BRroCMAN. — MERCURY UNDER PRESSURE. 399 



stant temperature, shown by the point B on the highest melting curve 

 of the diagram, until the eutectic point is reached, where complete 

 solidification sets in. This takes place at a pressure slightly higher 

 than the normal freezing pressure for the pure mercury, as shown ex- 

 perimentally above, but the precise amount will depend on the unknown 

 way in which the proportions of the eutectic mixture have changed. 

 To obtain the data desired by this method, A must initially lie to the 

 left of the eutectic point corresponding to the particular freezing 

 pressure instead of to the right, as in the diagram. The results evi- 

 dently cannot be accurate, and it was therefore not thought worth 

 while to push the investigation further at this time, particularly since 

 the destruction of the gauge mentioned before came as the direct result 

 of the high pressure reached above. An investigation of the change of 

 eutectic proportions under pressure would make an interesting research 

 for its own sake. 



The Equilibrium Data by the Change of Volume Method. 



The method was essentially that of Tammann, and gives both the 

 change of volume and the pressure at a given temperature. It consists 

 in measuring the displacement of the piston by which pressure is pro- 

 duced as a function of the pressure. Freezing is indicated by an inter- 

 ruption of the regular rise of pressure with advance of the piston. It 

 is at once obvious that an absolute essential for the success of the 

 method, and at the same time the hardest condition to maintain under 

 high pressures, is the entire freedom from leak of the moving piston 

 and the other parts of the apparatus. Tammann found this trouble at 

 pressures above 2000 kgm./cm.^, and avoided it in part by using a very 

 heavy oil for transmitting pressure. In this present work a new pack- 

 ing device was adopted which secured entire freedom from leak even 

 with so thin a fluid as kerosene or mercury. Data will be given in 

 support of this. 



The data needed at any point in addition to the equilibrium pres- 

 sure and temperature are the amount of the advance of the piston 

 during constant pressure, the quantity of liquid involved in the 

 change, and the cross-section of the advancing piston. The first two 

 may evidently be measured directly, but the last changes with pressure 

 by an amount not susceptible to direct measurement. Here, as in all 

 pressure measurements, the distortion of the containing vessel enters 

 as a disturbing factor. But it should be pointed out that here the 

 effect of the distortion is a minimum. It has been quite impossible up 

 to the present to make direct determinations of compressibility, for ex- 



