Slice of volcanic gla.s.s 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 gla.s.s of two separate periods of crystallisation--the period when the large crystals grew in the liquid ma.s.s 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 tiny cavities 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 gla.s.sy lava before they have time to grow further.

If you will think a little for yourselves about these wonderful discoveries made with the magic-gla.s.s, 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.

CHAPTER VI

AN HOUR WITH THE SUN

[Ill.u.s.tration]

Before 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 called _World 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 t.i.tle "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 was a 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 star in 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 pa.s.s 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.

[1] _Fairyland of Science_, Chapter II.

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 gla.s.s inside the eye-piece, you will find that this shining disc is really a round globe, and moreover, although the object-gla.s.s of your telescopes measures only 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.

[Ill.u.s.tration: Fig. 45.

Face of the sun projected on a sheet of cardboard C. T, Telescope. _f_, Finder. _og_, Object-gla.s.s. _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-gla.s.s 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 pa.s.sed the eye-piece of the telescope through a large screen _s_, 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 the tube 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 pa.s.sing 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.

[Ill.u.s.tration: Fig. 46.

Photograph of the face of the sun, taken by Mr. Selwyn, October 1860, showing spots, faculae, 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 called _faculae_, or torches, which often appear in any region where a spot is forming, while the whole face of the sun appears mottled with bright and darker s.p.a.ces 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 the _photosphere_, 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 or _corona_, 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 s.p.a.ces 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.

[Ill.u.s.tration: 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.]

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 pa.s.ses on its way. These projections in the real sun are of a bright red colour, and they take on all manners of strange shapes, sometimes looking like ranges of fiery hills, sometimes like gigantic spikes and scimitars, sometimes even like branching fiery trees. They were called _prominences_ before 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 which comes 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 pa.s.s 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.[1]

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.

[1] Two rare earths, Erbia and Didymium, form an exception to this, but they do not concern us here.

[Ill.u.s.tration: 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[1] 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 gla.s.s 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.)

[1] A direct-vision spectroscope is like a small telescope with prisms arranged inside the tube. The object-gla.s.s end is covered by two pieces of metal, which slide backwards and forwards by means of a screw, so that a narrow or broad slit can be opened.

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 ma.s.s 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 those dark 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.

[Ill.u.s.tration: 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.]

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 hydrogen burning 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 lens _l_, and in front of this again the prism _p_. 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 a _dark line_ D 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 or _absorbed_ on the way, and consequently there was a blank, black s.p.a.ce in the spectrum where they should have been. If I could put a hydrogen flame cooler than the original light 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 vapour _of its own kind_ somewhere 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 that the 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 sh.e.l.l of luminous clouds around it, and in this sh.e.l.l, or _reversing layer_--as it is often called, because it turns light to darkness--we have proved that iron, lead, copper, zinc, aluminum, magnesium, pota.s.sium, 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.