BRIDGALiN. — WATER UNDER PRESSURE. 477 



is raised at constant displacement to the point C. Here the pressure 

 is lowered to D, the reaction now running with sufficient speed because 

 of the higher temperature. From D, two excursions to low and then 

 back to high temperatures are made exactly as from (6), the water 

 now being in the form of ice I. The result of all these manoeuvres is 

 that we have sufficient data to find the displacement corresponding 

 to any temperature and pressure, and so the displacement on opposite 

 sides of the equilibrium line corresponding to the two phases I and III, 

 The difference of this displacement at any temperature is evidently the 

 same as if the reaction had run at that temperature, but the slowness of 

 the reaction is avoided. The change of volume is calculated directly 

 from the difference of displacements as usual. It was in general 

 necessary to measure the displacement at at least three points on each 

 side of the equilibrium line, since the relation between displacement 

 and pressure is not sufficiently linear to allow a linear extrapolation 

 from two points to the equilibrium line. For greater security, four or 

 five points were usually taken. 



The data are such that they contain internal evidence of their self- 

 consistency. After a complete cycle like the above, the starting point 

 ought to be reached again, if there has been no change in the appara- 

 tus in the meantime. This condition was always approximately ful- 

 filled, the piston returning within a few thousandths of an inch to the 

 original position. Failure to return exactly to the starting point is 

 probably due partly to elastic after-effects in the steel cylinders, and 

 partly to wearing away of the rubber packing on the end of the piston. 

 There was never any leak. The slight discrepancy was corrected for 

 by assuming that it had grown uniformly with the time. It will be 

 noticed that the cycle as described above is such that the effect of any 

 such error is at a minimum, since the two paths nearest the equilibrium 

 curve on either side were described in succession. 



The method is applicable here because of the fact that the equilib- 

 rium lines run so nearly parallel to the lines of constant displacement 

 that it is possible to approach very near to the equilibrium line over 

 the entire range. The method evidently would not apply without 

 modification to points on the I-L curve, for example. 



The slowness of the reaction which made necessary a modified method 

 of determining the change of volume is also evidently going to have its 

 effect on the equilibrium determinations. At the low temperatures, 

 the reaction runs so slowly that it requires a very long time for pressure 

 to return to the equilibrium values, if it has once been changed. The 

 behavior was such as to give the impression that the equilibrium pres- 

 sure might never be reached, there being a domain of indifference 



