Fig. 39.Fig. 39. Diagrammatic section of an active volcano. a, Central pipe or funnel. b, b, Walls of the crater or cup. c, c, Dark turbid cloud formed by the ascending globular clouds d, d. e, Rain-shower from escaped vapour. f, f, Shower of blocks, cooled bombs, stones, and ashes falling back on to the cone. g, Lava escaping through a fissure, and pouring out of a cone opened in the mountain side.Diagrammatic section of an active volcano.a, Central pipe or funnel.b, b, Walls of the crater or cup.c, c, Dark turbid cloud formed by the ascending globular cloudsd, d.e, Rain-shower from escaped vapour.f, f, Shower of blocks, cooled bombs, stones, and ashes falling back on to the cone.g, Lava escaping through a fissure, and pouring out of a cone opened in the mountain side.
a, Central pipe or funnel.b, b, Walls of the crater or cup.c, c, Dark turbid cloud formed by the ascending globular cloudsd, d.e, Rain-shower from escaped vapour.f, f, Shower of blocks, cooled bombs, stones, and ashes falling back on to the cone.g, Lava escaping through a fissure, and pouring out of a cone opened in the mountain side.
In the funnelawhich passes down from the crater or cupb, b, white-hot lava was surging up, having a large quantity of water and steam entangled in it. The lava, or melted rock, would be in much the same state as melted iron-slag is, in the huge blast-furnaces in which iron-rock is fused, only it would have floating in it great blocks of solid rock, and rounded stones called bombs which have been formed from pieces of half-melted rockwhirled in air and falling back into the crater, together with clinkers or scoriæ, dust and sand, all torn off and ground down from the walls of the funnel up which the rush was coming. And in the pipe of melted rock, forcing the lava upwards, enormous bubbles of steam and gasd, dwould be rising up one after another as bubbles rise in any thick boiling substances, such as boiling sugar or tar.
In the morning before the eruption, when only a little smoke was issuing from the crater, these bubbles rose very slowly through the loaded funnel and the half-cooled lava in the basin, and the booming noise, like that of heavy cannon, heard from time to time, was caused by the bursting of one of these globes of steam at the top of the funnel, as it brought up with it a feeble shower of stones, dust, and scoriæ. Meanwhile the lava surging below was forcing a passagegfor itself in a weak part of the mountain-side and, just at the time when our attention was called to Vesuvius, the violent pressure from below rent open a mouth or crater ath, so that the lava began to flow down the mountain in a steady stream. This, relieving the funnel, enabled the huge steam bubblesd, dto rise more quickly, and to form the large whitish-grey cloudc, into which from time to time the red-hot blocks, scoriæ, and pumice were thrown up by the escaping steam and gases. These blocks and fragments then fell back again in a fiery showerf, feither into the cup, to be thrown up again by the bursting of the next bubble, or on to the sides of the cone, making it both broader and higher.
Only one feature in the diagram was fortunatelyabsent the evening we went up, namely, the rain-showere. The night, as I said, was calm, and the air dry, and the steam floated peacefully away. The next night, however, when many people hurried down from Rome to see the sight they were woefully disappointed, for rain-showers fell heavily from the cloud, bringing down with them the dust and ashes, which covered the unfortunate sight-seers.
This was what happened during the eruption, and the result after a few days was that the cone was a little higher, with a fresh layer of rough slaggy scoriæ on its slopes, and that on the side of the mountain behind the Hermitage a new lava stream was added to the many which have flowed there of late years. What then can we learn from this stream about the materials which come up out of the depths of the earth, and of the manner in which volcanic rocks are formed?
The lava as I saw it when coming first out of the newly-opened crater is, as I have said, like white-hot iron slag, but very soon the top becomes black and solid, a hard cindery mass full of holes and cavities with rough edges, caused by the steam and sulphur and other gases breaking through it.[1]In fact, there are so many holes and bubbles in it that it is very light and floats on the top of the heavier lava below, falling over it on to the mountain-side when it comes to the end of the stream. Still, however, the great mass moves on, so that the streamslides over these fallen clinkers or scoriæ. Thus after an eruption a new flow consists of three layers; at the top the cooled and broken crust of clinkers, then the more solid lava, which often remains hot for years, and lastly another cindery layer beneath, formed of the scoriæ which have fallen from above (see Fig. 40).
