BRIDGMAN. — MERCURY UNDER PRESSURE. 435 



The evidence of the present data of significance for this question 

 as to the existence of either a critical point or a maximum are the 

 slope of the freezing curve, the variation of volume, and the variation 

 of the latent heat. 



The freezing curve is concave toward the pressure axis, which is 

 curvature in the direction demanded by the presence of a maximum. 

 This direction of curvature is the same as that shown by all the melt- 

 ing curves of Tammann, who uses it as an argument tending to show 

 the existence of a maximum. But the curves liquid- vapor also invar- 

 iably show the same curvature, and here there is a critical point. It 

 is also easy for a curve to run to infinite temperature and pressure, 

 still being concave toward the pressure axis. The direction of curva- 

 ture would seem to give absolutely no presumptive evidence, therefore, 

 one way or the other. 



The change of volume and the latent heat together may be consid- 

 ered. The change of volume alone invariably decreases with rising 

 temperature. This merely means that the phase with the smaller vol- 

 ume is less compressible. Tammann 's argument consists in showing 

 that the change of volume decreases along the equilibrium curve, 

 while the latent heat usually increases, or if it shows a decrease, the 

 decrease is slight in comparison with the decrease in the change of 

 volume. The change of volume becomes zero first, therefore, and we 

 have a maximum freezing temperature. 



The curves for A V and the latent heat of this present work on mer- 

 cury have been already shown. A V decreases with rising temperature, 

 as it does universally. The latent heat curve is the significant curve, 

 because it increases apparently to a maximum and seems about to 

 descend. Tammann's argument would be perfectly valid for the first 

 6000 or 7000 kgm., but at higher pressures this reversal of the latent 

 heat curve invalidates the whole thing. There is no reason to antici- 

 pate that the decrease in A^ beyond 11,000 might not be rapid enough 

 to make it vanish with A V. 



It is interesting in this connection to plot the difference of internal 

 energy between the solid and the liquid against temperature. The 

 difference is given by 



AE=ATI — pAv. 



^E shows the part of the heat of transformation which has been made 

 potential inside the mass. The Test of this heat goes into performing 

 mechanical work. The results are shown, with the steps of the calcu- 

 lation in Table XIV. and in Figure 21, in direct comparison with the 

 changes of volume. The energy decreases from the start along the 



