17. It is then evident that the primitive state of the softness or fluidity of the Earth must have been caused by the action of heat, not by water. The Earth was then originally an incandescent mass. In consequence of the caloric rays, it became liquefied. It has been gradually cooled, and the cooling process has naturally commenced on the surface, which has become hardened, whilst the interior has remained in a fluid state. One can thus compare the Earth to a block of coal coming red from the furnace, the surface cooling by contact with the air, although, if one breaks it, the interior is found to be yet burning.