the only way open to us if we wish to consider the chemical structure of the sun itself. If the elements were identical we should not be justified in imagining any great 1 Rede Lecture, Cambridge, 1871. 172 THE CHEMISTEY OF THE SUN. [CHAP. departure from the terrestrial distribution. But here again the facts were all the other way. How came it that the total chemical composition of the atmosphere of the sun, which we were taught to look upon as the exemplar of what must have once existed in the case of our own planet, varied so enormously from the composition of the crust of our earth ? Among all the metalloids known to chemists only one of them or one substance classed as such, hydrogen was present in the solar atmosphere, and that in overwhelming quantity; whereas the efforts of Angstrom, Kirchhoff, and others, could not trace such substances as oxygen, chlorine, silicon, and other common metalloidal constituents of the earth's crust. It was difficult to imagine a stronger difference to exist between any two masses of matter than that found between the incandescent sun, and the earth which is now cooling. Even the elements in the sun were in an unnatural order according to the received chemical views . To give an instance : The layer produced by what was taken to be gaseous magne- sium round the sun, a layer indicated by the brightest member of the 5 group, was always higher always gave us longer lines than that other layer which was brought under our ken by the bright line D seen in the spectrum of sodium. Here was a distinct inversion of the chemical order. The atomic weight of sodium being 23, and of magnesium 24, the sodium ought to have been higher than the magnesium ; but the contrary was the fact, and that fact has been established by twelve years of observation. 3. The Chemical Nature of the Sun unlike that of many of the Stars. If then the sun and earth were chemically unlike, it was of the highest importance to learn how the facts stood in the case of the stars a point of view very soon forced upon solar XIIL] INTENSITIES OF LINES. 173 observers by the success which had attended the labours of Rutherfurd, Miller and Huggins, Secchi, and other observers in recording the spectra of stars. This most interesting inquiry naturally enabled us to know whether the stars gave spectra quite like the sun, and if it happened that they did not give spectra like the sun or each other, then such a result would not only define the points of difference but would be sure to give us some excellent working suggestions. How striking the differences actually were, not only between sun and star, but between star and star, will be shown in the sequel. 4. Divergences between the Lines in the Spectra of Vapours and the Fraunhofer Lines. Kirchhoff in his memorable paper of 1861, which may justly be regarded as the basis of all subsequent work, was careful to state that the sixty iron lines in the sun to which he referred, 1 agreed, "as a rule" in intensity with those observed in the electric spark. Those who have given an account of his work have not always been so cautious. Indeed, I find my friend Professor Roscoe 2 running beyond the record in the following sentence : " In order to map and determine the positions of the bright lines found in the electric spectra of the various metals, Kirchhoff, as I have already stated, employed the dark lines in the solar spectrum as his guides. Judge of his astonishment when he observed that dark solar lines occur in positions connected with those of all the bright iron lines ! Exactly as the sodium lines were identical with Fraunhofer's lines, so for each of the iron lines, of which Kirchhoff and Angstrom have mapped no less than 460, a dark solar line was seen to correspond. Not only had each line its dark representative 1 Researches on the Solar Spectrum and the Spectra of the Chemical Elements, Roscoe's translation, part i. p. 19. 2 Spectrum Analysis, 3rd edit. p. 240. 174 THE CHEMISTRY OF THE SUN. [CHAP. in the solar spectrum, but the breadth and degree of shade of ttie two sets of lines were seen to agree in the most perfect manner, the brightest iron lines corresponding to the darkest solar lines." This statement (the italics are mine) was made to prove the absolutely identical nature of the iron vapour in the sun's atmo- sphere and in the electric spark. Minute study however revealed the fact that the intensities of the various lines really vary very greatly from vapour to sun. I wrote in 187i : l " It is obvious that greater attention will have to be given to the pre- cise character as well as to the position of each of the Fraunhof er lines, in the thickness of which I have already observed several anomalies. I may refer more particularly at present to the two H lines 3933 and 3968 belonging to calcium, which are much thicker in all photo- graphs of the solar spectrum " (I might have added that they were by far the thickest lines in the solar spectrum) " than the longest calcium line of this region (4226'3), this latter being invariably thicker than the H lines in all photographs of the calcium spectrum, and remaining, moreover, visible in the spectrum of substances containing calcium in such small quantities as not to show any traces of the H lines. " How far this and similar variations between photographic records and the solar spectrum are due to causes incident to the photo- graphic record itself, or to variations of the intensities of the various molecular vibrations under solar and terrestrial conditions, are questions which up to the present time I have been unable to discuss." The progress of the work showed that the differences here in- dicated are not exceptions, but are truly typical when the minute anatomy of the solar spectrum is studied. 5. Divergences between the Spectra of Vapours and the Lines visible in Spots and Prominences. We have dealt as yet with the spectroscopic difficulties so far as the ordinary spectrum of the sun is concerned, but in 1874 a 1 Phil. Trans, vol. clxiv. part ii. p. 807. xiii.] SPOTS AND FLAMES. 175 great advance had been made in the localization of solar pheno- mena ; in short, there was now another kind of work a newer kind of work going on. Observers began to give attention to the bright lines of solar prominences, and the lines thickened in solar spots. I shall here limit myself to the general state- ment that the divergence between the spectra of the different substances as observed in the sun and in our laboratories was very much intensified as facts were accumulated from these new regions. Very many of the lines observed in prominences were lines with no terrestrial equivalents, and the spot-spectrum often contained lines much thickened which were either not represented at all, or only feebly, among the Eraunhofer lines. As the work of tabulating the lines went on, and the more complex outpourings of vapours from the sun's interior were studied, it was found that the prominence lines coincident with those seen in the spectra of iron, calcium, and so forth, were by no means the brightest lines by no means the most important or most prominent lin.es in the known spectra of those sub- stances, but lines which really we had very great difficulty in recognising as characteristic of any particular spectrum. There* they certainly were, however, mapped as very fine lines by the most industrious observers. Similarly with the spots, there was an absolute inversion of the thicknesses of the lines of any one substance. 6. Motions indicated ~by Different Lines in Spots and Flames. Closely allied to these results we had another extraordinary fact. We could quite understand why in a spot the change of refrangibility of the magnesium lines when there was a storm going on in the sun should be different from the change of refrangibility of, say, the iron lines. The natural explanation was, of course, that the magnesium gas was going at one rate, and 176 THE CHEMISTRY OF THE SUN. [OHAP. the iron gas at another rate. But it was soon found that the differences which could be sharply seen between the spectrum of a particular mass of magnesium vapour and a particular mass of iron vapour extended to the iron vapour itself. There were just as many variations in the refrangibility of the lines of iron itself, for instance, as there were between the lines of iron and of other substances : that is to say, we had in the one case magnesium going at one rate and iron going at another rate ; but when we came to deal with the iron lines alone we found one iron line told us the iron vapour was going at one rate, and another iron line told us that the iron vapour was going at another rate, i.e. a thing which could not be divided was going at two rates at the same time. Further. The lines on which these determinations of the relative motions of the vapour depended were found to go in sets. In a spot, for instance, we would generally see movement indicated by one set of iron lines, whereas in a prominence we would always see a different set a set in a different part of the spectrum altogether registering this movement for us. Here again was considerable food for thought. That was stated very roundly a good many years ago in 1869. I will quote what I then wrote on this subject : 1 " Altera- tions of wave-length have been detected in the sodium, magnesium, and iron lines in a spot spectrum. In the case of the last substance, the lines in which the alteration was detected were not those observed when iron (if we accept them to be due to iron alone), is injected into the chromosphere." That caveat with regard to iron arose from the fact that of the 460 lines recorded, only three lines had up to that time been seen bright in the solar prominences. It will, I think, be clearly gathered from the foregoing, that the more observations were accumulated the more the spectroscopic difficulties increased. 1 Proc. Roy. Soc. No. 115, 1869. xiii.] COMMON LINES. 177 7. Spectral Lines common to two or more Spectra. We now pass to difficulties of another order met with in the work of comparing the lines of the different elementary bodies with the Fraunhofer lines work done chiefly in the first instance by Kirchhoff, Angstrom, and Thalen. Kirchhoff was not long before he found that to say that each substance had a spectrum entirely and specially belonging to that particular substance was not true. He says, 1 " If we compare the spectra of the different metals with each other, several of the bright lines appear to coincide." Now Kirchhoff was working with Bunsen as his collaborateur, and therefore, as we may imagine, this was not said lightly. Similarly Angstrom, who was working with the assistance of the Professor of Chemistry at Upsala, was driven to exactly the same conclusion. He says, 2 I translate his words " Of all bodies iron has certainly produced the greatest number of lines in the solar spectrum. . . . Some of these seem to be common with those of calcium. " Thalen carried on this inquiry, and if one compares the magnificent tables which we owe to his untiring skill and industry, one is perfectly astonished to find the number of coincidences which he has so carefully tabulated. It might be imagined naturally that these lines were due to feeble dis- persion or to impurities, but there was no spectroscopic method then of establishing the latter assumption. 8. Spectral Lines vary their Intensities by Temperature. We now come to the discrepancies between the spectra on varying the temperature which soon forced themselves upon the attention of observers. 1 Researches on the Solar Spectrum and the Spectra of the Chemical Elements, Roscoe's translation, p. 10. - RechercJies sur le Spectre Solairc, p. 36. N 178 THE CHEMISTRY OF THE SUN. [CHAP. Angstrom, in that exceedingly important memoir which accompanies his Atlas, states : 1 " In increasing successively the temperature I have found that the lines of the spectra vary in intensity in an exceedingly compli- cated way, and consequently new lines even may present themselves if the temperature is raised sufficiently high." Kirchhoff and Bunsen, indeed, as early as 1860, seem to have got a glimpse of the same thing. They wrote : 2 " If the intensity of the light ... be increased new lines appear . . . and the relation of the brightness of the old ones becomes altered. In general an indistinct line becomes brighter, upon increasing the illumination, more rapidly than does a brighter line, but not to such an extent that the indistinct line ever overtakes in intensity the brighter one. A good example of this is seen in the two lithium lines. We have only observed one exception to this rule, namely in the line Ba y, which by light of small intensity is scarcely visible, whilst Ba y appears plainly, but by light of greater intensity becomes more visible than the latter." Touching these variations I may remark that Kirchhoff did not agree that the temperature upon which Angstrom laid such strong stress was really the cause at work. 3 He attributed those variations rather to the mass and the thickness of the vapours ex- perimented upon nay, he went further : at a time when scarcely any facts were at his command he broached a theorem which went to prove this ; arid yet what had Kirchhoff himself done ? He had traversed his own theorem. He states that his observa- tions (those referred to in Chap. VI.), were made by means of a coil, using iron poles one millimetre in thickness. Now the thickness of a short spark taken from iron poles one millimetre in thickness would probably be two millimetres. Next Kirchhoff located the region where the absorption which produces the 1 Recherches sur le Spectre Solaire, pp. 38, 39. 2 Kirchhoff and Bunsen, Phil. Mag. vol. xx. s. 4, p. 94 Augtist. 1860. 3 Recherches sur le Spectre Solaire, pp. 38, 39. xin.] MULTIPLE SPECTRA. 179 reversal of the iron lines takes place at a considerable height in the atmosphere of the sun, and he imagined the atmosphere of the sun to be an enormous mass represented by the old drawings of coronas, so that on his view the thickness of the iron vapour which reversed the iron spectrum must have been, at a moderate estimate, 10,000 miles, and yet he said that the spectrum of that, and of the light given by the electric spark two millimetres in thick- ness, were absolutely identical ; that is to say, that the fact was that the variation of thickness from two millimetres to 10,000 miles made no difference. That was on the one hand ; on the other hand he gave us his theorem, showing that a slight variation of thickness would produce all the changes which Angstrom and others had observed up to that time, and which, it may be added, have been observed since in still greater number. We now know that in the case of hydrogen a difference in thickness of a million miles and of a millimetre makes no difference. 9. The same Substance may have more than one Spectrum,. At the time that KirchhofF announced the conclusion that the terrestrial elements as known to chemists existed in the sun, the general idea was that each element had only one spectrum, and that that spectrum was the same whether observed in the sun or in our laboratories. Underlying these general notions was, as I have before stated, the assumption that the " chemical atom," a thing with a definite weight given in all chemical text-books, once got, even the solar temperature was insufficient to simplify it. Soon after Kirchhoff had published his papers, three eminent Germans Pliicker and Hittorf 1 and the younger Mitscherlich found that in the case of a great many simple substances, what are called fluted spectra, as well as line spectra, were to be observed. 1 Phil. Trans. 1865, vol. civ. pp. 1-29, N 2 180 THE CHEMISTHY OF THE SUN. [CHAP. The accompanying diagram (Fig 71) of the fluted spectrum of iodine will show the difference between these fluted spectra and the line spectra on which we have been exclusively occupied up to the present. We observe that the chief novelty is an absolute rhythm in the spectrum ; instead of lines irregularly distributed over the on: FIG. 71. Fluted spectrum of iodine. (Thalen.) spectrum, we have groups which are beautifully regular in their structure. The next diagram (Fig 72) shows us the radiation spectrum of a particular molecular grouping of carbon vapour ; that also is beautifully rhythmic, the rhythm of each of the elementary flutings strongly resembling that of iodine. FIG 72. Carbon flutings. These observations were among the first to suggest the idea that the same chemical element could have two completely dis- tinct spectra. They were eminently suggestive, for if two, why not many ? In the reference to the " long and short " method of observation it was stated that it enabled us to note what happens when a xin.] AN ANALOGY. 181 known compound body is decomposed. With ordinary com- pounds, such as chloride of calcium and so on, one can watch the precise moment at which the compound is broken up when the calcium begins to come out ; and we can then determine the relative amount of dissociation by the number and brightness of the lines of calcium which are produced. Similarly with regard to these flutings we can take iodine vapour, which gives us a fluted spectrum, and we can then increase the temperature suddenly, in which case we no longer get the fluted spectrum at all ; or we may increase it so gently that the tme lines of iodine come out one by one in exactly the same way that the lines of calcium become visible in the spectrum of the chloride of cal- cium. We end by destroying the compound of calcium and its spectrum in the one case, and by destroying the fluted spectrum of iodine in the other, leaving, as the result in both cases, the bright lines of the constituents in the one case calcium and chlorine : in the other case iodine itself. I have by no means exhausted the list of difficulties which were gradually presented to us when we considered that both in the sun and in our laboratories spectrum analysis brought before us phenomena due to the vibrations of unique, absolutely similar " chemical atoms." Not only were there differences, but the differences worked in different ways, whether we passed from low to high temperatures in laboratory work, or from the general spectrum to the prominence- or spot-spectrum in the sun. But I have said enough for my present purpose ; details on some points I have referred to must be gone into in the next- chapter. CHAPTER XIV. DETAILS OF SOME OF THE DIFFICULTIES. 1. The Spectrum of the Prominences. DURING the accumulation of the observations of the bright chromospheric lines described in Chapter IX., many phenomena difficult to understand presented themselves. The following is a list 1 of the lines the positions of which were known at the end of 1869 : Kirchhoff's numbers. C F Near G h Near D 1989-5 2031-2 1474 1515-5 1529-5 Wave-lengths. 6561-8 4860-6 4340-1 4101-2 5874-9 ( 5895-0 ) { 5889-0 j 4933-4 4899-3 5183-0 5172-0 5168-3 5166-7 5315-9 5275-0 5263-9 Element in the spectrum, of which corresponding lines had been recorded. Hydrogen. Sodium. Barium. > Magnesium. Nickel. Magnesium, Iron. () Bright line. 1 " Spectroscopic Observations of the Sun," No. V., Proc. Roy. Soc. vol. xvii. pp. 74 and 118, July, 1869. CH. xiv.] FIRST LIST OF LINES. 183 Element in the spectrum, of KirchholFs numbers. Wave-lengths. which corresponding lines had been recorded. 1567-5 5233-6 (V) 1613-8 5197-9 (?) 1867-0 5017.6 Iron. 1871-5 5014-8 Bright line. 2001-5 4923-1 Iron. 2003-4 4921-3 (?) 2054-0 4880-5 (?) Band or line near black line, very delicate. When this list was communicated to the Koyal Society I remarked : " I refrain from dwelling on this list at present, except to point out that taking iron as an instance, and assuming that the iron lines mapped by Angstrom and Rirchhoff are dice to iron only, I have only been able up to the, present time to detect three lines out of the total number (460) in the spectrum of the lower regions of the chromo- sphere a fact full of promise as regards the results of future laboratory work. The same remark applies to magnesium and barium." It will be seen at a glance how variously the known chemical substances were spectroscopically represented, and what a large percentage of lines there was about which we knew absolutely nothing. It is true we had all the then known lines of hydrogen, but sodium was only represented by 2 lines out of 9 ; magne- sium by 3 out of 7 ; barium by 2 out of 26 ; and iron by 3 out of 460 ! Further, while the lines of sodium and magnesium were the brightest in the spectra of those elements, the lines of the other known bodies were not. 1 1 The two barium lines contained in my list are the two strongest barium lines in the F to b region, but are not the strongest barium lines. The line of nickel seen is one of the brightest nickel lines visible in the arc, but not the brightest seen in the spark, as mapped by Thalen. 184 THE CHEMISTKY OF THE SUN. [CHAP. Nor was this all ; some of the bright lines in the spectrum of the photosphere itself were continued into the prominence spectrum. Strange and unexpected as these observations were, they were soon endorsed. Thus Vogel gave the following list. 1 "Wave-length. u 6561-8 Hydrogen. D 3 5874-3 DJ 5895-0 Sodium. ; ix, 5889-0 5315-5 Iron(l) &! 5183-0 Magnesium. *a 5172-0 b 3 5168-0 Nickel. # 4 5167'0 Magnesium. 5017-6 Iron. 4923-1 F 4860-6 Hydrogen. 4340-0 About the same time Lorenzoni directed his attention to this subject, and not only indicated several new lines,^but gave frequencies of appearance. His new lines, like some of the old ones, had no counterparts among the Fraunhofer lines ; notably one, which he indicated by /, at wave-length 4471 '2, and another between B and c. In twenty-six protuberances seen in one month we have/ appearing 19 times against hydrogen 26 ; and the unknown substance which produces D 3 26 times against magnesium 8 and sodium 6. Three other lines, one between B and c, and two between F and 5, were only seen once. Fortunately, Lorenzoni was not the only one who was utilising the clear sky of Italy for this work. Tacchini saw a great number of lines brightened in his observations of prominences extending from 1871 to 1873. 1 H. C. Vogel, Beobachtungen, 1872, pp. 36, 37. 2 Mcmoire della Societa degli spettroscopisti Italiani, vol. i. 1872. xiv.] YOUNG'S WORK. 185 Especially noticeable were two lines at w.l. 4923*1, 5017'6, given in my list, which were seen with great frequency till 1873, when they disappeared, and two new lines, about which nothing is even yet known, came in their place. 