EVOLUTION IN GENERAL 273 



tendencies is displayed here also. When the lithosphere (and 

 hydrosphere and atmosphere) were formed certain physical 

 changes in a system of parts, or elements (the vaporous constituents 

 and their energies), occurred and the consequence of these was 

 the appearance of the envelope (say lithosphere). Now we may 

 divide up the whole earth-crust into blocks (of, say, 10,000 cubic 

 miles of volume each), and it will be easy to show that each such 

 block is, 071 the earth-scale, ver^' similar, in chemical and physical 

 nature, to any other block taken at random. That is to say, the 

 probability of uniformity of aspect of the parts of the lithosphere 

 is great and tends always to increase. 



So also with the hydrosphere and atmosphere. Thus the 

 tendencies of the changes in an initial physical-chemical system 

 that have led to the appearance of the earth-envelopes are similar 

 to those that apply to stellar evolution. 



89^. Chemical Evolution. What has occurred in the forma- 

 tion of the earth-envelopes is essentially what occurs when 

 chemical substances react with each other. W^hen, for instance, 

 coal burns carbon and hydrogen combine, or react, with oxygen 

 and heat is evolved. It used to be said that carbon and oxygen, 

 hydrogen and oxygen combined to form COo and OHo because 

 the carbon and hydrogen " had affinity " with oxygen. We say 

 now that the condition for such combinations is that energy 

 dissipates. W^hen C and O, unite, the reactants, C and O2, 

 have initially more internal energy than the resultant, COo, and 

 the balance of energy is represented by the heat given off during 

 the combustion. Initially the energy may be regarded as con- 

 centrated in the C and O2, but after the reaction it is emitted to 

 the surroundings, when the distribution of the resultant (the gas, 

 CO 2) and the energy (heat) has become more probable. That is, 

 if we sample the region all round the place where the reaction has 

 occurred it is much more probable that we shall find the elements 

 of the system (C, O, and energy) than before the reaction. 



The chemical constituents of a system thus evolve towards 

 states in which the elements and the involved energies tend to 

 more probable states of distribution. In the course of this 

 evolution the elements gradually attain chemical equilibrium, 

 when they cease to react. 



Chemical atoms themselves evolve in that they disintegrate by 

 radio-active transformations. This is notably the case with 



T 



