Electric and galvanic currents, circulating in the stratified crust, are not to be overlooked. The experiments of Mr. R. W. Fox and Mr. Robert Hunt suggest that, in passing long-continued galvanic currents through masses of moistened clay, there is a tendency to produce cleavage and a semi-crystalline arrangement of the particles of matter.[31] [31] Report of the Royal Cornwall Polytechnic Society for 1837. Robert Hunt, in “Memoirs of the Geological Survey of Great Britain,” vol. i., p. 433. THE BEGINNING. The theory which has been developed, and which considers the earth as an extinct sun, as a star cooled down from its original heated condition, as a nebula, or luminous cloud, which has passed from the gaseous to the solid state--this fine conception, which unites so brilliantly the kindred sciences of astronomy and geology, belongs to the French mathematician, Laplace, the immortal author of the “Mécanique Céleste.” The hypothesis of Laplace assigns to the sun, and to all bodies which gravitate in what Descartes calls his _tourbillon_, a common origin. “In the primitive state in which we must suppose the sun to be,” he says, “it resembles one of those nebulæ which the telescope reveals to us, consisting of a more or less brilliant central _nucleus_, surrounded by luminous clouds, which clouds, condensing at the surface, become transformed into a star.” It has been calculated that the centre of the earth has a temperature of about 195,000° Cent., a degree of heat which surpasses all that the imagination can conceive. We can have no difficulty in admitting that, at a heat so excessive, all the substances which now enter into the composition of the globe would be reduced to the state of gas or vapour. Our planet, then, must have been originally an aggregation of aëriform fluids--a mass of matter entirely gaseous; and if we reflect that substances in their gaseous state occupy a volume eighteen hundred times larger than when solid, we shall have some conception of the enormous volume of this gaseous mass. It would be as large as the sun, which is fourteen hundred thousand times larger than the terrestrial sphere. In Fig. 12 we have attempted to give an idea of the vast difference of volume between the earth in its present solid state and in its primitive gaseous condition. One of the globes, A, represents the former, B the latter. It is simply a comparison of size, which is made the more strikingly apparent by means of these geometrical figures--one the twentieth part of an inch in diameter, the other two inches and three quarters. [Illustration: VI.--The Earth circulating in space in the state of a gaseous star.] [Illustration: Fig. 12.--Comparative volume of the earth in the gaseous and solid state.]