BRIDGMAN. — MERCURY UNDER PRESSURE. 431 



reading, the several points on the vertical part of the curve at decreas- 

 ing pressure differing among themselves by only 0.05 mm. of bridge 

 wire, better than the limit of reading, corresponding to pressure differ- 

 ences of 1 kgm./cm.^. The points plotted are the mean of two readings, 

 each to 0.1 mm. The same possibility of subcooling was found during 

 measurements by the change of resistance method. 



Attempts to measure the amount of subcooling would have had no 

 absolute value, for its amount depends on many extraneous factors, 

 such as the size and shape of the glass capillary, the presence of small 

 particles of grit in the fluid transmitting pressure, and most important 

 of all, the element of time. The greatest amount of subcooling found 

 from the resistance measurements was 230 kgm. at 10°. 2, equivalent to 

 about l°.l. The mercury remained liquid under these conditions for 

 about 30 minutes, and did not freeze until the pressure was still fur- 

 ther increased. 



This impossibility of superheating a crystal has been shown to be so 

 universally true that it is coming to be regarded in the light of a natu- 

 ral law. It holds whether the liquid melts with increase or decrease 

 of volume. No satisfactory explanation has yet been given of the fact, 

 however. Nearly all the explanations suggested, applicable at atmos- 

 pheric pressure, have attempted to make it in some way a surface 

 phenomenon, the melting beginning at the free surface and running 

 in toward the interior. In view of the fact shown above that super- 

 heating is also impossible under very high pressures, it would seem 

 somewhat doubtful whether this is the correct method of attack on the 

 problem. Under high pressures the surface of separation between the 

 solid and the surrounding liquid (mercury and kerosene, for example) 

 cannot be thought of properly as a free surface. The molecules of 

 mercury would seem no more free to assume their natural positions at 

 this surface than they would in the interior of the mercury itself. This 

 is particularly probable when it is remembered that the mercury melts 

 with increase of volume. 



Conclusion. 



In this conclusion an attempt will be made to show what bearing the 

 data may have on the theories of the liquid state and on our knowl- 

 edge of the change of state liquid-solid. As giving the best general 

 survey of all the data, a diagram is presented showing the isothermal 

 lines for the liquid and the sudden change on passing to the solid 

 state. (Plate.) The data from which this figure was constructed 

 are shown in Table XIII. Within the accuracy of this work the isother- 

 mals may be taken as equi-spaced, as already explained. The first 



