HOW LIFE CAME TO EARTH 185 



earth is definitely and sharply fixed by 

 temperature. At a white heat such as exists 

 in the sun's atmosphere, we have seen that 

 only elements can exist, and many of these 

 are decomposed into proto-elements. At a 

 somewhat lower temperature binary com- 

 pounds, such as the oxides, can remain in 

 equilibrium, in incomplete combination, 

 becoming more and more complete as the 

 temperature falls, and as soon as their exist- 

 ence becomes possible these oxides do exist. 

 Lower still in the scale of temperature, 

 saline compounds, such as chlorides of the 

 alkalies, and mutually neutralized acidic and 

 basic oxides combined together, can stand 

 the heat. Such bodies as the carbonates of 

 calcium and magnesium can now be present 

 in an incomplete state of combination, 

 partially as oxide and partially as carbonate, 

 in labile balance as the temperature fluctu- 

 ates up or down, and the pressure of carbon 

 dioxide in the atmosphere changes. When- 

 ever the environmental conditions make their 

 presence possible, these more complex forms 

 must promptly make their appearance by 

 chemical law. 



But it is only at a very much lower tempera- 

 ture that compounds at all complicated in 

 chemical structure can exist in equilibrium, 



