CHAPTER IV. 



THE CRYSTALLISATION OF WATER : SNOW AND ICE. 



During the greater part of the year, in this country, 

 the atmospheric moisture is condensed in a liquid state, 

 partly as rain and partly as dew. But, when the temperature 

 of the air falls below the freezing-point, the water, unable 

 any longer to retain this Hquid condition, is reduced to a 

 solid form ; and the moisture is consequently precipitated 

 as snow instead of rain, and as hoar-frost instead of dew. 

 It is of much importance to have some knowledge of the 

 manner in which this great change in the physical condition 

 of water is brought about. 



Daily observation shows that almost everything gets 

 smaller as it is cooled. Reduction of temperature, as a 

 rule, causes the particles of which a given body is composed 

 to be brought more closely together, and the substance 

 consequently shrinks in bulk. Suppose a quantity of air is 

 confined in a vessel standing over water or quicksilver, and 

 that it is found at a given temperature to occupy a certain 

 bulk ; then, on lowering its temperature, the air will shrink in 

 volume, or occupy less space, so that the water or quick- 

 silver will tend to rise in the vessel, and thus fill the space 

 which would otherwise be left by contraction of the air. 



