108 BRIDGMAN. 



shape of the indifferent region makes it impossible to obtain equilib- 

 rium values on the ascending branch by shutting the pressure within 

 two limits approached from above and below. The best that could be 

 done here was to use the method of varying temperature at constant 

 volume, varying the temperature by small steps, and relying on the 

 relatively small amount of superheating to get a fairly good value for 

 the transition temperature. The change of volume can be determined 

 from these data by methods already used. Evidently the failure of 

 this method to give any results at 2250 kgm. on the first trial men- 

 tioned above was because the temperature was not raised sufficiently 

 high. 



The shape of the region of indifference gives rise to curious effects 

 in the neighborhood of the maximum. Thus at room temperature, 

 the pressure was once raised to 6500, the temperature was then raised 

 to 165°, and pressure was raised, with a transition at the expected 

 place, but on releasing pressure no reverse transition was found on 

 crossing either branch of the curve. The location of the indifferent 

 region evidently explains this. On another occasion, temperature was 

 raised at 6600 completely through the indifferent band, the transition 

 running as to be expected on emerging at 180°, but on lowering tem- 

 perature to 175° and reducing pressure, no reverse transition could be 

 foimd. 



The existence of the maximum was further established by the 

 following runs. First, temperature was raised to 190° at about 2500 

 kgm., giving the regular transition point at about 165°. At 190°, 

 pressure was increased to 10,000 with no transition; at 10,000 tem- 

 perature Avas lowered to 120° with no transition, and at 120° the tran- 

 sition was found again at the point to be expected on lowering pressure. 

 Again, starting at 120° and 9000 kgm., temperature was raised to 

 195° with the transition at the expected point; at 195° pressure was 

 lowered to 4600 with no transition; at 4600 kgm. temperature was 

 lowered in steps of 10° to 120°, passing a transition with almost im- 

 perceptibly small change of volume at 134°, which therefore marks the 

 lower limit of the indifferent region, and at 120°, pressure was raised 

 again, the volume returning to its initial value when the initial pres- 

 sure was reached. To make still more certain that the initial con- 

 ditions had been recovered, pressure was increased beyond its initial 

 value at 120°, and the regular transition found both from above and 

 below. That is, we have here the experiment of describing a complete 

 circuit and coming back to the starting point, with practically only 

 one discontinuous change of volume. If the pressure of decreasing 



