COMPKESSIBILITY OF METALS. 



203 



incident at 30°; the points all lay smoothly and there was no percep- 

 tible permanent deformation after the run. At 75°, however, which 

 is much nearer the melting point of this very soft substance, there was 

 permanent set after the run, and it was obvious from the shape of the 

 curve that the set had taken place during decreasing pressure, at the 

 lower end of the pressure range. This set may easily have been due 

 to viscosity in the transmitting medium. The last points at 75° were 

 therefore discarded. Except for these points the mean departure of 

 all the observed points from a smooth curve was 0.22%, and the maxi- 

 mum deviation from linearity 7.4% of the maximum pressure effect. 

 The deviation from linearity was perceptibly greater at the higher 

 temperature. Furthermore, the deviation is not symmetrical about 

 the mean pressure, and it is not possible to represent the change of 

 volume by a two constant formula in the pressure. In fact the devia- 

 tion from linearity is so marked that I have thought it best to com- 

 pute the changes of volume directly from the original data at every 

 thousand kilogram interval at 30° and 75°. The values so computed 

 are listed in the following table. The change of volume at each 



TABLE II. 



Change of Volume of Sodium under Pressure. 



temperature is computed in terms of the volume at atmospheric 

 pressure and 20° (room temperature) as unity. 



For comparison with the initial compressibility of Richards we may 



