This day's work differs from that of the preceding and succeeding days, in the fact that its sphere was without the earth, which was only indirectly influenced by it, and consequently the geological records give us no direct information upon the subject, though in two points they tally with the Mosaical account. In the first place, the deposits of coal, which preceded this period, indicate a time when a nearly uniform temperature, and that a high one, prevailed throughout the world. The coal beds are found not only in tropical regions, but in very high latitudes. Not only is the vegetation of which these coalfields are the result, analogous to that which is now found in warm climates only—(this might be the case, and yet we should not be justified in drawing the inference that the actual species of plants were tropical, for it often happens that different species of the same genus, having considerable external resemblance, are very different in their habits, some requiring tropical heat, while others flourish only in temperate climates)—but the marked feature is the astonishing luxuriance of this vegetation, which could only have been developed under the most favourable circumstances of warmth and moisture. Now the heat which any particular portion of the earth's surface receives from the sun depends entirely upon the latitude. hence it is impossible that a uniform high temperature could exist in a world which derived its heat wholly or chiefly from that source. Whether the high temperature which prevailed on the earth during the deposition of the coal measures was derived from internal heat it is impossible to say; it is evident that the temperature of the earth's surface has been in past times, and perhaps is now, modified by causes which no scientific research has been enabled to detect [Footnote: Since the sun's secular motion has been known, astronomers have suggested that the solar system has been carried through portions of space having variable temperatures. Geologists, however, do not seem inclined to accept this as a sufficient reason for the phenomena observed.]. But we may safely conclude that during the third day the earth did not derive its heat from the sun. The second point, the barrenness of the geological records of this period, will be noticed hereafter.
The record of the fourth day's work admits of two interpretations, it may describe things merely as they appeared, or as they actually occurred.
1. It is possible that the events of the fourth day may be described phenomenally—that up to this period the state of things on the earth had been to a great extent similar to that which we have reason to believe is still existing in the planet Jupiter- that the atmosphere was so charged with vapour that no direct rays from the heavenly bodies could penetrate it; but that at this time, owing to the declining heat, a great part of the aqueous constituents of this vapour had been precipitated in the form of rain, while other vapours had entered into chemical combinations with other elements to form the various minerals of the earth's surface, and the atmosphere had become first translucent, and then transparent. While this process was going on, no direct light from the sun, supposing it to be already in existence, could penetrate the veil. Diffused light only could reach the earth's surface, but when the atmosphere became clear the sun, moon, and stars would become visible.
Against this view several objections may be brought. In the first place, as has been already noticed, we cannot treat the account of the Creation as derived from ordinary human sources. Either it is a revelation from the Creator or it is nothing. Now we can readily admit that a man, speaking of an event which lie had witnessed, but did not understand, would describe it as it appeared to him, but we cannot admit this supposition when the work is described by the Great Artificer Himself. In the next place, the temperature of the earth's surface must in this case have been affected by the sun, and must therefore have been more or less dependent upon latitude—and in the third place the distinction between day and night must have come into operation, whereas the narrative implies that it was yet incomplete.
2. The other possible interpretation is, that at this period the concentration of light and heat in the sun was so far completed that he became the luminary of the system, which had hitherto derived its light and heat from other sources. Probably, for a long time, the internal heat of the planets may have been so great that they were a light to themselves. This state of things, however, must have come to an end before animal or vegetable life could have existed on their surface, but other ways exist, and are in operation in other parts of the universe, by which light and heat might have been supplied independently of the sun. That light which is now gathered up in the sun might for a long time have existed as a nebulous ring, similar to the well-known Ring Nebula in Lyra. Any planets existing within such a ring would probably derive from it sufficient light and heat. Or the nebulous matter, in a luminous state, while slowly advancing to concentration, might as yet have been so diffused as to fill a space in which the earth's orbit was included. In either case the earth would have received a uniform diffused light, without any alternations of night and day. It is of course impossible that we should be able to say whether there are any worlds in which such a state of things prevails at present. Up to this time, with one possible exception, [Footnote: "Sirius is accompanied by a 10 mag. star, whose existence was suspected (like that of Neptune), long before its discovery by Alvan Clark in 1861, from the irregular movements of its primary. But though it appears so small, its disturbing effects can only be accounted for on the supposition that its mass is at least half that of Sirius, in which case its light must be very faint, possibly wholly reflected." (Webb's Celestial Objects, p. 202.)] the only worlds which the telescope has revealed to us, beyond the limits of our own system, are self-luminous. No reflected light is strong enough to make its existence perceptible at such enormous distances in the most powerful telescope which has yet been constructed.
