ACTION OF WATER. 45 



(ii.) By alternate frost and thaw. Ice, as is well known, 

 occupies more space than the water from which it is 

 formed. The increase in volume amounts to about 

 10%, and the force exerted by water in freezing is 

 almost irresistible. Indeed, freezing cannot take place 

 without this expansion, and if it be prevented the 

 water remains liquid, though its temperature be 

 reduced much below 0C. It is found that if an 

 additional atmosphere of pressure be exerted upon 

 water its freezing point is lowered by -0075 C. 

 The bursting of water pipes in winter is a familiar 

 consequence of this expansion in freezing. 



In nature the disintegration of rock is greatly 

 aided by this action of water. During the warm part 

 of a winter's day the crannies and crevices of a rock 

 become filled with water. As the temperature falls 

 the w r ater begins to freeze, at first on the outside, so 

 that every crevice becomes stopped with a plug of 

 ice, the still fluid water behind the plug continues to 

 lose heat, and therefore tends to solidify. This it 

 can only do if it can increase its bulk by about 10 %. 

 In order to do this it must either w r iden or lengthen 

 the crevice which contains it. When the next thaw 

 comes, the widened or deepened crevice again fills 

 with water, and in the next frost repeats the action 

 described. This process, going on at hundreds of 

 places on the surface of a rock, soon breaks it up into 

 smaller fragments, and these in turn are subjected to 

 the same action. The process is necessarily confined 

 to the outermost layer and is only of much importance 

 when frost and thaw alternate rapidly. Long con- 

 tinued frost appears to protect rocks from weathering, 

 the ice formed cementing the whole surface together. 



(iii.) As glaciers. Glaciers have played a very important 

 part both in grinding and wearing down rocks into 

 the finest powder and also in transporting the 

 materials, in some cases for hundreds of miles. The 

 water which issues from the snout of a glacier is 

 always heavily laden with the finest mud, and huge 



