It is quite evident, from what we have said, that the spots, faculz and prominences are all results of one series of operations. This fact has been brought out in a very interesting way by the spectroheliograph. This is an instrument invented by George E. Hale of the Mount Wilson Observatory, which enables a photo graph of the sun to be taken by the light of a single ray of the spectrum, the light from all the other rays being cut off. A ray frequently used for the purpose is one emitted by calcium. When a photograph is taken with this ray, we have not a general photograph of the sun by all its light, which is what the ordinary camera would give us, but a photograph in which no light is allowed to enter except that emitted by the vapor of calcium. A photograph thus taken is found to be extremely variegated, some parts of the sun's disc being much brighter than others. The interesting feature is that, in the general average, the brightest parts of the disc are those where the spots, faculm, and prom inences are most numerous. But these bright patches may appear on any part of the sun. The general conclusion is that the three classes of objects we have described are all produced by processes going on in the sun which result in the throwing up of great masses of calcium vapor.
Having thus described what the sun is in a general way, and shown its appearances, we pass on to particulars respecting its constitution and surroundings. Our conclusions must be drawn step by step from the phenomena ex hibited by the photosphere, combined with rea soning upon the laws of force and the prop erties of matter. A conclusion most easily reached is that the sun is not a solid body throughout. If it were, its enormous radiation of heat would result in the surface rapidly cool ing off, so that, in a very short time, it would cease to emit light. Moreover, a study of the surface shows changes constantly going on in compatible with the idea of solidity. It follows that the heat radiated from the surface must be continually supplied, either by the rising up of hot material from the interior, or by radia tion from within outward. It is the latter which is believed to be acting. For the material of the sun is probably transparent, and it is sup posed that the enormous heat is absorbed and as we pass outward from the center.
The absorption and re-radiation from succes sive layers is almost instantaneous, the velocity of heat transference approaching 180,000 miles a second.
In drawing our conclusions the intensity of gravitation on the sun must be borne in mind. It has nearly 28 times the force of gravity on the earth. A man of ordinary size would weigh two tons at the surface of the sun, and would, therefore, be instantly crushed to death by his own weight, were it possible for him otherwise to exist there. Consequently, the pressure to
which the vapors of the sun are subject in creases with enormous rapidity below the surface.
The average specific gravity of the ma terials composing the sun can be determined by astronomical theory with great exactness. It is known that the mean specific gravity is about 40 per cent greater than that of water, and one-quarter that of the earth. It is doubt less much smaller than this at the surface, and, therefore, increases toward the centre. A cal culation of the resulting pressure shows that near the centre of the sun the pressure produced by the enormous mass and gravitation of the matter composing the solar orb amounts to about 5,000,000 tons per square inch. This pressure is so far beyond any that we can produce at the earth's surface that we are un able to say what effect it would have upon matter.
Yet another unknown factor is the tem perature of the interior. At no great distance toward the centre the temperature exceeds our •wers of determination — it may even be 1,000, is V'. As the highest temperature which it is possible to produce artificially probably does not amount to it is impossible to say what effect such a temperature would have upon matter. Thus we have two opposing causes, the one an inconceivable degree of heat, such that, were matter exposed to it on the surf ace of the earth, it would explode with a power to which nothing within our experience can be compared, and a pressure thousands of times any we can produce, tending to con dense and solidify this intensely heated matter. One thing which we can say with confidence as to the effect of these causes is that no chemical combinations can take place in matter so circumstanced. The distinction between liquid and gaseous matter is lost under such condi tions. Whether the central portions are com pressed into a solid, or remain liquid, it is im possible to say.
Modern research shows that the sun, as a whole, is a complex body, the various parts of which are in very different conditions. Begin ning at the centre and passing outward, we have first the vast, invisible interior which forms the globe itself, and which our sight can never pene trate. Surrounding this interior is the visible photosphere, or seeming surface, which we see with the naked eye or the telescope, the appear ance of which has been fully described. So far as ordinary direct observation could show, this would be the whole of the sun. But the spec troscope, as well as eye observation during total eclipses, has shown most complex surroundings of the sun, which would otherwise have been invisible. The surroundings are formed of two envelopes, the chromosphere and the corona.