BRIDGMAN. — MERCURY UNDER PRESSURE. 389 



sure, but always the mark was overshot a little. It would have been 

 impossible to land exactly on the mark, in the first place because of 

 the mechanical difficulty of raising the pressure exactly the right amount, 

 in the second place because of the initial effect of heat of compression 

 which would keep the mercury liquid even if the pressure corresponding 

 to the bath temperature were reached, and in the third place, because 

 of the possibility of subcooling the liquid into the region of stability of 

 the solid. With increasing pressure it was always necessary, therefore, 

 to run past the point by from 200 to 400 kgm. Freezing was indicated 

 by a sudden drop in the resistance. This drop was not instantaneous, 

 but might continue for several minutes. Everything points to the 

 spontaneous formation of a crystalline kernal at some point in the 

 capillary, and then the advance of the surface of separation of solid 

 and liquid through the capillary at a rate depending on the amount 

 subcooling, etc.^® Freezing, when it had once started, usually ran to com- 

 pletion. This was because the volume of the mercury was so small that 

 the change of volume on passing to the solid state was not sufficient 

 to lower the pressure to the equilibrium value. To find the equilib- 

 rium pressure, the pressure on the solid phase was lowered until the 

 reaction to the liquid started, and then by rapid work at the pump the 

 pressure was so adjusted that the resistance was in a state of unstable 

 equilibrium, changes of pressure too small to measure on the gauge 

 sufficing to produce a very large increase or decrease of resistance. 

 With a little practice, one could keep the mercury in a partly melted 

 condition for any desired length of time. This was necessary in order 

 to avoid the effect of heat of compression, which might sometimes be 

 so large as to mask the effect sought. The heat of compression of 

 kerosene is so high that it is possible for a decrease of pressure in the 

 kerosene to bring about a freezing of the mercury, the adiabatic drop 

 of temperature with pressure for the kerosene being more rapid than 

 the drop of pressure with temperature on the freezing curve of mercury. 

 To be absolutely certain that there was no such effect the mercury was 

 always kept in the partially melted condition for twenty minutes be- 

 fore recording the equilibrium pressure. To avoid any possibility of 

 error from impurities absorbed by the mercury, pains were taken to find 



** To measure the rate of advance of the surface would have been interest- 

 ing, but to be of value would have demanded some other disposition of ap- 

 paratus from that used. Evidently the rate of advance of the surface depends 

 more than anything else on the rate at which the heat of transformation ia 

 conducted away, which in turn will depend on the size of the glass capil- 

 lary, and the thermal conductivity of the surrounding liquid. Any results 

 obtained with the apparatus without modification would have had significance 

 only for this piece of apparatus. 



