48 FREEZING OF WATER. 



this equilibrium, or the equal pressure in all directions, 

 we do not feel its weight. On plunging the hand 

 into a pailful of water, we feel very little of its 

 weight, only in fact that of the small portion which 

 we displace, because it is much heavier than air is ; 

 but if we attempt to lift up the whole pailful, we then 

 feel the weight of the whole bulk of water. 



61. Flame seems to ascend in the air ; this is not 

 because heat has any tendency to ascend, but because 

 it expands the surrounding air, makes it larger, and 

 therefore specifically lighter than the cold air is, and 

 the latter, therefore, displaces it and causes it to rise. 

 Hot air, though it rises in the air, nevertheless has 

 weight, though it is not so heavy as cold air. A piece 

 of cork falls through the air towards the surface of 

 the earth, it has weight ; yet the same piece of cork 

 rises upwards through water till it reaches the surface. 

 As the cork does not fall downwards in water, but 

 rises upwards, so heated air ascends in the atmosphere; 

 not because it has no weight or gravity, but merely 

 because, bulk for bulk, it has less weight than the sur- 

 rounding cold air. It is for precisely the same reason 

 that a balloon rises in the atmosphere. 



62. It is, in fact, a general rule that, when sub- 

 stances are heated, they expand ; when water is heated, 

 it becomes larger and lighter, and consequently rises 

 through the cooler portions above it. Just the re- 

 verse of this happens when substances are cooled; 

 they then become smaller and heavier. 



63. There is one remarkable exception to this rule 



