200 THE ATMOSPHERE 



Gases diffuse with a velocity inversely as the sq. rt. of their 

 densities, but diffusion is so slow a process that it would take 

 ages to restore uniformity to a perfectly still atmosphere if this 

 were once effectually disturbed. The ceaseless convection currents 

 due to the sun's heat keep the atmosphere mixed and practically 

 uniform in composition. 



Deoxygenated or impure air only lurks in mines, tunnels, wells, 

 and brewers' vats, and large public halls, and these require 

 artificial methods of ventilation. In small rooms the ventilation 

 which takes place through the walls is usually sufficient, but in 

 large rooms as the air content increases as the cube while the 

 wall surface increases as the square, the ventilation is imperfect, 

 and stagnant air fills the central parts (Haldane). 



When a mixture of gases dissolves in a liquid, each component 

 dissolves according to its own partial pressure (Dalton's law). 

 There are divergences from these laws in the case of very soluble 

 gases and at great pressures. The chief gases of the atmosphere 

 are sparingly soluble, are neutral gases forming neither acidic nor 

 basic substances by their union with water. The higher the 

 temperature and the less the partial pressure, the more the gas 

 molecules by their increased mobility escape from their solvent. 



The air respired by aquatic animals is that dissolved in the 

 water, and as the respective solubilities of N 2 and 2 at C. 

 are "02 and '04 respectively, water holds in proportion to nitrogen 

 more oxygen than the atmosphere. The amount of these gases 

 dissolved in the sea appears to be uniform at all depths thus : 



Depth. Vol. O 2 per Litre. Vol. N 2 per Litre. 



6 : l-7'8 11-3-14-1 



3219 m. 6-7 12*9 



The C0 in the sea is combined as bicarbonate with calcium 

 carbonate, of which there is always an excess (CaC0 3 + H 2 C0 3 ). 



The composition of the atmosphere probably has been almost 

 constant throughout the more recent geological ages. It is 

 kept so by the metabolic antagonism of the plant and animal 

 world. The food-stuffs of green plants, C0 2 , H 2 0, nitrates, and 

 other mineral salts are saturated oxygen compounds. They 

 possess no more potential energy than a stone lying on the 

 earth. The energy of the sunlight activates by means of 

 chlorophyll the plant protoplasm, and this sundering oxygen from 

 carbon and hydrogen builde compounds which are themselves 



