THE PHYSICS OF ICE. 



411 



is shown the source of the Arveiron and the sublime view of the foot 

 of the glacier whence it issues. Of this M. Rendu says, " It is a vast 

 portico more than a hundred feet high, let into an immense fa^ade^ 

 and surmounted by lofty pyramids of ice. ^N'othing is more astonish- 

 ing than this work of the elements, of which Xature alone has con- 

 ceived the plan, and achieved the construction." 



Fig. 10. 



Source of the Akveirox 



Ice presents, under pressure, many phenomena of great interest 

 other than those mentioned, and to which we can only refer. The 

 prismatic or crystalline form, so beautifully developed in lake-ice, 

 is more or less destroyed in glaciers by the unceasing fracture and 

 regelation which takes place from pressure, and the mass assumes 

 a granular structure. The same phenomenon occurs with ice in a 

 mould. In glaciers are veins which reflect a deeper tint of blue, in- 

 dicating where from local causes greater or more persistent press- 

 ure has cleared it of bubbles of air. Glacier, and probably other ice, 

 under similar conditions of pressure, becomes laminated, or develops 

 planes of cleavage, resembling those of slate-rock in the quarry ; 

 and this structure is shown in the decay of the ice, as its prismatic 

 structure is shown in the decay of that on lakes and rivers. 



By the physical properties we have noticed, ice becomes a dynamic 

 agent of tremendous power. The play of forces and plasticity of 

 structure which make it a toy in the laboratory, have changed the 

 aspects of N'ature, and modified the surface, as they have the distri- 

 bution of life, upon a large portion of the globe. Rocks are broken 

 by its expansive energy, but they are also crushed by its weight, and 

 ground to dust in its irresistible motion. A "sheet of ice a mile in 