1 About this time an American astronomer, Professor Young, made an important series of observations during an expedition to Sherman, a point 8,000 feet high in the Eocky Moun- tains. He saw an immense number of bright lines in the prominences, but a study of them only served to show our ignorance. Thus the longest iron line in the region between F and b was only seen once, while a much fainter iron line at 49231 was seen forty times. Another iron line at 491 8 '2 was seen twenty times, while an equally strong line at 491 9'8 was not seen at all. In my observations I only saw three iron lines out of 460, and this, as I have shown, was confirmed by Vogel and Tacchini. Young, with much greater advantages as regards climate, was able to see 110 lines. He also saw the H and K calcium lines with a frequency of seventy-five and fifty re- spectively, while the much stronger blue calcium line at w.l. 4226*3 was seen only three times. 2 In the reduction of these observations he pointed out the fact that the lines which he had seen most frequently were lines common to two or more elements. He writes : 3 " Two explanations suggest themselves. The first, which seems rather the more probable is, that the metals operated upon by the observer who mapped their spectra were not absolutely pure either the iron contained traces of calcium and titanium, or vice versd. If this supposition is excluded, then we seem to be driven to the conclusion that there is some such similarity between the mole- cules of the different metals as renders them susceptible of cer- tain synchronous periods of vibrations a resemblance, as regards the manner in which the molecules are built up out of the 1 Tacchini, Memoire della Societa degli spettroscopisti Italiani, vol. i. p. 89 ; vol. ii. pp. 55, 59, 60, 62, 95 ; vol. iii. p. 95 ; vol. iv. pp. 81 et seq. " Young, United States Coast Survey Report, 1872. 3 Nature, vol. vii. p. 17 et seq. 186 THE CHEMISTKY OF THE SUN. [CHAP. constituent atoms, sufficient to establish between them an important physical (and probably chemical) relationship." If I understand Professor Young aright, his last words refer to what have been generally termed physical coincidences, that is, cases in which the common lines, being proved not to be due to impurity, are ascribed to common vibrations of dissimilar molecules. It will thus be seen that the further the work was pushed the greater became the difficulty, and it will, I think, be gathered that in these observations of the lines visible in the sun's chromosphere, by the new method, the idea that we witnessed in solar storms the ejection from the photosphere of vapours of metallic elements with which we are familiar on the earth became more and more improbable. The work, in short, of which I have given only the germs, re- vealed the most striking anomalies ; nevertheless, loyal to the old views, we have all of us continued to talk of " injections of iron into the chromosphere," " magnesium prominences," and the like. 2. Phenomena presented by the Stars. Since the sun is after all but a star the nearest star to us, it is of primary importance for our purpose that the spectra of the more distant ones should be compared with it. This is now, in part, possible, for although it was Fraunhofer who at the beginning of the century laid the foundations of a science which we may christen Comparative Stellar Chemistry, it is only in our own day that the work has been begun in earnest by Eutherfurd, Miller, Huggins, Secchi, Vogel, and others. Dealing with the knowledge already acquired, in 1873, along this line, we may say roughly that there were four genera of stars recognisable by their spectra. We have first the brightest and presumably hottest stars, and of these the spectrum is marvellously simple so simple, in fact, that we say their atmospheres consist in the main of only a very xiv.] HUGGINS' WORK. 187 few substances a statement founded on the observation that the lines in their spectra are matched by lines which we see in the spectra of hydrogen, magnesium, and perhaps of sodium too, but the faintness of the indication of these two latter substances only intensifies the unmistakable development of the phenomena by which the existence of the former is indicated. So much for the first class : now for the second. In this we find our sun. In the spectra of stars of this class, the indica- tions of hydrogen are distinctly enfeebled, and, accompanying this change, we find all simplicity vanished from the spectrum. The sodium and magnesium indications have increased, and a spectrum in which the lines obviously visible may be counted on the fingers is replaced by one of terrific complexity. The complexity which we meet with in passing from the first class to the second is one brought about by the addition of the lines produced by bodies of chemical substances of moderate atomic weight. The additional complexity observed when we pass from the second stage to the third is brought about by the addition of lines due in the main to bodies of higher atomic weight. And this is a point of the highest importance at the third stage the hydrogen, which existed in such abundance in stars of the first class, has now disappeared. Dr. Huggins' drawings of the spectra of Aldebaran and a Orionis (Fig. 73) show how much care has been devoted to this inquiry. Below the stellar spectra are mapped the lines of the various elements with which they were compared, and from the coincidence of some of these lines with those in the stellar spectra the existence of the corresponding elements in the atmospheres of these stars was inferred. It will be seen that in the spectrum of Aldebaran, the hydrogen lines c and F are present, while they are absent from a Orionis. No less than seventy lines have been observed in the spectra of these two stars, and Mr. Huggins and Dr. Miller have detected in Aldebaran the following elements: Hydrogen, sodium, 188 THE CHEMISTRY OF THE SUN. [CHAP. a 21 O 0> OH OH l II II II II II bCbf bO - 28 3 3975 U Zr Yt "354 i Va 4 ... Va Ba 2 Va I Pt ,.. Cc i '< I '" 3 Mn Ce 3 4 Os Vt i " 2 M 3 ...Rh Ta 2 Ce n \ 2 r B i 4 2 C M . -- 3 o Th 2 4026 3 4422 4 4720 2 5012 2 5160 2 5210 3 5423 4 6215 3 6571 6662 Va 5 a Yt 5 "" " 3 Di C e 2 Ru i 3 W u I M j 3 o i>t ^ * i Y t , C e Dl 5 Zr 3 " 2 Th 2 2 7555 3 7578 4 76F5 2 8083 Os Ta Cr 2 - 4 2 Di A T a 2 Ti 4 2 8320 Cr l 1 9520 3 9750 3 Ril 3 Mo 2 3 234 THE CHEMISTRY OF THE SUN. [CHAP. after all this work had been accomplished. It gives a list of the iron lines in a small part of the spectrum, which, after making every allowance for the existence of impurities, we found to coincide with lines in other substances. It will be seen, to take some instances from the above table, that the two short lines 390600 and 395423 coincided, the first with short lines in uranium, zirconium, and yttrium, the second with short lines in uranium, molybdenum, and tungsten. Similarly there are two short-line coincidences with zirconium, and no less than six with vanadium, and so on. The total gives the coincidences of the lines of all the elements under the conditions that I have drawn attention to. So that the sum total of this really very laborious inquiry with regard to iron was, first, that in the region between 39 and 40, the region including H and K, where, before the introduction of photography, scarcely any iron lines had been seen, and where only five solar lines had been given in Angstrom's atlas, photography gave us a total of nearly 300 lines in the solar spectrum, and it gave us sixty- two lines of iron. Next, of those sixty-two lines of iron only eighteen were what we then considered normal ; by which I mean that the remainder had short-line coincidences with lines of other substances. So that the idea first thrown out by Kirchhoff, Angstrom, and Thalen of the possibility of the coincidence of lines among the metallic elements seemed entirely endorsed. It will be seen that it is the rule in the case of iron, and it might be the case also in other substances. The fact of a line not being coincident with a line in another substance was the exception, and not the rule. The ratio in the case of iron being as 44 to 18 over the region examined. I now proceed to give the results in the case of titanium. We got one case of three coincidences, five cases of two, and ten of one coincidence : XVII.] DETAILS OF COINCIDENCES. FINAL EEDUCTION TITANIUM. 235 |i il i 4 5 2 5 4 3 3 2 2 4 3 2 3 1 2 Wave-length an length of line. Coincidences with Short Lines. 39 0000 3 0048 3 1040 1360 1 5r "~4 i Mn Ce Di "" 4 5 3 Va 3 1915 8 2050 Ce 4 U La 4 3 2368 v '" 3 3 3 i Fe 2 3718 5 4775 1 5722 1 6175 Th 4 ' C 4 e 1 5r 2 3 1 U " 8 Ta 2 6335 2 8083 Di 3 ' 5 Fe 1 8152 2 Mo 2 8922 Mn 1 9798 longest 4 4 Va ' longest Since therefore these lines which were common to two or more spectra could not be traced to impurities, what was their probable origin ? Their number was so great that to attribute them to physical coincidences, and to rest and be thankful accordingly, would have been to take the very pith and marrow out of the science of spectrum analysis, which we have heard so often is based absolutely upon different substances giving us spectra with special lines for each. The matter then was worthy of serious investigation. Lines of one kind we could explain, on the hypothesis that the elements are truly elementary, by supposing that in the case, 236 THE CHEMISTRY OF THE SUN. [CH. xvn. let us say, of coincident lines in the spectrum of iron and cobalt, the common line was due to an impurity either of iron in the cobalt or of cobalt in the iron. Most spectroscopic workers were of the true faith in this matter ; they accepted the dicta of the chemist, and not only was the work which had shown how the phenomena observed might be thus explained received with favour, but no one, so far as I know, inquired whether there was any other " might be " in the matter. The other set of lines was as different as possible. Of them there was, on the impurity hypothesis, no possible explanation forthcoming without changing ground. In fact, the separation of the coincidences into two classes was brought about by this very circumstance, since all the coincidences which, in accord- ance with a general law established for a constant temperature some years before, could be attributed to impurity had, as a matter of fact, been eliminated from the maps at a prior stage of the investigation. Further, be it noted that all the photo- graphs represented the work of similar temperatures, for they were all taken with electric arcs, for the production of which the same number of Grove's cells was used in all cases. It is amusing to go back to the old observation books, and to see with what pertinacity for the first two years we stuck to the possibility that the solar line or the metallic line we were dealing with was a double line, and then, after we had to give that idea up, as the coincidences became of three- four- five- and six-fold complexity, we came to the conclusion that we were dealing with a common impurity. That of course was a point we could not settle until we had gone through all the chemical elements which were known to us, and it was the discussion of the spectra of so many substances which took up so much time. CHAPTER XVIII. DISCUSSION OF THE DISSOCIATION HYPOTHESIS. 1. Variation of Intensity. SEEING that such an abundance of new facts had now been collected, it really did seem time to discuss the question to which reference has already been made. I approached it in the following manner, relying upon the FIG. 86. Hypothetical furnaces. The figures between the hypothetical spectra point to the gradual change in the intensities of the lines as the spectrum is observed near the temperature of each of the furnaces. spectroscopic behaviour of compound bodies already stated on p. 159, in order to keep as near to facts as possible : 1 Let us assume a series of furnaces A D, of which A is the hottest (Fig. 86). 1 Proe. Roy. Soc., No. 191, 1878. 238 THE CHEMISTRY OF THE SUN. [CHAP. Let us further assume that in A there exists a substance a, competent to form a compound body ft } by union with itself or with something else, when the temperature is lowered. Then we may imagine a furnace, B, in which this compound body exists alone. The spectrum of the compound ft would be the only one visible in B, as the spectrum of the assumed elementary body a would be the only one visible in A. A lower temperature furnace, c, will provide us with a more compound substance 7, and the same considerations will hold good. Now, if into the furnace A we throw some of this doubly- compounded body 7, we shall get at first an integration of the three spectra to which I have drawn attention ; the lines of 7 will first be thickest, then those of B ; finally a will exist alone, and the spectrum will be reduced to one of the greatest simplicity. This is not the only conclusion to be drawn from these considerations. Although we have, by hypothesis, ft, 7, and 8, all higher, that is more compound, forms of a, and although the strong lines in the diagram may represent the true spectra of these substances in the furnaces B, c, and D respectively, yet, in consequence of incomplete dissociation, the strong lines of ft will be seen in furnace c, and the strong lines of 7 will be seen in furnace D, all as thin lines. Thus, although in c we have no line which is not represented in D, the relative intensities of the lines in c and D are entirely changed. Here is another diagram representing the facts on the sup- position that the furnace A, instead of having a temperature sufficient to dissociate ft, 7, and 8 into a. is far below that stage, although higher than B. It will be seen from the diagram (Fig. 87) that then the only difference in the spectra of the bodies existing in the four furnaces ivould consist in the relative thicknesses of the lines. I confess that the result of this simple projection of what XVIII.] AN HYPOTHETICAL DIAGRAM. 239 might happen to elementary bodies if they behaved like com- pound ones, struck me with surprise. It seemed to make everything so clear. In the first place it presented us with such a simple and sufficient cause for the differences in intensities of lines, not only on passing from one temperature to another in the laboratory, to which Kirchhoff and Angstrom were the first to refer, but also on passing from laboratory to sun. It was in 1874 that I first glimpsed the idea that the line spectrum of a substance was probably produced by molecules of different degrees of fineness into which the substance was driven by the temperature employed. 1 3 c s FIG. 87. Hypothetic furnaces. In this case the temperature of A is not so great as in the former one. Now these molecules of different finenesses are represented by a, fi, 7, and 8, in the hypothetical diagrams given above ; indeed, it is quite easy to see that, if we change the temperature of the furnaces in such a manner as to produce in turn the lines of each of these hypothetical molecules in their most intense and wide condition owing to the great quantity of vapour rich in that particular kind of molecule, the strong lines produced at these different temperatures would vary; the strongest line 1 Proc. Hoy. Soc., vol. xxii. p. 380, 1874. 240 THE CHEMISTRY OF THE SUN. 3 [CHAP. produced in furnace D would not be the same as th% strongest line produced in furnace A; so that in that way we can imagine a very high temperature giving a very strong line in the spectrum of a particular substance, which may yet at a lower temperature only appear as an exceedingly feeble one. Evidence furnished ly Calcium, Magnesium, and Lithium. This being premised, let us now go back to page 194, and study what was there said regarding the variation of spectral Fia. 88. The varying intensities of the lines of Calcium as seen under different conditions. FIG. 89. The varying intensities of the lines of Calcium with increasing temperatures. lines from this new point of view. I there gave the facts touch- ing the variations in the spectra of calcium, magnesium, and xviii.] CALCIUM. 241 lithium. I reproduce Fig. 75. The question is, can we arrange these spectra along any one line ? and if so, does this line represent a change of temperature merely ? Fig. 89 shows the result of this inquiry. It will be seen how in all essentials it reproduces our hypothetical diagram, Fig. 87. The effect of temperature on the spectrum of calcium may be stated as follows : (1) At a low temperature we get a spectrum of calcium which contains no lines whatever in the blue ; (2) When we increase that temperature the temperature of a Bunsen burner is some- times sufficient we get a line in the blue at wave-length 4226 -3. (3) When we pass from a Bunsen burner to an electric lamp we get this blue line intensified, and at the same time we get two new lines in the violet, named H and K, at wave-lengths 3933 and 3968. (4) Using a still higher temperature in the arc, we thin the blue line, and at the expense of that line, so to speak, we thicken the two in the violet, so that the latter equal the blue line in thickness and intensity. (5) Passing to a large induction coil with a small jar we make the violet lines very much more prominent ; (6) and using a larger induction coil and the largest jar we can get, we practically abolish the blue line and get the violet lines alone. Note that we have simply produced these effects by varying the temperature. Figs. 90 and 91 enable us to see what happens in the case of magnesium ; how disorder is at once abolished, and how exactly the same order is brought about by the same progression as in the case of calcium, from flame through arc, spark, sun (general) to prominences, the last term in both series. Next comes the case of lithium (Figs. 92 and 93). As before, we find order brought about and the hypothetical diagram reproduced by the same series of increasing temper- atures, but here the last term in the series is wanting, for so far no line of lithium has been seen bright in the prominences. That in the case of the blue line we are really dealing with K 242 THE CHEMISTRY OF THE SUN. [CHAP. an effect of temperature, long ago recognised, may be seen from a notice of this lithium line in a discourse by Professor Tyndall, reprinted in the Chemical Neivs, and in a letter of Dr. Frank- land's to Professor Tyndall, dated November 7, 1861. This Sun. Flame of carbonic oxide. Arc. Flame of cyanogeu fed with oxygen. Bunsen. Spark. Prominences. FIG. 90. The various intensities of the lines of Magnesium as seen under different conditions Prominences. Sun (general). Spark. Flame of cyanogen fed with oxygtn. Flame of carbonic oxide. FIG. 91. The various intensities of the lines of Magnesium arranged in order of increasing temperatures. letter is so important for my argument that I reprint it entire from the Philosophical Magazine, vol. xxii. p. 472. ; ' On throwing the spectrum of lithium on the screen yesterday, I was surprised to see a magnificent blue band. At first I thought the lithic chloride must be adulterated with strontium, but on testing it XVIII.] LITHIUM. 243 with Steinheil's apparatus it yielded normal results without any trace of a blue band. I am just now reading the report of your dis- course in the Chemical News, and I find that you have noticed the same thing. Whence does this blue line arise ? Does it really belong to the lithium, or are the carbon points or ignited air guilty of its production ? I find these blue bands with common salt, but they have neither the definiteness nor the brilliancy of the lithium band. When lithium wire burns in air it emits a somewhat crim- son light ; plunge it into oxygen, and the light changes to bluish white? This seems to indicate that a high temperature is necessary to bring out the blue ray." Flame. Suiu Feeble spark. Arc. FIG. 92. The various intensities of the lines of Lithium under different conditions. FIG. 93.- -The various intensities of the lines of Lithium arranged in order of increasing temperatures. Postscript, November 22, 1861. "I have just made some further experiments on the lithium spectrum, and they conclusively prove that the appearance of the blue line depends entirely on the temper- ature. The spectrum of lithic chloride, ignited in a Bunsen's burner flame, does not disclose the faintest trace of the blue line ; re- place the Bunsen's burner by a jet of hydrogen (the temperature of which is higher than that of the Bunsen's burner) and the blue line appears, faint, it is true, but sharp and quite unmistakable. If oxygen now be slowly turned into the jet, the brilliancy of the blue B 2 244 THE CHEMISTRY OF THE SUN. [CHAP. line increases until the temperature of the flame rises high enough to fuse the platinum, and thus put an end to the experiment." It is remarkable that in the case of this body which goes through its changes at relatively low temperature, its compounds are broken up at the temperature of the Bunsen burner. Thus the spectrum of the chloride, so far as I know, has never been recorded. Stellar Evidence. I propose now to return to the case of calcium and see how the views put forward on solar and terrestrial evidence are borne out by the facts which are presented to us by the stars. There is no need to occupy much space in this, in fact reference need only be made to Dr. Huggins's paper which was communi- cated to the Eoyal Society in 1880, and with that paper we may compare some earlier writings. It was as early as 1864 that Dr. Huggins, who was then associated with the late Prof. Miller, called attention to the strong lines of hydrogen visible in the spectra of the hottest stars. 1 In this paper it was pointed out at the same time that other metallic lines associated with those lines of hydrogen were thin and faint. It has been already mentioned that, as we have independent evidence that these stars are hotter than our sun, we had strong grounds for believing that here we were in presence of a result brought about by a higher temperature, associated as it was with a simpler spectrum, and, therefore, presumably with simpler constituents. We need not stop now to discuss the objection which has been put forward by an ingenious person ignorant of the facts, that the broadening of these lines may not be due to an increase of temperature at all, but really to a very rapid equatorial rotation of the star. This is a fair sample of one of the classes of objections one has to meet. Of course it is at once put out of 1 "On the Spectra of some of the Fixed Stars," Proc. Roy. Soc. 1864, p. 242. xviii.] THE H AND K LINES. 