476 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



ness was sufficient to detect changes of 0°.03. The coil was tested 

 for permanent changes, elastic after-effects, etc., by frequent calibra- 

 tions in melting ice at 0°. That it was perfectly satisfactory is shown 

 by the fact that the zero has not changed by 0°.03 since the coil was 

 set up. 



A special method for determining the change of volume at the low 

 temperatures was made necessary by the extreme slowness of the reac- 

 tion. At the higher tempera- 

 tures, between —20° and —30°, 

 the reaction runs rapidly enough 

 so that the ordinary procedure is 

 available, but at lower tempera- 

 tures this is not possible. On one 

 occasion, such an attempt was 

 made at —70°. After four hours 

 waiting at a pressure several 

 hundred kgm. removed fi-om the 

 equilibrium pressure, the reaction 

 had not yet shown any signs of 

 drawing to completion, and the 

 attempt was given up. The new 

 method is made possible by the 

 fact that the equilibrium pressure 

 is very nearly constant. Figure 8 

 illustrates the method. The first 

 part of the process consists in- 

 getting the change of volume by 

 the ordinary method at some 

 temperature where the reaction 

 runs with convenient rapidity. 

 This consists in measuring the 

 piston displacement at constant 

 temperature at several points on 

 each side of the equilibrium line indicated at (1), (2), (3), (4), (5), (6). 

 After reaching (6) the piston displacement is kept constant, but the 

 temperature is reduced, the water now being in the form of ice III. 

 The pressure is measured at several points with decreasing temperature 

 until the point A is reached. The apparatus is so small that complete 

 temperature equilibrium is attained in at most 5 minutes after chang- 

 ing the bath temperature. At the point A the pressure is reduced at 

 constant temperature by changing the displacement, until the point B is 

 reached, still to the right of the equilibrium line. From B, temperature 



n 



> 



C 



n 



PRESSURE 



Figure S. Shows the cycles described 

 in finding the change of volume I-III. 



