236 The Realm of Nature CHAP. 



of the glacial period ( 352) subsequently modified by wind 

 and other agencies. The great German geologist, Professor 

 von Richthofen, who studied the deposit in China, came to 

 the conclusion that there it resulted from the gradual 

 accumulation of the fine dust carried by wind from Central 

 Asia, and brought to the ground by the moister air near 

 the coast. 



309. Water as a Sculpture Tool. Water is the agent 

 by which the Sun's energy is usually brought to bear upon 

 the land. The process consists in the Sun's heat evapora- 

 ting the surface of the hydrosphere and depositing it as 

 snow or rain on the land. The work done against gravity 

 in raising water-vapour to the height at which it condenses 

 to the liquid state, as rain, is converted into potential energy, 

 all of which would be restored in heat to the hydrosphere 

 if the rain fell without friction back to the sea again. Rain 

 evaporated before it reaches the sea has a new store of 

 potential energy imparted to it, like a clock wound up 

 before it has run down. The height to which a quantity of 

 water is raised by the Sun's heat is a measure of the dynamic 

 power which the water can exert in its descent ( 49). 

 This power in the case of raindrops is expended in heating 

 the air they fall through, and in friction against the channel 

 down which the water flows, in breaking off portions of 

 rock against the power of cohesion, and in dragging stones 

 or gravel along. The expended energy finally takes the 

 form of diffused heat in the water and rocks. The chemical 

 properties of water and its effects as a solvent are also 

 brought into action by sun-heat, which separates it from the 

 salts in the sea, shakes it with the gases of the atmosphere, 

 and pours this powerfully solvent and oxidising solution over 

 the rocks. The hydrosphere might be compared to a bee- 

 hive, whence the sunlight .attracts swarms of workers in the 

 form of raindrops, which after a longer or shorter journey 

 return laden with spoil from the land. 



310. Weathering. Rain, assisted by the dissolved 

 gases and surrounding air, acts chemically on rock surfaces, 

 producing changes known as weathering. Next to beds of 

 rock-salt and gypsum (calcium sulphate), limestone is the 



