58 SCIENCE PRIMERS. [MATERIAL 



brought about by heating water. At first, it ex- 

 pands gradually and slightly ; but, when it reaches the 

 boiling-point, it suddenly expands enormously, and is 

 no longer a liquid, but a gas. 



On the other hand, if warm water is allowed to 

 cool, it gradually contracts till it reaches the ordinary 

 temperature of the air in mild weather; but, if the 

 weather is very cold, or if the water is cooled 

 artificially, it goes on contracting only down to a 

 certain temperature (39), and then begins to expand 

 again. In this peculiarity water is unlike all other 

 bodies which are fluid at ordinary temperatures. 

 Hence the temperature of 39 is that at which pure 

 water has its greatest density or specific gravity, and 

 water at this temperature is heavier, bulk for bulk, 

 than the same water at any ether temperature. There- 

 fore if water at the top of a vessel is cooled down 

 to this temperature, it falls to the bottom, and if the 

 water at the bottom of a vessel is cooled below this 

 temperature it rises to the top. 



40. Water cooled still further becomes the 

 transparent brittle solid Ice. 



Our tumbler of water, if put out of doors on a cold 

 winter's night, would gradually cool until it assumed a 

 temperature of 39 throughout. Cooling below this 

 temperature, the water so cooled would gradually 

 accumulate at the surface by reason of its less density, 

 and its temperature would fall till the thermometer 

 placed in it marked 32. As soon as this upper water 

 cooled ever so little below 32, a film, like glass, would 

 form on its surface by the conversion of the coldest 

 fluid water into solid water, or ice. And if all the 



