168 THE FOKMS OF WATEE IN 



427. But if the ice could be caused to melt without this 

 supply of solar heat, a temperature lower than that of 

 ordinary thawing ice would result. When snow and 

 salt, or pounded ice and salt, are mixed together, the 

 salt causes the ice to melt, and in this way a cold 

 of 20 or 30 degrees- below the freezing point may be 

 produced. Here, in fact, the ice consumes its own 

 warmth in the work of liquefaction. Such a mixture 

 of ice and salt is called 6 a freezing mixture.' 



428. And if by any other means ice at the temperature 

 of 32° Fahrenheit could be liquefied without access of 

 heat from without, the water produced would be colder 

 than the ice. Now Professor James Thomson has proved 

 that ice may be liquefied by mere pressure, and his 

 brother, Sir William Thomson, has also shown that 

 water under pressure requires a lower temperature to 

 freeze it than when the pressure is removed. Professor 

 Mousson subsequently liquefied large masses of ice by a 

 hydraulic press ; and by a beautiful experiment Professor 

 Helmholtz has proved that water in a vessel from which 

 the air has been removed, and which is therefore relieved 

 from the pressure of the atmosphere, freezes and forme 

 ice-crystals when surrounded by melting ice. All these 

 facts are summed up in the brief statement that the 

 freezing point of water is lowered by pressure.* 



429. For our own instruction we may produce the 



* Professor James Thomson and Professor Clausius proved this inde- 

 pendently and almost contemporaneously. 



