IN all times human beings have been interested in seeking an explanation of the origin of the universe, the; creation., of the earth and the heavenly bodies which give it light. The first systems of cosmogony, that of Genesis for example, are simple, rudimentary, unscientific, and filled with a naive poetry. All of them were designed to serve as the bases of religions, and they attribute the causes of all phenomena to the direct intervention of Deity. The Greek and Latin philosophers attempted systems in which a little science was ingrafted on religious traditions, but Kant- was the first to invent a theory of the formation of the universe which rests on a solid foundation and is the basis of modern systems. In 1755 Kant wrote his »General Natural History and Theory of the Heavens.» ^ In this remarkable work the author, assuming with Lucretius, Epicurus and Democritus, that in the beginning was chaos, consisting of the fragments of all the heavenly bodies, imagines for the explanation of the origin of the solar system that a vast nebula composed of fine particles assumed a rotary motion. This motion, gradually becoming swifter, caused the formation of rings which broke and formed planetary nebulae which behaved like their parent, and thus planets and satellites were produced. Kant, with the little scientific knowledge of his period, often exceeded in his conclusions the logical result of then known laws. For example, in his theory of the formation of Saturn's ring, which he attributes to the separation of part of the vaporous atmosphere of that planet, he pictured the actual constitution of this ring as it is Shown to us by recent investigation, that is to say, a series of concentric rings of particles, in which each particle moves as an independent satellite. Forty years later, Laplace, in ignorance of the work of Kant, devised his theory of cosmogony. The hypothesis of Laplace supposes a primordial mass of gas, the condensation of which formed successive concentric rings as in Kant's theory. Laplace did not reach this conclusion at once. At first he supposed that the solar system originated in a stellar nebula which threw off rings, from which the planets were formed, and from this he advanced to the idea of a universal original nebula. The scope of this article does not allow us to develop these theories, in detail, nor to analyze those of Herschel (1811), Trowbridge (1864), Roche (1873), aijd others, the more so as the conclusions of these authors depend greatly upon auxiliary hypotheses, introduced in the course of calculation. In 1884 Faye modified Laplace's hypothesis by supposing that cyclones in the primordial mass produced spiral streams which were afterward converted into rings. All these theories show us a planet formed by condensation of a circular ring into a single mass. If this has actually occurred we should find among the nebulae various stages of these transformations. The recent advance of astronomical photography has rapidly increased our knowledge of the structure of nebulae and star clusters. None of these objects exhibits a perfectly circular form. In all nebulae the spiral is more or less indicated and this formation has certainly produced the condensation which produced stars. The star clusters have a similar formation. We possess also much spectroscopic information of double and multiple stars, from which we can infer that not all stellar systems are constructed on the model of our solar system. On the contrary, our system, with a single central sun surrounded by planets, is rather exceptional Most of the systems are double, having two suns of almost equal masses, although often of very dissimilar spectra, which indicates that they are at different stages of evolution. In the meanwhile the development of thermodynamics has given us new views of the constitution of matter. Matter is capable of assuming various states, of which the principal are the solid, liquid, and gaseous. To each state assumed by a certain mass of matter corresponds a certain quantity of energy, which is connected with that mass and is called potential energy. When this matter is transformed and gives out part of its potential energy, either in the form of radiation or by doing mechanical work, the energy thus liberated modifies neighboring portions of matter. Thus the energy contained in matter becomes gradually degraded and less available, and a system limited in space tends toward a condition of equilibrium, in which the energy which it contains is not able to undergo any further modification or to perform any work. Let us suppose two bodies in presence of each other. The bodyc which possesses the greater energy, that is to say, the hotter body, is constantly giving up energy to the cooler body, w;hich thus becomes heated, and the effect of the mutual radiation is to bring both bodies to the same temperature. After this is attained no further exchange of energy can take place between the two bodies which are in thermal equilibrium with each other. Let us assume a body at the lowest imaginable temperature. If we communicate energy to it it becomes heated. At first its appearance remains unchanged, but it expands, then it changes its state, and its structure is altered. It becomes a liquid, then a gas, the density of which continues to decrease with increase of temperature if the pressure is not changed. At a still higher temperature, it is dissociated or decomposed into other substances having other properties. At a still higher temperature these new bodies in turn become dissociated, and the process continues probably until the matter reaches the simplest state that it can assume and which we may suppose to be that of the gaseous nebulae. If the mass of gas thus obtained is allowed to cool by radiation it passes in an inverse order through all the phases above described until it reaches its original condition. Spectrum analysis reveals the existence in the gaseous nebulae of very few of the