The Birth Of The Earth

( Originally Published 1915 )

HOW did the little globe on which we live come into existence in the midst of the universe? Under what circumstances did it originate? What were the causes which determined its constitution, its primaeval form? In other words, what were the conditions of its conception and its birth?

These questions, which arise naturally at the beginning of this work, must always have been present to the mind of man. But it is only comparatively recently that sufficient light has been thrown upon them, if not to solve them completely, at any rate to enable us to obtain some idea of the forces which were at work, and the conditions which obtained, during the inception of our Earth, and during its formation as an individual world in the midst of space.

If, on a clear dark night, we look upwards at that expanse which we call the sky, and which seems to be a great dome supported upon the horizon, we see there a wonderful spectacle, which appears, at the first glance, absolutely in-extricable. Attentive observation of the heavens has, however, enabled astronomers to classify the phenomena in evidence, and, at the same time, to simplify the difficult problem which constitutes their study.

In the first place, among the innumerable brilliant points which stud the sky, there are some which scintillate and others which shine with a steady light. However great the magnification secured by the telescope utilised, the former appear only as geometrical' points; these are the stars which seem to rotate with a continuous movement about an imaginary line passing through a definite point in the heavens. On the other hand, the latter, which are the planets, appear larger with each increase of power of the instruments that we use to study them. Their movements differ from those of the stars; they revolve around a great and brilliant globe, the Sun.

The Sun itself appears to revolve about us, bringing the day when it is visible and leaving us to night when it disappears beneath the horizon, the truth being, however, that it is actually the Earth which moves, turning about its own axis.

A second globe which shines with brilliancy only during the night, and then only at certain times and under variable aspects, is more immediately de-pendent upon the Earth around which it revolves; this is the Moon.

Other phenomena, also, are exhibited by the celestial bodies. In some places, a large number of very small stars are gathered together so that, while they are all distinct from one another, they form a kind of luminous spot which is called a star-cluster; in other places are seen regions shining with a milky luminosity, distinct from the background of dark sky, having various forms, indefinite, spiral, or circular. These are the nebulae. An immense whitish track of light traverses and encircles the entire sky; this is called the Milky Way and is an aggregation of a very great number of little stars, of which our Sun doubtless forms one, gigantic for us, microscopic as compared to the immensity of space.

Transitory bodies appear suddenly in the heavens; these are the meteors which resemble fire-works, and which come often in considerable numbers and at fixed epochs; they are called " shooting stars." At certain times, special bodies appear with a luminous nucleus and a long tail. These are comets and their periods are often very long. Occasionally, new stars blaze up suddenly and take on temporarily the appearance of the fixed stars. Their brilliancy increases, remains for a time stationary, and then gradually decreases prior to their disappearance.

These are the simplest observational facts that astronomers have learned. If we now examine the heavenly bodies, not with an ordinary telescope, but with that wonderful apparatus known as the spectroscope, which, by splitting up light into its component rays, enables us to analyse it with great precision, we are able to prove that, at any rate, as far as the Sun and the Earth are concerned, they are composed, for the most part, of the same chemical elements. This fact, alone, is suggestive of their having had a common origin. Some elements which, formerly, were known to exist only in the Sun have since been discovered in the matter of which our Earth is formed.

Our globe, by the character of its movements, is analogous to the planets, which are all of a spheroidal form and are isolated in space, with a movement of revolution around the Sun. Now, voyages of circumnavigation, in the course of which the sailors have made the entire journey round the Earth, demonstrate to us that it also is isolated in space. Again, the circular form of the shadow which it throws upon the Moon during the eclipses of that body proves its globular form. There is, therefore, one conclusion that we can draw from our preliminary study. The Earth is a planet belonging to the Solar System, constituted of the same elements as the central Sun of that System.