Fig. 40.Fig. 40. Section of a lava-flow. (J. Geikie.) 1, Slaggy crust, formed chiefly of scoriæ of a glassy nature. 2, Middle portion where crystals form. 3, Slaggy crust which has slipped down and been covered by the flow.Section of a lava-flow. (J. Geikie.)1, Slaggy crust, formed chiefly of scoriæ of a glassy nature. 2, Middle portion where crystals form. 3, Slaggy crust which has slipped down and been covered by the flow.
1, Slaggy crust, formed chiefly of scoriæ of a glassy nature. 2, Middle portion where crystals form. 3, Slaggy crust which has slipped down and been covered by the flow.
You would be surprised to see how quickly the top hardens, so that you can actually walk across a stream of lava a day or two after it comes out from the mountain. But you must not stand still or your shoes would soon be burnt, and if you break the crust with a stick you will at once see the red-hot lava below; while after a few days the cavities become filled with crystals of common salt, sulphur or soda, as the vapour and gases escape.
Then as time goes on the harder minerals gradually crystallise out of the melted mass, and iron-pyrites, copper-sulphate, and numerous other forms of crystal appear in the lower part of the stream. In the clinkers above, where the cooling goes on very rapidly, the lavas formed are semi-transparent andlook much like common bottle-glass. In fact, if you take this piece of obsidian or volcanic glass in your hand, you might think that it had come out of an ordinary glass manufactory and had nothing remarkable in it.
Fig. 41.Fig. 41. A slice of volcanic glass showing the lines of crystallites and microliths which are the beginnings of crystals.A slice of volcanic glass showing the lines of crystallites and microliths which are the beginnings of crystals. (J. Geikie.)[2]
But the microscope tells another tale. I have put a thin slice under the first microscope, and this diagram (Fig. 41) shows what you will see. Nothing, you say, but a few black specks and some tiny dark rods. True, but these specks and rods are the first beginnings of crystals forming out of the ground-mass of glassy lava as it cools down. They are not real crystals, but the first step toward them, and by a careful examination of glassy lavas which have cooled at different rates, they have been seen under the microscope in all stages of growth, gradually building up different crystalline forms. When we remember how rapidly the top of many glassy lavas cool down we can understandthat they have often only time to grow very small.
The smaller specks are calledcrystallites, the rods are calledmicroliths.[3]Under the next microscope you can see the microliths much more distinctly (Fig. 42) and observe that they grow in very regular shapes.
Fig. 42.Fig. 42. A slice of volcanic glass under the microscope, showing well-developed microliths. (After Cohen.)A slice of volcanic glass under the microscope, showing well-developed microliths. (After Cohen.)
Our first slice, however (Fig. 41), tells us something more of their history, for the fact that they are arranged in lines shows that they have grown while the lava was flowing and carrying them along in streams. You will notice that each one has its greatest length in the direction of the lines, just as pieces of stick are carried along lengthways in a river. In the second specimen (Fig. 42) the microliths are much larger and the stream has evidently not been flowing fast, for they lie in all directions.
This is what we find in the upper part of the stream, but if we look at a piece of underlying lavawe find that it is much more coarse-grained, and the magnifying-glass shows many crystals in it, as well as a number of microliths. For this lava, covered by the crust above, has remained very hot for a long time, and the crystals have had time to build themselves up out of the microliths and crystallites.
Still there is much glassy groundwork even in these lavas. If we want to find really stony masses such as porphyry and granite made up entirely of crystals we must look inside the mountain where the molten rock is kept intensely hot for long periods, as for example in the fissureg, Fig. 39.
Such fissures sometimes open out on the surface like the one I saw, and sometimes only penetrate part of the way through the hill; but in either case when the lava in them cools down, it forms solid walls called dykes which help to bind the loose materials of the mountain together. We cannot, of course, examine these in an active volcano, but there are many extinct volcanoes which have been worn and washed by the weather for centuries, so that we can see the inside. The dykes laid bare in the cliffs of Somma are old fissures filled with molten rock which has cooled down, and they show us many stony lavas; and Mr. Judd tells us of one beautiful example of a ruined volcano which composes the whole island of Mull in the Hebrides, where such dykes can be traced right back to a centre. This centre must once have been a mass of melted matter far down in the earth, and as you trace the dykes back deeper and deeper into it, the rocks grow more and more stony, till at last they are composed entirely oflarge crystals closely crowded together without any glassy matter between them. You know this crystalline structure well, for we have plenty of blocks of granite scattered about on Dartmoor, showing that at some time long ago molten matter must have been at work in the depths under Devonshire.
