286 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



by continued precipitations that the exchange of heat between the cen- 

 ter and the periphery can no longer take place by means of currents, 

 the luminous envelope disappears for want of aliment, and it is only 

 after a sufficiently prolouged repose that the expansive force of the gas 

 succeeds in overcoming the resistance, and envelopes the entire globe 

 with a photosphere, which will be extinguished again when equilibrium 

 is established between the expansive force of the gas and the pressure 

 of the condensed matter. (Type, variable star, temporary star.) 



These phenomena may be repeated at intervals more or less consider- 

 able, but the effects of the cooling is manifested more and more, until 

 finally the luminous phenomena cease entirely. It is then the geological 

 life of the star commences. 



The sun is still far from the exhaustion of its calorific capacity, but 

 it has only a shadow of its former greatness. It extended, before the 

 formation of the planets, beyond the limits of our present system. The 

 equivalent of heat lost since that time in the generation of the force 

 necessary to divide the mass of the sun from the planets would make 

 it occupy a space whose radius would extend beyond the orbit of Nep- 

 tune. The attenuation of the solar matter must have been extreme 

 when our system formed one continuous whole, with the mass of the 

 other stars of the milky- way. 



We have considered the formation of the great centers in a general 

 way; let us now examine their particular development, which we can 

 study only in our solar system. 



We follow in this the theory of Laplace, suggested to the great 

 geometrician by the discovery of the rings of Saturn, in which, to use 

 his own expression, "man detected nature in the execution of one of 

 her mysterious creations." 



Our solar system had, before the creation of the planets, a form very 

 much flattened, in consequence of its movement of rotation and of its 

 gaseous nature. In fact, the centrifugal force and the expansive force, 

 which both increase with the distance from the center, counterbalanced 

 the attraction, and produced a considerable enlargement in the plane of 

 the equator. When a certain portion of the nebula, on account of the 

 equilibrium of these contrary forces, could no longer participate in the 

 general movement of contraction, a ring was formed, which circulated 

 around the central body with the velocity it had acquired as a portion 

 of the generating body, when the dimensions of the latter were the 

 same. 



Several rings were detached thus, successively, and continued to con- 

 dense independently of the central body, until, by an exterior influence, 

 or by the preponderating attraction of nuclei formed within their sub- 

 stance, they broke into fragments of unequal volume. The principal 

 nucleus, collected about a center of gravity the largest part of the 

 mass, which had formed the ring, and was then in the same condition as 



