600 BRIDGMAN. 



values for the change of volume between I and III or between II and 

 III at low pressures. Three sets of measurements were made around 

 the triple point I-II-III. The first gave the change of volume I-II 

 by the method of changing temperature at constant pressure. The 

 temperature could be raised so far beyond the transition line that the 

 reaction from II to I was surely complete, and of course it was not 

 ■difficult to be sure of starting with pure II at low temperature. But 

 this method does not give the temperature of the transition point, 

 and this transition temperature was especially needed in fixing the 

 position of the triple point. To fix this point, pressure and tempera- 

 ture values were obtained at low pressures on the I-III and the II-III 

 lines, by the method of varying pressure at constant temperature. It 

 was possible to shut the pressure within narrow limits, 20 to 30 kgm., 

 but the domain of subcooling was so wide that the reaction was not 

 completed even on reducing pressure to nearly atmospheric. The 

 directly found values of AV at these points were 10 to 15% too low. 

 These points were, therefore, discarded. The AV curves for I-III 

 and II-III at low pressures were determined by extrapolation from 

 the high pressure values together with the condition that the changes 

 of volume should check with that for I-II at the triple point. 



In spite of the comparatively large amount of lag, the width of 

 the band of indifference was not abnormal. No absolute value can 

 be attached to these figures, because of the disturbing effect of fissures. 

 The limits were notably wider on repeating a point, probal)ly because 

 of the fissures formed by the intervening transitions, and were very 

 much less in the low than in the high pressure apparatus. The figures 

 pretend only to give roughly a comparative estimate. The band was 

 widest for the II-IV curve, about 250 kgm.; on the II-III curve it 

 varied from 200 kgm. at the lowest to 100 kgm. at the highest tempera- 

 ture; on the I-III curve the variation with rising temperature was 

 from 100 to 30 kgm., and on the III-IV curve from 140 to kgm. 

 with rising pressure. On all the curves, the band becomes narrower 

 with rising temperature. The reaction, as usual, ran more rapidly 

 at the high temperatures, and at least on the II-III and III-IV curves 

 the transition ran more rapidly in the direction of falling pressure. 



The experimental values of pressure and temperature are shown in 

 Figure 10, the experimental values of AV in Figure 11, the computed 

 values of AH and AE in Figure 12, and the numerical results in Table 

 VI. All of the pressure-temperature determinations are shown on 

 the curves, but four of the AV points have been discarded for reasons 

 already given. 



