TEMPERATURE OF THE EARTH IN TIME AND SPACE 5 



and not by impact on one another, as the older hypothesis 

 of Sir George Darwin required. Evolution appears to 

 have consisted in the gathering of these small bodies, or 

 planetesimals, into planets and satellites. The meetings 

 and unions of planetesimals implied a relatively slow 

 evolution of nebula into a solar system but the end result 

 was inevitable. The planetesimal hypothesis therefore 

 signifies a relatively slow growth of the earth. 



What the temperature of the juvenile earth was is 

 still a matter of speculation. The hypothesis of Cham- 

 berlain requires a much lower temperature than that of 

 Laplace. Instead of consisting of a primitive molten 

 globe the earth probably originated in a nebulous knot 

 of solid matter on which fell scattered nebulous matter, 

 and the latter probably did not have a high temperature. 

 Liquefaction of the rocks only occurred locally as the 

 result of heat generated by increased pressure and by 

 radioactivity. This temperature problem thus becomes 

 fundamental because, according to the opinion of Cham- 

 berlain, the earth may have had a temperature suitable 

 to life long before it reached its present size — perhaps 

 when it had attained to the magnitude of Mars. 



The Primitive Atmosphere. — The primitive atmos- 

 phere was, by early geologists, held to be vast, hot, and 

 heavy, containing all the water of the globe, all the car- 

 bon dioxide now in carbonated rocks, all the oxygen which 

 has been added to the rocks by oxidation, as well as that 

 portion of all these constituents which is now found in 

 the atmosphere and in organic tissues. Such a condition 

 was assumed because heat promotes the expulsion of 

 gases from liquids or solids. The heat of the primitive 

 earth was believed to have been sufficient to drive out 

 all gases. As the earth cooled these gases were re- 

 absorbed. 