Another piece of information that the spectroscope affords us is that the Sun and the stars are self-luminous bodies, while the planets and the Moon are illuminated bodies which appear bright to us because they reflect the Sun's light and not because they emit light themselves. The self-luminous stars and the Sun must therefore be at a very high temperature, in the midst of celestial space, which the investigations of astrophysicists have shown to be very cold. Just as when the colour of a piece of iron heated in a forge passes, gradually as it cools, from dazzling white to yellow, then to orange, to bright red, and lastly to dull red, we can, from the colour, deduce the temperature of the iron, so from the colour of a star we can obtain information as to its temperature and age. The blue and white stars are in the height of their period of incandescence. The yellow stars have already cooled to some extent ; this is the condition of our Sun. The yellow and orange stars are already on the way towards the period of a more advanced cooling which, after the red phase, leads to the solidification of their surfaces, rendering these obscure and transforming them into extinct suns.

Among the facts observed in the sky and which seem to overwhelm the reason, we must place first that admirable mechanism which maintains the bodies isolated in space and which causes some of them to revolve around others.

This wonderful adjustment was a mystery to man, until Kepler formulated the laws under which the planetary orbits are described and Newton discovered and enunciated the law of universal gravitation, from which Kepler's laws can be deduced mathematically.

Kepler found that the planetary orbits are not exactly circular, but are ellipses of which the Sun occupies one of the foci. This is the first law, from which it follows that the distance from the centre of the planet to the centre of the Sun, the radius vector, to give it its mathematical name, is not a fixed length but varies according to the position of the body in its orbit. Kepler's second law deals with this variation in the length of the radius vector; it states that the area swept over by the radius vector in any given time is always the same, whatever the position of the planet in its orbit. This law governs the velocity with which a planet pursues its path; it moves more rapidly in proportion as it is nearer the Sun, and more slowly in proportion as it is farther from the Sun. The third law governs the time that the planets take to traverse their orbits entirely; the square of the period is proportional to the cube of the major axis of the elliptical orbit in the case of each planet.

From these laws Newton was able to read some-thing more than the principles of the movements that they enunciated. He divined from them the cause of these planetary movements and he gave the great and simple formula determining these in his law of attraction or universal gravitation.

"Any two bodies attract each other with a force directly proportional to the product of their masses and inversely proportional to the square of their distance apart."

This law in giving the principle of force gives also that of the movement produced by the force. It can be demonstrated by mechanics that Kepler's laws can be rigorously deduced from the law of gravitation.

We now know another fundamental law, the existence and the value of the pressure of radiation. It is by radiation, of which luminous radiation is the most obvious kind, that we are aware of the existence of the distant worlds with which infinite space is strewn. Apart from the luminous radiations perceived by the retina of the eye, there are other kinds which that organ is incapable of distinguishing. If a ray of sunlight be passed through a prism of quartz, and if the polychromatic image that is thus observed, and which is called a spectrum, be photographed it is found that the photographic image is prolonged some way beyond the extreme violet perceptible to the eye. There are thus ultra-violet rays of which our senses cannot inform us, but which the photographic plate registers. A very sensitive thermometer, placed in the region which precedes the red rays, discloses there infra-red rays imperceptible to the sight.

These radiations, to which must be added electric radiations and other forms that will probably be discovered in the future, are the means whereby the forces of the universe are transmitted. The great physicist Maxwell first discovered, in 1873, that radiation exerts a veritable pressure, the power of which is measured by the quantity of energy contained in a unit volume of the medium transmitting it. Maxwell feared that the smallness of this pressure might render its measure impossible, but to the Russian physicist Labedeff is due the credit of achieving this difficult measurement. If we imagine a black body placed against the Sun's surface, the radiation emitted by this surface exercises a pressure of 2.75 milligrammes [.o42 gr.] per square centimetre [.155 sq. in.] on the body. The illustrious Swedish physicist Arrhenius, to whom science owes a new outlook upon the formation of worlds, has been able to demonstrate mathematically that for a very small sphere, which is not transparent and of which the diameter is somewhat less than a micron, that is to say is less than a thousandth of a millimetre [.00003937 in.], situated in the neighbourhood of the Sun, the repulsive force resulting from the pressure of radiation would be greater than the attraction of the Sun's mass, and so the small body would be driven far away into space. If the diameter of the small sphere be supposed to be less, assuming its density to remain equal to i, the repulsive force would be increased, but such increase could not continue indefinitely, for, if the particle were much smaller than a wave-length of the light acting upon it, diffraction phenomena would be produced which would completely alter the nature of the light action. For particles having a diameter of .00015 millimetre, the repulsive force is ten times greater than the attractive one due to gravity.

