EXPANSION BY COLD. 25 



sure of the ice, although we, 01 course, are not made aware 

 of the fact until the ice melts again. If empty when the 

 frost sets in they do not suffer. 



But though we may grumble at the plumber's bill, or 

 perhaps at somebody's carelessness, we should be in a yet 

 worse plight if water did not expand on becoming ice, for in 

 that case the greater part of Europe would probably be 

 uninhabitable, and all the fishes in lakes and rivers would 

 die. The water at the surface, as it grew cooler, and, there- 

 fore, heavier, would sink to the bottom, while the warmer, 

 lighter water from beneath would take its place, and be 

 cooled in its turn, and again sink, and this exchange would 

 go on until the whole mass of water was congealed into 

 solid ice, which the summer heat would be powerless to 

 melt. As it is, however, no sooner has the whole of the water 

 in a pond been cooled down to 4 C. (39^ Fahr.) than this 

 vertical movement ceases ; and although the water on the 

 surface may continue to lose heat, instead of shrinking to- 

 gether, and so becoming heavier, as it has done up to this 

 point, it now does precisely the reverse. Below 4 C. it ex- 

 pands, becomes consequently lighter, remains on the surface, 

 and is frozen into a crust of ice. 



But how does all this affect the rocks ? for they are our 

 present concern. 



All rocks are more or less jointed, because all have 

 undergone drying or cooling, and have shrunk somewhat in 

 the process, and while some, like basalt, show regular lines 

 of division, others, such as chalk, clay, gravel, and even sand, 

 show certain irregular lines, along which water, of course, 

 makes its way more readily than in any other direction. But 