We see then that we can trace the melted rock of volcanoes right back—from the surface of the lava stream which cools quickly at the top, hurrying the crystallites and microliths along with it—down through the volcano to the depths of the earth, where the perfect crystals form slowly and deliberately in the underground lakes of white-hot rock which are kept in a melted state at an intense heat.
Fig. 43.Fig. 43. A piece of Dartmoor Granite, drawn from a specimen.A piece of Dartmoor Granite, drawn from a specimen.
But I promised you that we would have no guesswork here, and you will perhaps ask how I can be certain what was going on in the depths when these crystals were formed. A few years ago I could not have answered you, but now chemists, and especially two eminent French chemists, MM. Fouqué and Levy, have actuallymadelavas and shown us how it is done in Nature.
By using powerful furnaces and bellows they have succeeded in getting temperatures of all degrees, from a dazzling white heat down to a dull red, and to keep any temperature they like for a long time, so as to imitate the state of a mass of melted rockat different depths in the earth, and in this way they have actuallymadelavas in their crucibles. For example, there is a certain whitish rock common in Vesuvius calledleucotephrite,[4]which is made up chiefly of crystals of the minerals called leucite, Labrador felspar, and augite. This they proposed to make artificially, so they took proper quantities of silica, alumina, oxide of iron, lime, potash, and soda, and putting them in a crucible, melted them by keeping them at a white heat. Then they lowered the temperature to an orange-heat, that is a heat sufficient to melt steel. They kept this heat for forty-eight hours, after which they took out some of the mixture and, letting it cool, examined a slice under the microscope. Within it they found crystals ofleucitealready formed, showing that these are the first to grow while the melted rock is still intensely hot. The rest of the mixture they kept red-hot, or at the melting-point of copper, for another forty-eight hours, and when they took it out and examined it they found that the whole of it had been transformed into microliths of the two other forms of crystals, Labrador felspar and augite, except some small eight-sided crystals of magnetite and picotite which are also found in the natural rock.
There is no need for you to remember all these names. What I do want you to remember is, that, at the different temperatures, the right crystals and beginnings of crystals grew up to form the rock which is found in Vesuvius. And what is still moreinteresting, they grew exactly to the same stages as in the natural rock, which is composed ofcrystalsof leucite andmicrolithsof the two other minerals.
This is only one among numerous experiments by which we have learnt how volcanic rocks are formed and at what heat the crystals of different substances grow. We are only as yet at the beginning of this new study, and there is plenty for you boys to do by and by when you grow up. Many experiments have failed as yet to imitate certain rocks, and it is remarkable that these are usually rocks of very ancient eruptions, whenperhapsour globe may have been in a different state to what it is now; but this remains for us to find out.
Meanwhile I have still another very interesting slide to show you which tells us something of what is going on below the volcano. Under the third microscope I have put a slice of volcanic glass (Fig. 44) in which you will see really large crystals with dark bands curving round them. These crystals have clearly not been formed in the glass while the lava was flowing, first because they are too large to have grown up so rapidly, and secondly because they are broken at the edges in places and sometimes partly melted. They have evidently come up with the lava as it flowed out of the mountain, and the dark bands curving round them are composed of microliths which have been formed in the flow and have swept round them, as floating straws gather round a block of wood in a stream.
Such crystals as these are often found in lava streams, and in fact they make a great difference inthe rate at which a stream flows, for a thoroughly melted lava shoots along at a great pace and often travels several miles in a very short time; but an imperfectly melted lava full of crystals creeps slowly along, and often does not travel far from the crater out of which it flows.
Fig. 44.Fig. 44. Slice of volcanic glass under the microscope, showing large included crystals brought up from inside the volcano in the fluid lava. The dark bands are lines of microliths formed as the lava cooled. (J. Geikie.)Slice of volcanic glass under the microscope, showing large included crystals brought up from inside the volcano in the fluid lava. The dark bands are lines of microliths formed as the lava cooled. (J. Geikie.)
So you see we have proof in this slice of volcanic glass of two separate periods of crystallisation—the period when the large crystals grew in the liquid mass under the mountain, and the period when the microliths were formed after it was poured out above ground. And as we know that different substances form their crystals at very different temperatures, it is not surprising that some should be able to take up the material they require and grow in the underground lakes of melted matter, even though the rest of the lava was sufficiently fluid to be forced up out of the mountain.