chemical elements– hydrogen, helium, and a third gas not yet discovered on the earth. The matter which composes these nebulae is, therefore, at the highest point of dissociation, or nearly so, and its temperature must be very high. Its potential energy is consequently very great This energy is dissipated gradually by radiation, the temperature falls, the gas contracts, and when the point of dissociation is attained the first chemical reactions take place. The energy libefated in these reactions again heats the mass, maintains the Radiation and retards the process of cooling. To this energy is added that which is furnished by the contraction of the mass. Changes of physical state do not occur readily in the absence of particles in the final state, or, so to speak, germs of that state. For example, water may remain liquid below the freezing point, and phosphorus may remain liquid very much below its melting point. It must be the same in the nebula; the changes of state and the reactions no doubt take place around nuclei produced by the shattering of dead worlds and coming from the depths of space. We can even find here an explanation of the formation of double stars. It is only necessary to suppose that one of these stars, the cooler one, has been formed around a large nucleus, consisting of an extinct but still entire sun which has become entangled in the nebula at an epoch when the latter already possessed a central nucleus of condensation of the same order of magnitude. The sudden appearance of temporary stars is an example of this phenomenon. The most celebrated of these stars is that observed by Tycho Brah6, which appeared suddenly in the constellation of Cassiopeia, and the brightness of which surpassed that of Venus. Appearing in November, 1572, it gradually became less brilliant, and became invisible to the naked eye seventeen months later, in March, 1574. In 1604 a star almost equally bright appeared in Ophiuchus and remained visible about fifteen months. In 1866 a temporary star of the second magnitude appeared in Corona Borealis. This was the first temporary star to which spectrum analysis could be applied. The star showed a spectrum of dark lines, upon which was superposed a spectrum of bright lines of hydrogen and helium. It appears, then, that the apparition of a temporary star is due to an incandescent mass of hydrogen and helium suddenly enveloping an invisible star. The temporary star which appeared in Cyg- nus in 1876 showed a similar spectrum. In 1877 Lindsay observed that it resembled a nebula But the most interesting of temporary stars is certainly that which suddenly appeared on February 21, 1901, in the constellation of Perseus, at a point where on the previous day no -star brighter than the 12th magnitude existed. The new star, of the second magnitude on the day of its discovery, increased to the first magnitude on the following day, and then diminished in brightness, while its color changed gradually from blue to red. All the methods of modern astronomy, the most powerful telescopes and spectroscopes were employed in the study of this rare phenomenon, and very interesting results were obtained. At its first appearance, the star, to judge from its spectrum, was surrounded by an atmosphere of hydrogen and helium In the condition of vibration characteristic of a.very high temperature. Photographs taken subsequently indicated that the star became nebulous, and at the same time the nebular rays appeared in the spectrum. Around the star appeared, from time to time, nebular nhgs which seemed gradually to separate from the star. It was supposed that this appearance of rings was due to the illumination of the nebulous envelope by the star's intense light. All these observations taken together-lead to a simple explanation of temporary stars. A dead or dying sun, nearly or quite cool, encounters in its flight through spjac^ a nebular mass. The impact causes a loss of kinetic energy and a great disengagement of heat, which raises to a high temperature the nearest portions of the nebula. At the same time these gases condense around the star, which gradually becomes heated and finally dissolves in the surrounding nebular mass. Here we see one form of the resurrection of a world. Whether the primordial nebula originates in a formation similar to that of temporary stars or from a more violent collision between two dead or shattered stars, the mass of gas is not at first absolutely motionless. The impact which produces it gives it a rotary movement. The condensation caused by changes of state of portions of the nebula, the initial rotary motion and the attraction of gravitation determine the spiral arrangements observed in all nebulse. The spiral afterward condenses into a nucleus, which in turn becomes a small nebula and subsequently a sun. This sun is first blue and surrounded by a vast atmosphere of hydrogen and helium. Afterward it becomes white, and metallic vapors appear in its atmosphere, then yellow, red, and finally, when completely cooled, it becomes dark and passes slowly into the planetary condition. When the temperature of a sun has become sufficiently lowered, compound bodies, oxides, and salts begin to appear and the metallurgical operations which we commonly execute on earth are examples of the phenomena which may then take place. How is the solidification of an extinct sun brought about? By the gradual thickening of a superficial crust, as the majority of astronomers believe, or from the center (as matter is in general heavier in the solid than in the liquid state); or, finally, by a general crystallization similar to that which occurs in the case of most metals? The following facts speak in favor of the last hypothesis. Meteorites are composed chiefly of iron, alloyed with similar metals, and called meteoric iron. The spectroscope shows us this metal abounding in stars in the first stage of cooling; hence it is very probable that suns are largely composed of meteoric