garded as cool, but cool only in comparison with the more intense heat of the photosphere. In reality they are heated to luminosity, and but for the intense glare of the sun its atmosphere of gases and metallic vapors could be seen surrounding its disc as a ring of rosy light. It may actually be seen as such at the time of a total eclipse, during the few moments when the sun's light is cut off by the intervening moon.

The chromosphere, as this rose-red layer of atmosphere is called, is estimated to be some five thousand miles in depth. Although ordinarily invisible through a telescope, it can be observed at any time with a spectroscope properly adjusted. It is from this chromosphere that are projected the red "prominences," which are among the most interesting of the solar phenomena. They may be described as enormous flame-like protuberances, always to be found in greater or less number around the sun's limb, and which are often in a state of the most violent agitation, changing completely their forms in the course of a few minutes, shooting up to enormous distances-20,000, 40,000, even 80,000 miles being altitudes not unfrequently attained--flickering, breaking up into fragments, and slowly fading away. No brief description of these remarkable objects can convey more than a vague idea of the endless variety of forms, often fantastic, that they are seen to assume. They are among the most striking indications that the exterior and, presumably, the interior of the sun is in a continual state of commotion inconceivable in its violence. Occasionally these prominences attain to astonishing dimensions, and exhibit velocities that are enormous, even measured by a celestial standard. On June 17, 1891, M. Fenyi, at Kaloesa, in Hungary, observed a prominence which shot out vertically from the sun in a single mass and attained to an altitude of over 110,000 miles. The velocity of its ascent was estimated to be not less than 375 miles a second. The same prominence was seen by M. Trouvelot, at Paris, who estimated its velocity, taking into the account its motion in the line of sight, at over 600 miles a second! It can be calculated that matter moving at this enormous rate would be carried entirely away from the sun. There is, therefore, a mys

* Prominences, E. B.. Vol II. 789.

tery about these apparent motions which remains yet to be explained.

The spectroscope shows that these enormous tongues of flame are wholly gaseous, hydrogen and calcium being present through their whole extent, while at their bases are the heavier vapors of metals. Although they can be seen only around the edge of the sun's disc, they are, doubtless, continually flaring up over its entire surface.

Perhaps the most remarkable work accomplished with the aid of the spectroscope and photography is that done by Professor Hale, at the Kenwood Observatory, Chicago, by means of an instrument, invented by himself, which he has named the spectro-heliograph. It is impracticable to attempt to describe the instrument here; it can only be told what it does. Professor Hale photographs the sun, but by excluding from his camera all of the sun's light except that which comes from two selected points in the spectrum-the H and K lines of the spectrum, which are attributed to calcium he gets a picture which represents only objects that are gaseous. There is upon

his picture no trace of the dazzling photosphere. The only objects that appear upon it are the gaseous luminous clouds which float above the photosphere. They are those faculæ which appeared on our image of the sun as white blotches, and which are thus shown by the spectroheliograph to be gaseous, and, furthermore, they are now found to spread themselves over the whole face of the sun and not to be confined to the neighborhood of its limb. They probably float in the sun's atmosphere at a considerable elevation above the photosphere, and in this respect they may be likened to terrestrial clouds floating high above the earth's surface. The same instrument which seizes upon these gaseous luminous masses portrays also the solar promi

nences.

What connection there is, if any, between the prominences and the faculæ and between both and the dark spots, is a question still open. This much is known, however. The sun-spots are confined to two well-marked zones of the sun-zones which correspond, roughly speaking, to the two temperate zones of the earth. They are never found in either the equatorial or the polar regions. The faculæ, though not confined entirely

to the same zones, are, however, the most abundant over these parts of the sun. The prominences, on the contrary, are not confined to any particular region, but are found at any and every part of the sun's surface.

