HIGH PRESSURE BRIDGMAN. 



191 



sure resumed its regular rise with the displacement. This is illus- 

 trated by the curve in figure 4, in which the abscissae represent pres- 

 sures and the ordinates displacements. 



The pressure at which the piston falls into the cylinder without 

 producing a rise of pressure (that is, the vertical part of the curve in 

 fig. 1) is the pressure at which the water freezes to ice at the particu- 

 lar temperature of the experiment. For every temperature the pres- 

 sure at which the water freezes is different. When the ice is denser 

 than water the freezing temperature increases as the pressure in- 

 creases. In this way it is possible to find at what pressure water 



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i 



freezes for any given temperature, and so to construct so-called 

 "melting curves." 



It will be noted that the method given above, besides determining 

 the pressure at which water freezes at a given temperature, deter- 

 mines another factor. The amount by which the piston is pushed in 

 while the pressure remains constant evidently indicates the change of 

 volume in the water while freezing, from which the difference in 

 volume between the water and the ice can be computed. If we know 

 the density of the water, we can calculate immediately the density 

 of the ice. This is important data, since if both the temperature and 

 pressure at which the ice melts are known, together with the change 

 of volume, the amount of heat necessary to melt the ice can also be 

 computed. 



The method of experiment outlined above is not original with the 

 writer, and has, in fact, been employed by many other previous ex- 



