Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

Fig. 1.—Saturn, Jan. 26, 1889 (Antoniadi).

As if to prove that the planets and moons of the solar system were formed somewhat in the manner in which we have described it, one of these spheres, Saturn, retains a ring, or rather a band which appears to be divided obscurely into several rings which lie between its group of satellites and the main sphere. How this ring has been preserved when all the others have disappeared, and what is the exact constitution of the mass, is not yet well ascertained. It seems clear, however, that it can not be composed of solid matter. It is either in the form of dust or of small spheres, which are free to move on each other; otherwise, as computation shows, the strains due to the attraction which Saturn itself and its moons exercise upon it would serve to break it in pieces. Although this ring theory of the formation of the planets and satellites is not completely proved, the occurrence of such a structure as that which girdles Saturn affords presumptive evidence that it is true. Taken in connection with what we know of the nebulæ, the proof of Laplace's nebular hypothesis may fairly be regarded as complete.

It should be said that some of the fixed stars are not isolated suns like our own, but are composed of two great spheres revolving about one another; hence they are termed double stars. The motions of these bodies are very peculiar, and their conditions show us that it is not well to suppose that the solar system in which we dwell is the only type of order which prevails in the celestial families; there may, indeed, be other variations as yet undetected. Still, these differences throw no doubt on the essential truth of the theory as to the process of development of the celestial systems. Though there is much room for debate as to the details of the work there, the general truth of the theory is accepted by nearly all the students of the problem.

A peculiar advantage of the nebular hypothesis is that it serves to account for the energy which appears as light and heat in the sun and the fixed stars, as well as that which still abides in the mass of our earth, and doubtless also in the other large planets. When the matter of which these spheres were composed was disseminated through the realms of space, it is supposed to have had no positive temperature, and to have been dark, realizing the conception which appears in the first chapter of Genesis, "without form, and void." With each stage of the falling in toward the solar centres what is called the "energy of position" of this original matter became converted into light and heat. To understand how this took place, the reader should consider certain simple yet noble generalizations of physics. We readily recognise the fact that when a hammer falls often on an anvil it heats itself and the metal on which it strikes. Those who have been able to observe the descent of meteoric stones from the heavens have remarked that when they came to the earth they were, on their surfaces at least, exceedingly hot. Any one may observe shining meteors now and then flashing in the sky. These are known commonly to be very small bits of matter, probably not larger than grains of sand, which, rushing into our atmosphere, are so heated by the friction which they encounter that they burn to a gas or vapour before they attain the earth. As we know that these particles come from the starry spaces, where the temperature is somewhere near 500° below 0° Fahr., it is evident that the light and heat are not brought with them into the atmosphere; it can only be explained by the fact that when they enter the air they are moving at an average speed of about twenty miles a second, and that the energy which this motion represents is by the resistance which the body encounters converted into heat. This fact will help us to understand how, as the original star dust fell in toward the centre of attraction, it was able to convert what we have termed the energy of position into temperature. We see clearly that every such particle of dust or larger bit of matter which falls upon the earth brings about the development of heat, even though it does not actually strike upon the solid mass of our sphere. The conception of what took place in the consolidation of the originally disseminated materials of the sun and planets can be somewhat helped by a simple experiment. If we fit a piston closely into a cylinder, and then suddenly drive it down with a heavy blow, the compressed air is so heated that it may be made to communicate fire. If the piston should be slowly moved, the same amount of heat would be generated, or, as we may better say, liberated by the compression, though the effect would not be so striking. A host of experiments show that when a given mass of matter is brought to occupy a less space the effect is in practically all cases to increase the temperature. The energy which kept the particles apart is, when they are driven together, converted into heat. These two classes of actions are somewhat different in their nature; in the case of the meteors, or the equivalent star dust, the coming together of the particles is due to gravitation. In the experiment with the cylinder above described, the compression is due to mechanical energy, a force of another nature.

