NON-LIVING THINGS 2 09 



Pupils already know that water and other liquids have 



weight. Discuss with them the comparative weight of 



water, oil, molasses, mercury, and other 



Pressure of liquids. Construct a hydrometer as fol- 



Liquids and 



GaseS( lows: Fit a piece of light pine about the 



size of a lead pencil into a large cork, one 

 about an inch in diameter by an inch and a quarter long 

 will do. Load the bottom of the cork, so that when placed 

 in a deep vessel of water the hydrometer will float per- 

 pendicularly. When properly loaded, mark carefully upon 

 the stick the level reached when the instrument floats at rest 

 in pure water. Why does the instrument float? Because 

 it is lighter than the water that it displaces. That is, it is 

 lighter than the amount of water which would exactly equal 

 it in bulk. To prove this, fill a vessel holding a gallon or 

 more brimful of water. Set this in a larger vessel that will 

 catch the overflow. Select some object that is heavy enough 

 to give a considerable displacement and that does not water- 

 soak. A drinking glass will do. If too light at the bot- 

 tom to float upright, load it with a few nails or shot or even 

 sand. Place it in the full vessel, and have the class note 

 that as it sinks to its level the water overflows into the larger 

 vessel. Now weigh both the object used as a float and the 

 water that overflowed, and the weights will be found to be 

 equal. 



When pupils understand this simple demonstration of the 

 law of floating bodies as it applies to objects in water, dis- 

 cuss with them the density of different liquids. What 

 liquids are lighter than water? Those most easily secured 

 for experiment are alcohol and the various kinds of oils. 

 Liquids heavier than water are salt water, sugar solutions, 

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