WATER 75 



ena which he studies; 1 and the physiologist 

 has found that water is invariably the prin- 



14 '0f all the terrestrial agents by which the surface of 

 the earth is geologically modified, by far the most important 

 is water. We have already seen, when following livpogene 

 changes, how large a share is taken by water in the phenom- 

 ena of volcanoes and in other subterranean processes. Re- 

 turning to the surface of the earth and watching the opera- 

 tions of the atmosphere, we soon learn how important a part 

 of these is sustained by the aqueous vapor that pervades 

 the atmosphere. 



"The substance which we term water exists on the earth 

 in three well-known forms: (1) gaseous, as invisible vapor; 

 (2) liquid, as water; and (3) solid, as ice. The gaseous 

 form has already been noticed as one of the characteristic 

 ingredients of the atmosphere. Vast quantities of vapor 

 are continually rising from the surface of the seas, rivers, 

 lakes, snow fields, and glaciers of the world. This vapor 

 remains invisible until the air containing it is cooled down 

 below its dewpoint, or point of saturation, — a result which 

 follows upon the union or collision of two aerial currents of 

 different temperatures, or the rise of the air into the upper 

 cold regions of the atmosphere, where it is chilled by expansion, 

 by radiation, or by contact with cold mountains. Condensa- 

 tion appears only to take place on free surfaces, and the 

 formation of cloud and mist is explained by condensation 

 upon the fine microscopic dust of which the atmosphere is 

 full. At first minute particles of water vapor appear, which 

 either remain in the liquid condition, or, if the temperature 

 is sufficiently low, are frozen into ice. As these changes take 

 place over considerable spaces of the sky, they give rise to 

 the phenomena of clouds. Further condensation augments 

 the size of the cloud particles, and at last they fall to the sur- 

 face of the earth, if still liquid, as rain ; if solid, as snow or 

 hail; if partly solid and partly liquid, as sleet. As the 

 vapor is largely raised from the ocean surface, so iu great 



