120 THE FOEMS OF WATER IN 



experiment suffices to show the expansion of the water 

 by heat. 



307. We now take a common finger-glass and put 

 into it a little pounded ice and salt. On this we place 

 the flask, and then build round it the freezing mixture. 

 The liquid column retreats down the tube, proving the 

 contraction of the liquid by cold. We allow the shrinking 

 to continue for some minutes, noticing that the down- 

 ward retreat of the liquid becomes gradually slower, 

 and that it finally ceases altogether. 



308. Keep your eye upon the liquid column; it 

 remains quiescent for a fraction of a minute, and then 

 moves once more. But its motion is now upwards 

 instead of downwards. The freezing mixture noiv acts 

 exactly like the flame. 



309. It would not be difficult to pass a thermometer 

 through the cork into the flask, and it would tell us the 

 exact temperature at which the liquid ceased to contract 

 and began to expand. At that moment we should find 

 the temperature of the liquid a shade over 39° Fahr. 



310. At this temperature, then, water attains its maxi- 

 mum density. 



311. Seven degrees below this temperature, or at 32° 

 Fahr., the liquid begins to turn into solid crystals of ice, 

 which you know swims upon water because it is bulkier 

 for a given weight. In fact, this halt of the approach ing 

 molecules at the temperature of 39°, is but the prepara- 

 tion for the subsequent act of crystallisation, in which 