And here we must leave our lava stream. The microscope can tell us yet more, of marvellous tinycavities inside the crystals, millions in a single inch, and of other crystals inside these, all of which have their history; but this would lead us too far. We must be content for the present with having roughly traced a flow of lava from the depths below, where large crystals form in subterranean darkness, to the open air above, where we catch the tiny beginnings of crystals hardened into glassy lava before they have time to grow further.
If you will think a little for yourselves about these wonderful discoveries made with the magic-glass, you will see how many questions they suggest to us about the minerals which we find buried in the earth and running through it in veins, and you will want to know something about the more precious crystals, such as rubies, diamonds, sapphires, and garnets, and many others which Nature forms far away out of our sight. All these depend, though indirectly, upon the strange effects of underground heat, and if you have once formed a picture in your minds of what must have been going on before that magnificent lava stream crept down the mountain-side and added its small contribution to the surface of the earth, you will study eagerly all that comes in your way about crystals and minerals, and while you ask questions with the spectroscope about what is going on in the sun and stars millions of miles away, you will also ask the microscope what it has to tell of the work going on at depths many miles under your feet.
[1]For the cindery nature of the surface of such a stream see the initial letter of this chapter.
[1]For the cindery nature of the surface of such a stream see the initial letter of this chapter.
[2]This arrangement in lines is calledfluidal structurein lava.
[2]This arrangement in lines is calledfluidal structurein lava.
[3]Micros, little;lithos, stone.
[3]Micros, little;lithos, stone.
[4]Leucos, white;tephra, ashes.
[4]Leucos, white;tephra, ashes.
ornate capital d
efore beginning upon the subject of our lecture to-day I want to tell you the story of a great puzzle which presented itself to me when I was a very young child. I happened to come across a little book—I can see it now as though it were yesterday—a small square green book calledWorld without End, which had upon the cover a little gilt picture of a stile with trees on each side of it. That was all. I do not know what the book was about, indeed I am almost sure I never opened it or saw it again, but that stile and the title "World without End" puzzled me terribly. What was on the other side of the stile? If I could cross over it and go on and on should I be in a world which had no ending, and what would be on the other side? But then there could be no other side if it wasa world without any end. I was very young, you must remember, and I grew confused and bewildered as I imagined myself reaching onwards and onwards beyond that stile and never, never resting. At last I consulted my greatest friend, an old man who did the weeding in my father's garden, and whom I believed to be very wise. He looked at first almost as bewildered as I was, but at last light dawned upon him. "I tell you what it is, Master Arthur," said he, "I do not rightly know what happens when there is no end, but I do know that there is a mighty lot to be found out in this world, and I'm thinking we had better learn first all about that, and perhaps it may teach us something which will help us to understand the other."
I daresay you will wonder what this anecdote can have to do with a lecture on the sun—I will tell you. Last night I stood on the balcony and looked out far and farther away into the star-depths of the midnight sky, marvelling what could be the history of those countless suns of which we see ever more and more as we increase the power of our telescopes, or catch the faint beams of those we cannot see and make them print their image on the photographic plate. And, as I grew oppressed at the thought of this never-ending expanse of suns and at my own littleness, I remembered all at once the little square book of my childish days with its gilt stile, and my old friend's advice to learn first all we can of that which lies nearest.
So to-day, before we travel away to the stars, we had better inquire what is known about the one starin the heavens which is comparatively near to us, our own glorious sun, which sends us all our light and heat, causes all the movements of our atmosphere, draws up the moisture from the ground to return in refreshing rain, ripens our harvests, awakens the seeds and sleeping plants into vigorous growth, and in a word sustains all the energy and life upon our earth. Yet even this star, which is more than a million times as large as our earth, and bound so closely to us that a convulsion on its surface sends a thrill right through our atmosphere, is still so far off that it is only by questioning the sunbeams it sends to us, that we can know anything about it.
You have already learnt[1]a good deal as to the size, the intense heat and light, and the photographic power of the sun, and also how his white beams of light are composed of countless coloured rays which we can separate in a prism. Now let us pass on to the more difficult problem of the nature of the sun itself, and what we know of the changes and commotions going on in that blazing globe of light.
