BRIDGMAN. 



TIIEUMODYXAMIC PROPERTIES OF WATER. 



353 



for Cp. This quantity, the specific heat at constant volume, has more 

 theoretical sifjnifieance tlian the other specific heat, since this repre- 

 sents the heat going into the rise of internal energy of the liquid when 

 the temperature rises, and does not involve the work done against 

 external pressure in expanding the liquid. The external work in- 



123466789 10 U12 

 Pressure, kgm. / cm.^ x 10' 



Figure 12. The specific heat at constant volume of water as a function 

 of the pressure. 



volves in a complicated and at present unknown way the action of 

 the surface layer, while the specific heat at constant volume does 

 not contain this surface effect. This specific heat is therefore one of 

 the quantities mentioned in the beginning as having significance be- 

 cause it does not involve the unknown attractive forces between the 

 molecules as displaj^ed in the surface layer. In order to show this 

 independence of the surface layer, of course C„ should be plotted 

 against a variable not itself involving the action of the layer. It is 

 evidently not adequate, therefore, to plot C„ against the pressure as 

 as been done in Figure 12. C„ plotted against volume may be ex- 

 pected to show this independence of the action of the surface layer. 

 It is shown so plotted in Figure 13. The figure is of the same general 

 character as that in which it is plotted against pressure, but the 

 separation of the curves for the different temperatures is greater, 

 partl\- because the curves do not start from a common origin. The 

 minimum on the curves for 40° and 60° comes at a lower pressure 

 than it does in the former figure, and the upper end of the 80° curve 

 is perhaps a trifle steeper at the upper end, but there are no essential 

 differences. The entire behavior of the curves is not what one would 



