THE APPEARANCE OF LIFE ON WORLDS AND THE

HYPOTHESIS OF ARRHENIUS.1

By ALPHONSE BERGET,

Professor at the Paris Institute of Oceanography.

The problem of cosmogony is one of those that has most disturbed the mind of man. No question, indeed, is more perplexing than that of finding out whence comes the earth, whence comes the sun about which it gravitates with its sister planets, toward what goal is it carried by that slow evolution that it undergoes.

To this question Laplace was the first to give a scientific answer. Starting from the results of the observation of the nebulæ, different forms of which we can observe in the sky at different stages of their history, he formulated, by a true flash of genius, that wonderful theory that bears his name. According to his conception, an incandescent nebula, radiating its heat gradually toward cold space, would contract as it cooled; these successive contractions would by degrees have agglomerated the constituent matter of the nebula into a "nucleus," at first gaseous, then igneous fluid, the sun. In proportion as the dimensions of this revolving nucleus-the total mass of which remained the same-was diminishing, its momentum of inertia was also diminishing, its velocity of rotation was increasing, and consequently the centrifugal force was increasing at the same time. A depression was formed on the still plastic nucleus near the axis of rotation; the equator expanded and gradually a ring was detached from it, which on breaking gave birth to a new planet, by the condensing of the matter of which it was formed. This planet began to revolve around the nucleus and on itself, in the same direction as that of the rotary motion of the central nucleus. Thus must the earth have been born, thus must have been born the other planets, fragments detached from the central sun, and necessarily containing the same chemical elements.

When Laplace advanced this hypothesis, certain astronomical facts that are known to-day were yet unknown, notably the inverse rotation of the satellites of distant planets. Nothing was known of the existence of new forces that modern physics has just discovered

1 Translated by permission from Biologica, Paris, 2d year, No. 13, January 15, 1912.

in the course of the last few years. Perhaps it is fortunate that the famous astronomer did not know these "new facts." They would have destroyed the unity of the system of the universe as he saw it, and the complication introduced by instances of exception would doubtless have prevented him from formulating his hypothesis, so grand in its unaffected simplicity.

To-day we know that the Laplace theory must be modified in some points. As a whole, however, it is still in force; it is a citadel which in spite of everything resists all assaults, as H. Poincaré has so well said. It is enough, therefore, to reconcile it with the new conquests of science; that is what the illustrious physicist of Stockholm, Prof. Svante Arrhénius, has done.

The Swedish scientist introduced into the theory of the evolution of worlds a second force as necessary to consider as universal gravitation, that is, the pressure of radiation, the conception of which is due to J. Clerk-Maxwell, and the reality of which has been demonstrated by the experiments of Lebedeff. This pressure is exerted upon every surface exposed to a radiation by the very action of this radiation; it is equivalent, in the immediate neighborhood of the solar surface, to nearly 2 milligrams a square centimeter.

As the dimensions of a very small spherule of matter decrease, the importance of the surface in comparison with the mass increases at the same time. Now the attraction of gravitation is dependent on the mass, while the pressure of radiation is dependent on the extent of surface. One can readily conceive, therefore, that in the case of very tenuous particles the pressure of radiation may exceed the attractive force of gravitation; in the case of nontransparent spherules the 0.0015 of a millimeter in diameter the two forces are in equilibrium; and if the diameter of the particle falls below this amount, the repelling force is the stronger and the particle is driven away from the radiating body. On tiny particles whose diameter would amount to as little as the 0.00016 of a millimeter, the pressure of radiation would be ten times as great as the attracting force. These dimensions are realized in the spores of bacteria. The small mass of these microscopic granules increases the importance of their surface, and the resistance of the air has such force over them that this tiny mass. dropped into the air would not fall a hundred meters in a year. The slightest wind carries them off into the atmosphere and may take them to the limits of our gaseous envelope, where the pressure of the air is not more than a very small fraction of a millimeter of mercury; that is, to an altitude of 100 kilometers.

It is this pressure of radiation that Arrhénius has given a place in the formation of worlds. It drives away from the stars the fine "cosmic dust" which the constant eruptions of these incandescent stars throw out every moment; especially, it is this dust which con

stitutes the coronal atmosphere of the sun. These expelled particles bear a negative electrical charge. They are going to come in contact with these cold, gaseous masses of rarified molecules, containing helium and hydrogen, called "nebula." These nebulæ contain a very small number of molecules; hence their low temperature. When the electrically charged particles reach them, the former make the periphery luminous, and then these nebulæ are visible to observers on the earth; the dust which is agglomerated into meteorites, however, becomes centers of condensation for these nebulæ. Let a dark body, such as the moon is to-day, such as the sun will be later, happen to penetrate into such surroundings in the course of its peregrinations lasting myriads of centuries, it will become still more easily a center about which nebulous matter would accumulate while it becomes heated; the nucleus becomes incandescent, a sun will be born. Finally, let two dark suns collide in the infinity of space and time; the violence of the shock is enough to volatilize their matter; the breaking of their envelopes would release the igneous matter so long imprisoned beneath their cooled crusts; like two gigantic shells they "explode" and the endothermic components that their centers contain, held under enormous pressures, set free masses of gas that escape in spiral spirts. Then the stages of which Laplace conceived can begin to follow each other, generating planets; one or two "nuclei" exist in the midst of the nebulous spheres surrounding them; we have watched the resurrection of a world. These collisions are not idle hypotheses, we witness them in the heavens each time that a new star appears, like the "nova Persei," for example; we have seen a world born, but reborn from a dead world. It is a perpetual cycle that recommences in this manner, a cycle the mechanism of which has been pointed out for the first time by the brilliant genius of Arrhénius.