We will try first what we can see for ourselves. If you take a card and make a pin-hole in it, you can look through this hole straight at the sun without injuring your eye, and you will see a round shining disc on which, perhaps, you may detect a few dark spots. Then if you take your hand telescopes, which I have shaded by putting a piece of smoked glass inside the eye-piece, you will find that this shining disc is really a round globe, and moreover, although the object-glass of your telescopes measuresonly two-and-a-half inches across, you will be able to see the dark spots very distinctly and to observe that they are shaded, having a deep spot in the centre with a paler shadow round it.
Fig. 45.Fig. 45. Face of the sun projected on a sheet of cardboard C. T, Telescope. f, Finder. og, Object-glass. ep, Eye-piece. S, Screen shutting off the diffused light from the window.Face of the sun projected on a sheet of cardboard C.T, Telescope.f, Finder.og, Object-glass.ep, Eye-piece. S, Screen shutting off the diffused light from the window.
T, Telescope.f, Finder.og, Object-glass.ep, Eye-piece. S, Screen shutting off the diffused light from the window.
As, however, you cannot all use the telescopes, and those who can will find it difficult to point them truly on to the sun, we will adopt still another plan. I will turn the object-glass of my portable telescope full upon the sun's face, and bringing a large piece of cardboard on an easel near to the other end, draw it slowly backward till the eye-piece forms a clear sharp image upon it (see Fig. 45). This you can all see clearly, especially as I have passed theeye-piece of the telescope through a large screens, which shuts off the light from the window.
You have now an exact image of the face of the sun and the few dark spots which are upon it, and we have brought, as it were, into our room that great globe of light and heat which sustains all the life and vigour upon our earth.
This small image can, however, tell us very little. Let us next see what photography can show us. The diagram (Fig. 46) shows a photograph of the sun taken by Mr. Selwyn in October 1860. Let me describe how this is done. You will remember that there is a point in the telescope tube where the rays of light form a real image of the object at which the telescope is pointed (see p. 44). Now an astronomer who wishes to take a photograph of the sun takes away the eye-piece of his telescope and puts a photographic plate in the tube exactly at the place where this real image is formed. He takes care to blacken the frame of the plate and shuts up this end of the telescope and the plate in a completely dark box, so that no diffused light from outside can reach it. Then he turns his telescope upon the sun that it may print its image.
But the sun's light is so strong that even in a second of time it would print a great deal too much, and all would be black and confused. To prevent this he has a strip of metal which slides across the tube of the telescope in front of the plate, and in the upper part of this strip a very fine slit is cut. Before he begins, he draws the metal up so that the slit is outside thetube and the solid portion within, and he fastens it in this position by a thread drawn through and tied to a bar outside. Then he turns his telescope on the sun, and as soon as he wishes to take the photograph he cuts the thread. The metal slides across the tube with a flash, the slit passing across it and out again below in the hundredth part of a second, and in that time the sun has printed through the slit the picture before you.
Fig. 46.Fig. 46. Photograph of the face of the sun, taken by Mr. Selwyn, October 1860, showing spots, faculæ, and mottled surface.Photograph of the face of the sun, taken by Mr. Selwyn, October 1860, showing spots, faculæ, and mottled surface.
In it you will observe at least two things not visible on our card-image. The spots, though in a different position from where we see them to-day, look much the same, but round them we see also some bright streaks calledfaculæ, or torches, whichoften appear in any region where a spot is forming, while the whole face of the sun appears mottled with bright and darker spaces intermixed. Those of you who have the telescopes can see this mottling quite distinctly through them if you look at the sun. The bright points have been called by many names, and are now generally known as "light granules," as good a name, perhaps, as any other.
This is all our photograph can tell us, but the round disc there shown, which is called thephotosphere, or light-giving sphere, is by no means the whole of the sun, though it is all we see daily with the naked eye. Whenever a total eclipse of the sun takes place—by the dark body of the moon coming between us and it, so as to shut out the whole of this disc—a brilliant white halo, called the crown orcorona, is seen to extend for many thousands of miles all round the darkened globe. It varies very much in shape, sometimes forming a kind of irregular square, sometimes a circle with off-shoots, as in Fig. 47, which shows what Major Tennant saw in India during the total eclipse of August 18, 1868, and at other times it shoots out in long pearly white jets and sheets of light with dark spaces between. On the whole it varies periodically. At the time of few sun-spots its extensions are equatorial; but when the sun's face is much covered with spots, they are diagonal, stretching away from the spot-zones, but not nearly so far.
