PRESIDENTIAL ADDRESS. 131 
part is evaporated back into the air, and part flows off the 
land into the rivers—into the sea. The portion that has 
fallen on the soil, and entered it, carries some of its gases 
with it; the oxygen oxidises, the carbon dioxide forms car- 
bonates, and both may thus have become fixed for the time, 
constituting portions of solids. The part that has evaporated 
back into the air will carry, no doubt, some of its dissolved 
air-gases and some of the new gases that may have been 
produced in the meantime; the part finding its way to the 
sea may have passed over land such as limestone, which its 
gases have partially dissolved and carried into the sea; many 
other chemical changes may have been brought about by 
these air-gases on the way, resulting in the formation of 
other solid, liquid, or gaseous compounds. We know that 
such traffic is going on, and that, arrived at the sea, some of 
the products help to build up shells, and bones, and coral 
reefs, and new continents. Some of these air-gases may be 
returned to the air again shortly, or only after countless 
ages. The animal and plant life of the sea are just as 
dependent upon these gases as are the same lives upon the 
land, and for that reason nature has to provide by wind and 
wave an aerated ocean. But a rain-shower is not necessary 
to effect a great interchange of gases between air and sea and 
land. 
A sheet of water, or a moist surface, has the power of dis- 
solving gases, and air has the power of dissolving or absorb- 
ing water in the vapourous state. This interchange is con- 
stantly going on. The amount of air dissolved by water 
depends upon the composition, the temperature, and pres- 
sure. We have to remind ourselves that there is a larger 
proportion of oxygen and carbon dioxide in the air-gases 
dissolved in water than what exists in the air itself, thus 
increasing its chemical action upon rock and soil and in- 
habitant of the waters. 
As the pressure and temperature are constantly changing, 
so the exchange of gases must be constantly going on, for 
this reason. Indeed, each puff of wind must have its effect. 
But, even if temperature and pressure were constant, there 
would still be a continued exchange. 
Take arable land as an example of porous ground, The 
moist ground will, as we -have seen, absorb and give out air- 
gases; but the porous ground will have a constant inter 
change of air in at least two ways. As the pressure of the 
air increases, more air will be pressed into the ground; as 
the air. pressure falls, gases will be withdrawn—a simple case 
of expansion and contraction of gas. For the most part, 
