The most important conservative 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 condensed 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 checks radiation 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, determining 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 technically 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 temperature higher than at the surface. It is a well-known property of water to dissolve certain substances, and its efficiency in dissolving many rock-substances is greatly increased when the substances are subjected to pressure and heat; and, under these conditions, it also ionizes complex substancesi. e., separates them into their simple components or ions. Accordingly when moist rocks are subjected to strong pressure at high temperature, as is frequently the case deep in the earthcrust, the rock-matter is dissolved at the points and planes of greatest pressure and precipitated or redeposited at neighboring points and planes of less pressure ; so that, for example, a crystalline cube of wet and hot rock-matter may be permanently distorted by long-continued pressure on opposite faces, the crystals gradually yielding to the stress in the direction of pressure and elongating themselves in the orthogonal directions. Through its property as a dissolving and ionizing agent, that portion of the hydrosphere which penetrates and suffuses the lithosphere has determined the texture and structure of most of our rocks ; it has transformed the muds and sands and slimes of original deposition into shales, sandstones, and limestones ; in some instances it has reconverted or metamorphosed these rocks into schists, quartzites, and marbles ; still more significantly it has aided in remetamorphosing deep-seated rocks into lavas and other crystallines. This extreme effect of water is peculiarly instructive in that it reveals something of the character of the centrosphere, whose dense materials are brought within reach of observation only by water as a solvent and sublimant in the form of lavas, vein-stones, and other rocks of hypogean origin.
Aqueous vapor of the air