The pressing of the high-lying snow, so as to form solid ice or “glacier,” is concerned with the same property of snow as that which enables us to make snow “bind” into a snowball. You cannot make snowballs during very hard frost--the snow must be in air of a thawing temperature at the moment it is squeezed by the hand. The hand itself will not be warm enough to produce that temperature when the thermometer is below freezing-point. The snow commences to melt in the hand when one squeezes it, and then when the squeezing is stopped the water formed quickly freezes again and cements the snow particles together to form ice, enclosing innumerable minute bubbles. The heat of the sun and the pressure of the weight of the snow itself take the place in the mountains of the warmth and pressure of the human hand. The minute air bubbles make the newest glacier-ice white and opaque, especially when seen in a great mass; but gradually they get squeezed together, and the glacier ice becomes first “fibrous” in appearance, and then, after long years of pressure by its own weight, fairly clear. Ice in great masses has the properties of a viscous body, like pitch or soft sealing-wax, owing to the fact that wherever the solid mass breaks its particles melt a very little and then freeze again. Under increased pressure ice melts at a lower temperature than when it is not subjected to pressure. When the pressure is removed the water freezes again. Thus crushed ice or snow can be put into a “squeeze-mould” and pressed, so as to form a solid mass of ice of any shape you may choose. Four or five slabs of ice, placed one over the other, very soon become, owing to this property, one continuous solid mass. White glacier ice is so full of air bubbles as to be comparable in structure to sponge, or, more closely, to cork. A cube of such ice exposes, owing to its rough air-hole pitted surface, a much larger surface of contact to the atmosphere than does a cube of perfectly smooth clear ice. Consequently in a warm room or chamber the white ice melts much more quickly than does the clear, and hence you should choose clear ice rather than white ice if you wish for a block which will last.