245 court by the fact, also stated by Dr. Huggins, that, associated with the thick lines, are excessively thin ones. Any enormous equatorial velocity of the star should have made all the lines thick, or have obliterated them, but this is not so. Now we have only two lines in the solar spectrum at all comparable in thickness with these hydrogen lines in the hottest stars, taking Sirius and a Lyrae as types. In a paper communicated to the Eoyal Society in 1876 l I remarked that laboratory work indicated the possibility that line-spectra might, after all, really not result from the vibration of similar molecules ; and- at that time the evidence seemed to be so clear in the case of calcium that it was pointed out that the time had arrived when evidence touching the H and K lines of that substance ought, if possible, to be obtained from the stars by means, of course, of photography, because the part of the spectrum in question is exceedingly faint in the case of the stars. "Why, it may be asked, was it important to get this evidence from the stars ? I here give an extract from a book, 2 published some years ago, which puts this view forth : "It is abundantly clear that if the so-called elements, or, more properly speaking, their finest atoms, those that give us line-spectra, are really compounds, the compounds must have been formed at a very high temperature. It is easy to imagine that there may be no superior limit to temperature, and, therefore, no superior limit be- yond which such combinations are possible, because the atoms which have the power of combining together at these transcendental stages of heat do not exist as such, or rather they exist combined with other similar atoms at all lower temperatures. Hence the association will be a combination of more complex molecules as temperature is reduced, and of dissociation, therefore, with increased temperature there may be no end." 1 " Preliminary Note on the Compound Nature of the Line-Spectra of Elemen- tary Bodies," Proc. Roy. Soc. No. 168, 1876. 2 Studies in Spectrum Analysis, p. 196. 246 THE CHEMISTRY OF THE SUN. [CHAIV That was one point. Here is the next point which made an appeal to the stars so necessary. "We are justified in supposing that our terrestrial calcium once formed is a distinct entity, whether it be an element or not, and, therefore, by working at terrestrial calcium alone we shall never know, even if its dissociation be granted, whether the temperature produces a simpler form, a more atomic condition, of the same thing, or whether we are able to break it up into x + y, because in our terrestrial calcium, assuming all calcium to be alike, neither x nor y will ever vary ; but if calcium be a product of a condition of rela- tively lower temperature, then in the stars hot enough to enable its constituents to exist uncompounded, we may expect these constitu- ents to vary in quantity ; there may be more of x in one star and more of y in another ; and if this be so, then the H and K lines will vary in thickness, and the extremest limit of variation will be that we shall only have H, representing, say, x in one star, and only have K, representing, say, y in another. Intermediately between these extreme conditions we may have cases in which, though both H and K are visible, H is thicker in some and K is thicker in others." What, then, is the result of my appeal to the stars which Dr. Huggins's beautiful researches have rendered possible ? We have in the hottest stars a spectrum so regular, so rhythmic, that it seems impossible not to consider it as produced either by the same substances or by substances closely allied. Is it by mere accident that some of the least refrangible lines coincide with those of hydrogen ? Fig. 94 is a copy of Dr. Huggins's diagram, to which reference has been made. At the top is a portion of the solar spectrum in the violet and ultra-violet, and next is the spectrum of the hottest star, a Lyrse. This spectrum, it will be seen, is simpler even than the spectrum of the solar prominences, and not only is there this wonderful simplicity, but an exquisite rhythm by which the distance between the lines gradually increases as we go from one end of the XVIII.] DR. MUGGINS'S PHOTOGRAPHS. 247 PU Q. 5 !? S2 a) o 'S ^ S 2 ." <13 p It! si: $ $ 111 IP S *- 248 THE CHEMISTRY OF THE SUN. [CHAP. spectrum to the other. Note also that the least refrangible line shown on the diagram is coincident with h in the violet part of the solar spectrum, and that the next line is coincident with the line H, to which reference has been made in the notes I have quoted. Note also the relative intensities of the lines H and K in the sun, in which their intensities are about equal, and in t) Ursse Majoris, in which K is altogether 'absent. These are the first points in this diagram to which attention must be drawn. There will be other points as we proceed further. In descending from the general to the particular Dr. Huggins writes : 1 " The spectrum of Vega may be taken con- veniently as typical of the whole class of white stars, so that the distinctive features of the other stars of this class may be regarded as modifications or departures from this common typical form." He then adds : " There are principally three directions in which changes take place ; " one of these consists " in the presence or absence of K, and if present, in its breadth and ntensity relative to H." He goes on, " One of these modi- fications, which possess great suggestiveness, consists of the absence, or difference of character presented by the line K. In all the stars of this class this line is either absent or is very thin as compared with its appearance in the solar spectrum, at the same time that H remains very broad and intense. In the spectrum of Arcturus, a star which belongs to another class which includes our sun, this line K has passed beyond the condition in which it occurs in the solar spectrum, and even exceeds the solar K in breadth and intensity." Arcturus is given in the lower part of the diagram, and it will be seen that there K is relatively thicker than H ; and also that with this relative increase in the thickness of K we get a considerable complexity of spectrum, very much more approaching the solar spectrum in the number of lines that we have to contend with. But at the same time I should point out that the positions of 1 Phil. Trans. 1880. XVJIL] THE PREDICTION VERIFIED. 249 these lines vary from the positions of lines in the solar spectrum. "The spectra of these stars," Dr. Huggins continues, "may therefore be arranged in a continuous series, in which first we find this line to be absent. Then it appears as an exceedingly thin line. We then pass to another stage in which it is distinct and defined at the edges ; in the solar spectrum it becomes broad and winged, and lastly in Arcturus there is further pro- gress in the same direction, and the line, now a broad band, exceeds in intensity H." Absolute continuity we see is the story which Dr. Huggins brings us with reference to this concrete case of H and K in the details and in the general. Well might Dr. Huggins ask after this : " Do these modifications not repre- sent some of the stages through which our sun has passed ? " In another part of his paper he uses the term " life changes." Now the life of a star is its temperature, and all these changes must have been produced by the running down of temperature, and I think the simplest view to take, limiting ourselves to the concrete case of H and K, is that the substance which pro- duces K has been formed at the expense of the substance which produces H, and the reason that we see these two lines in calcium when a high temperature is employed is that we reveal the presence of these true root-forms. There may be very many more difficult explanations, but that I think is the simplest one to which one is driven by the logic of facts. The appeal to the stars, then, I think, amply justifies the prediction which I based upon the comparison of solar with terrestrial phenomena; and, therefore, tends to show that the basis on which those predictions were founded had at all events some little glimmering of truth about it. I think also that it increases the number of dissociation stages through which we must assume the vapours of our so-called elements to pass when higher temperatures are employed in succession. How rigid a test we have been able to apply to these views by means of Dr. Huggins' remarkably beautiful researches the 250 THE CHEMISTRY OF THE SUN. [CHAP. accompanying woodcut, Fig. 95, will show ; a perfect continuity exists between the old facts and the new ; the stars arrange themselves along the same line as do our terrestrial heat sources. The wide departure of stars hotter presumably than the sun (taking the absorption of the rhythmic series of lines, as the indication of temperature) from the solar type shows that there is much more work to be done in this field. The success of my former prediction emboldens me to make another one. FIG. 95. The changes in the spectrum of Calcium from the Bunsen flame to Sirius. It will in all probability be found that the remaining thick lines in stars of the Sirius type are represented in many cases ly the lines brightened in solar prominences. 2. Lines in the spectra of two or more substances. We have now to point out another direction on which the discussion of the hypothetical furnaces seemed to throw some light. The next conclusion was quite unexpected. Let us take the conditions represented in Fig. 87, and remember that in almost all cases the strongest line in a spectrum at any one temperature xviir.] AN UNEXPECTED RESULT. 251 is the longest. Although the spectrum of the substances as they exist in A would contain as many lines as would the spectrum of the substances as they exist in D ; each line would be thick at one temperature and thin at another. It would be therefore longest at one temperature and shortest at another. Hence, since the longest lines at one temperature will not le the longest at another, the ivhole fabric of " impurity elimination" based upon the assumed single molecular grouping falls to pieces. To take a concrete case. Let us suppose that in Fig.- 87 the four furnaces represent the spectra of iron broken up into different finenesses by successive stages of heat. It is first of all abundantly clear that the relative thicknesses of the iron lines observed will vary as the temperature resembles that of A, B, c, or D. The positions in the spectra we may assume to be the same, but the intensities will vary ; this is the point. The strongest, and, therefore, as a rule the longest, lines will vary as we pass from one temperature to another. But how does the whole fabric of impurity elimination fall to pieces ? In this way. Let us suppose that manganese is a compound body, and that one of its constituents is a form of iron represented in furnace B. Suppose the photograph of the spectrum of iron I compared with that of manganese is taken at the temperature represented by furnace D, and that the photograph of manganese is also taken at the same temperature ; now, to eliminate the impurity of iron, in the manganese, we look for the longest (strongest) lines in the iron photograph among the manganese lines. If we do not find them we say there is no impurity of iron ; but although the longest lines are absent we get the shorter familiar ones. It will be seen at once, then, that the consideration of the question gave as a natural consequence precisely the state of things detailed in page 232 et seq., which on the old view seemed so inexplicable. 252 THE CHEMISTRY OF THE SUN. [CHAP. This was one way of regarding the matter, there is another which perhaps is more simple. The analogy between simple and compound bodies which we have heretofore employed, has been that suggested by the manner in which, after a compound of calcium and chlorine has been exposed to the action of a sufficient temperature, the spectra of calcium and chlorine, become visible in addition to that of the compound. Now suppose we expose compounds of calcium with chlorine, bromine and iodine to a like temperature, we eventually get the calcium lines visible in all as the compound is broken up. This, then, furnishes us with a new analogy, since the common lines seen in the spectra of elementary bodies may be likened to the lines of calcium, common to all the salts of calcium when they are dissociated. I am particularly anxious to point out that there was abso- lutely nothing new imported into the consideration of the question. We had simply with regard to change of intensity taken as our guide the behaviour of a known compound body, and then pushed the reasoning three or four stages further. We had gone just the safest possible way, by the easiest possi- ble stages, from the known to the unknown. This had brought home to us another analogy furnished by a group of compound bodies with a common base. No new theory in fact was necessary. The appeal to the law of continuity was open to us, and it seemed to answer our question at once. What then was the first thing to be done ? In the first part of our inquiry we confined ourselves to the lines seen in the spectrum of one substance. It was obviously now our duty to study the lines seen in the spectra of different substances equally au fond. First, it was of importance to see whether an examination of photographs gave us similar results with regard to the coin- cidence of lines in other parts of the spectrum than that first reduced. The question of absolute coincidence being reserved, as XVIIL] VARIATION OF THE ATTACK. 253 I always have reserved it, we have to consider whether the lines are massed in certain regions of the spectrum. A rapid survey was made with this object, the result showing that the same thing did truly apply to these other regions with the dispersion we had employed. Still full of the " solar line double " and " common impurity " point of view, the attack was varied by an inquiry whether there was any special character connected with the coincident lines. In this way it was imagined that some light might be thrown upon the question as to whether they were chance coincidences. In short, the thing done was inquiring whether these coincident lines varied their behaviour in some other special manner from non-coincident lines taken at random. Supposing them to represent mere chance coincidences " physical coincidences," as they have been called, or again, lines so near together that our means cannot separate them there is no reason why they should behave differently from the other lines in a spectrum when the temperature is changed ; while, as our hypothetical furnaces had taught us, if they be truly common, they must vary with temperature. Further, they must vary in such a way that other conditions being equal, they shall become stronger when the temperature is increased, and fainter when the temperature is reduced. Now what was the best mode of attacking this problem ? I was unable to see a more expeditious one than that presented to us by the sun's atmosphere. The following considerations will show how we might hope for help in this quarter. Whatever be the chemical nature of this atmosphere, it will certainly be hotter at bottom that is, nearer the photo- sphere than higher up. Hence, if temperature plays any part in moulding the conditions by which changes in the resulting spectrum are brought about, the spectrum of the atmosphere close to the photosphere will be different from that of any higher region, and therefore from the general spectrum 254 THE CHEMISTKY OF THE SUN. [CHAP. of the sun, which practically gives us the summation of all the absorptions of all the regions from the top of the atmosphere to the bottom. Now as a matter of fact we have the opportunity, when we observe the spectrum of a sun-spot or a prominence, of deter- mining the spectrum of a practically isolated mass of vapours in the hottest region open to OUT inquiries, and seeing whether it is like or unlike the general spectrum of the sun. What then are the facts ? It is as unlike as possible : the intensities of the lines, as we have seen, are inverted to a wonderful extent. More than this : there is a constant difference between the spectra of sun-spots and the spectra of metallic prominences, though we see these phenomena generally at about the same niveau in the sun's atmosphere. To get an idea of this inversion, maps were prepared of the spectra of the chief chemical substances showing the behaviour of the various lines under the various conditions. The result is very striking ; indeed, it is striking to quite an unexpected degree. The whole character of the spectrum of iron, for instance, is changed when we pass from the iron lines seen among the Fraunhofer lines to those seen among the spot and prominence lines ; a complex spectrum is turned into a simple one, the feeble lines are exalted, the stronger ones suppressed almost altogether. Since then the spectra of spots and prominences are confes- sedly the spectra of the hottest regions of the sun available for our inquiries, we can test the nature of the common lines by seeing how they behave when we pass from the general solar to these special solar spectra. With special reference to this point the various observations recorded of the lines visible in solar disturbances at the sun's limb, and of those observed to be widened, brightened or other- wise modified in the spectra of solar spots were confronted. xvin.] LIMITS OF THE DISCUSSION. 255 The finest series of observations of this kind that we pos- sessed in 1878, was that collected by Prof. Young near the time of the last maximum of sun-spots, during his stay at Sherman, at a height of 8 ; 000 feet. The result which stares us in the face when we examine the work done by Young is most striking ; but although his observations of the chromospheric lines extend over the whole visible spectrum, the list of lines in the solar spots is limited to the less refrangible region ; the discussion was therefore limited to this region. As a basis for this discussion, the lines given in Thaleu's admirable tables were taken, comparing them with those shown in Angstrom's map, and indicating the intensities of the lines which are given in the tables, and which particular line occurs in the map only. A glance, then, shows which line is seen in spots and prominences, and how it is affected. In short we have in one view, for each metallic substance, exactly what happens to the lines of that substance which lines are not touched; those which are visibly affected both in spots and storms, or those recorded in one table and not in the other. Taking all the lines included in the discussion, the following statistics will show how they are distributed : Total number of lines in Thalen's list and map included in the discussion . ...... 345 Number of lines affected in spots 108 Number of lines bright in storms (prominences) 122 Number of lines common to spots and storms . 68 Number of lines seen in neither spots rior storms 183 So much for the list of lines as a whole. It is also necessary to show the number of lines assigned to each metal, and those among them which occur in both spots and storms, or only in one or the other. In order that this may be clearly shown the table overleaf is appended. 256 THE CHEMISTRY OF THE SUN. [CHAP. Number of Number of lines lines due to Metal. Number common to Unaf- of lines. spots and fected. storms. Spots. Storms. Sodium ... 8 4 6 6 Magnesium Barium ... 4 23 3 1 4 3 3 7 14 Calcium ... 25 7 15 10 7 Strontium 18 18 Nickel ... . 12 1 3 2 8 Cobalt ... . 19 3 3 3 16 Manganese 16 2 3 6 9 Cadmium 15 15 Chromium 14 3 3 5 9 Titanium 87 11 18 18 62 Iron 104 33 50 62 25 345 68 108 122 183 It will be seen that the ratio between the affected and un- affected lines is very variable. What strikes one, indeed, is the wonderful irregularity in the behaviour of the various lines ; there is no relation, for instance, between the widening of the lines in the spots and their appearances in the prominences. It may here be asked, " But what has this to do with the lines common to two or more spectra ? " I answer, it would have nothing to do with such lines if Thalen had not observed them ; but in his observations, which are the ne plus ultra of spectroscopic accuracy, he came across them abundantly. Among the 345 lines given ly ThaUn are 18 with identical readings in two spectra. They are, therefore, the exact equiva- lents of those lines found to be coincident in work on another part of the spectrum. Now, for the reasons above given, if the explanation of their basic character suggested by the consideration of the hypothetical furnaces be the correct one, then we should expect a con- siderable development of these lines in the spectrum of the XVIII.] KESULT OF THE INQUIHY. 