We can give a familiar illustration of gas in a state of extreme tenuity. Look at the beautiful incandescent light with which in these days our buildings are illuminated. How brilliantly those little globes shine! The globe has to be most carefully sealed against the outside air. If there were the smallest opportunity for access, the air from outside would rush in and the lamp would be destroyed. In the preparation of such a lamp elaborate precautions have to be taken to secure that the exhaustion of the air from the little globe shall be as nearly perfect as possible. Of course it is impossible to remove all the air. No known processes can produce a perfect vacuum. Some traces of gas would remain after the air-pump had been applied even for hours.
We must now imagine a globe, not merely two inches in diameter like one of these little lamps, but a globe 186,000,000 miles in diameter, a globe so large that the earth’s orbit would just form a girdle round it. Even if this globe had been exhausted, so that its density was only the twelve-thousandth part of the ordinary atmospheric density, it would still contain more material than is found in the sun in heaven. Thus our reasoning has conducted us to the notion of an epoch when the sun—or rather I should say the matter composing the sun—formed something totally different from the orb which we know so well. The matter in that very diffuse state would not dispense light and heat as a sun in the sense in which we understand the word. However vast might be the store of energy which it contained—a store indeed thousands of times greater than our present sun possesses—yet it would hardly be possessed of the power of effective radiation. It would assuredly not be able to warm and light a world associated with it, in the same way as the sun now provides so gloriously for our wants and comfort.
Fig. 21.—The Great Comet of 1882.
(Photographed on November 7th, 1882, by Sir David Gill, K.C.B.)
But it is certain that in those early days there was no earth to be warmed and lighted. Our globe, even if it can be said to have existed at all, was truly “without form and void.” At the time when the sun was swollen into a great globe of gas or rarefied matter, the elementary substances which were to form the future earth were in a condition utterly different from that of our present globe. The history of this earth itself involves another chapter of the argument. Let it suffice to notice, for the present, that our reasoning has led us to a time when the sun consisted only of a rarefied gaseous material, and let us give to the matter in this condition the name which astronomers apply to any object of a similar character wherever they may meet with it in the universe. Suppose that we could observe through our telescopes at the present moment an object in remote space which was like what the sun must have been at that early stage of its existence which we have been considering, I do not think that the object would be unfamiliar to astronomers. There is, indeed, no doubt that there are many objects visible at this moment, and nightly studied in our observatories, which are formed of matter just in the same state as the sun was in those early times. Examined with a good telescope, the object would seem like a small stain of light on the black background of the sky. The observer would at once call it a nebula. In these modern days he would probably apply the spectroscope to it, and this instrument would assure him that the object he was looking at was a mass of incandescent gas. Such an object would in all probability not greatly differ from many nebulæ now known to us.
This being so, why should we withhold from the sun of primitive days the designation to which it seems to be so fully entitled? Why should we not speak of it as a nebula? The application of the laws of heat has shown that the great orb of day was once one of those numerous objects which astronomers know as nebulæ, and perhaps it may not be too fanciful to suppose that a trace of the primæval nebula still survives in what we call the Solar Corona (Fig. [20]).
CHAPTER VIII.
THE EARTH’S BEGINNING.
The Earth to be Studied—A great Experiment—The Diamond Drill—A Boring upwards of a Mile Deep—A Mechanical Feat—The Scientific Importance of the Work—Increase of Temperature with the Depth—A special Form of Thermometer—Taking the Temperature in the Boring—The Level of Constant Temperature—The Rate of Increase of Temperature with the Depth—One degree Fahrenheit for every Sixty-six Feet in Depth—Temperatures at Depths above a Mile—Conclusions as to the Heat at very great Depths—The Heat developed by Tidal Action—This will not account for the Earth’s Internal Heat—The Earth must be continually Cooling—Inferences from the incessant loss of Heat from the Earth—The Earth’s Surface once Red-Hot, or Molten—The Earth must have originated from a Nebula—The Earth’s Beginning.
IN the last chapter we endeavoured to ascertain what can be learned from the radiation of the sun with regard to the history of the solar system. In this chapter we shall not consider any body in the heavens, but the condition of the earth itself. We have learned something of the history of the solar system from the celestial bodies; we shall now learn something about it in another way—from the condition of our globe at depths far beneath our feet.