EXPANSION BY HEAT. 27 



changes of temperature is enormous, the tops of our higher 

 hills being often covered to a great depth with their own 

 ruins ; while in Kamschatka both hills and table-lands are 

 crowded with the great square blocks and slabs which have 

 been forced off by the mighty energy of the frost. Even 

 the soil is made to feel its power ; fence-posts are sometimes 

 lifted out ot their places by the heaving occasioned by the 

 formation of ice in its interstices, and the farmer finds that 

 the clods on his heavy land are more effectually broken up 

 for him by a sharp frost than they could be by any human 

 implements. 



But heat also has a share in breaking down the rocks, 

 and we might burst our iron ball just as effectually by boil- 

 ing as by freezing the water contained in it. 



Between o C. (32 Fahr.) and 4 C. (39*- Fahr.) water, 

 as we have seen, expands when cooled and contracts when 

 heated ; but above and below these temperatures it follows 

 the usual law, and like any other body, whether solid, liquid, 

 or gas, the more it is heated the more room it wants, one 

 cubic inch of water being enough to produce 1,728 cubic 

 inches of steam. 



All bodies do not expand in the same proportion, how- 

 ever, and while most metals remain solid, i.e., frozen, even 

 under the fierce sun of the equator, quicksilver is never 

 anything but liquid, even during intense frost, and expands 

 so rapidly under the influence of heat, that thermometers 

 sometimes burst from simple exposure to a hot sun. 



Iron, though remaining solid, expands perceptibly, and 

 the difference in length of the 400 miles of rails laid down 

 on the line between London and Edinburgh is 338 yards, 



