Page:Scientific Memoirs, Vol. 1 (1837).djvu/147

Rh "The nearly spherical form of the earth and planets, and their flattening at the poles of rotation, evidently show that these bodies were originally in a fluid or perhaps in an aëriform state. Beginning from this initial state, the earth could not, wholly or partly, become solid, except by a loss of heat arising from its temperature exceeding that of the medium in which it was placed. But it is not demonstrated that the solidification of the earth could have commenced at the surface and been propagated towards the centre, as the state of the globe still fluid in the greatest part of the interior would lead us to suppose; the contrary' appears to me more probable. For the extreme parts, or those nearer to the surface, being the first cooled, must have descended to the interior and been replaced by internal portions which had ascended to cool at the surface and to descend again in their turn. This double current must have maintained an equality of temperature in the mass, or at least must have prevented the inequality from becoming in any way so great as in a solid body, which cools from the surface; and we may add that this mixture of the parts of the fluid, and the equalization of their temperatures, must have been favoured by the oscillations of the whole mass, which must have taken place until the globe attained a permanent figure and rotation. On the other hand, the excessively great pressure sustained by the central strata may have determined their solidification long before that of those nearer the surface; that is to say, the first may have become solid by the effect of this extreme pressure at a temperature equal or even superior to that of the strata more distant from the centre, and consequently subjected to a much less degree of pressure. Experiment has shown, for example, that water at the ordinary temperature being submitted to a pressure of 1000 atmospheres, experiences a condensation of about th of its primitive volume. Now let us conceive a column of water whose height is equal to one radius of the globe, and let us reduce its weight to half of that which we observe at the surface of the earth, in order to render it equal to the mean gravity which would exist along each radius of the earth upon the hypothesis of its homogeneity; the inferior strata of this liquid column would experience a pressure of more than three millions of atmospheres, or equal to more than three thousand times the pressure which would reduce water to ths of its volume; but without knowing the law of the compression of this liquid, and although we do not know in what manner this law may depend on the temperature, we may believe, notwithstanding, that so enormous a pressure would reduce the inferior strata of the mass of water to the solid state, even when the temperature is very high. It seems therefore more natural to conceive that the solidification of the earth began at the centre and was successively propagated towards the surface: at a certain temperature, which might be extremely high, the strata nearer the centre became at first solid, by reason of the excessive pressure which they experienced ; the succeeding strata were