257 hottest regions of the sun, which spots and storms enable us to study apart from the absorption going on at higher levels. It is not too much to say that the result of this inquiry was most striking. What came out in the strongest manner was the very remarkable fact that these common lines were always widened in the spots. However feebly the brighter lines of a chemical substance, taken as a whole, might be represented amongst the spot lines, yet the common lines among these, which are often of the second or third order of intensity and sometimes even of the fourth, are never absent. The same fact held almost equally true with regard to the storms. The comparison of Thalen's lines, recorded in two spectra, with those seen by Young in solar spots and storms shows this result : THALEN. YOUNG. Wave-length. Common to Intensity. Spots. Storms. Widen- ing. Fre- quency. Bright- ness. 5207-6 Fe Or 3 1 4 10 6 52037 Fe Cr 3 1 4 10 6 5340-2 Fe Mn 2 3 2 1 2 6064-5 Fe Ti 2 2 3 5 2 5661-5 Fe Ti 3 1 4 15 2 5403-1 Fe Ti 23 4 5 3 5396-1 Fe Ti 2 2 7 4 2 5352-4 Fe Co 4 3 2 4 2 5265-8 Fe Co 2 3 2 10 4 5168-3 Fe Ni 3 5 4 40 30 5166-7 Fe Mg i 2 1 2 30 20 5681-4 Fe Na 3 3 3 2 1 6121-2 Co Ca 1 3 4 5 3 5601-7 Ca Fe 4 1 2 5597-2 Ca Fe 3 1 2 5856-5 Ca Ni 3 4 2 5425-0 Ba Ti 3 3 4 6449-0 Ca Ba 2 3 2 So far as my own knowledge of these matters goes, I can imagine no severer test to apply to the hypothesis that s 258 THE CHEMISTRY OF THE SUN. [CH. xvm. the coincident lines in the above table are not produced by chance. We may conveniently sum up what has been advanced in this chapter as follows : Attempting to picture the spectral changes brought about by the action of a series of furnaces, we find that exactly such changes of intensity in spectral lines as have been recorded must be produced if dissociation takes place. This holds true, not only for metallic vapours, but for stars ; and some predictions which have been made on the strength of the hypothesis have been verified. Further, a continuation of the discussion of the hypothetical furnaces seems to give a vera causa for coincident lines, on the assumption that some of the elements have common bases, and a further consideration of the common lines observed indicate that they do not appear to depend upon the presence of impurities, and it is also shown that these coincident lines do somehow differ in other ways from non-coincident lines taken at random from the same spectra. CHAPTER XIX. DISCUSSION OF THE DISSOCIATION HYPOTHESIS. continued. IT will, I think, be generally conceded that the results referred to in the preceding chapter, " gave us prima facie ground for thinking that it was quite worth while to go on with the consideration of the hypothesis which had been advanced. We saw that dissociation did seem to explain the changes in the intensity of lines of spectra on the analogy of the dissocia- tion of known compounds. I was careful at the very outset to point out that the view advanced is based upon the analogies furnished by those bodies which by common consent, and beyond cavil and discussion, are compound bodies. Indeed, had I not been careful to urge this point, the remark might have been made that the various changes in the spectra to which I have drawn attention are not the results of successive dissociations, but are effects due to putting the same mass into different kinds of vibration or of producing the vibration in different ways. Thus the many high notes, both true and false, which can be produced out of a bell, with or without its fundamental one, might have been put forward as analogous with those spectral lines which are pro- duced at different degrees of temperature with or without the line due to each substance when vibrating visibly with the lowest temperature. To this argument, however, if it were brought forward, the reply would be that it proves too much. If it demonstrates that the li hydrogen line in the sun is produced by the same molecular grouping of hydrogen as that which s 2 260 THE CHEMISTRY OF THE SUN. [CHAP. gives us a green line only when the weakest possible spark is taken in hydrogen inclosed in a large glass globe, it also proves that calcium is identical with its salts. For we can get the spectrum of any of the salts alone without its common base, calcium, as we can get the green line of hydrogen without the violet one. Hence the argument founded on the overnotes of a sounding body, such as a bell, cannot be urged by any one who believes in the existence of any compound bodies at all, because there is no spectroscopic break between acknowledged com- pounds and the supposed elementary bodies. The spectroscopic differences between calcium itself at different temperatures, is as great as when we pass from known compounds of calcium to calcium itself. There is a perfect continuity of phenomena from one end of the scale of temperature to the other. We next saw that common lines had been observed in the spectra of various substances under conditions which seemed to put impurity out of the question, and that then another line of inquiry had been undertaken to see if we were not after all dealing with apparent coincidences only. The result of this inquiry was to show that the coincident lines really behaved differently in the sun from the non-coincident ones. The view of which we have now to continue the consideration, is, I think, after all but a slight expansion of the present- chemical view. Chemists regard matter as composed of atoms and molecules. The view now brought forward simply expands the series into a larger number of terms, and suggests that the molecular grouping of a chemical substance may be simplified almost without limit if the temperature be increased. A diagram (Fig. 96) will show exactly what I mean, and what, in fact, flows easily from the consideration of the hypothetical furnaces referred to in a preceding chapter. If we assume a very great difference in the temperature which can be brought to bear upon a substance, we may assume that at the highest temperature we have, for simplicity's sake say, a single XIX.] EVOLUTION DIAGRAM. 261 line represented by a single circle ; let us imagine the tempera- ture reduced, we shall then get another spectrum, which we can represent by a double circle, if we like to assume that the evolution is one which proceeds by con- stant additions of the original unit. Coming lower down, we get an- other substance formed with a more complex spec- trum represented by three circles ; lower down still we have one represented by four circles, another by live, another by six, and so on. We might take another supposition, easier perhaps to some minds, and sup- pose that evolution pro- ceeded, not by the constant addition of the initial unit, A + A, but by the constant doubling of the substance of the molecule itself. In- stead, therefore, of our circles increasing by one, we shall have one, two, four, eight, sixteen, thirty- two, and it will be readily understood that if there are a con- siderable number of stages of temperature, both within our ken and beyond our ken, and if some substances form them- selves perpetually by doubling, then the unit with which we can experiment at low temperature, call it the chemical atom 262 THE CHEMISTKY OF THE SUN. [CHAP. or the chemical molecule, or what you will, must be a very complex thing indeed. If the lower spectrum represents that of a complex body such as iron, or a salt of calcium, the upper spectra will represent those due to the finer groupings brought about by higher temperatures. We pass continuously, as in the sun and the stars, from complexity to simplicity, if we begin at the lower, and from simplicity to complexity if we begin at the higher, stages of temperature. As it is most important to obtain a clear mental view of the manner in which, on the principles of evolution, various bases may be formed, it is as well to point out here that although it does not seem unnatural that the bases should increase their com- plexity by a process of continual addition of like units, or even by continual multiplication of them, the factors being 2, 3, or some higher number ; still that these need not necessarily, or even pro- bably, be the only lines. Thus we may have increase of molecular complexity by the addition of molecules of different origins, and seeing that we must suppose these early forms to be produced in a mass of incandescent vapour, probably in a state of the wildest commotion, I should fancy that the greater number after the first would be formed in such fashion. A -f A would give place to A + B, and a variation of the process would consist in a still further com- plexity being brought about by the addition of another molecule of B, so that instead of (A. -f B] 2 merely, we should have A + B 2 . Now, two questions arise here which I think it is important to discuss. Are we playing fast and loose, in such an hypo- thesis as this, with the ordinary course of nature's operations, or are we in harmony with her ? Again ; is it contrary to the view expressed by the greatest minds which have studied chemical phenomena? That the view is not inharmonious with the theory of evolution which has been formulated, or other view which we have gathered from other regions of thought and work, is at once obvious; in fact, I think it derives its whole force from the fact that along many lines it runs parallel with the xix.] ASSUMED ASSOCIATION. 263 processes of development in the different kingdoms of nature. Of course we know that in the organic kingdom evolution always proceeds along many lines ; in the inorganic world the environ- ment was less complex, but to simplify the problem it is per- missible to take one of the simplest that I can think of. Let us assume that in a certain very hot star there shall be two substances, which we will call a and b. They will first at the transcendental temperature which I assume, exist as separate entities. The temperature being then reduced, they probably will combine, and, instead of two atoms, a and I, we shall have one group of a + b. If the temperature is still further reduced, we shall get b combining with b ; along that line we shall have a grouping consisting of a + 2 b. Let the same operation be per- formed again, and we shall then have a + 4&; we shall have what we can represent, in short, in chemical language by ab. Now, having got our ab^ having got our temperature reduced, let us assume that ab 2 is now the substance linked on to give a greater complexity, instead of b or 2b merely. We then have this series given in the table. TABLE I. HYPOTHETICAL SERIES. a b a + b a + bb a a G b u -j- ab 2 = ...... f &i = ab = 4- <*>b 2 = 264 THE CHEMISTRY OF THE SUN. [CHAP. Now, that is an ideal scale. The question is, Is it absurd ? How can we honestly answer that question ? By asking whether we are or are not on the lines on which nature works in the region of the known in the region which we can get at. We will now refer to another table ; we will pass from the ideal to the concrete, and it will be seen that there is, if one can invert the term in such a way, a distinct precedent for such a table as the last ; for here are the absolute facts with regard to one series of the combinations of carbon and hydrogen. TABLE II. CONCRETE SERIES OF HYDROCARBONS. C H C + H C 4- HH C + (HH)(HH) - ...CH 4 -hCH 2 = C 2 H 6 + CH 2 = ; ..C 3 H + CH - C r H 14 4- CH 9 - C r H 1( , 4- CH CH 9n + CH 9 - .... ...C n H f ! w 4- CH 9 ....( i & 4- OH., C H 4- CH 9 ^14 30 4- CH - .. 'h We have gases CH 4 , C 2 H 6 , C 3 H 8 ; followed by liquids from C 4 H 10 to C 15 H 32 ; each of them formed, not by the addition of my hypothetical aZ> 2 , but by a concrete CH 2 , and we have connected with that a beautiful order and exquisite regularity in the way in which the boiling-points and specific gravities of the successively more complex forms increase, as will be seen in the next table. X1X.J CONCRETE ASSOCIATION. 265 Gaseous Solid TABLE III. HYDROCARBON SERIES. re H 4 i&H, ICsX Marsh Gas. Ethane. Propane. f 'G\ H^ Tetrane or Diethyl ... C 5 H 12 Pentane C 6 H 14 Hexane or Dipropyl... C 7 H 16 Heptane C 8 H 18 Octane C 9 H 20 Nonane C 10 H 22 Decane C U H 24 Endecane C 12 H 26 Dodecane or Dihexyl. C 13 H 28 Tridecane C 14 H 30 Tetradecane , C 15 H 32 Pentadecane p TT (Hexdecane or Di- ^16^34 j octyl Boiling point. Specific gravity. o 600 at 38 '.'.'. 628 at 17 71 ... 669 at 16 99 ... 699 at 15 124 ... 726 at 15 148 ... 728 at 13-5 166 ... 739 at 13-5 180 ... 765 at 16 202 ... 774 at 17 218 ... 792 at 20 230 ... 258 ... 825 at 16 278 Melts at 21 There is no break in the general line of increase, and after we have gone through the gaseous stage, which stops at C 3 H 8 , and through the liquid stage, which stops at C 15 H 32 , we get the solid state, and there again the same series is represented. So that I think one is justified in saying that, dealing with this one simple case (and the only reason why I have taken it is that it is a line which has been thoroughly worked out by organic chemists), we are justified in saying that if nature, in the regions which we cannot get at, works in the same way as she does in the regions which we can get at, the view is not absurd, and in fact any one who wishes to dispute it in such a case as this has, I think, the onus probandi thrown upon him. He must show that either in a certain latitude or longitude, or at a certain temperature, or under some unknown condition the laws of nature are absolutely changed, and give place to new ones. That has not yet been found in any other region of natural philosophy. Indeed I think one 26G THE CHEMISTRY OF THE SUN. [CHAP. might go further and say that all the processes of development observed in different regions of thought, have such a oneness about them that to my mind one of the best mental images we can get of the conditions which determine the lines picked out for special prominence in solar spots and solar flames, is to consider the molecular groupings that produce them as re- sembling the roots of the present European languages which our ancestors brought from the cradle of the race in Asia. Now comes the second question, to which reference has been made. What is the opinion of those who have given the great- est attention to chemical philosophy ? I do not mean to chem- istry, I mean to chemical philosophy. I have already referred to Brodie's clear statement, but we can anticipate his time by a reference to Dalton. He says, " We do not know that any one of the bodies denominated elementary is absolutely indecom- posable." Graham also wrote, " It is conceivable that the various kinds of matter now recognised in different elementary sub- stances may possess one and the same element or atomic molecule existing in different conditions of movability. The essential unity of matter is an hypothesis in harmony with the equal action of gravity upon all bodies." The greatest chemical philosopher now living, M. Dumas, so long ago as 1836 published a series of lectures in which his views were very clearly stated indeed, and any one who reads them will see how convinced he was then of the considerable amount of evidence that had already been accumulated in favour of the non- elementary nature of a great number of substances then classed as elements. Then again we can pass to another chemical philosopher, Kopp. In his researches on specific heats he also gives evidence to show that that relationship is not to be depended upon to establish the received view. If, then, the three greatest English chemists of their time we can name, and the most eminent chemical philosophers in France and Germany, give their opinion xix.] SOME OPINIONS. 267 in behalf of the compound nature of the chemical elements, can these simpler forms be any other than those we detect by means of the spectroscope ? By the conditions of the problem and the absence of knowledge they are not decomposable in the laboratory; if they were they would cease to be elementary bodies at once, and would be wiped out of our tables. Nor do I think it possible that in the present stage of our knowledge they can be revealed to us in any other way than by the spec- troscope. It is unfortunate that none of these chemists who have given us this view have helped us by showing in what way the possibility, which all of them suggest, and which many of them intensely believe in, could be absolutely demonstrated ; but it is obvious that if dissociation is the thing which time out of mind has made compound bodies simpler, in their minds the condition of higher temperature must have been present. The only difficulty was the way in which the effects of that high temperature could be measured and weighed, and I think that if the spectroscope had been introduced earlier they would pro- bably have left some hints behind them which would have been of the greatest value to those who work with that instrument. Passing from the chemists to the physicists, there is one, at all events, who has appreciated exactly how this decomposability of the terrestrial elements could be established. I refer to the lamented Clerk Maxwell. In his article on Atoms in the Encyclopaedia Britannica, he says : " The discovery of a particular line in a celestial spectrum which does not coincide with any line in a terrestrial spectrum indicates either that a substance exists in the heavenly body not yet detected by chemistry on the earth, or" (and it is to the "or" I wish to draw attention) " that the temperature of the heavenly body is such that some substance undecomposable by our methods is there split up into components unknown to us in their separate states." Absolutely nothing could be clearer than this. But in endeavouring to discuss the question as to how far the 68 THE CHEMISTRY OF THE SUN. [CHAP. hypothesis of evolution of chemical forms is contrary to, or in accordance with the views of modern chemistry, we must draw a very wide distinction between chemical theory and chemical fact. When we compare the laws given in average chemical text- books with the laws which lie at the root, let us say, of astro- nomy, the candid mind cannot fail to be struck by the difficulty which chemists must have encountered in endeavouring to reduce the face of their science to order, on the hypothesis they bring before us. An outsider thinks, for instance, that the basis of chemistry, or a large part of the basis of chemistry at all events, lies in the fact that the chemist has determined the existence of a certain number of elementary bodies, each of these elementary bodies having a certain atomic weight, and that this atomic weight determines all the constants of that body. Yet we read in chemical text-books that this atomic weight is fixed according to no invariable rule ; indeed, with Kepler's laws and Newton's laws in the mind one comes to the conclusion that it is not too much to say that it is determined by a series of compromises. An outsider would, think that if any one of these elementary bodies were taken as a standard, the weight of an equal volume of vapour of another substance under equal conditions would bear some relationship of a definite character to the atomic weight. This however is not the case. Again, among the questions to be considered as de- termining the atomic weights taken, is an assumed limitation of combination power, a so-called atomicity, according to which one substance is a monad, because it will combine with that same relative proportion of hydrogen which exists in half a water-molecule. Another substance is called a dyad, because it will combine with the same relative proportion of hydrogen which exists in a whole water-molecule, and so on. When we thus begin to class the substances into monads, dyads, hexads, and so forth in fact, when we thus effect a re- classification xix.] THEORY VERSUS FACT. 269 of the elementary bodies, the solidarity at once breaks down ; we find that the classification after all is useless, because the same substance may behave as a dyad, a tetrad, a hexad, a pseudo-tryad, and a pseudo-octad ; in fact, one feels one is dealing with something that is more like a moral than a phy- sical attribute a sort of expression of free will on the part of the molecules. We are, I think, justified in asking whether these various attempts to formulate a science do not break down after a certain point, because they attempt to give a fixity to what is in truth variable. When we pass to the facts of the science, the key-note of which is variability from one end of the scale to the other, we find that the view of successive dissociation, the view of variable molecular groupings brought about under different conditions, is really more or less in accordance with them, where the laws based on fixity break down entirely. Thus, for instance, let us take the question of vapour densities. The view accounts fully for the so-called anomalous vapour densities, and in this way : it sug- gests that the elements may really be complex groups which break up into their constituent groups under suitable conditions of temperature, as phosphoric chloride and many other bodies do when obtained in the condition of vapour. We have dis- similar groups in the one case, and possibly similar groups in the other. In this way, that contradiction in terms, the " mon- atomic molecule," really becomes the evidence of a higher law. Let us pass to allotropic conditions. The explanation of these is that there are bodies which have a large molecular range within the ordinary temperatures at our command. The substances in which allotropism is most marked are all metal- loids which have not been found in the sun, and the allotropic forms give us in many cases different spectra spectra indicating a considerable complexity of the molecules which produce them about which something has been said already. In the passage from one allotropic condition to the other, energy, without any 270 THE CHEMISTRY OF THE SUN. [CHAP. known exception, is absorbed or given out. What is it doing ? if it is not in some way or other controlling the passage from one molecular group to another ? These allotropic conditions, occurring very obviously to us in certain limits at our ordinary temperature and pressure are, possibly, but special cases of group-condensation common to all bodies, represented by Dalton's law of multiple proportions. We can indeed imagine a condition of things in which the difference between iron in the ferrous and ferric chlorides (FeCl 2 and Fe 2 Cl 6 ) would be as obvious as the difference between ordinary and amorphous phosphorus. In certain classes of so-called organic substances this group- ing of simpler groups to more complex actually takes place, and is recognised under the term polymerisin for instance, with cyanogen compounds of oxygen we have a simple thing like cyanic acid (CNO) say, which will form a series of com- pounds, and we have its so-called polymers, C 2 lSr 2 2 , or C 3 N" 3 O 3 , which will each form a series of compounds, these groups of more complex nature forming, by their combination, group- individuals with related but not identical properties with the simplest or fundamental group. In many cases the amount of this condensation may be determined by the vapour densities. In others, again, a disso- ciation takes place at a certain limit of temperature, a simpler or fundamental group being the resolution product. The resemblance between these cases of polymerism and especially those elementary bodies which exhibit allotropism, is at least striking. In the one case, the organic complex bodies, the range of ex- istence is in most cases within our easy attainment ; in the so- called elementary stuffs it is less frequently the case. We can certainly convert ordinary phosphorus and sulphur into allo- tropic and most likely polymeric forms, but we do not know as yet how many atoms more are contained in the polymeric forms of these substances than in their simpler states. xix.] ATOMICITY. 271 And in other substances this range of condition of formation passes gradually out of our reach, but the phenomena are the same in kind up to the temperature of the sun. And again, when we can obtain the spectra of bodies like amorphous phos- phorus we can prophesy that the relative grouping of the atoms of phosphorus in this to the ordinary form will be exhibited. This brings us to the next point atomicity. What are the associated phenomena? Lowest melting-point, simplest spec- trum, lowest atomicity. Therefore we are justified, I think, in assuming that atomicity may after all be but the measure of the molecular groupings at work. In this way we can associate various atomicities, not with moral phenomena as regards the behaviour of the same molecule, but with different physical states different complexities of the same substance. Thus in the same substance the more complex or allotropic the molecular grouping, the higher the atomicity. Hence the substances in which the highest atomicities appear should, as a rule, be formed and broken up at the lowest temperature. This, I am informed, is really what happens in the majority of cases. I have ventured in these few remarks to touch upon the rela- tions of the new view to modern chemical facts, because I think such a discussion shows us that there are several chemical regions in which the views can be tested from a chemical point of view, although I have, of set purpose, dealt with them abso- lutely from the physical side. One such step of the highest interest has already been taken by Captain Abney. The language of Professor Eoscoe, the President of the Chemical Society, in de- scribing it, is so clear, and so admirably put, that it is impossible for any one to improve upon it. 1 Eeferring to the work which Captain Abney and Colonel Testing have done together, he says : This work "is no less than a distinct physical test of the exist- ence in organic compounds, of the organic radicals, and a means of recognising the chemical structure of an organic compound 1 Journal of the Chemical Society, May, 1881. 