There is reason for believing that all our planets, as well as the sun itself, and also the myriad other orbs of space, have all passed through the stages of a transition in which a continually concentrating vapour, drawn together by gravitation, became progressively hotter and more dense until it assumed the condition of a fluid. This fluid gradually parted with its heat to the cold spaces of the heavens, and became more and more concentrated and of a lower temperature until in the end, as in the case of our earth and of other planets, it ceased to glow on the outside, though it remained intensely heated in the inner parts. It is easy to see that the rate of this cooling would be in some proportion to the size of the sphere. Thus the earth, which is relatively small, has become relatively cold, while the sun itself, because of its vastly greater mass, still retains an exceedingly high temperature. The reason for this can readily be conceived by making a comparison of the rate of cooling which occurs in many of our ordinary experiences. Thus a vial of hot water will quickly come down to the temperature of the air, while a large jug filled with the fluid at the same temperature will retain its heat many times as long. The reason for this rests upon the simple principle that the contents of a sphere increase with its enlargement more rapidly than the surface through which the cooling takes place.

The modern studies on the physical history of the sun and other celestial bodies show that their original store of heat is constantly flowing away into the empty realms of space. The rate at which this form of energy goes away from the sun is vast beyond the powers of the imagination to conceive; thus, in the case of our earth, which viewed from the sun would appear no more than a small star, the amount of heat which falls upon it from the great centre is enough each day to melt, if it all could be put to such work, about eight thousand cubic miles of ice. Yet the earth receives only 1/2,170,000,000 part of the solar radiation. The greater part of this solar heat—in fact, we may say nearly all of it—slips by the few and relatively small planets and disappears in the great void.

The destiny of all the celestial spheres seems in time to be that they shall become cooled down to a temperature far below anything which is now experienced on this earth. Even the sun, though its heat will doubtless endure for millions of years to come, must in time, so far as we can see, become dark and cold. So far as we know, we can perceive no certain method by which the life of the slowly decaying suns can be restored. It has, however, been suggested that in many cases a planetary system which has attained the lifeless and lightless stage may by collision with some other association of spheres be by the blow restored to its previous state of vapour, the joint mass of the colliding systems once again to resume the process of concentration through which it had gone before. Now and then stars have been seen to flash suddenly into great brilliancy in a way which suggests that possibly their heat had been refreshed by a collision with some great mass which had fallen into them from the celestial spaces. There is room for much speculation in this field, but no certainty appears to be attainable.

The ancients believed that light and heat were emanations which were given off from the bodies that yielded them substantially as odours are given forth by many substances. Since the days of Newton inquiry has forced us to the conviction that these effects of temperature are produced by vibrations having the general character of waves, which are sent through the spaces with great celerity. When a ray of light departs from the sun or other luminous body, it does not convey any part of the mass; it transmits only motion. A conception of the action can perhaps best be formed by suspending a number of balls of ivory, stone, or other hard substance each by a cord, the series so arranged that they touch each other. Then striking a blow against one end of the line, we observe that the ball at the farther end of the line is set in motion, swinging a little away from the place it occupied before. The movement of the intermediate balls may be so slight as to escape attention. We thus perceive that energy can be transmitted from one to another of these little spheres. Close observation shows us that under the impulse which the blow gives each separate body is made to sway within itself much in the manner of a bell when it is rung, and that the movement is transmitted to the object with which it is in contact. In passing from the sun to the earth, the light and heat traverse a space which we know to be substantially destitute of any such materials as make up the mass of the earth or the sun. Judged by the standards which we can apply, this space must be essentially empty. Yet because motions go through it, we have to believe that it is occupied by something which has certain of the properties of matter. It has, indeed, one of the most important properties of all substances, in that it can vibrate. This practically unknown thing is called ether.

The first important observational work done by the ancients led them to perceive that there was a very characteristic difference between the planets and the fixed stars. They noted the fact that the planets wandered in a ceaseless way across the heavens, while the fixed stars showed little trace of changing position in relation to one another. For a long time it was believed that these, as well as the remoter fixed stars, revolved about the earth. This error, though great, is perfectly comprehensible, for the evident appearance of the movement is substantially what would be brought about if they really coursed around our sphere. It was only when the true nature of the earth and its relations to the sun were understood that men could correct this first view. It was not, indeed, until relatively modern times that the solar system came to be perceived as something independent and widely detached from the fixed stars system; that the spaces which separate the members of our own solar family, inconceivably great as they are, are but trifling as compared with the intervals which part us from the nearer fixed stars. At this stage of our knowledge men came to the noble suggestion that each of the fixed stars was itself a sun, each of the myriad probably attended by planetary bodies such as exist about our own luminary.