WATER 



present in it, and this applies to all other 

 substances. 



I think we are most familiar with water in 

 its liquid state ; iron, silver, gold, copper, and 

 other metals are best known to us as solids. 

 There is one metal, however, which exists as a 

 liquid at the temperature we live in, and that is 

 mercury, or quicksilver ; but it too can be made 

 to freeze into a solid like iron or gold if we can 

 arrange to take away a sufficient quantity of heat. 

 Air, and the gases which compose it, are, as we 

 can tell, generally about us as gases or vapours. 

 Still, some of us may have heard of people who 

 can make and have uses for liquid air and liquid 

 oxygen. 



When water evaporates, and is transformed 

 into vapour, it seems to disappear into the air, 

 and we do not think much about it again until 

 it reappears near us as rain, fog, snow, or dew. 

 As the air which contains the water-vapour 

 moves about, it often ascends into the colder 

 regions of the sky above. There it is chilled, 

 and in consequence some of the water-vapour 

 is deposited in minute drops, or tiny ice crystals, 

 so small that they can only fall very slowly 

 through the air, and thus float about and appear 

 as clouds. These little drops have often some 

 difficulty in forming, even when the air is suffi- 

 ciently cold to allow them to do so ; they seem 

 to want a start, and often begin collecting round 

 a speck of dust ; in hailstones, for instance, we 



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