272 THE CHEMISTRY OF THE SUN. [CHAP. xix. by means of the spectroscope." This result " is accomplished by photographing the absorption spectra of organic compounds in the infra-red part of the spectrum. In these invisible por- tions characteristic and distinct absorption lines and bands occur for each organic radical. The ethyl compounds all show one special ethyl band ; the methyl compounds a special methyl band ; and thus, just as a glance at the luminous portion of the spec- trum satisfies us of the presence of calcium, lithium, and rhu- bidium, so a simple inspection of these infra-red photographs enables us to ascertain the presence of the various organic radicals. This invention is still in its infancy, but one of greater importance to chemists has seldom if ever been communicated to the Society." I have been the more anxious to give these re- sults in Professor Eoscoe's own words, because it will be seen that, mutatis mutandis, these remarks touching the spectra of organic radicals are precisely identical with one statement I have been endeavouring to make with regard to inorganic radicals. It cannot therefore be said that the nature of the principle I bring forward is one with which chemists are not familiar. CHAPTER XX. SOME TEST EXPERIMENTS. ABSORPTION PHENOMENA. WE are now in a position to test the new hypothesis by a detailed reference to those solar phenomena and connected laboratory experiments which can bear upon it. But on the festina lente principle, which I have so far adopted, I propose in the first instance to state the results of several new modes of experimental investigation which I tried with the view of throwing light if possible upon the question of molecular complexity raised in 1873. I have shown in chap. xiv. when dealing with the difficulties connected with the old view, the growing importance of the question of multiple spectra, and how, as the work went on, the number of observers who ascribed them to impurities got less and less. Some of the new methods have to do with the question of multiple spectra, and a brief reference to them will show that their results are almost entirely in favour of the new view, and that the progress of spectrum analysis increases the possible number of molecular groupings. We began with line spectra and fluted spectra as the maximum of variation ; we end with line and fluted spectra both of complex origin, and new orders of spectra altogether, determined by absorption or radiation of a continuous sort in one or other part of the spectrum. These points may be discussed seriatim, and I propose to take the last first. T 274 THE CHEMISTRY OF THE SUN. [CHAP. When it first began to be conceded that the fluted spectrum of a substance was not due to an impurity, but to an allo- tropic state, it was thought that it was produced by molecules intermediate in complexity between thoie which in the case of any one body produced the continuous and line spectrum respect- ively. Some experiments which I undertook soon led me to the conclusion that this molecular grouping was not the only one between the extremes named. It was an inquiry which carried us into lower temperatures than had been employed before ; it was less wonderful therefore that it should show that more " orders of spectra," to use Pllicker's term, were necessary. These experiments were undertaken in consequence of the following considerations : I. Most solids when heated if they can be heated and yet retain their solid state give us continuous radiation spectra. II. Many of the metalloids in vapour give us fluted absorp- tion spectra at ordinary temperatures, and others do the same at higher temperatures. III. Most elements driven into vapour by the temperature of the voltaic arc give us line radiation spectra, with line absorption. IV. All elements driven into vapour by the induced current give us line radiation spectra. Now we have here, prima facie, good reason for supposing that the molecular structure of the vapours which give us such different effects is not the same. To take the lowest ground. If, in the absence of all knowledge on the subject, it could be shown that all vapours at all stages of temperature gave us spectra absolutely similar in chara^dter, then it would be more likely that all vapours were truly Homogeneous and similar among themselves, as regards molecular structure, than if the spectra varied in character, not only from element to element, but from one temperature to another in the vapour of the same xx.] NEW ORDERS OF SPECTRA. 275 element. Further, the continuous spectrum of -the solid ele- ment is observed in the case of some well-known compounds, whereas all known compounds are resolved by the high tension spark into their constituents. We have a right to assume, therefore, that an element in the solid state is a more complex mass than the element in a state of vapour, since its spectrum is the same as that of a mass known to be more complex. Again, when changes occur in the spectrum of the same sub- stance, they are always in the same direction; and, further, the spectra we obtain from elements in a state of vapour are similar in character to those we obtain from vapours of known compounds. So far we have continuous, fluted, and line spectra, but this is not all ; certain bodies, both simple and compound, when we study them by their absorption at ordinaiy temperatures, give us phenomena not included in the above statement. Here is what I said on this subject in 1874 l : " At ordinary temperatures in some cases, as in selenium, the more refrangible end is absorbed : in others the continuous spec- trum in the blue is accompanied by a continuous spectrum in the red. On the application of heat, the spectrum in the red disappears, that in the blue remains ; and, further, as Faraday has shown in his researches on gold leaf, the masses which absorb in the blue may be isolated from those which absorb in the red. It is well known that many Rubstances known to be compounds, in solution give us an absorption in the blue or blue and red ; and also that the addition of a substance known to be a compound (such as water) to substances known to be compounds which absorb the blue, super- adds an absorption in the red. " In those cases which do not conform to what has been stated, the limited range of the visible spectrum must be borne in mind. Thus I have little doubt that the simple gases at the ordinary conditions of temperature and pressure have an absorption in the ultra-violet, and that highly compound vapours and liquids are often 1 Proc. R. S. No. 153, 1874. T 2 27G THE CHEMISTRY OF THE SUN. [CHAP. colourless because their absorption is beyond the red, with or without an absorption in the ultra-violet. Glass is a good case in point ; others will suggest themselves as opposed to the opacity of the metals." The point in the new experiments, then, to which reference has now to be made was to determine what spectrum the vapours of the metallic bodies gave at a lower temperature than any at which they had been previously observed. It is evidently therefore a question of absorption phenomena. The first experiments were made, in December, 1873, upon zinc in a glass tube closed at each end with glass plates ; they were made by myself in conjunction with my friend, Dr. Russell. We studied the absorption of the light of the positive pole of the electric lamp placed at one end of the tube by the vapour in the tube. A one-prism spectroscope was used for the ob- servations. A stream of pure hydrogen was allowed to pass through the tube, which was heated in a Hoffmann's gas furnace. This plan, however, would not work for several reasons among them, the glass melted. In the next attempt we used an iron tube some four feet long, closed with glass at the ends as before, with its central part in a charcoal furnace. The air in the tube was replaced by hydrogen and the substance of which the low temperature absorption spectrum was to be examined was dropped, by a special arrange- ment, into the central portion. There was an electric lamp at one end of the tube and a spectroscope at the other, as in the first arrangement. The object of keeping the voltaic arc out- side the tube was of course to enable us to deal with the vapour at the temperature of the furnace merely. In most cases, when the reduction of the density of the vapour enabled any light to be seen, the light was green, and the spectroscope indicated that this was due to the absorption of the red and blue light. Various experiments indicated that the blue and x*.] RESULTS. 277 red absorptions were independent of each other, for they varied with temperature. We found that the temperature reached by the tube could be marked by the following phenomena : I. When the continuous spectrum of the tube extended to D this line not being visible. II. When it extended beyond D, D being bright. III. When it extended into the green, D being very bright. IV. When it extended beyond the green, D being invisible. Before I detail some of the results of this inquiry I should state that Professor Roscoe, who had seen some of the earlier ex- periments, also took the matter up, and commanding a higher temperature than was at first at our disposal, observed some previously unrecorded spectra in the case of sodium and potas- sium. These I shall call the new spectra. They are always easily visible when a sufficient temperature has been reached. The results obtained were as follows : Hydrogen Nitrogen ' NQ &b Qn Mercury Bismuth Zinc. Cadmium, i . , , . \ Continuous absorption in the blue. Antimony, f Phosphorus. Sulphur. Fluted spectrum (previously observed by Salet). Iodine. Fluted spectrum in the green, and intense band of general absorption in the violet where, at the ordinary temperature, the vapour transmits light. Sodium. There were observed either separately or together a. D absorbed. /?. Continuous absorption breaking up in the middle into red and blue absorption. y. The new spectrum. . 278 THE CHEMISTRY OF THE SUN. [CHAP. Potassium. There were observed either separately or together a. Line absorption more refrangible than D. ft. As in Sodium. y. The new spectrum. An expansion of this research by Professor Roberts and myself added great weight to the views expressed above and con- siderably increased the number of bodies possessing fluted spectra. The experimental conditions were varied as follows : The temperature was greatly increased by the employment of an oxyhydrogen blowpipe and a lime still, such as that employed by Stas, but modified so that the metallic vapour might be conducted into a lime-tube or funnel heated to redness, and so placed that a beam from an electric lamp would readily traverse it. 1 The general results of this investigation may be stated as follows : New fluted spectra. Silver. Manganese. Chromium. Bismuth. Selenium. Absorption in the blue. Silver. Manganese (red also). Copper. Aluminium. Cadmium (red also). Iron. Cobalt. Nickel. 1 Full details of the arrangements will be found in Proc. R. S. No. 160, 1875, and also in Studies in Spectrum Analysis. xx.j THE CASE OF GOLD. 279 Absorption in the blue (continued.} Tin (red also), Lead (red also). Antimony. Gold (red also). Palladium. A discussion of these observations at the temperature of the oxyhydrogen flame with those made at the lower temperatures seemed to prove beyond all question that the molecules which absorbed the blue were broken up to produce the one with the fluted spectrum, and similar reasoning indicated that the blue molecules were produced at the expense of the red ones. The reasoning was the same as that which had been held to demonstrate the different complexities of the molecules which gave line and fluted spectra respectively. Unfortunately we were in both cases almost beyond chemical and other physical inquiry almost, but not quite ; and where other inquiry was possible, it gave no uncertain sound. It is important that these other inquiries should be referred to in some detail. In the case of gold it is possible to separate the blue and red molecules, which I contend produce the colour of gold leaf, and to get them into different bottles that ne plus ultra of chemical separation. The beautiful researches of Faraday on this substance led him to the conclusion that we were by the colour phenomenon brought really into presence of a different molecular structure, but he worked in the prespectroscopic days, and each different colour to him represented a molecule. Hence he held that the molecules existed " of intermediate sizes or proportions." l I hold, however, that a twofold complexity is sufficient to explain all the phenomena. We have absorp- tion of the blue giving us red gold, absorption of the red giving us blue gold, and absorption both of blue and red giving us green gold. 1 Researches in Chemistry, p. 417. 280 THE CHEMISTRY OF THE SUN. [CHAP. Again, it is known that by compression we can turn green chlorine vapour green because it absorbs the blue into yellow liquid chlorine yellow because an absorption in the red is added ; and the changes in colour of sulphur and phosphorus are also in accordance with the hypothesis. There is also another proof behind of a novel character, to which a slight reference has already been made on p. 211. In my work on spectrum photography I had been struck with the possible con- nection between the continuous absorption in the blue recorded in the above experiments, and the continuous action in the blue of the salts of silver, and it seemed probable that further study would provide us with continuous action in the red, if we could either complicate the molecular structure of the salt of silver or use some other metal altogether. I invited my friend Dr. Russell to join me in this research, but other work made it impossible. Still, however, we had not long to wait, and I have already recorded with what success Captain Abney has worked in this direction, having discovered a method of obtaining the silver salt of the same chemical nature but more complex in its molecular arrangement. It may be argued that because we observed more absorption in the red with the oxyhydrogen flame than we did in the cooler tube, that the red absorption may have been produced by the breaking up of the blue-absorbing molecule. But this is not so, because the blue was produced at last at the expense of the red, and the fact that the red-absorbing vapour is produced more richly at higher temperatures is in accordance with what we know of hydrocarbon vaporizing, if we assume it to be more complex. In organic work the higher the temperature used to break up a compound the richer in complex forms is the vapour first given off. So much then for the independent confirmation of the exist- ence of red and blue molecules, and of the fact that the one xx.] VAPOUR DENSITY. 281 which absorbs the red is more complex than the one which absorbs the blue light. Have we the like information in the case of the passage from the blue molecule to the fluted one ? I think we have in the case of sulphur ; and here we approach a question of great interest to us in our inquiries that of vapour densities, on which something must be said before the question of sulphur is discussed. Chemists, by their vapour-density determinations, endeavour, so far as the elements are concerned, to ascertain the weight of say a cubic inch or a litre of any permanent gaseous element or of the vapour of any liquid or solid element; and in order that relative statements may be made they take the weight of one of the permanent gases, hydrogen, as the unit, because they have found that this is the lightest. Thus, in the case of the permanent gases hydrogen, oxygen, and nitrogen, we may construct the following table, showing also the atomic weights, hydrogen being taken as the unit in that case also Vapour density. Atomic weight. Hydrogen 1 1 Nitrogen 14 14 Oxygen 16 16 In addition to the permanent gases, there are three other bodies which readily lend themselves to such determinations ; these are chlorine, bromine, and iodine. Tabulating out the results, we get Vapour density. Atomic weight. Chlorine 35 "5 35 -5 Bromine 80 80 Iodine 127 127 As before, we find the vapour density on all fours with the atomic weight, and represented by the same values. Hence with chemists the " normal " state is that the vapour density and 282 THE CHEMISTRY OF THE SUN. [CHAP. atomic weight as referred to hydrogen shall be represented by the same values. Unfortunately, however, in the case of most of the elementary bodies which exist generally in the solid state, the determination of vapour densities is either very difficult or impossible, so that in addition to those we have named the facts have only been ascertained in the case of cadmium, mercury, phosphorus, arsenic, and sulphur, and all these are " anomalous/' that is, a "law" founded on six cases fails in the next five tried. In the first four of these we have two distinctly opposed results Vapour density. Atomic weight. Cadmium 56 112 Mercury 100 200 Phosphorus 62 31 Arsenic 150 75 That is to say, we have cases in which the density is half, others in which it is double, the atomic weight. To bring out the facts in the preceding table, a very concrete illustration may be pardoned. Suppose the complexity of the bodies in the solid state to be represented by something higher than a penny, the pennies being changed into halfpennies, or even into farthings, by various dissociating agencies. Next, let us assume that the molecular fineness produced by chemical action and represented by the weighed atom in each case is represented by a halfpenny. In the case of cadmium and mercury the temperature needed to produce the vapour has done more than chemical action, we get farthings. In the case of phosphorus and arsenic temperature does less, and we get pennies. Now as a matter of fact cadmium and mercury are sluggish chemically, while phosphorus and arsenic are active. The interesting point about these determinations is that where there is chemical confusion there is also spectroscopic xx.] SPECTROSCOPIC CONFUSION. 283 confusion. If we can attach certain spectroscopic characteristics to certain molecular finenesses (as many chemists were prepared to do before they found out where it would lead them), then when we find ourselves in presence of the true chemical molecule in each case we should expect to find similar spectra. Thus, to give an instance, it was at first suggested that, since "atoms " were supposed to give line spectra and " molecules '' fluted spectra, cadmium and mercury vapour, the vapour densities of which lead chemists to believe that they are dealing with the " atoms " of these substances, should give us line spectra. This, however, they do not do. The spectroscopic confusion will be gathered from the follow- ing table : Hydrogen . . .No absorption. Nitrogen ... No absorption. Oxygen .... Flutings. Chlorine . . . Absorption in the blue, and flutings. Bromine . . . Flutings. Iodine .... Flutings, with or without continuous absorp- tion in the violet according to temperature. Cadmium . . . Absorption in the blue. Mercury . . . No absorption. Phosphorus . . Probable flutings. Arsenic . . . Probable flutings. Evidently therefore, so far as spectroscopic evidence is con- cerned, these vapours do not exist in comparable molecular conditions, and this at once explains the " anomalies." We are now in a position to consider the case of sulphur. This body, which is well within the range of experiment, gives us two vapour densities, which we can tabulate as follows : Vapour density. Atomic weight. Sulphur Above 1000 32 ) q9 Below 1000 96 I 284 THE CHEMISTRY OF THE SUN. [CHAP. Now to the receptive mind there is nothing " anomalous " about this : a higher temperature brings about simplification, viola tout ! And what says the spectroscope ? On passing from the density of 96 to that of 32, the vapour changes its spectrum from one giving continuous absorption in the blue to one of flutings. Here then we have, if vapour density determinations be worth anything, an undoubted change of molecular grouping ac- companied by a spectroscopic change exactly in the direction required by other considerations. Have we then similar evidence having regard to the change from the fluted spectrum to the line spectrum ? We have, and it is precisely of the same nature as that afforded by sulphur, except that the temperature employed is higher. For many years the sulphur result stood alone, but abundant evidence has now been brought forward to show that iodine and bromine behave exactly in the same way, that is they have two different vapour densities at two different temperatures. Vapour density. Atomic weight. Bromine Above 2000 5 40 \ ^ Below 2000 80 j Iodine Above 2000 63-5 \ Below 2 000 127 j This change of vapour density occurs at the temperature at which the fluted spectra give place to line spectra. The absence of sodium and potassium from the above lists may have been remarked, since they vaporize at low tempera- tures. Very great difficulties have presented themselves in experiments on these bodies, and various values have been recorded of the density of their vapours. It is the opinion, how- ever, of many, that it changes perhaps twice with the tempera- xx.] CONCURRENCE OF TESTIMONY. 