The self-luminous bodies, the Sun and stars, thus have the power of driving away into space those very small particles of matter of low density, and these particles constitute the cosmic dust that permeates interstellar space. Particles similarly repulsed constitute the tails of comets, which are always directed away from the Sun, as if under the action of wind from the latter body. In all probability, the Solar Corona itself is composed of such minute fragments of matter. The dust particles that are thus repelled and flung into space by the stars are negatively electrified, the star remaining positively charged. Such of those emanating from the Sun, which reach the Earth, produce by their negative charges important electric effects, as we shall see later on.

In addition to the suns which shine in the heavens and which are bodies radiating heat, there are cold bodies in interstellar space whose useful purpose seems to be to arrest and absorb the heat energy radiated by the suns. If these did not exist, the infinity of stars scattered through-out space would give to the sky the aspect of a fiery vault, an incandescent dome, and their aggregate radiation would annihilate all manifestation of life at the surface of the habitable globes. These cold bodies are the nebulae 1, they exist everywhere scattered through space. [However, where the stars are most numerous, in the Milky Way, the visible nebulae are very scarce, and conversely the nebulae abound where the stars are fewest.—Ed.] They can be seen in the clear dark sky as milky patches of light, 2 some without definite form or contour (Fig. I), others with a roughly circular shape in which can almost always be traced a tendency to form a more or less sharply materialised spiral. [This form, in fact, predominates to a great degree. Ed.] In some further cases the contour is still more definite. These are the "planetary" nebulae1 which consist of a central nucleus surrounded symmetrically by an "atmosphere of light." Spectrum analysis enables us to study the composition of the nebula; their spectra are composed of bright lines similar to those which are obtained in our laboratories from incandescent gases. [This refers only to the so-called green nebula. The predominating type, the spiral nebulae, known spectroscopically as white nebulae, present spectra radically different.—Ed.] The characteristic rays given by hydrogen and by helium, a gaseous element originally discovered in the Sun and recently [1895. —Ed.] extracted from terrestrial sources, can be recognised, also rays from an unknown substance, not yet found in the Earth, to which the name nebulium has been assigned. Helium is formed by the molecular disintegration of the radioactive substances which exist in the solid crusts of the planets; possibly all the constituents of these, that is, all forms of matter, are radioactive. Both hydrogen and helium diffuse into space, where their thinly scattered molecules constitute the rarefied nebulous medium which only requires the coming of a nucleus for condensation to commence there. Helium and hydrogen, therefore, might appear to be the ultimate products of the disintegration of matter.'

Whatever degree of precision the outlines of the spiral nebulae show, streams of matter appear to diverge from a central point and to become more or less blended as they spread out, thus indicating a general movement of rotation of the matter composing the nebula. [The rotation of a spiral nebula was proved, in the spring of 1914, by spectrum photographs made at the Lowell Observatory. —Ed.] But one of the chief features of these objects is the existence in them of nuclei, more brilliant than the remaining part, apparently centres of condensation, around which the nebulous matter accumulates as it becomes more dense, thus giving birth to stars. The great spiral nebula in Canes Venatici is a remarkable example, in which there is also a second nucleus apart from the principal one (Fig. 2).

How do these condensation centres arise? Three explanations have been advanced. First, the contraction due to cooling through many ages suffices to increase the density at the centre, in consequence of which the velocity of rotation and, therefore, also the resulting centrifugal force, is augmented, thus causing the detachment of annular masses from the further parts of the nebula. This is what scientists, following Laplace, thought at the beginning of the nineteenth century. [It is now known, however, that nebula cannot be at a high temperature.—Ed.] Secondly, there is the explanation of modern astrophysicists, following Arrhenius, viz.: that, in the course of those innumerable centuries which are only an instant in the eternal history of worlds, "dead suns" enter the nebula and serve as nuclei for condensation. Thirdly, bodies, such as the Earth is now and as the Sun will be in the future, superficially cooled, but nevertheless containing in their heated internal masses a tremendous reserve of energy, have given birth, by collision with each other and the consequent partial volatilisation due to the heat disengaged by the shock, to a nebula for which the remaining portions of the original bodies serve as a nucleus, thus giving rise to a new star and so exemplifying the resurrection of a world. [The collision, or even very close approach, of two large absolutely cold bodies would produce practically the same result.—Ed.]

