360 PROCEEDINGS OF THE AMERICAN ACADEMY. 



above zero under high pressure, it is impossible by this method to push 

 the compressibility determinations of mercury to the freezing curve of 

 mercury, as was desired. The difference between the freezing pressure 

 of water and mercury is not very great, comparatively speaking, being 

 about 1500 kgm./cm.^ at 15°. By pushing the pressure into the un- 

 stable region for water, it is possible to come closer than this to the 

 freezing curve for mercury. At 0° the approach was made to within 700 

 kgm., and at 22° to within 1000 kgm. Attempts to come closer than 

 this failed. On several occasions the two piezometers, one filled with 

 water and the other with mercury and water, were subjected to pressure 

 simultaneously beyond the freezing pressure of water. The water 

 alone successfully withstood this subcooling, but the water with the 

 mercury always froze. Even when freezing took place, a rough value 

 for the compressibility could be found by using in the calculations the 

 change of volume of the water on freezing. A compressibility for the 

 mercury was thus found lying on a smooth curve with the values at 

 lower pressures, but of course any such procedure as this is question- 

 able. In view of the strong probability that the compressibility of 

 mercury shows no unusual features in the unreached region, it seemed 

 unnecessary to take the measures that would be needed to explore this 

 region, such as the employment of some auxiliary fluid other than 

 water whose freezing point is higher than that of mercury. 



It may be mentioned as confirming these views as to the part played 

 by internal strain in making the results irregular, that the irregularity 

 was always much greater after the freezing had taken place after high 

 super-pressures ; that is, after the freezing presumably had been most 

 rapid, and the excess pressure on the outside with its resulting defor- 

 mation greatest. 



It will be noticed that the method is essentially an integrating ar- 

 rangement for measuring the total increase of pressure during the 

 process ; that is, at every increase of pressure mercury is forced into the 

 piezometer whether the previous maximum is thereby exceeded or not. 

 In using the method it is essential, therefore, that pressure should be 

 increased continuously to the maximum with no retrogressions. The 

 form of pressure apparatus used made this particularly easy to accom- 

 plish. Pressure was produced by a hydraulic ram, the larger piston of 

 which was 6 inches in diameter and the smaller 11/8 inches. The 

 6-inch piston was actuated by the pressure pump of the Soci^t^ 

 Genevoise, the pressure being run as high as 600 kgm./cm.^ 



The motion of the piston was very slow because of the large volumes 

 involved. About 50 strokes of the pump were necessary for an in- 

 crease of pressure on the high pressure side of 1000 kgm./cm.^. Fur- 



