Gases from the Atmosphere 43 



Suspended organic solids likewise contribute nothing 

 to the food supply as long as they remain undissolved. 

 But when they decay their substance is restored to 

 circulation. Only the dissolved substances that are 

 in the water are at once available for food. The soil 

 and the atmosphere are the great storehouses of these 

 materials, and the sources from which they were all 

 originally derived. 



Gases from the atmosphere — The important gases 

 derived from the atmosphere are two : carbon dioxide 

 (CO2) and oxygen (O). Nitrogen is present in the 

 atmosphere in great excess (N, 79% to O, nearly 21%, 

 and CO2, .03^), and nitrogen is the most important 

 constituent of living substance, but in gaseous form, 

 free or dissolved, it is not available for food. The 

 capacity of water for absorbing these gases varies with 

 the temperature and the pressure, diminishing as 

 warmth increases (insomuch that by boiling they are 

 removed from it), and increasing directly as the pres- 

 sure increases. Pure water at a pressure of 760 mm. in 

 an atmosphere of pure gas, absorbs these three as 

 follows : 



At double the pressure twice the quantity of the gas 

 would be dissolved. Natural waters are exposed not 

 to the pure gas but to the mixture of gases which make 

 up the atmosphere. In such a mixture the gases are 

 absorbed independently of each other, and in propor- 

 tion to their several pressures, which vary as their 

 several densities: the following table* shows, for 



*Abridged from a table of values to tenths of a degree by Birge and Juday 

 in Bull. 22, Wise. Geol. & Xat. Hist. Survey, p. 20. 