There are some facts connected with our own system which make it appear not improbable that up to the time of which we are speaking the light which is now gathered up in the sun was diffused over a space in which at all events the earth's orbit was included. It is now a recognized fact that all the light of the system is not as yet wholly concentrated in the sun, as we generally recognize it, but that to some extent the sun is still a nebulous star. Under ordinary circumstances we see only that circular disc, which we usually recognize as the sun. Its surpassing brightness overpowers every thing else, whether we view it with the unaided eye or through the telescope. But when the actual disc is hidden from us by the moon in a total eclipse, other regions of light surrounding the disc, make their appearance, and in them the most wonderful processes are continually going on. The simultaneous discoveries of Messrs. Lockyer and Janssen, in 1868, have enabled some of these processes to be continuously watched when the sun is not eclipsed, but others can as yet only be seen during the few minutes (never amounting to seven) which a total eclipse lasts, so that as yet we know very little of them.
Immediately surrounding the disc of the sun, which is visible to the naked eye, is a brilliant ring of light, known now as the chromosphere or sierra. This is the region which till 1868 could be seen only during total eclipses, but can now be watched at all times by means of the spectroscope. In it symptoms of intense action are from time to time witnessed. For many years past, whenever a total eclipse occurred, there were observed on the edge of this ring certain red prominences. The spectroscope has revealed their nature. They consist chiefly of enormous volumes of hydrogen, ejected from the surface of the sun with a velocity almost inconceivable, and at the same time revolving about their axis after the fashion of a cyclone. [Footnote: Popular Science Review, January, 1872, p. 150; Look. Byer's Lecture on the Sun, at Manchester, 1871.] A very remarkable instance of this was observed in America in September 1871, by Professor Young. A mass of incandescent hydrogen was propelled to a height of 200,000 miles above the visible disc; of these the last 100,000 miles were passed through in 10 minutes. Such events, though not commonly on so vast a scale, are continually occurring on the surface of the sun, and they seem to be in close connexion with the magnetic phenomena occurring on the earth.
Beyond the chromosphere lies the corona. The spectroscope has not yet rendered this visible at all times, and consequently we are dependent upon the information to be obtained during the few minutes of total eclipses, when alone it is visible. Consequently during recent solar eclipses this has been the point to which the attention of astronomers has been especially devoted. The eclipse of December, 1870, decided one point, that the corona was a truly solar phenomenon, and not, as some astronomers imagined, an optical phenomenon, produced by our own atmosphere. The corona presents the appearance of nebulous light, fading as it becomes more remote from the sun, of very irregular outline, at some points not extending more than 15', at others as much as 60' or 70' from the sun's disc, or, in other words, reaching to distances from the sun's surface varying from 400,000 to 1,800,000 miles. More important information has been obtained from the eclipse of December 12,1871. It is now ascertained that the corona comprises not only gaseous elements, especially hydrogen, but also solid or fluid particles, capable of giving a continuous though very faint spectrum with dark lines, indicating the existence of matter capable of reflecting light. The character of the coronal spectrum very much resembles that of the Nebula in Draco, No. 4373. The ascertained extent of the corona exceeds a million of miles above the surface of the sun, and it seems probable that the Zodiacal light is only a fainter extension of it. [Footnote: Popular Science Review, April, 1872, pp. 136-146.]
On a clear evening in the early spring months, as soon as twilight is completely ended, a conical streak of light may be sometimes seen, arising' from the western horizon, and extending through an arc of 60 or 70 degrees, nearly in the direction of the Ecliptic, and finally terminating in a point. This is the Zodiacal light. In tropical climates it is seen much more frequently, [Footnote: Humboldt, Kosmos, vol. i. p. 126 (Bohu's edition).] and is much more brilliant than in England. This then is probably an envelope of still fainter light than the corona. It must extend beyond the orbit of Venus, as the maximum elongation of Venus is 47 degrees, while the Zodiacal light has been traced for 70 degrees, and probably farther. It is very possible that the earth is occasionally involved in it, and that from it we derive that diffused light which, though faint, is very serviceable to us on a starless evening, and of which no other account has as yet been given. The light we receive in this way is often as powerful as that which we should receive from the stars if they were not hidden by clouds.
These phenomena seem to point to the conclusion that the condensation of light in the sun has been a very gradual process, which is even yet incomplete. If we suppose that at the time of the formation of the coal measures it was not far advanced, but that a diffused light extended beyond the orbit of the earth, similar in some respects to the present Zodiacal light, but equal in intensity to the light which we now see in the corona, the phenomena of the third day will be satisfactorily accounted for. There is, however, still an enormous amount of mystery connected with the sun. It is the centre from which an inconceivable amount of force in the shape of light, heat, actinism, and probably other manifestations, is hourly poured forth. If the whole of that force were divided into two thousand million parts, the portion received by the earth would be represented by one of those parts, and the whole amount received by all the planets would fall short of twelve of them. All the rest is radiated away into space, and so far as we know at present lost to the system. The question then arises, "How is this enormous expenditure supplied?" Various sources of heat have been suggested, but none of them seem satisfactory. One conceivable source there is, but that lies out of the domain of science. Then again, metals, which only our most powerful furnaces will even melt, exist in the sun's atmosphere in the state of vapour. What must be the intensity of the heat which underlies that metallic atmosphere? and what can be the solid or fluid substances which, from the continuity of the spectrum, we know must exist there?