285 ture as one would expect, and we have already referred to the series of spectra given by these bodies at comparatively low temperatures, from which we should have predicted these changes of vapour density. To sum up, then, this concurrent testimony, we do find evidence, other than spectroscopic, of the existence of mole- cular groupings of the elementary bodies in different degrees of fineness; and if we endeavour to explain the "anoma- lous " vapour density of some of the substances to which atten- tion has been drawn by continuing the same line of argument* it is easy to see that it can be easily and simply explained in this way. If the molecule present in the vapour at the time it is weighed is that same molecule to which the mass of the element is most broken down by chemical affinity, the density will be normal. If the molecule is finer, we shall have the condition of things represented by cadmium and mercury ; if coarser, the state of things represented by phos- phorus and arsenic. It is further interesting to note that, of the permanent gases, the one most easily driven into the line stage, hydrogen, is a monad ; while sodium, lithium, potassium, chlorine, bromine, and iodine, bodies most easily driven into vapour, are also monads. I wish here to give in confirmation of my own work a reference to the work of others on the possible complex origin of line spectra. One of the most exhaustive inquiries of this nature was published by Dr. Schuster in the Philosophical Transactions in 1879. The subject was Oxygen. He distinguishes no less than four spectra of this substance, two of them being true line spectra. In order to make the reference complete I give his description of all four. I. The Elementary Line-Spectrum. This is the spectrum which appears at the highest temperature to which we can subject oxygen. That is, whenever the jar and air-break are introduced into the 286 THE CHEMISTRY OF THE SQN. [CHAP. electric circuit. It consists of a great number of lines, especially in the more refrangible part of the spectrum. It has been called elementary line-spectrum to distinguish it from the other line- spectrum, because, according to one hypothesis which has been suggested to explain the variability of spectra, the molecule which gives this spectrum is in a simpler or more elementary state than that which gives the other so-called compound line-spectrum. We may, however, adopt the nomenclature independently of any hypo- thesis that may have suggested it. II. The Compound Line-Spectrum. This spectrum appears at lower temperatures than the first. It consists of four lines : one in the red, two in the green, and one in the blue. With the exception of the blue line all the lines in this spectrum widen very easily, and, with an increase of pressure, more easily even than the hydrogen lines. They do not widen out equally on both sides, but more towards the red than towards the violet. This fact is especially noticeable in the more refrangible of the two green lines. The blue always remains sharp. III. The Continuous Spectrum of Oxygen. This spectrum appears at the lowest temperature at which oxygen is luminous. The wide part of a Pliicker tube, filled with pure oxygen, generally shines with a faint yellow light, which gives a continuous spectrum. Even at atmospheric pressure this continuous spectrum can be obtained by putting the contact-breaker of the induction coil out of adjust- ment, so that the spark is weakened. According to Becquerel an excess of oxygen in the oxyhydrogen flame produces a yellow colour, which is very likely due to this continuous spectrum. The continuous background which often accompanies the elementary line-spectrum must not be confounded with this spectrum. IY. The Spectrum of the Negative Glow. This spectrum, which was first accurately described by Wiillner, is always seen in the glow surrounding the negative electrode in oxygen. It consists of five bands : three in the red and two in the green. The least refrangible of the red bands is so weak that it easily escapes ob- servation ; the two other red bands are rather near together, and may be taken for a single band if the dispersion applied is small. The two green bands, which appear of the same brightness throughout, with pretty sharply -defined edges, are resolved into a series of lines when looked at with high optical powers. xx.] DR. SCHUSTER ON OXYGEN. 287 Dr. Schuster gives the following description of the appearance of a vacuum-tube filled with pure oxygen, as it undergoes gradual exhaustion, in order to give an idea of the way in which the spectra of oxygen gradually diffuse into each other : At first the spark has a yellow colour, and the spectrum is perfectly continuous. Almost immediately, however, four lines are seen in the capillary part above the continuous spectrum. One of these lines is in the red, two in the green, and one in the blue. The discharge still passes as a narrow spark throughout the length of the tube. In the wide part the spectrum remains continuous, and it extends more towards the red and blue than in the capillary part. It seems as if the four lines had taken away part of the energy of the continuous spectrum. As the pressure diminishes these lines increase considerably in strength, the spark spreads out in the wide part of the tube, and the intensity of the continuous spectrum is therefore considerably diminished, while it still forms a prominent part in the spectrum of the capillary part. When the pressure is small the continuous spectrum decreases in intensity. At the same time the negative glow, with its own characteristic spectrum, gradually extends through the negative half of the tube into the capillary part. The continuous spectrum has now entirely disappeared, and the bands of the negative pole and the four lines stand out on a perfectly black background. It is under these con- ditions that the change from the compound line-spectrum to the elementary line-spectrum is best studied. The mere insertion of a Leyden jar makes hardly any difference ; the jar does not seem to be charged at all. If, in addition to the jar, we insert a moveable air- brake which can be opened or closed at will, while we look through the spectroscope, we shall be able to see alternately, two perfectly distinct spectra. If the air-brake is closed, the four lines of the compound-spectra only are seen, if the air-brake is opened, these four lines will disappear entirely, and the elementary line-spectrum will come out. We have here as complete a transformation as we have from the band to the line-spectrum of nitrogen, taking place under exactly the same circumstances. 1 The following extract from Dr. Schuster's paper will show 1 Phil. Trans, clxx. 51. 288 THE CHEMISTRY OF THE SUN. [CHAP. that he accepts the view of molecular simplification that I advocate : " According to [one] view, liquid and solid bodies give generally continuous spectra, not because they are liquid or solid, but because in these states the molecules have a more complicated structure than in the gaseous state. Experiment has to decide between the two theories, the theory of molecular disturbance and the theory of molecular structure. I think the facts are decidedly in favour of the latter theory. Mr. Lockyer's investigations have shown that most bodies give us a continuous spectrum, as a gas, before they condense, and many at a considerable temperature above the boiling point. Mr. Lockyer has rightly drawn the conclusion from this fact, that the atomic aggregation of the molecules is the cause of the different orders of spectrum. If we observe the changes in a spectrum which gradually take place on heating or cooling a vapour, we find that the continuous spectrum is produced, not by a widening of the bands, but by a direct replacement, which is some- times sudden and sometimes gradual, and which leaves no doubt in the observer's mind that he has to deal with two vibrating systems, and not simply with a disturbed one ', I do not, of course, mean to say that the impacts of other molecules have no observable influence. If the hydrogen lines widen through increased pressure, it is very likely that the alteration is produced by impacts ; but the change from a line-spectrum to a continuous-spectrum, as a rule, is quite different from the change which takes place with hydrogen. Accord- ing to the theory of molecular aggregation, it seems quite possible that a liquid should give the same spectrum as its vapour, and this indeed seems to be true in some cases." 1 In a subsequent note Dr. Schuster adds " That the discontinuous spectra of different orders (line and band spectra) are due to different molecular combinations I consider to be pretty well established ; and analogy has led me (and Mr. Lockyer before me) to explain the continuous spectra by the same cause ; for the change of the continuous spectrum to the line or band spectrum takes place in exactly the same way as the change of spectra of different orders into each other." 2 1 Phil. Trans. Part I. 1879, p. 38. * Loc. cit. p. 3. xx.] BERTHELOT'S VIEW. 289 This chapter may fitly end by a quotation from Professor Berthelot, who has been studying elementary chemical substances from quite another standpoint. "L'etude approfondie des proprietes physiques et chemiques des masses elementaires, qui constituent nos corps simples actuels, tend chaque jour davantage & les assimiler, non a des atonies in- divisibles, homogenes et susceptibles d'e"prouver seulement des mouvements d'ensemble, il est difficile d'imaginer un mot et une notion plus contraires a 1'observation ; mais a des edifices fort complexes, doues d'une architecture specifique et animes des mouve- ments intestins tres varies." Berthelot, Comptes Rendus, 1880, vol. 90, p. 1512. CHAPTER XXI. SOME TEST EXPERIMENTS. TRIAL OF NEW METHODS. THERE was still however a difficulty in many minds in as- cribing changes in spectra to temperature alone, and therefore to a possible dissociation. It thus appeared to be one's duty to find an experimentum crucis if possible. It seemed as if the results of experiments based on the following considerations ought to be accepted as throwing light on the question. First Consideration. At a low temperature some substances give us few lines while at a high one they give us many. Vapours, therefore, already glowing with few lines at a low temperature, say in a flame, should give us all their lines when the vapour is suddenly sub- jected to a high one, say by the passage of a high tension spark. On the bell hypothesis the spectrum should change with the mode of striking. On the dissociation hypothesis this should only happen for the lines of those molecular groupings which are from other considerations held to be more simple. If the flame has brought the substance to its lowest state, the passage of the most powerful spark should not cause the flame spectrum to vary. Now what are the " other considerations " above referred to ? This necessitates a slight digression. Of the short lines (see ante, p. 213), the explanation generally COMPLEX ORIGIN OF SHORT LINES. 291 CHAP. XXI.] given and accepted was that they were produced by a more complex vibration imparted to the " atom " in the region of greatest electrical excitement, and that these vibrations were obliterated, or prevented from arising, by cooling or by admixture with dissimilar " atoms." Subsequent work, however, has shown 1 that the different behaviour of these lines seemed to suggest the probability that not all of the short lines of spectra were } in reality, true products of high temperature. Now if not all but only some of the short lines are products of high temperature, we are bound to think that the others are remnants of the spectra of those molecular groupings first to disappear on the application of heat. a o ff d f g Jt kirn, A B C 11(1 FIG. 97. A. Highest temperature. C. Lowest temperature. At any particular heat-level, then, some of the short lines may be due to the vibrations of molecular groupings 'produced with difficulty by the temperature employed, while others may represent the fading out of the vibrations of other molecular groupings produced on the first, application of the heat. In the line of reasoning advanced in Chap. 18, 2 both these results are anticipated, and are easily explained. Slightly vary- ing Fig. 86, we may imagine furnace A to represent the tempe- rature of the jar spark, B that of the Bunsen burner, and C a temperature lower than that of the Bunsen burner (Fig. 97). Then in the light of the hypothesis the lines b and c if seen 1 Proc. R. Soc. vol. xxviii. p. 159. 2 Ibid. p. 162. u 2 292 THE CHEMISTRY OF THE SUN. [CHAP. in the spectrum of B would be truly produced by the action of high temperature, while a,f y and / would be but remnants of the spectrum of a lower temperature. So much then by way of explanation ; it is clear that to make this reasoning valid we must show that the spark, or better still the arc, provides us with a summation of the spectra of various molecular groupings into which the solid metal which we use as poles is successively broken up by the action of heat. We are not limited to solid metals ; we may use their salts. In this case it has already been shown 1 that in very many cases the spectrum is one much less rich in lines. Second Consideration. Since at different temperatures the brilliancy of the spectral lines of the same substances as ordinarily observed changes enormously, it is important to see if these changes can be pro- duced at the same temperature by employing those experimental conditions which will be most likely to bring about different molecular conditions if such exist. This experiment is founded on the behaviour of compound bodies when they are distilled at different temperatures. If we take, for instance, a mixture of hydrocarbons, some of them very complex in their nature, and others more simple ; when a Low temperature is employed it is found that the simpler hydro- carbons come over in the shape of vapour in greater abundance, when a higher temperature is employed more complex forms come over in greater abundance. If therefore we were fortunate enough to be able to observe the spectra of these different vapours, assume that the series of hydrocarbons, for instance, shown in the accompanying diagram (Fig. 98), had each of them a distinct spectrum, we should be able to follow spectro- scopically the effect of each change of temperature, and we could in that way associate the known fact of the greater 1 Phil. Trans. 1873, p. 258. xxi.] EXPERIMENTS. 293 abundance of dense vapour which conies over at a higher temperature with a spectrum of a certain kind. Now in our experiment we deal not with a compound body in the ordinary sense, but with a so-called elementary body. The question is, Will this make any difference ; or, rather, shall we get similar differences ? FIG. 98. Hypothetical spectra obtained on distilling a successively increasing temperatures a mixture of light and heavy hydrocarbons. The experimental work has followed two distinct lines. I shall refer in detail to the results obtained along each. Experiments based on the First Consideration. The first method of investigation adopted consists in volatilising those substances which give us flame spectra in a Bunsen flame and passing a strong spark through the flame, first during the process of volatilisation, and then after the tempera- ture of the flame has produced all the simplification it is capable of producing. The results have been very striking; the puzzles which a comparison of flame spectra and the Fraunhofer lines has set us find, I think, a solution ; while the genesis of spectra is made much more clear. 1 To take an instance, the flame spectrum of sodium gives us, 1 I allude more especially to the production of triplets, their change into quartets, and in all probability into flutings, and to the vanishing of flutings into lines, by increasing the rate of dissociation. 294 THE CHEMISTRY OF THE SUN. [CHAP. as its brightest, a yellow line, which is also of marked import- ance in the solar spectrum. The flame spectra of lithium and potassium give us, as their brightest, lines in the red which have not any representatives among the Fraunhofer lines, although other lines seen with higher temperatures are present. Whence arises this marked difference of behaviour ? From the similarity of the flame spectrum to that of the sun in one case, and from the dissimilarity in the other, we may imagine that in the former case that of sodium we are dealing with a body easily broken up, while lithium and potassium are more resistant ; in other words, in the case of sodium, and dealing only with lines recognised generally as sodium lines, the flame has done the work of dissociation as completely as the sun it- self. It is easy to test this point by the method now under con- sideration, for if this reasoning be correct (1) the chief lines and flutings of sodium should be seen in the flame itself, and (2) the spark should pass through the vapour after complete volatilisation has been effected without any visible effect. Observation and experiment have largely confirmed these predictions. Using two prisms of 60 and a high-power eyepiece to enfeeble the continuous spectrum of the densest vapour pro- duced at a high temperature, the green lines, the flutings recorded by Roscoe and Schuster, and another coarser system of flutings, _so far as I know not yet described, are beautifully seen. I say largely, and not completely, because the double red line and the lines in the blue have not yet been seen in the flame, either with one, two, or four prisms of 60, though the lines are seen during volatilisation if a spark be passed through the flame. Subsequent inquiry may perhaps show that this is due to the sharp boundary of the heated region, and to the fact that the lines in question represent the vibrations of molecular groupings more complex than those which give us the yellow and green lines. The visibility of the green lines, which are short, in the flame, taken in connection with the fact that they have been xxi.] MORE EXPERIMENTS 295 seen alone in a vacuum tube, is enough for my present purpose. With regard to the second point ; the passage from the heat- level of the flame to that of the spark after volatilisation is complete produces no visible effect, indicating that in all proba- bility the effects heretofore ascribed to quantity have been due to the presence of the molecular groupings of greater complex- ity. The more there is to dissociate, the more time is required to run through the series, and the better the first stages are seen. Experiments lased on the Second Consideration. The second experimental method deals with vapours of ele- mentary bodies volatilised at different temperatures in vacuum tubes. Many of the phenomena are quite new, and lines thus seen alone and of surpassing brilliancy, are those seen as short and faint in ordinary methods of observation. The novelty of the method consists in the use of the luminous electric current as an explorer and not as an agent for the supply of the vapours under examination ; that is to say, the vapours are first produced by an external source of heat, and are then rendered luminous by the passage of the current. The length and bore of the tube therefore control the phenomena to a certain extent. A form of apparatus which I have found to answer very well is shown in the accompanying woodcut (Fig. 99). A i s the tube or retort containing the metal experimented on in its lower extremity, and having a platinum wire sealed into it at a distance of about two inches from the lower end, the other e nd being drawn out and connected by a mercury joint to an ordinary Geissler tube, which is connected by another mercury joint to the Sprengel pump c. Another form of tube which I have used is prepared by in- serting two platinum poles into a piece of combustion tubing 296 THE CHEMISTRY OF THE SUN. [CHAP. sealed at one end, and after inserting the metal to be experi- mented on, drawing out the glass between the platinums to a capillary^ tube. FIG. 99. 1st Form. Position of spectroscope when observing vapours close to the metal. I have also tried inserting the platinum pole at the end of the retort, so that the spark passes from the surface of the metal, but this arrangement did not answer at all. XXI.] METHOD OF WOKK. 297 Some other modifications have been tried, but the first form I have described is that which I have found to answer best, so far as the trials have yet gone. A is the tube containing the metal ; B, the capillary ; c, the connection with the pump. FIG. 100. 2nd Form. Spectroscope and lens in position for observing spectrum of capillary. In both forms D is the spectroscope, a direct vision being used in the first case. E is the lens used for focusing the image of the Geissler tube on the slit in the second case. F is the spirit-lamp for heating the retort. 298 THE CHEMISTRY OF THE SUN. [CHAP. H is the battery. K and L are the wires connected with the coil. In the second form (Fig. 100) the method of observing the spectrum of the vapours close to the surface of the metal is indicated. For determining the exact positions of the lines in the spec- trum of the vapour in any part of the retort, a larger spectro- scope, with its illuminated scale, was used in the place of the direct-vision spectroscope. The secondary wires of the coil were connected, one with the pole in the upper bulb at B, and the other with the platinum at A. B is an ordinary Geissler tube with two bulbs separated by a capillary tube. The great advantage of this arrangement is that this capillary portion can be used for ascertaining what gases or vapours are carried over by the pump without any in- terference with the retort, both wires being connected with the Geissler tube. If, for example, we are working with sodium which contains an impurity of hydrocarbon, the moment at which it begins or ceases to come off can be found by examining the spectrum of this capillary tube. I now give an account of the phenomena observed when we were working with sodium, in order to indicate the changes observed. After a vacuum has been obtained the retort is heated gradu- ally. The pump almost immediately stops clicking, and in a short time becomes nearly full of hydrogen. The spectrum of the capillary then shows the hydrogen lines intensely bright. After some time the gas comes off far less freely, and an ap- proach to a vacuum is again obtained. Another phenomenon now begins to show itself: on passing the current a yellow glow is seen, which gradually fills the whole space between the pole in the retort and the metal ; its sjpectrum consists of the lines of hydrogen and the yellow line of sodium, the red and green xxi.] , RESULTS WITH SODIUM. 