les, 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 below 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
layers, 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; the moment it reaches a temperature
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
action 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 extremity.

Ice of this kind, resulting from the direct congelation of water, is
easily recognized under all circumstances by its regular
stratification, the alternate beds varying in thickness according to the
intensity of the cold, and its continuance below the freezing-point
during a longer or shorter period. Singly, these layers consist of
irregular crystals confusedly blended together, as in large masses of
crystalline rocks in which a crystalline structure prevails, though
regular crystals occur but rarely. The appearance of stratification is
the result of the circumstances under which the water congeals. The
temperature varies much more rapidly in the atmosphere around the earth
than in the waters upon its surface. When the atmosphere above any sheet
of water sinks below the freezing-point, there stretches over its
surface a stratum of cold air, determining by its intensity and duration
the formation of the first stratum of ice. According to the alternations
of temperature, this process goes on with varying activity until the
sheet of ice is so thick that it becomes itself a shelter to the water
below, and protects it, to a certain degree, from the cold without. Thus
a given thickness of ice may cause a suspension of the freezing process,
and the first ice-stratum may even be partially thawed before the cold
is renewed with such intensity as to continue the thickening of the
ice-sheet by the addition of fresh layers. The strata or beds of ice
increase gradual