Fig. 47.Fig. 47. Total eclipse of the sun, as drawn by Major Tennant at Guntoor in India, August 18, 1868, showing corona and the protuberances seen at the beginning of totality.Total eclipse of the sun, as drawn by Major Tennant at Guntoor in India, August 18, 1868, showing corona and the protuberances seen at the beginning of totality.
And besides this corona there are seen very curious flaming projections on the edge of the sun, which begin to appear as soon as the moon covers the bright disc. In our diagram (Fig. 47) you see them on the left side where the moon is just creeping over the limits of the photosphere and shutting out the strong light of the sun as the eclipse becomes total. A very little later they are better seen on the other side just before the bright edge of the sun is uncovered as the moon passes on its way. These projections in the real sun are of a bright red colour, and they take on all manners of strange shapes, sometimeslooking like ranges of fiery hills, sometimes like gigantic spikes and scimitars, sometimes even like branching fiery trees. They were calledprominencesbefore their nature was well understood, and will probably always keep that name. It would be far better, however, if some other name such as "glowing clouds" or "red jets" could be used, for there is now no doubt that they are jets of gases, chiefly hydrogen, constantly playing over the face of the sun, though only seen when his brighter light is quenched. They have been found to shoot up 20,000, 80,000, and even as much as 350,000 miles beyond the edge of the shining disc; and this last means that the flames were so gigantic that if they had started from our earth they would have reached beyond the moon. We shall see presently that astronomers are now able by the help of the spectroscope to see the prominences even when there is no eclipse, and we know them to be permanent parts of the bright globe.
This gives us at last the whole of the sun, so far as we know. There is, indeed, a strange faint zodiacal light, a kind of pearly glow seen after sunset or before sunrise extending far beyond the region of the corona; but we understand so little about this that we cannot be sure that it actually belongs to the sun.
And now how shall I best give you an idea of what little we do know about this great surging monster of light and heat which shines down upon us? You must give me all your attention, for I want to make the facts quite clear, that you may take a firm hold upon them.
Our first step is to question the sunlight whichcomes to us; and this we do with the spectroscope. Let me remind you how we read the story of light through this instrument. Taking in a narrow beam of light through a fine slit, we pass the beam through a lens to make the rays parallel, and then throw it upon a prism or row of prisms, so that each set of waves of coloured light coming through the slit is bent on its own road and makes an upright image of the slit on any screen or telescope put to receive it (see Fig. 21, p. 52). Now when the light we examine comes from a glowing solid, like white-hot iron, or a glowing liquid, or a gas under such enormous pressure that it behaves like a liquid, then the images of the slit always overlap each other, so that we see a continuous unbroken band of colour. However much you spread out the light you can never break up or separate the spectrum in any part.[2]But when you send the light, of a glowing gas such as hydrogen through the spectroscope, or of a substance melted into gas or vapour, such as sodium or iron vaporised by great heat, then it is a different story. Such gases give only a certain number of bright lines quite separate from each other on the dark background, and each kind of gas gives its own peculiar lines; so that even when several are glowing together there is no confusion, but when you look at them through the spectroscope you can detect the presence of each gas by its own lines in the spectrum.
Plate I.TABLE OF SPECTRA. Plate I.TABLE OF SPECTRA.
Plate I.
To make quite sure of this we will close the shutters and put a pinch of salt in a spirit-flame. Salt is chloride of sodium, and in the flame the sodium glows with a bright yellow light. Look at this light through your small direct-vision spectroscopes[3]and you see at once the bright yellow double-line of sodium (No. 3, Plate I.) start into view across the faint continuous spectrum given by the spirit-flame. Next I will show you glowing hydrogen. I have here a glass tube containing hydrogen, so arranged that by connecting two wires fastened to it with the induction coil of our electric battery it will soon glow with a bright red colour. Look at this through your spectroscopes and you will see three bright lines, one red, one greenish blue, and one indigo blue, standing out on the dark background (No. 4, Plate I.)
Think for a moment what a grand power this gives you of reading as in a book the different gases which are glowing in the sky even billions of miles away. You would never mistake the lines of hydrogen for the line of sodium, but when looking at a nebula or any mass of glowing gas you could say at once "sodium is glowing there," or "that cloud must be composed of hydrogen."
