THE SUN'S LIGHT
By Sir Robert Ball,
Lowndean Professor of Astronomy and Geometry at Cambridge, England; formerly Royal Astronomer of Ireland.
HE light of the great orb of day emanates solely from a closely fitting robe of surpassing brightness. The great bulk of the sun which lies within that brilliant mantle is comparatively obscure, and might at first seem to play but an unimportant part so far as the dispensing of light and heat is concerned. It may indeed be likened to the coal-cellar from whence are drawn the supplies that produce the warmth and brightness of the domestic hearth; while the brilliant robe where the sun develops its heat corresponds to the grate in which the coal is consumed. With regard to the thickness of the robe, we might liken this brilliant exterior to the rind of an orange, while the gloomy interior regions would correspond to the edible portion of the fruit. Generally speaking, the rind of the orange is rather too coarse for the purpose of this illustration. It might be nearer the truth to affirm that the luminous part of the sun may be compared to the delicate filmy skin of the peach. There can be no doubt that if this glorious veil were unhappily stripped from the sun, the great luminary would forthwith lose its powers of shedding forth light and heat. The spots which we see so frequently to fleck the dazzling surface, are merely rents in the brilliant mantle through which we are permitted to obtain glimpses of the comparatively non-luminous interior.
As the ability of the sun to warm and light this earth arises from the peculiar properties of the thin glowing shell which surrounds it, a problem of the greatest interest is presented in an inquiry as to the material composition of this particular layer of solar substance. We want, in fact, to ascertain what that special stuff can be which enables the sun to be so useful to us dwellers on the earth. This great problem has been solved, and the result is extremely interesting and instructive; it has been discovered that the material which confers on the sun its beneficent power is also a material which is found in the greatest abundance on the earth, where it fulfils purposes of the very highest importance. Let us see, in the first place, what is the most patent fact with regard to the structure of this solar mantle possessed of a glory so indescribable. It is perfectly plain that it is not composed of any continuous solid material. It has a granular character which is sometimes perceptible when viewed through a powerful telescope, but which can be seen more frequently and studied more satisfactorily on a photographic plate. These granules have an obvious resemblance to clouds; and clouds, indeed, we may call them. There is, however, a very wide difference between the solar clouds and those clouds which float in our own atmosphere. The clouds which we know so well are, of course, merely vast collections of globules of water suspended in the air. No doubt the mighty solar clouds do also consist of incalculable myriads of globules of some particular substance floating in the solar atmosphere. The material of which these solar clouds are composed is, however, I need hardly say, not water, nor is it anything in the remotest degree resembling water. Some years ago any attempt to ascertain the particular substance out of which the solar clouds were formed would at once have been regarded as futile; inasmuch as such a problem would then have been thought to lie outside the possibilities of human knowledge. The advance of discovery has, however, shed a flood of light on the subject, and has revealed the nature of that material to whose presence we are indebted for the solar beneficence. The detection of the particular element to which all living creatures are so much indebted is due to that distinguished physicist, Dr. G. Johnstone Stoney.
In the whole range of science, one of the most remarkable discoveries ever made is that which has taught us that the elementary bodies of which the sun and the stars are constructed are essentially the same as those of which the earth has been built. This discovery was indeed as unexpected as it is interesting. Could we ever have anticipated that a body ninety-three millions of miles away, as the sun is, or a hundred million of millions of miles distant, as a star may be, should actually prove to have been formed from the same materials as those which compose this earth of ours and all which it contains, whether animate or inanimate? Yet such is indeed the fact. We are thus, in a measure, prepared to find that the material which forms the great solar clouds may turn out to be a substance not quite unknown to the terrestrial chemist. Nay, further, its very abundance in the sun might seem to suggest that this particular material might perhaps prove to be one which was very abundant on the earth.
THE SUN'S CORONA.
From a photograph taken by Professor Schaeberle, at Mina Bronces, Chili, in April, 1893, and kindly loaned by Professor E.S. Holden, director of the Lick Observatory.
I had occasion to make use of the word carbon in a lecture which I gave a short time ago, and I thought when I did so that I was of course merely using a term with whose meaning all my audience must be well acquainted. But I found out afterwards that in this matter I had been mistaken. I was told that my introduction of the word carbon had quite puzzled some of those who were listening to me. I learned that a few of those who were unfamiliar with this word went to a gentleman of their acquaintance who they thought would be likely to know, and begged from him an explanation of this mysterious term; whereupon he told them that he was not quite sure himself, but believed that carbon was something which was made out of nitro-glycerine! Even at the risk of telling what every schoolboy ought to know, I will say that carbon is one of the commonest as well as one of the most remarkable substances in nature. A lump of coke only differs from a piece of carbon by the ash which the coke leaves behind when burned. As charcoal is almost entirely carbon, so wood is largely composed of this same element. Carbon is indeed present everywhere. In various forms carbon is in the earth beneath our feet, and in the air which we breath. This substance courses with the blood through our veins; it is by carbon that the heat of the body is sustained; and the same element is intimately associated with life in every phase. Nor is the presence of carbon merely confined to this earth. We know it abounds on other bodies in space. It has been shown to be eminently characteristic of the composition of comets. Carbon is not only intimately associated with articles of daily utility, and of plenteous abundance, but with the most exquisite gems of "purest ray serene." More precious than gold, more precious than rubies, the diamond itself is no more than the same element in crystalline form. But the greatest of all the functions of carbon in the universe has yet to be mentioned. This same wonderful element has been shown to be in all probability the material which constitutes those glowing solar clouds to whose kindly radiation our very life owes its origin.
