Evolution of the Earth. 105 



coldest winter weatlu r. The assumption of absolute uniformity 

 of diffusion would be contrary to all the evidences at present 

 around us. If a single point is even infinitesimally denser than 

 the matter surrounding it, this becomes a center of attraction. 

 The aggregation of matter involves the arrest of motion; for par- 

 ticles moving toward the center are there stopped, but their en- 

 ergy is not destroyed. Mass motion is changed into that motion 

 of molecules which is manifested as temperature. The greater 

 the aggregation, the higher is the resulting temperature. Kadia- 

 tion also begins as soon as temperature is raised, but the rates of 

 increase and loss of heat are not necessarily the same. Even af- 

 ter there ceases to be any sensible increase of matter aggregated 

 from without, that which has been brought together tends to set- 

 tle upon the center; and this continues the transformation of en- 

 ergy already manifested. The ill-defined mass of gas would 

 gradually become viscous, until a condition of solidity would be 

 reached, fixst at either the center where the pressure is greatest, 

 or at the surface where radiation is fastest; most probably at the 

 center. We have thus great central density with intense heat; 

 around this the solid passes by insensible gi'adations into a %is- 

 cous, tarry envelope, almost equally hot; this in turn as we pass 

 outward becomes less viscous until a condition is reached like the 

 average of what we now call liquids; but differing in this respect, 

 that at first there is no well defined boundary between it and the 

 densely gaseous materials which surround it. These in turn be- 

 come thinner as we pass outward, the thinnest of all, hydrogen, 

 forming the exterior envelope, Avhich is devoid of any distinct 

 bounding surface. Throughout the entire mass the heat is so great 

 as to prevent molecules of different kinds from coming within the 

 range of chemical attraction. They are all beyond the limit of 

 chemical association. But gravity is operative, and hence the ele- 

 ments of highest specific gra\'ity, such as platinum, gold, mer- 

 cury, etc., tend to aggregate at the center, while nitrogen, oxygen, 

 and hydrogen remain near the boundary. We would thus have 

 a nucleus of heavy, unoxidized material. 



Cooling by radiation goes on most rapidly at the outer surface, 

 so that here is the first place at which chemical activity begins to 

 come into play. Hydrogen and oxygen unite to produce steam, 

 but this is far beyond its condensing point, and is separated from 

 outer cold space by a bed of still uncombined hydrogen. Long 

 remaining in the gaseous state, it slowly penetrates downward bv 

 diffusion, mixing thus with the heavier nitrogen below. Sulpliur 

 also unites with oxygen, and chlorine with hydrogen, forming sul- 