Such is, too briefly summarized, the Swedish physicist's principle of the theory of cosmogony. But he has not been content with explaining the evolution of "cosmic" matter. He has asked himself-and it is this that will interest the reader of Biologica more especially-how life could appear on a world thus created; he has tried to find out whether living germs, having left a world where they found their conditions of existence realized, can endure the long journey through intersidereal space and bring to another world the germ of life which is in themselves, becoming the starting point of a series of living beings brought slowly, by an evolution parallel to that of the planet that sustains them, to gradually increasing degrees of perfection; in a word, to "higher" states.

Svante Arrhénius answers this question by the elegant, original, and seductive form that he has known how to give to the doctrine.

of Panspermy, adapting it to the most recent advancement of modern physics.

The doctrine of Panspermy is not new; Richter was the first to advance it, about 1865. Later it received the distinguished support of the illustrious English physicist, Lord Kelvin, and in Germany Helmholtz lent it the aid of his great authority.

In its first form, this doctrine assumed that meteorites, fragments resulting from the collision between two dark bodies of the heavens, come in contact with a sun and bring there germs that the explosion has not had time to destroy, as, when one blows up a quarry with dynamite, certain pieces of rock may roll to the bottom of the mountain, remaining covered with vegetation, with living germs that have stayed intact. Under these conditions meteorites could admit of organic "inclusions," which could carry life to celestial bodies yet devoid of it.

However, examination of this hypothesis in this very simple form raises objections, the principal of which is the stupendous temperature to which the germs would be immediately subjected. Merely the sudden stopping of the earth in its motion, even without the intervention of a collision, would suffice to volatilize its matter as a result of the quantity of heat liberated; if, in addition, there should be a collision of two celestial masses, with the liberation of the igneous matter composing their respective nuclei, it is almost certain that not a living organism would escape this thermic manifestation, which would reduce them to their gaseous elements. It is, then, very difficult to admit of the conveyance of germs by meteorites considered as "fragments" from a celestial cataclysm.

Arrhénius has completely modified the hypothesis of Panspermy by adapting it to the demands and achievements of modern physics. He has considered the possibility of the conveyance of germs themselves, independently of all mineral aid, and this by bringing into play the "pressure of radiation" of which we have spoken in the beginning of this article, when we explained in broad outline the cosmogonic hypothesis of the Swedish physicist.

We have said that by direct measurement the pressure of radiation on a spherule the 0.00016 of a millimeter in diameter (or 0.16 of a micron) might be 10 times as strong as the attractive force resulting from universal gravitation. Now germs of these reduced dimensions do exist. Botanists know for a certainty that the spores of many bacteria have a diameter of 0.3 to 0.2 of a micron, and that beyond doubt there exist some even much smaller; the progress of the ultramicroscope is beginning to enable us to see these germs of the order of one-tenth of a micron in size.

Let us imagine such a microorganism swept off the surface of the earth by a current of air that carries it as far as the higher atmosphere,

say to the altitude of approximately a hundred kilometers. When it has reached that point it is subjected to another category of forces susceptible of acting on it; these are forces of an electrical kind.

It is, indeed, at about that altitude that radiations produce polar auroras. These auroras are caused by the arrival into the atmosphere of the earth of cosmic dust coming from the sun and driven from it by the pressure of radiation. This dust is charged negatively, and its discharge makes luminous the region of the atmosphere in which it is. Under these conditions, if a spore coming from the earth's surface is also negatively charged by contact with the electrically charged dust, it may be repelled by the latter, which will drive it toward intersideral space as a result of the electrostatic repulsion of two charges of the same sign. Calculation shows that an electrical field of 200 volts a meter is enough to produce on a spherule the 0.16 of a micron in diameter a repulsion greater than gravitation; now, the field usually observed in the atmospheric air is greater. Electrostatic repulsion of germs that have reached the higher atmosphere is, then, not only qualitatively, but even quantitatively possible.

We have our germ, then, started on its intersideral journey. Let us put aside for a time the conditions of existence and destruction that it may encounter among the stars, circumstances that we shall study in a moment. We are going to find out first of all the conditions of time of such a journey, effected under the influence of the pressure of radiation which acts on the germ as soon as it is at a sufficient distance from the earth. On its way it will be caught, in the neighborhood of a celestial body, by some larger particle of the order of size of a micron, which forms a portion of that dust scattered profusely around the solar systems. Once carried away by this particle, which, because of its greater size, is more subject to the action of attraction than to that of the repelling force, it can then penetrate into the atmosphere of the planets that it will happen to encounter.

If we assume that this traveling germ has a density equal to that of water, which is obviously accurate for living germs, we find that it will need nearly 20 days for it to reach the planet Mars, 80 to reach Jupiter, 15 months to get to the distant planet of Neptune. These are only planets forming part of our own solar system. If we try to find the time necessary for this germ to reach the solar system nearest to ours, that is, the system whose central sun is the star a of the constellation of the Centaur, we will find the duration of the journey to be approximately 9,000 years.

How will our germ, living at the time of its departure, act in the course of this long journey?

Interstellar space has a very low temperature; it is near the absolute zero of the physicists, which is 273° C. below the temperature