The careful study of the solar phenomena has very clearly established the fact that none of the sun's envelopes, from the reversing layer to the corona itself, is in any sense an atmosphere. The combination of enormous gravitative force with an amount of heat which turns all the elements into the liquid or gaseous state, leads to consequences which it is difficult for us to follow or comprehend. There is evidently constant internal movement or circulation in the interior of the sun, resulting in the faculæ, the sun-spots, the intensely luminous photosphere, and the chromosphere with its vast flaming coruscations and eruptive protuberances. But it seems impossible that this incessant and violent movement can be kept up without some great and periodical or continuous inrush of fresh materials to renew the heat, keep up the internal circulation, and supply the waste. Perhaps the movement of the sun through space may bring him into contact with sufficiently large masses of matter to continually excite that internal movement without which the exterior surface would rapidly become cool and all planetary life cease. The various solar envelopes are the result of this internal agitation, uprushes, and explosions, while the vast white corona is probably of little more density than comets' tails, probably even of less density, since comets not unfrequently rush through its midst without suffering any loss of velocity. The fact that none of the solar envelopes are visible to us until the light of the photosphere is completely shut off, and that they all vanish the very instant the first gleam of direct sunlight reaches us, is another proof of their extreme tenuity, as is also the sharply defined edge of the sun's disc. The envelopes therefore consist partly of liquid or vaporous matter, in a very finely divided state, driven off by explosions or by electrical forces, and this matter, rapidly cooling, becomes solidified into minutest particles, or even physical molecules. Much of this matter continually falls back on the sun's surface, but a certain quantity of the very finest dust is continually driven away by electrical repulsion, so as to form the corona and the zodiacal light. The vast coronal streamers and the still more extensive ring of the zodiacal light are therefore in all probability due to the same causes, and have a similar physical constitution with the tails of comets.
As the whole of our sunlight must pass through both the reversing layer and the red chromosphere, its colour must be somewhat modified by them. Hence it is believed that, if they were absent, not only would the light and heat of the sun be considerably greater, but its colour would be a purer white, tending towards bluish rather than towards the yellowish tinge it actually possesses.
The Nebular and Meteoritic Hypotheses
As the constitution of the sun, and its agency in producing magnetism and electricity in the matter and orbs around it, afford us our best guide to the constitution of the stars and nebulæ, and to their possible action on each other, and even upon our earth, so the mode of evolution of the sun and solar system, from some pre-existing condition, is likely to help us towards gaining some knowledge of the constitution of the stellar universe and the processes of change going on there.
At the very commencement of the nineteenth century the great mathematician Laplace published his Nebular Theory of the Origin of the Solar System; and although he put it forth merely as a suggestion, and did not support it with any numerical or physical data, or by any mathematical processes, his great reputation, and its apparent probability and simplicity, caused it to be almost universally accepted, and to be extended so as to apply to the evolution of the stellar universe. This theory, very briefly stated, is, that the whole of the matter of the solar system once formed a globular or spheroidal mass of intensely heated gases, extending beyond the orbit of the outermost planet, and having a slow motion of revolution about an axis. As it cooled and contracted, its rate of revolution increased, and this became so great that at successive epochs it threw off rings, which, owing to slight irregularities, broke up, and, gravitating together, formed the planets. The contraction continuing, the sun, as we now see it, was the result.
For about half a century this nebular hypothesis was generally accepted, but during the last thirty years so many objections and difficulties have been suggested, that it has been felt impossible to retain it even as a working hypothesis. At the same time another hypothesis has been put forth which seems more in accordance with the facts of nature as we find them in our own solar system, and which is not open to any of the objections against the nebular theory, even if it introduces a few new ones.
A fundamental objection to Laplace's theory is, that in a gas of such extreme tenuity as the solar nebula must have been, even when it extended only to Saturn or Uranus, it could not possibly have had any cohesion, and therefore could not have given off whole rings at distant intervals, but only small fragments continuously as condensation went on, and these, rapidly cooling, would form solid particles, a kind of meteoric dust, which might aggregate into numerous small planets, or might persist for indefinite periods, like the rings of Saturn or the great ring of the Asteroids.
Another equally vital objection is, that, as the nebula when extending beyond the orbit of Neptune could have had a mean density of only about the two-hundred millionth of our air at sea level, it must have been many hundred times less dense than this at and near its outer surface, and would there be exposed to the cold of stellar space—a cold that would solidify hydrogen. It is thus evident that the gases of all the metallic and other solid elements could not possibly exist as such, but would rapidly, perhaps almost instantaneously, become first liquid and then solid, forming meteoric dust even before contraction had gone far enough to produce such increased rotation as would throw off any portion of the gaseous matter.
Here we have the foundations of the meteoritic hypothesis which is now steadily making its way. It is supported by the fact that we everywhere find proofs of such solid matter in the planetary spaces around us. It falls continually upon the earth. It can be collected on the Arctic and Alpine snows. It occurs everywhere in the deepest abysses of the ocean where there are not sufficient organic deposits to mask it. It constitutes, as has now been demonstrated, the rings of Saturn. Thousands of vast rings of solid particles circulate around the sun, and when our earth crosses any of these rings, and their particles enter our atmosphere with planetary velocity, the friction ignites them and we see falling stars. Comets' tails, the sun's corona, and the zodiacal light are three strange phenomena, which, though wholly insoluble on any theory of gaseous formation, receive their intelligible explanation by means of excessively minute solid particles—microscopic cosmic dust—driven outward by the tremendous electrical repulsions that emanate from the sun.
Having these and other proofs that solid matter, ranging in size, perhaps, from the majestic orbs of Jupiter and Saturn down to the inconceivably minute particles driven millions of miles into space to form a comet's tail, does actually exist everywhere around us, and by collisions between the particles or with planetary atmospheres can produce heat and light and gaseous emanations, we find a basis of fact and observation for the meteoritic hypothesis which Laplace's nebular, and essentially gaseous, theory does not possess.