THE FORMATION OF GLACIERS. 
215 
being in a measure transparent to beat, the water 
below an) r frozen surface is nearly as susceptible 
to the elevation of the temperature without as if 
it were in immediate contact with it. Such 
changes of temperature produce air-bubbles, which 
float upward against the lower surface of the ice 
and are stranded there. At night there may 
come a severe frost ; new ice is then formed be- 
low the air-bubbles, and they are thus caught and 
imprisoned, a layer of air-bubbles between two 
layers of ice, and this process may be continued 
until we have a succession of such parallel lay- 
ers, forming a body of ice more or less permeated 
with air. These air-bubbles have the power also 
of extending their own area, and thus rendering 
the whole mass still more porous ; for, since the 
ice offers little or no obstacle to the passage of 
heat, such an air-bubble may easily become heated 
during the day 5 the moment it reaches a temper- 
ature above thirty-two degrees, it melts the ice 
around it, thus clearing a little space for itself, 
and rises through the water produced by the ac- 
tion of its own warmth. The spaces so formed 
are so many vertical tubes in the ice, filled with 
water, and having an air-bubble at the upper ex- 
tremity. 
Ice of this kind, resulting from the direct con- 
' gelation of water, is easily recognized under all 
circumstances by its regular stratification, the 