299 lines being both absent until the experiment has gone on foi some time. - As the distillation goes on, the yellow glow increases in brilliancy, and extends to a greater distance above the pole, and the red and green lines presently make their appearance as very faint lines. * .' The upper boundary of the yellow is quite sharp, the lines and fluted spectrum of hydrogen appearing above it. After the yellow glow-giving vapour (which does not attack the glass) has been visible for some time, the pump is stopped and the metal heated more strongly. On passing the current a little while afterwards, a very brilliant leaf-green vapour is seen underlying the yellow one, and connected with it by a sap-green vapour. The spectra then visible in the tube at the same time are Leaf -green ...Green and red lines of sodium and c of hydrogen ; D absent. Sap-green ...Green, red, and yellow sodium lines of equal brilliancy, and c of hydrogen. Yellow ...p alone and c. Bluish-green. ...c and F and hydrogen structure. To observe the green sodium line alone it is necessary to point the direct-vision spectroscope just above the surface of the metal where the green is strongest. It is also necessary to guard against internal reflections from the glass, as this may sometimes cause the D line to be seen by reflection from the surface. This method of inquiry has been tried also with potassium, calcium, and some other metals, and with metallic salts. With potassium and calcium we get the same inversion of phenomena ; the yello \v-green lines of potassium being seen without the red; while in the case of calcium the blue line alone was seen. We get, as before mentioned, vapours which 300 THE CHEMISTRY OF THE SUN. [CHAP. at one and the same time exhibit different colours and different spectra at different levels in the tube. It may be advantageous to dwell on the details observed in the case of potassium. The tube is very perfectly ex- hausted, and then gently warmed with a spirit-lamp, the exhaustion going on during the whole process. On passing a current between the platinum electrodes we see a beautiful green glow in the tube, and obtain a certain spectrum. On re- placing the spirit-lamp by a Bunsen burner we find as the result of this increased temperature that the colour in the tube changes to blood-red, and the spectrum is entirely different. The spectrum of potassium is one which requires a very great deal of study, for the reason that it varies very much under different experimental conditions. If the potassium, as is well known, is thrown into a Bunsen burner, the chief line that one gets is a red one. Kirchhoff, in the early days of solar chemical investigation, pointed out that this red line is not to be found among the Fraunhofer lines. The flame also gives us a line in the blue. If we examine the spectrum of potassium by means of an induction-coil we find the blue line which we also see in the flame, but it is intensified in the spark. We also see some strong lines in the green and yellow, which are barely visible in the flame which are in fact not generally recorded in the flame-spectrum of potassium, although they are really visible when considerable dispersion is employed. These lines in the yellow and green become prominent lines. Now, it so happens that some of these lines in the green do, it is believed, corre- spond with Fraunhofer lines, and we are, therefore, justified in assuming that they represent a something, whatever it may be, in the potassium, which can withstand the heat of the sun, while the red lines represent something which is broken up at the temperature of those regions of which we can determine the absorption. The interesting point of the experiment, therefore, is this : assuming for a moment that the red line does xxi.] RESULTS WITH POTASSIUM. 301 represent a complex something which cannot withstand the temperature of the sun, and that the yellow line represents a something finer which can withstand the temperature of the sun, what happens when we try to drive off the vapour of this potassium at the lowest temperature at which we can get it to volatilise at all, is that if the experiment is carefully performed it gives precisely those lines which are reversed in the solar spectrum alone ; and of that line which is the strongest line at the temperature of the Bunsen burner we see absolutely nothing at all. Referring to the spectrum which we get in the lilac and yellow-green part of the tube, two out of the three lines visible at all events are seen in the sun, whereas the other lines which we get in the flame and some of them which we get with an induction coil are not represented in the fine vapour which was produced at the lowest possible temperature. While heated with the Bunsen burner some very exquisite colour-effects are seen in the tube, and especially a beautiful blood-red colour which might be imagined to be the product of that molecule which gives the red line seen in the Bunsen flame ; but that is not the fact. The line seen in the flame of the Bunsen burner is not visible as a rule in the vapour when heated in this way, the lines actually seen being more refrangible. The experiment then comes to this. If we assume potas- sium to be a compound body and that its finer constituent molecules are those which resist the solar temperature, then it behaves exactly like a mixture of hydrocarbons is known to do, that is, the finer vapours come off in greatest quantity at the lowest temperature, and the more complex ones as the tempera- ture is raised. The result of the application of this new method indicates that in the case of a considerable number of chemical sub- stances not only is the line spectrum compound in its origin, as I suggested many years ago, but that a large number of the lines is due to molecular groupings of considerable complexity, 302 THE CHEMISTRY OF THE SUN. [CHAP. xxi. which can be kept out of the reaction by careful low tempera- ture distillation. Not only do these new results indicate an almost undreamt-of variability in spectra, but so far as I can see they are simply and sufficiently explained on the assumption that the elementary bodies in the solid state con- sist of various sets of molecules which behave like mixtures of organic compounds. CHAPTER XXII. THE SOLAR ATMOSPHERE ON THE NEW HYPOTHESIS. BEFORE we attempt to apply further tests we must thoroughly consider the change of front in solar research necessitated by the introduction of the view of the possible dissociation of elementary bodies at solar temperatures. We must first consider what solar facts we may expect on the two hypotheses to com- pare with the result of laboratory experiments. On the old hypothesis the construction of the solar atmo- sphere was imaged as follows : x (1.) We have terrestrial elements in the sun's atmosphere. (2.) They thin out in the order of vapour density, all being represented in the lower strata, since the temperature of the solar atmosphere at the lower levels is incompetent to dissociate them (see p. 168.) (3.) In the lower strata we have especially those of higher atomic weight, all together forming a so-called "reversing layer " by which chiefly the Fraunhofer spectrum is produced. The new hypothesis necessitates a radical change in the above views. According to it the three main statements just made require to be changed as follows : (1.) If the terrestrial elements exist at all in the sun's atmosphere they are in process of ultimate formation in the cooler parts of it. 2 1 See ante, Chapter XIII. 2 See ante, p. 201 at bottom. 304 THE CHEMISTRY OF THE SUN. [CHAP. (2.) The sun's atmosphere is not composed of strata which thin out, all substances being represented at the bottom ; but of true strata, like the skins of an onion, each different in compo- sition from the one either above or below. Thus, taking the sun in a state of quiescence and dealing only with a section, we shall have (as shown in fig. 101) C, say, containing neither D nor B, and B containing neither A nor C. FIG. 101. Layers in solar atmosphere. (3.) In the lower strata we have not elementary substances of high atomic weight, lut those constituents of the elementary bodies which can resist the greater heat of these regions. The conditions under which we observe the phenomena of the sun's atmosphere have not, as a rule, been sufficiently borne in mind, and it is quite possible that the notion of the strata thinning out has, to a certain extent, been based more upon the actual phenomena than upon reasoning upon the phenomena. XXII.j LAYERS. 305 I have referred to this point before, p. 164, but recur to it here for greater clearness. FIG. 102. Lines produced by layers are of different lengths. Take three concentric envelopes of the sun's atmosphere, A, B,C (fig. 102), so that C extends to the base of A, and B also to Sun's Spectrum FIG. 103. But lines of different length do not necessarily indicate that the sub- stances producing the longer of them extend down to the photosphere. the base of A, that is, in both cases to the photosphere. Then, whether we deal with the sphere or with a section of it, the x 306 THE CHEMISTRY OF THE SUN. [CHAP. lengths of the lines in the spectrum of the strata C, B, A, will give the heights to which the strata extend from the sun, and show where B and A respectively thin out. As the material is by hypothesis continuous down to the sun, the lines will be continuous down to the spectrum of the sun seen below as shown. Now take three concentric envelopes, A, B, C (fig. 103), so that only A rests on the photosphere, B rests on A, and C on B. The phenomena will in the main be the same as in the former case, i.e., the line C .will still appear to rest on the spectrum -of the photosphere, for it will be fed, so to speak, from C' and C", though absent along the line CBA at B and A. So also with B. Thus much having been premised with regard to the obser- vations as conditioned by the fact that we are observing a sphere, we can now proceed to note how the two hypotheses deal with the facts. Old Hypothesis. New Hypothesis. 1. The spectrum of each element as The spectra should not resemble each seen in our laboratories should be other. exactly represented in the solar spec- trum. FACT. There is a very wide difference between the spectra. See ante, pp. 174 and 230. 2. The spectrum of the base of the The spectrum of the liase should solar atmosphere should most resemble least resemble the Fraunhofer spectrum, the ordinary Fraunhofer spectrum. because at the base we only get those molecules which can resist the highest temperatures. FACT. When we leave out of consideration the lines of hydrogen, calcium, and magnesium, those seen at the base, as a rule, are either faint Fraunhofer lines, or are entirely absent from the ordinary spectrum of the sun. See p. 182. xxii.] THE HYPOTHESES CONTRASTED. 307 3. The spectra of prominences should The spectra of prominences should consist of lines familiar to us in our even be in many cases unfamiliar, be- laboratories, and should have the same cause prominences represent what is intensities, because solar and terres- going on at a temperature hot enough trial elements are the same. to prevent the coming together of the atoms of which our chemical elements are composed. FACT. When we leave out of consideration the lines of hydro- gen, calcium, magnesium, and sodium, which are seen in the hottest stars, most of the lines are either of unknown origin or are feeble lines in the spectra of known elements. See p. 185. 4. Qud the same element the lines Qud the same element the lines widest in spots should always be the widest in spots should vary enor- same. mously, because the absorbing material is likely to originate in and to be carried to different depths. FACT. There is immense variation, as will be seen in the sequel. 5. The spectrum of iron in a pro- The spectrum of iron in a promi- mrnence should be the same as the nence should be vastly different from spectrum of iron in a sun-spot. the spectrum of iron in a sun-spot because the spot is cooler than the prominence. FACT. The spectra, as will be seen in the sequel, are as dissimilar as those of any two elements. 6. The spectra of spots and promi- The spectra should vary, because nerices should not vary with the sun- the sun is hotter at maximum. spot period. FACT. They do vary, as will be shown in the sequel. 7. Motion in the iron vapour, e.g. Motion should be unequally indi- in a spot or a prominence, should be cated, because the lines are due to indicated by the contortion of all the divers constituents which exist in dif- irori lines equally. ferent strata according as they can resist the higher temperatures of the interior regions. FACT. The indications show both rest and motion. See post, chap. xxiv. From the above sketch, hasty though it be, it is, I think, easy to gather that the new view includes the facts much better than x 2 308 THE CHEMISTRY OF THE SUN. [CHA*. the old one, and in truth demands phenomena, and simply and sufficiently explains them, which were stumbling-blocks and paradoxes on the old one. But we go still more into detail. Let us first suppose, to take the simplest case, that the sun when cold will be a solid mass of one pure element, i.e., that the evolution brought about by reduction of temperature shall be along one line only. Let us take iron as the final product. FIG. 104. Hypothetical section of solar atmosphere. Then the sun's atmosphere on the new theory qud this one element may be represented as follows (see fig. 104) : Assume strata A L. Then > (1.) The Fraunhofer spectrum will integrate for us the absorption of all strata from A to L. (2.J The darkest lines of the Fraunhofer spectrum will be those absorbed nearest the outside of the atmosphere. xxii.] RESULT OF EVOLUTION. 309 (3.) We shall rarely, if ever, see the darkest lines affected in spots and prominences. (4.) The germs of iron are distributed among the various strata according to their heat-resisting properties, the most complex at L, the least complex at A. (5.) Whatever process of evolution be imagined, as the tem- perature runs down from A to L, whether A, 2 A, 4A; or A-hB, 2(A+B); or A+B+C; the formed material or final product is the work of the successive associations rendered possible by the gradually lowering temperature of the successive strata, and can therefore only exist at L. CHAPTER XXIII. MOllE TESTS. THE SPECTRA OF SUN-SPOTS.' IN the last chapter when endeavouring to show how the new hypothesis fits the facts better than the old one there were some points on which I promised to give further information depending upon later work. This I now proceed to do with regard to those which refer to spot-spectra, or to the comparison of such spectra with those of the prominences. The first attempt I made 1 to get light out of this inquiry was one which dealt with a long catalogue of lines observed by Professor Young in his memorable expedition to Mount Sherman, where, at the height of between 8,000 and 9,000 feet, with per- fect weather and admirable instrumental appliances, about a month was employed in getting such a catalogue of lines as had never been got before. But it was found that, although the result of this inquiry was to strengthen the evidence in favour of the inversions referred to at p. 175, still, after all, one wanted more facts. In this changeable climate I soon found that it would not do to proceed as I began to attempt to observe all the lines acted upon in a solar spot. The excessive complication, and the great variation of a spot-spectrum from the ordinary solar spectrum, cannot be better shown than by the accompanying copy of a spectrum of one of the sun-spots observed at Greenwich. 1 Proc. R. S. No. 197, 1879, page 251 and seq. CH. XXIII.] METHOD ADOPTED. 311 The figure (Fig. 105) shows a limited part of the solar spectrum, and the lines thickened in the spot-spectrum. It will be seen therefore that to tabulate the existence, thickness and intensity of these lines over the whole of the solar spectrum would be a work which it would be difficult to accomplish in a single day even if the day were absolutely fine. So that was given up in favour of a limited inquiry over a small part of the solar spec- trum ; limited further by this, that we only note the twelve lines most affected in each spot on each day. In this way we insure a considerable number of absolutely comparable observations nil 1 mli in null i ii ilini Mil UN Luiliiiilimlm Illllllll ul 1 1 I'l 1 ' I II II I linn FIG. 105. Part of the spectrum of a sun-spot observed at Greenwich. and we can more easily compare the spot results with those which have been obtained in the observation of the brightest lines in prominences; because when one begins to observe lines in the solar prominences one naturally observes the brightest lines first. So that by observing the darkest lines first in the case of spots one has a fairer comparison. The work in question has now been going on uninterruptedly since 1879. From the first I felt it was crucial, and that I was in duty bound to proceed with it to see if it would supply us wuth any additional tests. The work enables us to discuss the lines of several chemical elements most widened in 700 spots observed at Kensington. The period of observation commenced November, 1879, and ex- tended to August, 1885. It includes, therefore, the sun-spot curve from a minimum to a maximum and some distance beyond. 312 THE CHEMISTRY OF THE SUN. [CHAP. I may begin by stating the way in which the observations have been made. The work, which has been in large part done by my assistants and chiefly by Messrs. Lawrance and Greening, simply consists of a survey of the two regions F I and b D. The most widened line in each region not the widest line, but the most widened, is first noted ; its wave-length being given in the observation books from Angstrom's map. Next the lines which most nearly approach the first one in widening are recorded, and so on till the positions of six lines have been noted, the wave-lengths being given for each region, from Angstrom's map. It is to be observed that these observations are made without any reference whatever to the origin of the lines ; that is to say, it is no part of the observer's work to see Avhether there are metallic coincidences or not ; this point has only been inquired into in the reductions. In this way perfect absence of all bias is secured. It may further be remarked that the number of lines widened at any time during a sun-spot period is about the same, so that the conditions of observation vary very little from month to month, or from year to year. The absolute uniformity of the results obtained in the case of the chemical elements investigated indicates, I think, that the observations have been thoroughly well made ; and, as a matter of fact, they are not difficult. It was not only important to record these observations for their own sake, but also for the comparison they allowed us to make with other observations of different phenomena, especially of those presented by the prominences. Side by side with the sun-spot work, therefore, went on an elaborate mapping of every prominence line observed by Tacchini since 1870. The reason of this is obvious, but will bear repetition here. The spots, as everybody agrees, are caused by down currents when a disturbance in the solar atmosphere brings vapour down from the cooler ex- terior regions. The prominences, on the other hand, are either xxin.] GENERAL STATEMENTS. ^313 ejections from the most highly heated part of the sun below the photosphere, or are caused by the dissociation of the descending material when it has reached the point of highest temperature. This being premised we pass on to the results obtained by the spot observations, including with them the results of the comparison with the prominence spectra. I will begin with the results obtained by the complete discussion of the first hundred observations taken between November, 1879, and September, 1880. Each line observed in each spot was carefully mapped on a large scale on sheets on the top of which were shown the Fraun- hofer lines with their true intensities in the region explored. General Statements regarding Spots. 1. The spot-spectra are very unlike the ordinary spectrum of the sun, some Fraunhofer lines are omitted, new lines appear and the intensities of the old lines are changed. 2. The next point was that in the case of each chemical element, even those with many lines among the Fraunhofer lines, only a very few lines, comparatively speaking, were seen to be most widened. It was as if on a piano only a few notes were played over and over again, always producing a different tune. 8. An immense variation, from spot to spot, was observed between the most widened lines seen in the first hundred observations. Change of quality or density will not account for this variation. To investigate this point I had the in- dividual observations of lines seen in the spectrum of iron plotted out on strips of paper, and I then tried to arrange them in order, but I could not succeed, for even when the observations were divided into six groups about half of them were left outstanding. 4. If we consider the lines' of any one substance, there is as much inversion between these lines as between the lines of any two metals. By the term inversion I mean of any three lines, 314 THE CHEMISTRY OF THE SUN. [CHAP. A, B, C, that we may get A and B without C, A and C without B, B and C without A, and so on. One was struck with the marvellous individuality, so to speak, of each of the lines. They did not go in battalions, or companies, or corporal's guards, but in single unities. Each in turn is seen without the other. We get as. much inversion of lines in the case of one element as we could do between the lines of different elements ; by which I mean that the lines of nickel, say, are just as much varied in different spots as the lines of iron, nickel or calcium would be in spots in which the proportions of these substances very greatly varied. 5. Very few lines, indeed, are strongly affected at the same time in the same spot. A great many lines of the same sub- stance may be affected, of course, besides the twelve recorded as most widened on each day ; but a small number relatively altogether are affected in this manner. 6. Many of the lines seen in the spots are lines seen at low temperatures (some of them in the oxyhydrogen flame), and none of them are those brightened or intensified when we pass from the temperature of the electric arc to that of the electric spark. 7-. Certain lines of a substance have indicated rest, while other adjacent lines seen in the spectrum of the same substance in the same field of view have shown change of wave-length. 8. A large number of the lines seen in spots are common to two or more substances w r ith the dispersion employed. 9. The lines of iron, cobalt, chromium, manganese, titanium, calcium, and nickel seen in the spectra of spots are usually coin- cident with lines in the spectra of other metals, with the dis- persion employed ; whilst the lines of tungsten, copper, and zinc seen in spots are not coincident with lines in other spectra. 10. The lines of iron, manganese, zinc, and titanium most frequently seen in spots are different from those most frequently seen in flames, whilst in cobalt, chromium, and calcium the lines seen in spots are the same as those seen in flames. XXIIL] GENERAL STATEMENTS. 11. Towards the end of the first series a few lines appeared among the most widened ones which are not represented, so far as is known, among the lines seen in the spectra of terrestrial elements. This change took place when there was a marked increase in the solar activity. A second series was begun on 29th September, 1880, and came down to October, 1881. The second series, like the first one, consisted of 100 observations, these observations being either of different spots or of the same spots seen on different days, and therefore in different stages of development. The following results were obtained : 12. The number of new lines seen amongst the most widened lines steadily increased. Many of these lines are very faint in the solar spectrum, and are unrecorded by Angstrom, while they are wide and dark in the spot-spectrum. 