PHYSICAL PARADOXES. 



the top of the colder. To prevent the ice from 

 rising, a perforated metal disc, or bundle of 

 folded wire, or wire gauze, freely permeable to 

 water, is placed over the ice before the water is 

 poured in. This ice remains for the most part 

 unmelted at the bottom while the water is 

 briskly boiling above. 



5. Expansion and its Opposite both produced 

 by the Same Cause. 



In the last section reference was made to 

 the fact that water, like other things, expands 

 under the influence of heat. It is only another 

 way of saying this, to say that it contracts 

 with cold. Water, however, expands in a very 

 paradoxical manner, by which it is distin- 

 guished from most other substances except 

 iron, which exhibits a similar paradox. 



If a flask, filled with water, be closed by a 

 perforated stopper, to which is fitted a long 

 glass tube, the whole arrangement (see Fig. 30) 

 can be made to act as a thermometer. 



When the flask is heated, the heat makes 

 the water expand and over-fill the flask. The 

 spare quantity runs up the stem, and presently 

 overflows at the top, clearly showing the influ- 

 ence of heat in causing expansion. 



If now the flask be put into a vessel of ice 

 broken small, and covered up to the neck with 

 the ice, the temperature will rapidly begin to 



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