as an unexplained fact, is toward concentration. The evolution of heat is rather the check put upon this tendency, and, in so far as it exerts any influence, it exerts it in a direction the reverse of gravitation. There is a perpetual and rhythmic antagonism between the forces of integration and disintegration. When for any reason the former acquires an impetus which carries it to great lengths, it is resisted with increasing violence by the antithetical force evolving great heat, and eventually restoring the normal equilibrium. It seems altogether probable, therefore, that in the process of contraction of a nebulous mass, and its resolution into a system of worlds, the amount of heat radiated is in the end equal to the amount produced by condensation, which disposes entirely of the supposition that there must exist an incandescent nebula at the outset. The so-called "cooling off" is only apparent, and, while at times the amount of heat may be diminished, at other times it will be correspondingly increased. If the radiation of heat from the surface of a body into space tends to cool it off, so does the constant diminution of its volume without loss of mass tend to heat it, and throughout its career these two influences must antagonize each other. It is only after the limit to possible contraction, due to the nature of matter itself, begins to be reached that the amount of radiation of heat comes greatly to exceed the amount of its generation, and that the body actually begins to cool off.
During the greater part of the history of an evolving system, the central mass must possess an enormously high temperature. This is required by chemistry as well as by physics. Throughout nearly the whole of this period, all the matter of the system must exist in the form of gas. But there exist in our globe many substances whose existence in the gaseous state presupposes great heat. The degree of heat required to volatilize the metals is immense, and there are certain other substances, such as silicon, for which still greater temperatures are demanded. It would, however, be a violent assumption to suppose that the parent nebulæ, out of which the solar system was formed, contained from the outset in this diffused state all the substances which are found on the earth. It is much more reasonable, and our hypothesis permits us, to assume that these substances, requiring so great heat to liquefy and volatilize them, have been created, i. e., developed, during the progress of the formation of the system out of materials already existing in other forms and states of aggregation. On the supposition that during the earlier part, and perhaps during all but the very latest period, of this process the temperature of the nascent system was increasing, it is reasonable to assume that the intense heat would cause the breaking up of some of the molecular aggregates which were capable of maintaining the gaseous form at low temperatures, and would at the same time cause the formation of new aggregates only capable of maintaining that form under the high temperatures to which they were subjected at the time of their for-
