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Parr. Sucr. ii. §5.] ACTION OF FROST. 401 
into ice. In this condition it performs a series of important 
geological operations before being again melted and relegated to 
the general mass of liquid terrestrial waters. Five conditions under 
which ice occurs on the land deserve notice, viz., frost, frozen rivers 
and lakes, hail, snow, and glaciers. 
Frost.—Water in freezing expands. If it be confined in such 
a way that expansion is impossible, it remains liquid even at tem- 
peratures far below the freezing point; but the instant that the 
pressure is removed this chilled water becomes solid ice. There is a 
constant effort on the part of the water to expand and become solid, 
very considerable pressure being needed to counterbalance this 
expansive power, which increases as the temperature sinks. At 30° 
Fahr. the pressure must amount to 146 atmospheres, or the weight 
of a column of ice a mile high, or 138 tons on the square foot. 
Consequently when the water freezes at a lower temperature its 
pressure on the walls of its enclosing cavity must exceed 138 tons on 
the square foot. Bombshells and cannon filled with water and 
hermetically sealed have been burst in strong frosts by the expansion 
of the freezing water within them. In nature the enormous pressures 
_which can be obtained artificially occur rarely or not at all, because 
the spaces into which water penetrates can hardly ever be so securely 
closed as to permit the water to be cooled down considerably below 
32° Fahr. before freezing. But ice forming at even two or three 
degrees below the freezing point exerts an enormous disruptive 
force. 
Soils and rocks being all porous, and usually containing a good 
deal of moisture, have their particles pushed asunder by the freezing 
of this interstitial water. Stones, stumps of trees or other objects 
imbedded in the ground are squeezed out of it. Whena thaw comes, 
the soil seems as if it had been ground down in a mortar. Water 
freezing in the innumerable joits and fissures of rocks exerts great 
pressure upon the walls between which it lies, pushing them asunder 
_as if a wedge were driven between them. When this ice melts, the 

separated masses do not return to their original position. Their 
centre of gravity in successive winters becomes more and more 
displaced, until the sundered masses fall apart. In mountainous 
districts, where the winters are severe, and in high latitudes, much 
waste is thus produced on exposed cliffs and loose blocks of rock. 
Some measure of its magnitude may be seen in the heaps of angular 
rubbish which in these regions so frequently lie at the foot of crags 
_and steep slopes. At Spitzbergen and on the coast of Greenland the 
observed amount of destruction caused by frost is enormous. The 
_ short warm summer, melting the snow, fills the pores and joints of 
the rocks with water, which when it freezes splits off large blocks, 
launching them to the base of the declivities, where they are further 
broken up by the same cause. 
Frozen Rivers and Lakes.—In countries such as Canada the 
_ lakes and rivers are frozen over in winter with a cake of ice 14 to 24 
2D 
