DENSITY 



35 



also, a sandy soil is " warmer " than a clay soil, and a room with damp 

 walls is chilly. 



Water vapour is an athermanous gas, i.e. it does not readily allow the 

 radiation of heat through it, unlike the other gases of the atmosphere, which 

 are diathennanous. The atmosphere of Great Britain being humid, the 

 intensity of the sun's heat in summer, and the radiation of the earth's heat 

 into space in winter, is less than in drier countries in the same latitude, and 

 the climate is consequently more temperate. 



Water contracts when it cools from ioo° to 4° C, but, unlike other liquids, 

 expands from 4° to 0°, and, on freezing, further expands to the extent of 

 9 per cent, of its volume. These changes may be studied by filling a large 

 thermometer tube and bulb with hot water (Fig. 10), and gradually cooling 

 to a temperature below the freezing point. They explain the necessity for 

 observing the temperature in calibrating burettes (p. 12), and in deter- 

 mining the density of solids and liquids (p. 31). They are of import- 

 ance ( I ) in the production of ocean currents, another factor having a great 

 influence on the climate of Great Britain ; (2) in the preservation of animal 

 life in lakes and ponds ; (3) in the rupture and decay of vegetable tissue ; 

 (4) in the heating of buildings by hot water ; (5) in the bursting of water 

 pipes during frost ; and (6) in the disintegration of rocks and soil by frost. 



The physical properties of water are compared with those of mercury and 

 alcohol in the following table. 



Physical Properties of Liquids, 



