BRIDGMAN. — MERCURY UNDER PRESSURE. 433 



which we are familiar for the more compressible liquids under lower 

 pressures. The compressibility and the dilatation both decrease with 

 rising pressure. This has as a consequence that the effect of rising 

 temperature is as usual to increase the compressibility. The behavior 

 of the specific heats is rather unusual, approximate constancy for Gp 

 and increase of C^ with pressure. For water, Cp — C^ increases instead 

 of decreasing. As a little surprising, but not unusual, it may be men- 

 tioned that the internal energy decreases along an isothermal with in- 

 creasing pressure, but increases, as is normal, along an adiabatic. This 

 means that along an isothermal the attractive force between the mole- 

 cules is still the dominant factor in the situation, even up to 12,000 

 kgm. At higher pressures a reversal of the effect is indicated, where 

 the repulsion between the molecules would become the important part. 



The data are more suggestive in the light they throw on the change 

 from the liquid to the solid state. In this connection it may be useful 

 to outline briefly the present state of the theory, and the points at issue. 



For the change of state liquid-vapor, the physical facts have been 

 worked out pretty fully until we now have a fairly definite understand- 

 ing of the nature of the process. This increase of knowledge has come 

 about almost entirely from the experimental side, the one thermody- 

 namic relation not being of much assistance. In particular, the existence 

 of the critical phenomena, and the possibility of the continuous pas- 

 sage from the liquid to the vapor were facts which were unexpected 

 before the actual experimental proof. There is no thermodynamics 

 which would predict the existence of such a point. Our present knowl- 

 edge of the nature of the equilibrium between liquid and solid is in 

 much the same state as the knowledge of the transformation liquid- 

 vapor before the discovery of the critical point. The fundamental 

 question as to whether a critical point exists or not is therefore of 

 greatest immediate interest. 



At present, both the most important theory of the equilibrium between 

 solid and liquid and the most far reaching experimental work are due 

 to Tammann.61 The essential part of his theory, as opposed to Ost- 

 wald,52 Poynting,53 and others, is that for the transition solid-liquid 

 there is no critical point like that for the transition liquid-vapor. The 

 reason for this fundamental distinction is to be found in the essential 

 similarity between a liquid and a vapor on the one hand, and the essen- 



'*■ Tammann, loc. cit. 



"^ Ostwald, Lehrbufh dor Allgcmeinen Chemie, IF^, Verwandtschaftslchre, 



3S9 (Engolmann, Leipzig, 1902). 



" PojTiting, Phil. Mag. (.5), 12, 32 (1881). 



VOL. XLVII. — 28 