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THREE VIEWS OF AN ERUPTIVE PROMINENCE OF THE SUN.
From photographs taken at Kenwood Observatory, Chicago, March 25, 1895, and kindly loaned by Professor George E. Hale, of the Chicago University.
In the ordinary incandescent electric lamp, the brilliant light is produced by a glowing filament of carbon. The powerful current of electricity experiences so much resistance as it flows through this badly conducting substance, that it raises the temperature of the carbon wire so as to make it dazzlingly white-hot. Indeed the carbon is thus elevated to a temperature far in excess of that which could be obtained in any other way. The reason why carbon is employed in the electric lamp, in preference to any other substance, may be easily understood. Suppose we tried to employ an iron wire as the glowing filament within the well-known glass globe. Then when the current was turned on that iron would of course become red-hot and white-hot; but ere a sufficient temperature had been attained to produce the requisite illumination, the iron wire would have been fused into drops of liquid, the current would have been broken, and the lamp would have been destroyed. Nor would the attempt to make an incandescent lamp have proved much more successful had the filament been made of any other metal. The least fusible of metals is the costly element platinum, but even a wire of platinum, though it would stand much more heat than a wire of iron or of steel, would not have retained the solid form by the time it had been raised to the temperature necessary for an incandescent lamp.
There is no known metal, and perhaps no substance whatever, which demands so high a temperature to fuse it as does the element carbon. A filament of carbon, and a filament of carbon alone, will remain unfused and unbroken when heated by the electric current to the dazzling brilliance necessary for effective illumination. This is the reason why this particular element is so indispensable for our incandescent electric lamps. Modern research has now taught us that, just as the electrician has to employ carbon as the immediate agent in producing the brightest of artificial lights down here, so the sun in heaven uses precisely the same element as the immediate agent in the production of its transcendent light and heat. Owing to the extraordinary fervor which prevails in the interior parts of the sun, all substances there present, no matter how difficult we may find their fusion, would have to submit to be melted, nay, even to be driven off into vapor. If submitted to the heat of this appalling solar furnace, an iron poker, for instance, would vanish into invisible vapor. In the presence of the intense heat of the inner parts of the sun, even carbon itself is unable to remain solid. It would seem that it must assume a gaseous form under such circumstances, just as the copper and the iron and all the other substances do which yield more readily than it to the fierce heat of their surroundings.
The buoyancy of carbon vapor is one of its most remarkable characteristics. Accordingly immense volumes of the carbon steam in the sun soar at a higher level than do the vapors of the other elements. Thus carbon becomes a very large and important constituent of the more elevated regions of the solar atmosphere. We can understand what happens to these carbon vapors by the analogous case of the familiar clouds in our own skies. It is true, no doubt, that our terrestrial clouds are composed of a material totally different from that which constitutes the solar clouds. The sun evaporates the water from the great oceans which cover so large a proportion of our earth. The vapor thus produced ascends in the form of invisible gas through our atmosphere, until it reaches an altitude thousands of feet above the surface of the earth. The chill that the watery vapor experiences up there is so great that the vapor collects into little liquid beads, and it is, of course, these liquid beads, associated in countless myriads, which form the clouds we know so well.
We can now understand what happens as the buoyant carbon vapors soar upwards through the sun's atmosphere. They attain at last to an elevation where the fearful intensity of the solar heat has so far abated that, though nearly all other elements may still remain entirely gaseous, yet the exceptionally refractory carbon begins to return to the liquid state. At the first stage in this return, the carbon vapor conducts itself just as does the ascending watery vapor from the earth when about to be transformed into a visible cloud. Under the influence of a chill the carbon vapor collects into a myriad host of little beads of liquid. Each of these drops of liquid carbon in the glorious solar clouds has a temperature and a corresponding radiance vastly exceeding that with which the filament glows in the incandescent electric lamp. When we remember further that the entire surface of our luminary is coated with these clouds, every particle of which is thus intensely luminous, we need no longer wonder at that dazzling brilliance which, even across the awful gulf of ninety-three millions of miles, produces for us the indescribable glory of daylight.
Sir Robert Ball will contribute a series of articles on "The Marvels of the Universe." Six or eight of these articles may be expected during the coming year.
