X907.] AND CONTRACTION OF THE EARTH. 195 



all the heat of condensation escaped from the surface by radiation 

 through the atmosphere, thus leaving the slowly-accumulated nucleus 

 within at low temperature. In fact this last view seems to be 

 effectively contradicted by the observed internal heat of the globe; 

 and if heat was retained in the layers just beneath the crust, it must 

 likewise have been accumulated throughout the whole nucleus. 



It is much more likely that the agglomeration went on with 

 moderate rapidit}-, so that the primitive globe in formation cooled 

 about as rapidly as a condensing mass of monatomic gas would do 

 under like conditions. Such a gaseous globe retains a little over 

 one half of the heat of condensation. There are many indications 

 that the sun is such a gaseous globe, and the constitution of Jupiter 

 and Saturn probably is more or less similar. It seems likely that 

 in the formation of the earth the meteorites and finer planetary dust 

 came together at such a rate that the whole new planet thus consti- 

 tuted was made to glow with maximum temperature at the centre, 

 as in the case of a sphere of monatomic gas in a state of equilibrium 

 under its own gravitation. Then as the planet grew in mass by the 

 addition of other meteoritic materials, the pressure augmented also, 

 and eventually became so great as to prevent circulation of convec- 

 tion currents. At length the surface gradually cooled by radiation, 

 grew dense and solid, and thus finally arose the encrusted earth. 



It must be remembered that until the earth attained a mass of 

 considerable magnitude, the matter falling into it would not be 

 heated to a very high temperature ; for in the early stages of the 

 planet the central temperatures would not be high, nor would the 

 force of gravity be intense enough to condense greatly the central 

 nucleus. But as the mass gradually grew in size, the force of grav- 

 ity would naturally augment, and the temperature rise accordingly. 

 The rise of temperature would take place throughout the central 

 mass by the condensation of the nucleus under increasing pressure; 

 and the deeper down the matter which condensed, the less of the 

 resulting heat would escape to the surface. Thus when the impacts 

 became more and more violent, under increasing gravity, and th? 

 surface temperature was much augmented, the central temperature 

 would rise in nearly the same proportion ; so that the central tem- 

 perature would always be the maximum. Yet it is clear that as 