Our subject cannot be dismissed without a reference to the corona;* but so little is known about it at the present time that it can be disposed of in a few words. What is known as the corona is a halo of light that is seen about the sun at the time of a total eclipse, and which can be seen at that time only. Except in the immediate neighborhood of the sun its light is so exceedingly faint and nebulous. that observers who have attempted to depict it with the pencil have rarely agreed as to its appearance or as to the distance from the sun to which it was traceable. It is, however, agreed by all observers that it is very irregular in outline and that it presents certain details of structure, jets and sprays of various forms, which indicate that whatever it is, whether it consists of matter ejected from the sun or of meteoric matter, or of meteoric matter, which has been gathered by the attraction of the sun, its particles are in a state of motion. It appears to be partly gaseous and self-luminous and partly to shine by reflected solar light.

The intensity of the sun's light was estimated by Herschel to be 146 times that of the calcium light, and a more recent estimate makes it four times that of the electric light. As to the sun's temperature, the estimates have ranged from thousands to millions of degrees, according to the law of the increase of temperature which different investigators have thought the most applicable to the case. The latest investigations make it somewhere from 5,000 to 7,000 degrees centigrade. Perhaps the most that can be said with confidence upon this point is that the temperature of the sun at its surface undoubtedly far transcends the greatest heat which it has yet been possible to obtain in the laboratory with the electric arc. Its interior must, on any reasonable supposition, be fully as heated in its exterior. It is the general opinion among astronomers that its elemental substances are all in a gaseous condition, although in its exterior, owing to the enormous compression to which they

*Corona, E. B. Vol. II., 789.

must be subjected, these gases must be in a condition very unlike the free gases with which we are familiar. Very likely they are so compressed as to be in a pasty condition. In the interior of so hot a furnace none of the chemical compounds could exist, but all of the solar elements are probably in a state of "dissociation." Some solar physicists-chief among them Mr. J. Norman Lockyergo farther and hold that it is probable that many of the "elements" of terrestrial chemistry are brokeu up and become still more elemental in the laboratory of the sun.

The source of the sun's heat supply has been a subject of much speculation. The science of geology furnishes conclusive evidence that for tens of thousands of years, perhaps for millions of years, the sun has furnished the earth with about the same annual supply of heat as it does now, Yet had it been a mere cooling body, radiating its heat into space as lavishly as it does to-day, it would have cooled down to opaqueness in a few thousand years. Whence, then, does it renew its supply of expended heat? The theory which is now held to afford the most complete and satisfactory answer to this question was first advanced by Helmholz. It is that the sun is now and has been for ages past slowly contracting its dimensions, and that the heat evolved by this mechanical action is precisely that which it is continually radiating into space. Sir William Thompson calculated that a contraction which diminished the sun's diameter no more than four miles in a century would fully account for its heat-expenditure, enormous as it is. The sun might contract at this rate for several thousand years before any diminution of its size would become apparent.

But this process cannot go on forever. A time must come when the sun will have become so dense a body that it can contract no farther. Its source of heat failing, it must begin to cool off. Its light will pale. It will become in time a red body, its dense atmosphere having become charged with watery vapors, and finally it will cease altogether to be luminous. We may, however, comfort ourselves with the assurance of Sir William Thompson that it will probably shine with little diminution of its light and heat for twenty million years to come.

HE answers to SELF CULTURE questions for March will be found in Vol. VIII. of the new Encyclopædia Britannica.

March 1. When did the science of electricity first attract experimenters, and what is its present state? 3, and 616 Supplement.

March 2. In what respects do the Eskimo differ from the other aborigines of America, the so-called Indians? 543 March 3. How old is the art of embroidery? 160 March 4. What are the distinct epochs of English history? 263 March 5. What are the chief structural peculiarities of the normal eye? 816

March 6. To what extent was Euclid a discoverer and to what a compiler of geometrical truths? 655

March 24. What is the history of the English Bible?

March 8. Why and how did the Egyptians embalm their dead? 158

March 9. What successes in war caused Prince Eugene to be esteemed the greatest general of his age?

381 March 25. What distinguished Erasmus above all contemporary workers in his field? 512

659

466 March 27. Wherein did Euripides, the celebrated Greek dramatist, specially excel? 673 March 28. In what respects does encaustic painting differ from oil painting?

185