ch. viii.] THE BEGINNINGS OF LIFE. 431 



When our earth, refusing to follow in their retreat the 

 heavier portions of the solar nebula, began its independent 

 career as a planet, its surface was by no means so hetero- 

 geneous as at present. We may fairly suppose that the tem- 

 perature of that surface cannot have been lower than the 

 temperature of the solar surface at the present time, which is 

 estimated at three million degrees Fahrenheit, or some four- 

 teen thousand times hotter than boiling water. At such a 

 temperature there could have been no formation of chemical 

 compounds, so that the chief source of terrestrial hetero- 

 geneity did not exist ; while physical causes of heterogeneity 

 were equally kept in abeyance by the maintenance of all 

 things in a gaseous state. We have now to note how the 

 mere consolidation and cooling of this originally gaseous 

 planet must have given rise to the endless variety of struc- 

 tures, organic as well as inorganic, which the earth's surface 

 now presents. The origination of life will thus appear in its 

 proper place, as an event in the chemical history of the earth. 

 Let us see what must have been the inevitable chemical 

 consequences of the earth's cooling. 



In a large number of cases heat is favourable to chemical 

 union, as in the familiar instance of lighting a candle, a gas- 

 jet, or a wood-fire. The molecules of carbon and oxygen, 

 which will not unite when simply brought into juxtaposition, 

 nevertheless begin rapidly to unite as soon as their rates of 

 undulation are heightened by the intense heat of the match. 

 In like manner the phosphoric compound with which the end 

 of the match is equipped refuses to take up molecules of atmo- 

 spheric oxygen, until its own molecules receive an increment 

 of motion supplied by the arrested molar motion of the match 

 along a rough surface. So oxygen and hydrogen do not com- 

 bine when they are simply mingled together in the same 

 vessel ; but when sufficiently heated they explode, and unite 

 to farm steam. In these, and in many other cases, a certain 

 amount of heat causes substances to enter into chemical 



