COMMON THINGS. WATER. 



1. NEXT to air water is the most common of natural substances. It 

 is less universally present, and although the uses to which it 

 subserves are not less numerous and important, the want of it on 

 the part of the animal and vegetable creation cannot be regarded 

 as so incessant. 



"Water, according to certain varying physical conditions, may 

 exist either in the solid, liquid, or vaporous state. It is perhaps in 

 the last state that it is most universally diffused over the surface 

 of the globe ; but not being so obvious to the senses as it is when 

 in the former two states, it is not recognised except by those who 

 are familiar with the scientific tests of its presence. 



It is therefore in the liquid form that we are most familiar 

 with it. 



2. At ordinary temperatures, and exposed to common atmos- 

 pheric conditions, pure water is a colourless and tasteless liquid, 

 having great transparency. 



3. Its weight in relation to its bulk is very easily remembered, 

 for it has been found that a cubic foot weighs almost exactly a 

 thousand ounces, the temperature being 60, the ordinary tem- 

 perature of the atmosphere in these climates. 



It may also be easily remembered that an imperial gallon of 

 pure water, at this temperature, weighs 10 lb., and consequently 

 that an imperial pint or the eighth part of a gallon weighs 1 lb. 



4. All liquids expand or swell when heated, and contract when 

 cooled. This is a general fact with which every one is familiar. 

 "Water is not an exception to this. A gallon of boiling water will 

 be less than a gallon when it becomes cold, and a gallon of cold 

 water will be more than a gallon when it is heated. 



"Water is therefore rendered more dense, that is to say, heavier 

 in a % given bulk, by cooling it, and less dense, that is lighter in a 

 given bulk, by heating it. 



5. There is, however, at a certain point in the thermal scale, a 

 very striking exception to this general law in the case of water. 

 If it be gradually cooled, its dimensions will continually contract, 

 and it will become denser and denser until its temperature is 

 reduced to SS -^ of Fahrenheit's thermometer. But when it is 

 cooled below that point, instead of contracting, it is found to 

 expand ; instead of becoming denser and heavier, it becomes less 

 dense and lighter. 



"Water, therefore, .bulk for bulk, is heavier and denser at the 

 temperature of 38-^ than at any other temperature, whether 

 higher or lower. 



This is therefore called the " temperature of greatest density." 



6. "When the temperature is reduced to 32 water becomes solid. 

 This change from the liquid to the solid is called congelation 



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