tag="{http://www.w3.org/1999/xhtml}a">[6]

“The huge inert globe of permanently combined elements below, and the equally unchanging realm of the ether above,” offer no home for the living organism; least of all for the highest of such organisms—Man. Both must be tempered to a condition which will permit and favour continual change, the metabolism which is the essential feature of life.

“When the earth had to be prepared for the habitation of man, a veil, as it were, of intermediate being was spread between him and its darkness, in which were joined, in a subdued measure, the stability and the insensibility of the earth, and the passion and perishing of mankind.

“But the heavens, also, had to be prepared for his habitation. Between their burning light,—their deep vacuity, and man, as between the earth’s gloom of iron substance, and man, a veil had to be spread of intermediate being;—which should appease the unendurable glory to the level of human feebleness, and sign the changeless motion of the heavens with the semblance of human vicissitude. Between the earth and man arose the leaf. Between the heaven and man came the cloud. His life being partly as the falling leaf and partly as the flying vapour.”[7]

The leaf and the cloud are the signs of a habitable world. The leaf—that is to say, plant life, vegetation—is necessary because animal life is not capable of building itself up from inorganic material. This step must have been previously taken by the plant. The cloud, that is to say water-vapour, is necessary because the plant in its turn cannot directly assimilate to itself the nitrogen from the atmosphere. The food for the plant is brought to it by water, and it assimilates it by the help of water. It is, therefore, upon the question of the presence of water that the question of the habitability of a given world chiefly turns. In the physical sense, man is “born of water,” and any world fitted for his habitation must “stand out of the water and in the water.”

CHAPTER III

THE SUN

The Sun is, of all the heavenly bodies, the most impressive, and has necessarily, at all times, attracted the chief attention of men. There are only two of the heavenly bodies that appear to be more than points of light, only two that show a surface to the naked eye, and the Sun, being so much the brighter of the two, and the obvious source of all our light and heat, and the fosterer of vegetation, readily takes the premier place in interest. In the present day we know too much about the Sun for anyone to suppose that it can be the home of organic life; but it is not many years since its habitability was seriously suggested even by so high an authority as Sir William Herschel. He conceived that it was possible that its stores of light and heat might be confined to a relatively thin shell in its upper atmosphere, and that below this shell a screen of clouds might so check radiation downward that it would be possible for an inner nucleus to exist which should be cool and solid. This fancied inner globe would then necessarily enjoy perpetual daylight, and a climate which knew no variation from pole to pole. To its inhabitants the entire heavens would be generally luminous, the light not being concentrated into any one part of the vault; and it was supposed that, ignorant of time, a happy race might flourish, cultivating the far-spread solar fields, in perpetual daylight, and in the serenity of a perpetual spring that was distracted by no storm.

The picture thus conjured up is a pleasing one, though probably, to the restless sons of Earth, it would seem to suffer somewhat from monotony. But we now know that it corresponds in not a single detail to the actual facts. The study of solar conditions carried on through the last hundred years has revealed to us, not serenity and peace, but storm, stress, and commotion on the most gigantic scale. But though we now can dismiss from our minds the possibility that the Sun can be inhabited, yet it is of such importance to the maintenance of life on this planet, and by parity of reasoning to life on any other planet, that a review of its conditions forms a necessary introduction to our subject. Further, those conditions themselves will bring out certain principles that are of necessary application when we come to consider the case of particular planets.

The distance of the Sun from the Earth is often spoken of as the “astronomical unit”; it is the fundamental measure of astronomy, and all our information as to the sizes and distances of the various planets rests upon it. And, as we shall shortly see, the particular problem with which we are engaged—the habitability of worlds—is directly connected with these two factors: the size of the world in question, and its distance from the Sun.

The distance of the Sun has been determined by several different methods the principles of which do not concern us here, but they agree in giving the mean distance of the Sun as a little less than 93,000,000 miles; that is to say, it would require 11,720 worlds as large as our own to be put side by side in order to bridge the chasm between the two. Or a traveller going round the Earth at its equator would have to repeat the journey 3730 times before he had traversed a space equal to the Sun’s distance.

But knowing the Sun’s distance, we are able to deduce its actual diameter, its superficial extent, and its volume, for its apparent diameter can readily be measured. Its actual diameter then comes out as 866,400 miles, or 109·4 times that of the Earth. Its surface exceeds that of the Earth 11,970 times; its volume, 1,310,000 times.

But the weight of the Sun is known as well as its size; this follows as a consequence of gravitation. For the planets move in orbits under the influence of the Sun’s attraction; the dimensions of their orbits are known, and the times taken in describing them; the amount of the attractive force therefore is also known, that is to say, the mass of the Sun. This is 332,000 times the mass of the Earth; and as the latter has been determined as equal to about

6,000,000,000,000,000,000,000 tons

that of the Sun would be equal to

2,000,000,000,000,000,000,000,000,000 tons.

It will be seen that the proportion of the volume of the Sun to that of the Earth is greater than the proportion of its mass to the Earth’s mass—almost exactly four times greater; so that the mean density of the Sun can be only one-fourth that of the Earth. Yet, if we calculate the force of gravity at the surfaces of both Sun and Earth, we find that the Sun has a great preponderance. Its mass is 332,000 times that of the Earth, but to compare it with the attraction of the Earth’s surface we must divide by (109·4)², since the distance of the Sun’s centre from its surface is 109·4 times as great as the corresponding distance in the case of the Earth, and the force of gravity diminishes as the square of the increased distance. This gives the force of gravity at the solar surface as 27·65 times its power at the surface of the Earth, so that a body weighing one ton here would weigh 27 tons 13 cwt. if it were taken to the Sun.[8]

This relation is