GLACIERS AND ICE. 
43 
To answer this question let us trace the history of the 
glacier itself from its birth-place in the snow-fields above, to its 
dissolution in the valleys below. The surface of the snow- 
field is melted by the daily action of the sun, but being subse- 
quently refrozen, a crust is produced sufficiently strong, before 
the sun melts it, to bear the weight of the climber. Below this 
platform the snow is dry and incoherent ; it will not “ bind 33 
when squeezed together; it is like so much white dust or 
flour : nevertheless this is the material, which, by the unceasing 
action of directed force, is fashioned into a glacier. It is by 
the repetition of the sun’s action just described, that the first 
change is wrought. The liquefaction of the superficial portions 
of the snow-crust creates little rivulets, which, piercing into 
the under snow, deliver their warmth to the mass around as 
they themselves become frozen. In this way the external 
heat is conveyed to the interior of the snow, in the end 
elevating’ its temperature to 32° Fahr., the freezing point of 
water. If the snow be now examined it can be rolled or 
squeezed into a coherent mass ; in fact, we have here dis- 
covered what every schoolboy knows, that snowballs can only 
be made when a thaw sets in. As the snow lies on the sides 
of the mountain the action of gravity tends to pull the mass 
downward. The pressure thus developed acts like the squeeze 
of our hands on a snowball, it binds together the snow-slope 
in those , parts where the temperature is not below 32°, and 
throughout the whole it brings the particles of snow into 
closer contact, gradually ejecting the air which lies between 
the granules. 
Snow is white, not by itself, but from its state of division ; 
sugar and salt are white from the same reason. Pound a 
transparent lump of sugar-candy or rock-salt, and the 
powder is opaque and white like snow. The transparency of 
the solid is due to its particles being in perfect contact ; the 
opacity of the powder is caused by the breaking up of this 
continuity. In the former case the light readily glides 
through ; in the latter, from a well-known optical law, a 
little light is pitched back to the eye as it passes and emerges 
from particle to particle. A luminous beam is thus unable to 
struggle through the entanglement of air and solid, for though 
each alone is transparent, their intermixture becomes opaque 
from the incessant “ echoing 33 of light that is aroused. From 
this cause when glass, alum, or ice is crushed, an opaque 
white powder results ; and conversely, when sugar, spermaceti, 
stearine, or snow is melted or powerfully squeezed together, 
a translucent substance is produced. 
The mountain-snow is thus squeezed by the superjacent 
portions ; it yields, and as some of the air entrapped in the 