Now, opening the shutters, look at the sunlight through your spectroscopes. Here you have something different from either the continuous spectrum of solids, or the bright separate lines of gases, for while you have a bright-coloured band you have also some dark lines crossing it (No. 2, Plate I.) It is thosedark lines which enable us to guess what is going on in the sun before the light comes to us. In 1859 Professor Kirchhoff made an experiment which explained those dark lines, and we will repeat it now. Take a good look at the sunlight spectrum, to fix the lines in your memory, and then close the shutters again.
Fig. 48.Fig. 48. Kirchhoff's experiment, explaining the dark lines in sunlight. A, Limelight dispersed through a prism. s, Slit through which the beam of light comes. l, Lens bringing it to a focus on the prism p. sp, Continuous spectrum thrown on the wall. B, The same light, with the flame f containing glowing sodium placed in front of it. D, Dark sodium line appearing in the spectrum.Kirchhoff's experiment, explaining the dark lines in sunlight.A, Limelight dispersed through a prism.s, Slit through which the beam of light comes.l, Lens bringing it to a focus on the prismp.sp, Continuous spectrum thrown on the wall. B, The same light, with the flamefcontaining glowing sodium placed in front of it. D, Dark sodium line appearing in the spectrum.
A, Limelight dispersed through a prism.s, Slit through which the beam of light comes.l, Lens bringing it to a focus on the prismp.sp, Continuous spectrum thrown on the wall. B, The same light, with the flamefcontaining glowing sodium placed in front of it. D, Dark sodium line appearing in the spectrum.
I have here our magic-lantern with its lime-light, in which the solid lime glows with a white heat, in consequence of the jets of oxygen and hydrogenburning round it. This was the light Kirchhoff used, and you know it will give a continuous bright band in the spectroscope. I put a cap with a narrow slit in it over the lantern tube, so as to get a narrow beam of light; in front of this I put a lensl, and in front of this again the prismp. The slit and the prism act exactly like your spectroscopes, and you can all see the continuous spectrum on the screen (sp, A, Fig. 48). Next I put a lighted lamp of very weak spirit in front of the slit, and find that it makes no difference, for whatever light it gives only strengthens the spectrum. But now notice carefully. I am going to put a little salt into the flame, and you would expect that the sodium in it, when turned to glowing vapour, causing it to look yellow, would strengthen the yellow part of the spectrum and give a bright line. This is what Kirchhoff expected, but to his intense surprise he saw as you do now adark lineD start out where the bright line should have been.
What can have happened? It is this. The oxyhydrogen light is very hot indeed, the spirit flame with the sodium is comparatively weak and cool. So when those special coloured waves of the oxyhydrogen light which agree with those of the sodium light reached the flame, they spent all their energy in heating up those waves to their own temperature, and while all the other coloured rays travelled on and reached the screen, these waves were stopped orabsorbedon the way, and consequently there was a blank, black space in the spectrum where they should have been. If I could put a hydrogen flame cooler than the originallight in the road, then there would be three dark lines where the bright hydrogen lines should be, and so with every other gas.The cool vapour in front of the hot light cuts off from the white ray exactly those waves which it gives out itself when burning.
Thus each black line of the sun-spectrum (No. 2, Plate I.), tells us that some particular ray of sunlight has been absorbed by a cooler vapourof its own kindsomewhere between the sun and us, and it must be in the sun itself, for when we examine other stars we often find dark lines in their spectrum different from those in the sun, and this shows that the missing rays must have been stopped close at home, for if they were stopped in our atmosphere they would all be alike.
There are, by the bye, some lines which we know are caused by our atmosphere, especially when it is full of invisible water vapour, and these we easily detect, because they show more distinctly when the sun is low and shines through a thicker layer of air than when he is high up and shines through less.
But to return to the sun. In your small spectroscopes you see very few dark lines, but in larger and more perfect ones they can be counted by thousands, and can be compared with the bright lines of glowing gases burnt here on earth. In the spectrum of glowing iron vapour 460 lines are found to agree with dark lines in the sun-spectrum, and other gases have nearly as many. Still, though thousands of lines can now be explained, by matching them with the bright lines of known gases, the whole secret of sunlight is not yet solved, for the larger number of lines still remain a riddle to be read.
We see then that the spectroscope teaches us thatthe round light-giving disc or photosphere of the sun consists of a bright and intensely hot light shining behind a layer of cooler though still very hot vapours, which form a kind of shell of luminous clouds around it, and in this shell, orreversing layer—as it is often called, because it turns light to darkness—we have proved that iron, lead, copper, zinc, aluminum, magnesium, potassium, sodium, carbon, hydrogen, and many other substances common to our earth, exist in a state of vapour for a depth of perhaps 1000 miles.