13. In the months of May and June (1881), there was a great change in the spectra of the spots, the old lines dying out and new lines appearing. 14. When series of observations, consisting of ten consecutive observations of the spectra of spots, taken from the commence- ment of the first series in November, 1879, and from the end of it on 27th September, 1880, were compared with those made towards the end of the second series on 18th July, 1881, it was found that the lines widened in each set were markedly distinct from those in the other sets. To illustrate this, I give the diagram on page 316. At the top are some of the principal Fraunhoferic lines in the region F to D, the lengths representing the intensities. The lower part of the diagram is divided into three sections by strong lines ; the first of these (1 10) contains the observations made be- tween November 12, 1879, and January 20, 1880, the second (11 20) the observations made between September 27 and October 1, 1880, and the third those made between July 18 and "July 29, 1881. "S'o g-a I'- ll 3s o o S3 "^ O ^ .S 53 ^b ^ ^SL S 11 >i 1s- ? sf OH. xxm.j SOME STATISTICS. 317 15. At the commencement of October, 1881, there was a change in the spectra of the spots similar to that which took place in May and June, bat much more abrupt, for only one of the old lines remained. This is exactly analogous to variations observed by Tacchini in the spectra of the prominences in the region F to I, in December, 1872. 16. In the first hundred observations the total number of most widened lines in the region F to b was fifty- seven, forty of which were due to iron, whilst in the second hundred the total number of lines seen was 104, and the iron lines faded away gradually, the last disappearing on 26th July, 1881. By November of that year more than 75 per cent, of the most widened lines are not represented in the spectra of terrestrial elements. The result of this inquiry with regard to chemical substances which has been most carefully worked out, is indicated in the accompanying table, giving the result of the work for two years from 200 spots. STATISTICS OF THE MOST WIDENED LINES SEEN IN 200 SUN-SPOTS. Total number of lines Total number in part of spectrum of lines discussed. widened. Iron .... .... 172 . 72 Titanium .. . .... 120 . 38 Nickel . . . .... 24 9 . . . . 19 5 Cobalt . . . .... 17 3 Calcium . . .... 17 7 Chromium .... 15 r-; j 9 Molybdenum .... 14 1 Tungsten . . .... 14 2 Manganese . .... 13 4 Platinum . . .... 12 1 Barium . . . .... 10 , 'V J 1 Copper . . . .... 10 1 Sodium . . . 7 . . . a I. 2 318 THE CHEMISTRY OF THE SUN. [CHAP. In these 200 spots, out of 172 lines of iron which we might have seen only 72 were observed altogether ; out of 120 lines of titanium which we might have seen only 38 were seen ; and then the number goes on decreasing : 24 in the case of nickel, of which 9 were seen ; 19 in the case of zinc, of which 5 were seen; 13 of magnesium, of which 4 were seen; 12 of platinum, of which 1 was seen, and so on. The final upshot is, therefore, that at the spot-level we do not see the Fraunhofer spectrum, as we ought to do on the old theory. What we do see is a small percentage of the lines, and we see them under conditions which are entirely unexpected. These, though the earlier results, are not the only ones which we may hope to get by going on with the work. At present the dates of the spots have alone been recorded. But this is not enough ; we must know the actual positions of the spots on the sun. We must note whether each particular spot is in the northern hemisphere or in the southern hemisphere, with the view of determining whether there is any chemical difference between the north part of the sun and the south p'art ; and then again we must compare the latitudes of spots, with the view of determining whether there is any difference in the chemistry of the spots according to the latitude. This particular point is just now being worked up, and it really does look as if the sudden changes in the spectra noted from time to time may have been due to the fact that the spots compared were spots varying very considerably in latitude, and it would not surprise me to find that spots which are very like each other in their spectra will be found to be situated more or less in the same degree of latitude, whether the same degree of latitude north or south we do not know. And there is another question, too. I pointed out that there is a considerable number of lines seen in the spectrum of the arc which are left out of the spectrum of the spark. Now, will that help us at all in our inquiries ? I xxiii.] EXPLANATION OF TABLES. 319 think perhaps it may in time, and that we shall ultimately be able, in the way indicated, to classify spots according to their temperature. Although the reduction of the whole 700 observations is not yet complete, some of the general results obtained from the whole of the observations up to 1885 may be briefly referred to in this place. I first give tables (A, B, C) showing that for each of the chemical elements so far considered iron, nickel, and titanium the number of lines seen in the aggregate in each hundred observations is reduced from the sun-spot minimum to the maxi- mum, and this result holds good for both regions of the spectrum. I give another table (D) showing that during the observations the lines recorded as most widened near the maximum period of sun-spots have not been tabulated amongst metallic lines by either Angstrom or Thalen, and that many of them are not among the mapped Fraunhofer lines, though some of them may exist as faint lines in the solar spectrum when the observing conditions are best. The result of the observations of 700 spots observations extending over six years along this line, may be thus briefly stated. As we pass from minimum to maximum, the lines of the chemical elements gradually disappear from among those most widened, their places being taken by lines of which at present we have no terrestrial representatives. Or, to put the result another way at the minimum period of sun-spots when we know the solar atmosphere is quietest and coolest, vapours containing the lines of some of our terrestrial elements are present in sun-spots. The vapours, however, which produce the phenomena of sun-spots at the sun-spot maximum are entirely unfamiliar to us. 320 THE CHEMISTRY OF THE SUN. [CHAP, 7th HUNE 19th Feb., 1 24th Augusi ill "dS W &Sg 5th HUND 30th August, 23rd June, Pi 81 (ji 1st HUNDl 12th Nov., 1 29th Sept., ,_, 00 Sj MOO S8 S" ' V M^* & # ' r' ' 8" 4863*2 4870*2 4871*3 4875-5 4877-4 4884-2 4886-5 48>8*0 4h90*0 4907*0 4909*5 1 491S*0 4919*8 4956*7 4981*8 4982*5 4983*5 4984*8 5004-9 5005-2 5006-5 5011 "5 5014*2 5019-2 5026*2 5027*2 5038*2 5040*1 5041*2 5047*8 5049-4 5051*0 5068*1 5071*0 5074*0 5075 "S 5077*9 5078*8 5082*2 5090*2 5096*3 5098*2 5107*0 5109-8 5121*0 5123*2 5126*5 5133*0 5136*8 5138*6 5141*8 5145*7 5150*0 5151*2 5158*5 5161*5 5165*8 I xxin.] TABLES. TABLE B. NICKEL. List of most Widened Lines observed. 321 CO iO 05 CO rH (0 U t C ?. 5 3 c > * J O 1 9 O 3 C ; i - c- > c 3 - h o- > s o 4 ^ 9 u 1 C 5 (N o 41 ^ i ? y 3 i - I & j C -( 5 - H ^ . C n ^ H r- > c/ > O 1 T H --i J C ^ ) 5 r- J5 > X ' c l.- 1 if : i i U ^ i- ^1 - C 3 C 1$ o a 3 ". ^ D C- ^ ir ?s 1 r- > l : il 4 ^ D : ?s H r- 3 1- > C i tt H ?i - -J > C u li 3 T- * 32 H i- s u 3 - 4 -5 -i i- au H 1 T H r- 1 ^ - I ( t< US H I 1 } 1st hundred spots 2nd hundred spots 3rd hundred spots 4th hundred spots 5th hundred spots 6th hundred spots 7th hundred spots 3 STo li IK' 8. Y 322 THE CHEMISTRY OF THE SUN. [CHAP. TABLE D. Unknown "Widened Lines observed. 1st Hundred. 2nd Hundred. 3rd Hundred. 4th Hundred. 5th Hundred. 6th Hundred. 7th Hundred. 4865 1 4885 ... ... ... 1 4888-3 1 ... ... 4891-8 ... ... 1 4910 "2 ... 4944 1 5017-2 1 ... ... ... 5028-9 1 ... 5030 1 5034-8 11 3 ... 5037 1 5038-9 1 'i ... ... 5042 ... 3 5042-3 4 5043 1 5044-6 3 ... ... 5061 .... 2 3 5061-5 ... ... 2 5062 ,-. 5 5062-4 ... ... 2 5062-8 ... 3 ... 2 5065 ... 8 ... 5067 1 ... ... ... ... 5069-5 ... 1 5070-8 1 ... -5077 ... i 5079-5 ... 2 5080 "i 5081-5 ... ... ... 3 5082 2 5083 2 5083-3 1 "2 3 _ 5084 ... 1 ... ... 3 5084-5 ... 2 5086 17 i 5086-8 ... 1 50877 1 5088-1 ... 1 5088-6 1 5089-0 ... 1 5101 " 1 5103-5 ... 1 . 5112-1 6 22 4 2 1 5115-5 ... ... ... 9 5116 3 6 24 3 5116-2 ... 7 5118 4 14 5127 ... 1 5127-5 1 ... XXIII.] TABLES. 323 1st Hundred. 2nd Hundred. 3rd Hundred. 4th Hundred. 5th Hundred. Oth Hundred. 7th Hundred. 5128-8 1 5129-6 17 19 4 ... 5130 1 ... 5132 14 21 6 5132-5 1 5132-8 ... "3 5133-5 "i 1 '3 17 5133-8 ... 30 47 43 62 3 27 5134 ... ... ... 12 41 10 5134-4 ... ... ... ... 19 5135 ... 16 36 11 5135-5 ... 33 15 53 36 20 5135-8 ... 37 52 13 2 5136 ... 4 ... 9 22 27 5136-5 3 1 5137 ... ... 2 2 1 5137-5 4 72 79 22 5137-8 12 35 64 13 10 3 5138 ... ... 1 3 5139 ... ... ... 1 1 5139-4 1 2 3 5140-4 2 5142-2 13 4 i 5142-8 ... 21 7 19 2 5143 20 5143-2 ... ... "2 5144-2 1 3 ... 2 5144-5 ... 1 5145-5 ... 1 5146 ... ... 36 12 5146-5 2 5148 ... ... ... "i 5148-8 ... 1 2 5149 '2 32 31 36 4 35 5149-2 ... ... ... 1 5149-5 4 ... 29 5149-8 ... "s 2 8 8 5150 ... ... 1 5151-8 ... ... i 5153-8 ... i 5154 ... ... 'i 5155-4 ... "i 5156 1 12 37 74 32 91 95 5156-5 ... a 5157-2 "4 5159 "i 8 13 11 41 5159-5 1 31 59 80 86 57 5160 1 4 9 5160-4 1 5 4 5162 "9 7 61 67 62 5162-2 1 ... 23 49 21 30 5175 ... 3 Y 2 324 THE CHEMISTRY OF THE SUN. [CHAP. The disappearance of the lines of iron, nickel, and titanium, and the appearance of unknown lines as the maximum is reached is shown by curves in Fig. 107 given on the next page. The important results thus obtained from the preliminary discussion of the whole of the 700 spots enable us to see that the strange changes noted in the second series (see page 315) were but the indications of the effects of a general law which connects the lines affected in spots with the sun-spot period, and obviously with the varying temperatures proper to each phase of the period. Hence we may now generalise the last six statements given on page 316 as follows : 12. The most widened lines in sun-spots change with the sun-spot period. 13. At and slightly after the minimum, the lines are chiefly known lines of the various metals. 14. At and slightly after the maximum, the lines are chiefly of unknown origin. 15. On the hypothesis under discussion the change indicates an increased temperature in the spots at the sun-spot maximum. The results, in my opinion, amply justify the working hypothesis as to the construction of the solar atmosphere given in chapter xxii. I am quite content, therefore, to believe that iron, titanium, nickel, and the other substances very nearly as complex as we know them here, descend to the surface of the photosphere, in the downrush that forms a spot at the period of minimum ; but that at the maximum, on the contrary, only their finest con- stituent atoms can reach it. It may also be remarked that these particles which survive the dissociating energies of the lower strata are not the same particles among the constituents of the chemical elements named which give the chromospheric lines recorded by Tacchini, Kicco, and myself. CURVES SHOWING RESULTS. Most Widened Lines YEARS 1879-60 1880-1 !88l- 1882 FIG. 107. Number of appearances of known and unknown lines. CHAPTER XXIV. SPECIAL TESTS WITH REGARD TO IRON. 1. Youngs Work. IN the last chapter I gave the sun-spot observations in their most general form, represented by Fig. 106, in which we had the Fraunhofer lines compared with the lines widened in spots. Now if I had contented myself with such maps as that a portion of which was selected to engrave the figure in question, it might, and it certainly would have been said that nothing was more natural, that in fact the only result which had been got out, was that there was more of one chemical element in one spot and more of another in another ; and 'that the inversions were simply due to this cause. I have already shown, how- ever, that we have got far beyond that, and that the inversions are truly inversions of lines of the same metal. This is one of the points to be strengthened in the present chapter. These results have been mapped in all their details in another long series of maps of a different kind, the difference being that instead of considering all the Fraunhofer lines and all the lines recorded in spots and prominences, we have a map for each hundred observations, strictly limited, both as regards sun- spot and prominences, to one metal. I propose in this chapter to refer specially to this latter series of maps. Part of the work which has been undertaken in connection with this special branch of the investigation in order to enable CH. xxiv.] YOUNG'S WORK. 327 us to add laboratory results to the solar ones, has been a careful inquiry into the changes brought about in the spectrum of substances by exposing them to widely different temperatures. In the case of iron our last work has been to note the lines visible when fine iron gauze is burned in the flame produced by a mixture of oxygen and coal gas. The research is a very laborious one, and it may be that some day we shall get a very much better record than that which my assistants and myself have produced ; what we have been able to do we have done over the region of the spectrum already worked over in the spots and flames. In any case it is clear that when such facts as these have been studied in the laboratory, we can then compare both solar and laboratory evidence. When these observations are taken into account, we can not only deal with the sun-spot observa- tions comparing them with the spectra of prominences, but with the changes of spectra observed in our laboratories when various temperatures are employed. And further, we here limit ourselves to one spectrum, that of iron, for the reason that it has been studied with the greatest care by several eminent observers. In this way we shall be best brought face to face with the actual phenomena, and with very definite statements of facts over a small area. Dolus latet in generalibus. Many of the points brought out strongly by the recent spot observations may be gathered, though not certainly in all their fulness, from a comparison of the spot and prominence observa- tions made by Young at Sherman so far back as 1872. I will therefore give them here. I have brought both sets of observa- tions together in the following table, and it will be seen what small relation there is between the intensities of the lines seen in our laboratories and the number of times they are seen, either in spots or storms; while the fact that many lines affected in spots are not seen in prominences, and vice versa, comes out very clearly. 328 THE CHEMISTRY OF THE SUN. [CHAP THALEN. YOUNG. I 2 | Spots. Prominences. f I "to s 1 >, . < i J a 1 a 9 3 I " 1 i 2 i "> |l I 1-1 r M W M 6489-8 3 6407-0 Map ... 6399-0 1 Yes 5 2 6392-6 Map Yes 5 1 6357-7 Map Yes 4 6300-3 3 6245'4 2 Yes 8 5 6231-5 Map ... Yes 5 1 6229-7 2 6199-6 Map Yes 2 2 6190-5 2 ... Yes 2 Yes 10 2 6148-1 Map Yes 3 2 6135-6 2 ... Yes 3 Yes 2 1 6064-5 2 Ti Yes 3 Yes 5 2 6023-0 3 6019-1 4 6007-5 4 Yes * 6002-1 4 5986-2 4 5984-2 4 5982-8 4 5976-1 4 5974-6 4 5913-2 Map Yes 2 1 5883-0 Map ... Yes 2 1 5761-9 3 5708 3 3 Yes 1 1 5705-1 Map Yes "4 / 5681 -5 Map Yes "2 1 1 15681-4 3 Na ... 1 5661-5 3 Ti Yes 4 Yes 15 2 5657-6 1 5654-4 3 Yes "2 1 5623-2 3 Yes "2 Yes 2 1 5614-5 1 ... Yes 3 Yes 2 1 5601-7 1 Ca Yes 2 5597-2 1 Ca Yes 2 5591-2 2 Yes 2 j 5585-6 1 f 5585-5 Map Yes 3 Yes 2 1 5583-7 Map Yes 4 5574-9 2 5571-7 1 Yes 2 5568-5 2 * Double nearly invisible in spot spectrum. XXIV.] TABLES. 329 THALEN. YOUNG. - 4 Spots. Prominences. 1 8 1 li > * ! d 'o o r2 R ' < g |s I ^ h-t "o "* ^ [> ^ -" _tc a a W^ 5531-6 Map Yes 2 5525-9 Map ... Yes 40 5 5505-9 3 5500-5 3 Yes 2 Yes 2 1 5496-6 3 ... Yes 2 Yes 2 1 5486-8 4 Yes 3 5462-3 Map .!. Yes 1 1 5454 7 1 Yes 3 Yes 10 4 5445-9 1 Yes 4 Yes 10 4 5433-0 Map Yes 4 Yes 2 2 5428 8 1 Yes 3 Yes 8 3 5423-6 Map Yes 3 5414-5 Map ... Yes 3 Yes 2 2 5410-0 Map Yes 2 1 5404-8 2 Yes '4 Yes 2 1 5403-1 2 Ti Yes 4 Yes 5 3 5396-1 2 Ti Yes 7 Yes 4 2 / 5392-3 3 \5392-2 Map ... Yes 2 1 5382-3 3 5370-5 1 Yes 4 Yes 10 3 5369-0 3 Yes 4 Yes 1 1 5366-5 3 ... Yes 1 1 5364-0 3 ... Yes 1 1 5361-9 4 Yes 20 10 5352-4 4 Co Yes 2 Yes 4 2 5348-6 4 5340-2 2 Mn Yes 2 Yes 1 2 5339-2 2 Yes 2 5327-6 Map Yes 5 2 j 5327-3 1 Yes 2 \5327-l Map Yes 5 2 5323-4 2 5315-9 2 Yes * Yes 90 50 5306 5 3 5301-5 3 5282-6 2 5280-9 3 5269-5 1 Yes 3 Yes 15 4 5268-5 1 Yes 3 Yes 12 3 5265-8 2 Co Yes 2 Yes 10 4 5262-4 4 5254-1 Map ... Yes 1 2 5249-7 - Map ... Yes 3 1 Distinctly weakened and sometimes reversed. 330 THE CHEMISTRY OF THE SUN. [CH. xxiv. THAL^N. YOUNG. Spots. Prominences. 9 | 'BE II 55 1 . fcb K i 2 1 a 3 5 yj I o S a 43 g Cj I 1 i > ^.9 i CM P ~ W jB m 5246-3 Map Yes 3 i 5239-0 Map Yes 4 2 5232-1 1 Yes "2 Yes 1 3 5229-0 Map Yes 2 5226-2 1 Yes 2 Yes 10 3 5216-5 Map .. Yes 2 1 5215-5 Map Yes 3 2 5214-4 Map Yes 2 1 5207-6 3 Cr Yes 4 Yes 10 6 52037 3 Cr Yes 4 Yes 10 6 5201-5 4 Yes 2 Yes 5 3 5197-9 Map Yes 3 Yes 1 1 5194-1 3 Yes 2 2 51917 2 Yes 2 5190 5 4 Yes 2 5185-1 Map Yes 5 2 51711 4 5168-3 3 Ni Yes 4* Yes 40 30 5166-7 2 Mg Yes 2* Yes 30 20 5161-6 4 5151-3 Map Yes 2 * Sometimes reversed. The above tables then clearly indicate the considerable differences which have been noted by former observers touch- ing the various appearances and intensities of the lines of iron as seen in sun-spots and prominences respectively ; but it might be imagined that when we use the highest tempera- ture available here and contrast the spectrum thus obtained with that given us by the prominences, that here at all events we should get a closer agreement. The map given on the next page will show that this is not so, and that there are the greatest divergences of intensity between lines seen under these two conditions ; that many of the lines seen in our laboratories when a chemical element is studied and therefore Iron in the Spark and in Prominences. uno A 332" YOUNG'S WORK. [CHAP. called by us lines of that chemical element, are absent from this part of the solar record. I show elsewhere that many of the bright lines seen in prominences are very probably lines due to descending cooler vapours, but even if we make allowance for this the divergences observed by Professor Young must still be regarded as extraordinary. To show this in connection with Professor Young's Sherman observations of prominences ; in 1879 I prepared maps of the spectra of calcium, barium, iron, and manganese. In these the lengths of the lines in the spectra of the metallic elements represent the intensities given by Thalen, whose lines and wave-lengths I have followed in all cases, while those of the lines visible in prominences, represent the number of times each line has been seen in the spectrum of the chromosphere by Professor Young. An inspection of these maps is sufficient to show that there is no connection whatever beyond that of wave-length between the spectra ; it will be gathered from them how the long lines seen in our laboratories are sup- pressed and the feeble lines exalted in the spectrum of the chromosphere. Of these spectrum maps I give only that of iron, although if space had permitted I should have been glad to add those of calcium and barium, because in some cases the changes of intensity in those spectra are even more striking than in iron. 2. Some Details of the Changes of Intensity in the Iron Spectrum. In studying the iron spectrum we may not only consider the visible portion, but we can also include in our inquiry that recorded on my photographic plates, between H and G. It may be described as a very complicated spectrum, so far as the number of lines is concerned, in comparison with such xxiv.] TRIPLETS. bodies as sodium and potassium, lead, thallium, and the like ; unlike them, again, it contains no one line which is clearly and unmistakably reversed on all occasions. Compared, however, with the spectrum of such bodies as cerium and uranium, the spectrum is simplicity itself. My attention was first directed to the variation in the relative intensity of the iron lines under different conditions by the differences easily recognised in the case of some of the triplets with which the spectrum is crowded. These give us beautiful examples of those repetitions of structure which we meet with in the spectra of almost all bodies, some of which have already been pointed out by Mascart, Cornu and myself. In many photographs in which iron has been com- pared with other substances, and in others again in which iron has been photographed as existing in different degrees of impurity in other substances, these triplets have been seen almost alone, and the relative intensity of them, as compared with the few remaining lines, is greatly changed. In this these photographs resemble one I took ten years ago, in which a large coil and jar were employed instead of the arc ; they necessitated an exposure of an hour instead of two minutes. In this the triplet near G is very marked ; the two adjacent more refrangible lines near it, which are seen nearly as strong as the triplet itself in some of the arc photographs I possess, are only very faintly visible, while dimmer still are seen the lines of the triplet between H and h. In the spark photograph, then, the more refrangible triplet is barely visible, while the one near G is very strong. Fig. 109, which is copied from a photograph, will show another difference. The lines at wave-lengths 4325 '0, 43007, 427 I/O, composing the less refrangible triplet, are three of the strongest iron lines in the arc spectrum, and those at 4071-0, 4063-0, 4045'0, composing the more refrangible ones, are less strong than the others. It will be seen that in the solar spectrum the last triplet is much more 334 THE CHEMISTRY OF THE SUN. [CHAP. important, much thicker, and much darker than the first, so that here is an absolute inversion in the intensity of the lines. I appeal to the photograph because it is perfectly impartial ; it has no view, no anxiety therefore to intensify one particular part of it at the expense of the other, and it is referred to as the exemplar of many similar reversals which we see whenever such observations are made. One word here on the probable origin of these triplets. If one molecular grouping alone were in question, their relative intensity should always be preserved, however much the absolute intensity of the compound system might vary ; but if it is a question of two molecules, we might expect that, in some s