BREDGMAN. — WATER UNDER PRESSURE. 



491 



rium pressure becoming zero at this temperature. Because of the 

 slowness of the reaction it would be well nigh impossible to verify this 

 conjecture experimentally. Except for the constant pressure error al- 

 ready mentioned, the curve as found by Tammann is of approximately 

 the same shape as that found here. These figures give a drop in the 



-80 



-70 



-00° -50' 



TEMPERATURE 



40° 



30" 



Figure 14. The change of volume when II passes to I. 



equilibrium pressure of 360 kgm. between —40° and —80°, whereas 

 Tammann gives 340 kgm. Tammann does not draw the line as straight, 

 however, but prefers to make it slightly concave toward the tempera- 

 ture axis. This would bring the equilibrium temperature at atmos- 

 pheric pressure up somewhat higher than the absolute zero. 



The changes of volume, found by the method already described, are 

 given in Table XIV., and shown graphically in Figure 14. Because of 

 the fairly large difference in slope between the equilibrium curve and 

 the lines of equal volume, the actual cycles described in measuring A V 

 did not have the exact location shown in Figure 8. The lower parts 

 of both DE and GF, which were fairly close together, projected over 

 the equilibrium line into the region of stability of II. The upper end 

 of the line CB was managed so as to just clear reaching into the region 

 of I. This evidently introduces no essential error, since the compres- 

 sibility and dilatation of one phase do not change on passing into the 

 region of another. The difference merely means a somewhat wider 

 extrapolation to determine the displacement on the equilibrium curve 

 corresponding to the phase II, but this disadvantage is offset by the 

 fact that what was an extrapolation for the phase I has now become an 

 interpolation. 



The change of volume is seen to be linear with temperature (or pres- 

 sure) within a maximum error of 1/2 per cent. Only one point, that 

 at —35.7°, shows a discrepancy as much as this, the average departure 

 from linearity being in the neighborhood of 1/8 per cent. 



Tammann gives three values for the change of volume which he lists 

 as I-II, and two values listed as I-III. One of the supposed I-II values 



