532 THE POPULAR SCIENCE MONTHLY. 



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 gravita- 

 tion. 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 con- 

 densation, 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 tinies 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 con- 

 traction, 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 w^hose 

 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 nebulse, 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 j^rocess 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- 



