THE GEOSPHERES 443 



radiation of its heat into space it would soon be refrigerated ; 

 we know too that in this case the temperature of its surface 

 would be determined solely by two factors, viz : (1) the temper- 

 ature of the ball itself, and (2) the temperature of interstellar 

 space. Now, on examining our planet as an actual thing and not 

 as a figment of the imagination, we do not find that the tem- 

 perature of its surface is determined, or even perceptibly affected, 

 by its own proper heat ; we do find that the temperature of the 

 external earth is determined by the heat received from the sun. 

 It follows, of course, that the earth is not merely a cooling ball 

 suspended in cold space. On- examining more critically the 

 conditions determining our temperature we find there are two, 

 viz : (1) insolation or accession of solar heat, and (2) conserva- 

 tion of a considerable part of this heat for a time by a terrestrial 

 mechanism. This mechanism resides chiefly in interrelations 

 among the exterior geospheres. The most important conserva- 

 tive agency is the aqueous vapor of the air, which not only stores 

 quantities of heat to be given off on condensation, but serves to 

 check radiation from the earth into space. When the sun shines 

 on the ocean, a film of water is evaporated to be borne high in 

 the clouds and carried far over the mainland ; when it is con- 

 densed a part of its heat is- employed in raising the temperature 

 of surrounding air, water, and rock; so that water, chiefly in 

 the form of vapor, stores heat more effectively than any other 

 substance with which we are acquainted. Still more efficient is 

 aqueous vapor as a blanket checking evaporation ; dry air is 

 diathermous, but vapor-laden air checks radiation from the 

 earth as a garment checksradiation from the body. Since there 

 is no part of the earth, even on the deserts and polar ice-fields, 

 in which there is not an appreciable quantity of aqueous vapor 

 in the air, this substance forms a clothing for the earth, deter- 

 mining its temperature, rendering it habitable, and making it 

 what it is today, the stage of human activity. 



There is another class of special relations between the geo- 

 spheres which I should like to bring before you, partly as a new 

 discovery. As before pointed out, the rocks of the earthcrust or 

 lithosphere are permeated by water in the form known techni- 

 cally as ground water or phreatic water. Now one of our most 

 distinguished geologists, Professor Van Hise of the University 

 of Wisconsin, has recently shown that this ground water plays 

 an important role in changing the texture and structure of 

 rocks, especially at depths where the pressure is great and the 