iron. At a temperature of about 1,200.or l,5°. G (2,192 to 2,732 deg. F.) this Iron would become crystallized as a solid nucleus, which may in clude a cavity, and would be covered by a gangue or slag of fusible matter, as cast iron is covered in a blast furnace. At this temperature water could not exist, as it would be dissociated into hydrogen and oxygen. At a slightly lower temperature certain reactions could take place, forming chlorides, iodides, and carbides in the planetary crust, and at a still lower temperature the vapor of water would appear in the atmosphere. Below 300 deg. C (572 deg. F.) the soil of the planet would become moist and aqueous reactions would occur. All these reactions taken together suffice to explain the volcanic and geological phenomena observed on the earth. Very little is known of the crust of the earth. The thickness of the paper wrapped around an orange of medium size is at least six times greater in proportion to the orange than the deepest of our borings is in proportion to the earth. It would be a very bold insect which ^ould pretend to know the composition of an orange because it had perforated the paper envelope. Hence we are at liberty to make any supposition which does not contradict observed facts. Now, celestial mechanics show us the earth rigid as steel. The volcanic regions are relatively near large masses of water and the center of disturbance in earthquakes is generally about 10 miles below the surface. These facts, as well as the variation of the geothermic gradient (the rate at which the temperature increases in going downward) found in various borings, are in perfect accordance with our hypothesis, and all volcanic phenomena can be explained at least as simply by the effect of water upon anhydrous materials of deep-lying rocks, as by the existence of a central fire. This theory of the formation of planets accounts also for a fact which is very embarrassing in other theories, the differences of density of the planets. These planets and the sun may be arranged in order of increasing density as follows: Saturn 0.7, Uranus 1.07, Jupiter 1.33, the sun 1.39, Neptune 1.65, Mars 3.91, Venus 4.44, the earth 5.50, Mercury 6.45. The order is not regular, but in general the inner planets are denser than the outer planets. In 1870 Flammarion observed that the density is a function of the time of rotation, for all planets whose rotation is known. This should be the case on our hypothesis, as a central cavity is formed by the centrifugal force in combination with the force of crystallization. Attempts have been made to explain this variation of density by the gravitation of the denser materials to the center of the system in the process of formation, but the composition of the gaseous nebula is uniform and the planetary condensations, like the central condensation, are simply portions of the original homogeneous mixture. The spectra of gaseous nebulse prove the truth of this assertion. If we examine the arrangement of stars on the celestial sphere we see that they are grouped around a great circle, along which lies the Milky Way. The stars are collected in a flat disk and our solar system is situated.near the center of this disk. Powerful telescopes show that certain whitish spots which were formerly assumed to be nebulse are really star clusters. Many of these clusters show a spiral arrangement, similar to that of nebulse, and the preceding observations lead to the conclusion that our sun is a member of a spiral cluster which constitutes the Milky Way. The study of double and multiple stars indicates that our solar system is not the model of all stellar systems, but that a great diversity exists in these. Nevertheless there is no reason for refusing to admit the existence of planets in these systems. A planet is an extinct sun and we see on the earth how life is developed on the surface of such a body. Why should not the same thing occur in the other planets of the solar system? Doubtless life is not at the same stage on all planets. Jupiter is still probably in the Silurian or the Carboniferous period, and the human race on Meirs is possibly at the point of extinction. But why should the earth alone of the similar bodies be able to support life? When an extinct sun has become a planet, how does life begin on its surface– spontaneously when conditions permit, or by the reception of germs proceeding from the disruption of dead worlds and transported by meteorites? We are able to conceive space only as infinite. Our reason refuses to admit a limit beyond which nothing exists. The portion, of space which' surrounds us and is perceptible to our senses contains nebulse, worlds in process of formation, and star clusters. One of these star clusters, the Milky Way, envelopes us and we are part of it, and our nearest neighbor in this cluster is so far away that its light, though moving 186,330 miles per second, occupies four and one-half years in coming to us. The luminous rays propagated by the vibrations of the ether bring to us from immense distances the energy radiated by the heavenly bodies. Nothing forbids our believing that the reach of our vision is unlimited and that some of the rays of light which we perceive come to us from an infinite distance, or perhaps the hypothetical ether which we have imagined as a medium for luminous vibrations does not fill infinite space, and our universe is, as certain bold minds have supposed, a bubble of ether, a simple cell, floating in the infinite and beyond which all is unknown to us. When all the energy contained in the bubble shall have become degraded and' equilibrium of temperature shall have been established throughout, the cell will be dead. Then probably will be realized the intuitive prevision of our great poet. Our finite universe will unite with another cell and give birth to daughter cells which will develop into new worlds.– Translated for the Scientific American Supplement from La Science au XXme Siécle.