Whatever the original cause, the fact is almost certainly established that stars, that is to say, suns, are produced from nebulae by condensation of the matter forming these. When a fragment of cosmic dust penetrates into the midst of the nebulae, it falls towards the centre of gravity of the whole, and the more the condensation progresses the greater the rise of temperature. The imperishable fame of Laplace lies in his having been the first to indicate the way in which our Solar System was derived from its original nebula.

Nebulae, in their early stages, are composed of gases in a state of extreme rarefaction, of which the contents of a Crookes tube give us some idea. They retain what reaches them of the cosmic dust thrust outwards from the suns by the pressure of radiation. The nebulae at first have the properties of gaseous masses in adiabatic equilibrium; that is to say, when they receive heat from neighbouring suns their temperature does not rise, but falls. Thus, according to Arrhenius, they have a negative specific heat. Since the cosmic dust is electrified a charge accumulates in the outer layers of the rarefied gaseous mass. It should be noticed that the temperature of a nebula must be very low, on account of its rarefaction, which implies an absence of internal movements and therefore of molecular collisions giving rise to heat. In all probability, the temperature of such a nebula is about 5o° C. [900 F.] above the absolute zero of the physicists, which zero has been definitely established by Amagat to be 273°C. [491.4°F.] below the temperature of melting ice, this latter being the zero of our thermometer [centigrade] for practical purposes. It is this recognition of the low initial temperature of the nebula which constitutes one of the most essential modifications that modern science has made in Laplace's theory of the evolution of the Solar System. He assumed that the nebula was originally at a high temperature.

Now, in spite of the low temperature we have been led to attribute to it, the nebulous mass emits light, and so is visible to us by reason of the luminosity with which its constituent materials shine. It seems remarkable how this state of incandescence can be maintained in these circumstances. It is due to the fact that, in proportion as the accumulating, electrified dust particles add an increasing quantity of electricity to the periphery of the nebula, the strain increases little by little, and ends by becoming sufficient for a discharge, analogous to that which takes place in a Crookes tube. This illuminates the entire mass, thus rendering it visible against the dark background of the sky. It should, therefore, be noticed that we cannot see any such nebula in which the electric stress is not yet great enough to have produced discharge luminosity in the gaseous masses which compose its outer layers, so that the number of known nebula must be vastly, perhaps almost infinitely, increased if this number is to represent all that actually exist. In all cases, the production of luminosity is the first stage in the life of a nebula, hitherto, figuratively speaking, inert.

The second stage is the formation of a nucleus. Possibly a cooled body like the Moon or Earth comes in the course of ages and penetrates into the nebula, or denser masses of particles agglomerated together as meteorites similarly enter. Or perhaps the moving gaseous molecules collect in one part from some cause. Condensation at once begins around these intruded masses, as they may be called, which are therefore the means of starting the condensation process. This process sets heat free, and the nucleus, which grows continuously, gradually becomes incandescent, after having captured the greater part of the rarefied matter which constituted the original nebula. The system has now reached the stellar phase. As the condensation continues, the pressure at the centre increases and soon becomes very large.

The original hydrogen and helium, the residue of the disintegration of the matter of other stars, now become the origins of, or points of departure for, the integration of the matter of a new star.

