THE PHTSICS OF ICE. 



409 



in the one case liquefaction is produced by pressure, in the other by 

 solar heat. Gradually the under4ayers become incipient glacier-ice. 

 Movement of the mass originates in its gravity, and the direction 

 must be down the slope on which it lies. Many streams in this way 

 blend into immense rivers of ice, often several hundred feet in depth. 

 The snow when first consolidated is filled with air-bubbles, and is 

 white and opaque. Its whiteness disappears by expulsion of the air- 



FiG. 8. 





Showing the Confluents of the Mek de Glaoe, 



bubbles from pressure as the glacier moves down the valley. At its 

 termination the ice is transparent, and its exquisite tints of blue indicate 

 the extreme minuteness of the reflecting surfaces which linger in it. 

 The physical properties of ice by which it flows need not be re- 



