438 THE GEOSPHERES 



Let us now consider the relations between the geospheres. 



In the first place, the matter of the geospheres is unlike in 

 state or physical condition. The atmosphere is almost wholly- 

 gaseous; the hydrosphere is for the most part liquid, though in 

 part solid and in small part gaseous; the lithosphere is almost 

 wholly solid, though a minute part is gaseous (chiefly as im- 

 purities in the air), while a small part may be liquid under 

 temporary and local circumstances ; for the present the centro- 

 sphere may be considered a transolid. Thus the four geo- 

 spheres represent the three well-known states of matter, together 

 with a fourth state which is not certainly known from direct 

 observation. It is the marvelously delicate interrelation between 

 the three exterior geospheres that gives character to the earth 

 as the theater of life and the home of humanity ; for plant and 

 beast and man are alike dependent on the lithosphere for the 

 solid part of their bodily substance, on the hydrosphere for the 

 greater part of their sustenance, and on the atmosphere for the 

 breath of life. 



In the second place, the exterior geospheres at least are, de- 

 spite the differences in physical condition, in some degree inter- 

 mixed. The greater part of the atmosphere floats over the 

 waters and lands of the earth as a thin mantle growing more 

 and more tenuous outward ; an early estimate of its thickness 

 was forty-five miles, but the American physicist Woodward has 

 recently shown that the outer portion is much less dense than 

 at first supposed, and that the total thickness of the mantle ex- 

 ceeds the radius of the solid earth. A small part of the atmos- 

 phere is intermixed with the waters of the hydrosphere, especially 

 the running waters of rivers and brooks; another part pushes 

 down into the lithosphere, filling interstices in the rocks and 

 playing an important role in the chemical and physical changes 

 ever proceeding in the earth-crust. In like manner, while the 

 greater part of the hydrosphere exists in the oceans, lakes, 

 rivers, snow-fields and glaciers, a considerable volume rises far 

 into the atmosphere in the form of aqueous vapor, and a much 

 greater volume permeates the lithosphere as ground water or in 

 still more intimate combination with the solid earth-substance. 

 So, too, the material of the lithosphere is in small part dis- 

 solved or suspended in the waters or afloat in the air; at the 

 same time there is an obscure interrelation between the litho- 

 sphere proper and the centrosphere, manifested in volcanic and 

 other phenomena and perhaps in the presence of metals among 