It may also happen, as has been said above, that two dead suns clash together in the course of their journeyings through space, at some time or other. If they are constituted similarly to the Earth, their frail envelopes would be broken by the force of the shock and, independently of the enormous amount of heat disengaged by the impact, the fiery material set free by the rupture of the containing crust would rush out into space, being, for the most part, volatilised owing to the sudden decrease of pressure. 1 Two jets of spiral form would be produced, the whole rotating by reason of the obliquity of the impact of the two bodies. A new nebula may thus be created out of two dead suns. At its centre would be a new sun, or perhaps two or even more suns. This is an explanation of the appearance of those nova or new stars which so strongly arouse the interest of astronomers. In all observed cases a spiral nebula has come into being with one or several incandescent nuclei. Minor centres of condensation, arising perhaps from masses thrown out during the original collision, are found in the spirals. These secondary suns originate immediately after the primary one, drawing to them a part of the en-compassing cosmic material; they gravitate around the central and more important sun, and so a planetary system is given birth.

The planets, at the commencement of their history, are formed of practically the same elements as the central nucleus. Carried round by the initial movement of rotation, they all generally revolve the same way, except in the cases where a strange body previously rotating in a contrary way may have penetrated into the exterior limits of the nebula, and so come into the field of attraction exercised by the new sun, the satellites of this body retaining their primitive rotatory sense on account of its mass. This is perhaps what occurred with regard to the outermost planets of our Solar System, Uranus and Neptune.' It is not necessary to assume that adventitious bodies entered the nebula to provide planets for the primitive central body. Laplace held that the centrifugal force would suffice to detach successive equatorial rings from the mass of the principal nucleus, the speed of rotation of which increases in proportion as it contracts by cooling; that each of these rings afterwards would become a planet by the process of agglomeration of its material at one point ; and that, in its turn, the planet might produce one or more satellites by an exactly similar method. [That the planets and their satellites could have been, and probably were, all built from the parent body or bodies and their particles and gases is accepted by the scientist of today. The equatorial ring theory of Laplace, is, however, no longer credited.—Ed.]

We have now, therefore, attained to the conception of a nebula having a principal centre of condensation, that is to say having a central sun and also secondary centres, whether formed by the intrusion of adventitious bodies or arising from the condensations and agglomerations of matter detached from the principal mass first, possibly by the initial catastrophe or later by the agency of centrifugal force. The subsidiary centres begin to gravitate around the chief one, describing elliptical orbits, the form of which was defined by Kepler, who also first stated the laws of the planetary movements. The secondary nuclei begin to rotate on their axes in consequence of the initial movement of rotation of the primitive nebula. We shall here only consider one of these bodies, the Earth.

From the time when it was separated from the central nebulous mass, the Earth's individuality commenced, but it had not yet become what may be called the terrestrial globe. Before doing so, it had to cool and consequently to contract. It can be shown by mechanics that the velocity of rotation increased as the diameter diminished. Centrifugal force caused a mass of matter to be detached from the Earth's equator, the Earth having been previously flattened to the shape of an orange by the same agency, [and later, just before the mass broke loose, drawn out into a somewhat pear-shaped form. Ed.] This detached body took the normal spheroidal form round a nucleus, the small mass of which permitted a more rapid cooling. Thus, the Moon was formed and has subsequently continued its revolution about the Earth.

The temperature of the detached Earth fell much more quickly than that of the central Sun, which, on account of its enormous mass, 325,000 times that of the Earth, cooled with extreme slowness, just as of two pieces of iron heated red hot in the same fire the greater remains warm a much longer time than the smaller. The mass constituting the Earth, therefore, passed gradually from the gaseous to the liquid state, and then to a viscous condition. Its rotation, which implied a concomitant centrifugal force, then caused the equator to bulge out and, also, the polar regions to become flattened. [It was succeeding this that the Moon was cast off.—Ed.] In proportion to the extent of the cooling, some of the gaseous elements which constituted its atmosphere, dissociated and kept from combination by the high initial temperature, were enabled to condense, as, for example, metals that were originally vaporised, and others to combine together when they arrived at a sufficiently low temperature. As the cooling steadily proceeded during this time, the globe came to solidify at its exterior surface, and there-fore to be covered with a crust which, although very thin at first, gradually thickened until it served to maintain a kind of equilibrium between the escaping internal heat, which it transmitted badly, on account of its feeble conductivity, and the external heat received from the central Sun.