You will easily understand that when the spectroscope had told so much, astronomers were eager to learn what it would reveal about the prominences or red jets seen during eclipses, and they got an answer in India during that same eclipse of August 1868 which is shown in our diagram (Fig. 47). Making use of the time during which the prominences were seen, they turned the telescope upon them with a spectroscope attached to it, and saw a number of bright lines start out, of which the chief were the three bright lines of hydrogen, showing that these curious appearances are really flames of glowing gas.
In the same year Professor Jannsen and Mr. Lockyer succeeded in seeing the bright lines of the prominences in full sunlight. This was done in a very simple way, when once it was discovered to be possible, and though my apparatus (Fig. 49) is very primitive compared with some now made, it will serve to explain the method.
Fig. 49.Fig. 49. The spectroscope attached to the telescope for the examination of the sun. (Lockyer.) P, Pillar of Telescope. T, Telescope. S, Finder or small telescope for pointing the telescope in position. a, a, b, Supports fastening the spectroscope to the telescope. d, Collimator or tube carrying the slit at the end nearest the telescope, and a lens at the other end to render the rays parallel. c, Plate on which the prisms are fixed. e, Small telescope through which the observer examines the spectrum after the ray has been dispersed in the prisms. h, Micrometer for measuring the relative distance of the lines.The spectroscope attached to the telescope for the examination of the sun. (Lockyer.)P, Pillar of Telescope. T, Telescope. S, Finder or small telescope for pointing the telescope in position.a,a,b, Supports fastening the spectroscope to the telescope.d, Collimator or tube carrying the slit at the end nearest the telescope, and a lens at the other end to render the rays parallel.c, Plate on which the prisms are fixed.e, Small telescope through which the observer examines the spectrum after the ray has been dispersed in the prisms.h, Micrometer for measuring the relative distance of the lines.
P, Pillar of Telescope. T, Telescope. S, Finder or small telescope for pointing the telescope in position.a,a,b, Supports fastening the spectroscope to the telescope.d, Collimator or tube carrying the slit at the end nearest the telescope, and a lens at the other end to render the rays parallel.c, Plate on which the prisms are fixed.e, Small telescope through which the observer examines the spectrum after the ray has been dispersed in the prisms.h, Micrometer for measuring the relative distance of the lines.
When an astronomer wishes to examine the spectrum of any special part of the sun, he takes off the eye-piece of his telescope and screws the spectroscope upon the draw-tube. The spectroscope ismade exactly like the large one for ordinary work. The tubed(Fig. 49) carries the slit at the end nearest the telescope, and this slit must be so placed as to stand precisely at the principal focus of the lens where the sun's image is formed (seei,i, p. 44). This comes to exactly the same thing as if we could put the slit close against the face of the sun, so as to show only the small strip which it covers, and by moving it to one part or another of the image we can see any point that we wish and no other. The light then passes through the tubedinto the round of prisms standing on the trayc, and the observer looking through the small telescopeesees the spectrum as it emerges from the last prism. In this way astronomers can examine the spectrum of a spot, or part of a spot, or of a bright streak, or any other mark on the sun's face.
Now in looking at the prominences we have seen that the difficulty is caused by the sunlight, between us and them, overpowering the bright lines of the gas, nor could we overcome this if it were not for a difference which exists between the two kinds of light. The more you disperse or spread out the continuous sun-spectrum the fainter it becomes, but in spreading out the bright lines of the gas you only send them farther and farther apart; they themselves remain almost as bright as ever. So, when the telescope forms an image of the red flame in front of the slit, though the glowing gas and the sunlight both send rays into the spectroscope, you have only to use enough prisms and arrange them in such a way thatthe sunlight is dispersed into a very long faint spectrum, and then the bright lines of the flames will stand out bright and clear. Of course only a small part of the long spectrum can be seen at once, and the lines must be studied separately. On the other hand, if you want to compare the strong light of the sun with the bright lines of the prominences, you place the slit just at the edge of the sun's image in the telescope, so that half the slit is on the sun's face and half on the prominence. The prisms then disperse the sunlight between you and the prominences, while they only lessen the strong light of the sun itself, which still shows clearly. In this way the two spectra are seen side by side and the dark and bright lines can be compared accurately together (see Fig. 50).