BRIDGMAN. — MERCURY UNDER PRESSURE. 387 



and shown in Figure 14. The curvature of Figure 14 is so slight, 

 however, that it seems probable that at a high enough pressure the 

 derivative will vanish and then become positive, so that ultimately 

 the internal energy will increase along an isothermal. 



The Change of State Liquid-Solid. 



The quantities involved thermodynamically in the passage of a sub- 

 stance from one phase to another at a given pressure and temperature 

 are the change of volume and the latent heat. From these quantities 

 the change of freezing temperature with pressure may be calculated by 



dr tAV 



Clapeyron's equation, -y =-rTr- -^'^^ ^^^ experimental difficulties 



of determining the latent heat are almost prohibitive. This is because 



in order to withstand high pressures the mass of the steel containing 



vessel must be much greater than the mass of the substance under 



experiment. Another method of finding these quantities based on 



Clapeyron's equation was therefore used, as in practically all the work 



done previously in this field. If the course of the freezing curve is 



dr 

 known, -j- may be found at any point, which, together with the change 



of volume at this point and the temperature of transformation, gives 

 the latent heat. 



The data were obtained by three methods. The first gives simply 

 the course of the freezing curve. This was found by measuring the 

 electrical resistance at constant temperature as a function of the pres- 

 sure, freezing being indicated by a sudden drop in the resistance to 

 about one third its value for the liquid. The second method, by far 

 the most important, gives both the melting pressure and the change of 

 volume. It is the same as that used by Tammann,^^ and consists in 

 measuring the volume as a function of the pressure at constant tem- 

 perature. At the freezing pressure, the volume suddenly changes, so 

 that by plotting volume against pressure, the point of discontinuity 

 gives both the equilibrium pressure and the change of volume. The 

 third method is concerned with a single isolated quantity on the freez- 

 ing curve, the change of volume on freezing at atmospheric pressure. 

 The existing data were not accurate enough, so this quantity had to 



^* Tammann, loc. cit., p. 204. 



