334 pkoceedings of the american academy. 



Dilatation at High Pressures. 



The determinations of the thermal dilatation at the higher pressures 

 were made on four occasions. The first two of these were preliminary, 

 during which was discovered the necessity of seasoning for tempera- 

 ture as well as for pressure, and also the necessity for the secondary 

 pressure seasoning over the small range of pressure accompanying 

 the changes of temperature. These first two determinations, while 

 confirming the results of the two later ones, were not given much 

 weight in selecting the final value. The method of computation 

 adopted in finding the thermal expansion from the data requires 

 mention. At first an attempt was made to apply the same graphical 

 method which has been already explained in its application to the 

 determinations at the lower pressures. This method involves the 

 drawing of a curve of the same general slope as the compressibility 

 curve through the two points giving piston displacement against 

 pressure at each temperature. But it was found that even after the 

 seasoning for the small pressure range involved here, the points were 

 too irregular to give good results by this method. The irregularities 

 may be due to residual hysteresis, but are more probably due to 

 slight irregularities brought about by the motion of the piston itself. 

 These irregularities are too minute to have any effect on the com- 

 pressibility determinations. The best way to avoid them is to utilize 

 in the computations only those readings during which the piston 

 remains stationary. This means that only the change of pressure 

 accompanying a change of temperature is used in making the computa- 

 tions, the second reading at any temperature by which the pressure is 

 brought back to the mean value being ignored. The change of 

 volume at constant pressure for the given change of temperature is 

 then computed from the known change of pressure at constant volume 

 and the previously determined change of volume with pressure at 

 constant temperature. In making this computation it is generally 

 necessary to make two corrections; one to bring the temperature 

 interval to the exact 20° desired for the final results, and the second 

 to correct for the very slight change of measured piston displacement 

 accompanying the change of temperature. This change of displace- 

 ment is seldom over 0.003". It is probably not due entirely to actual 

 motion of the piston, but partly to temperature changes in the bars 

 of the press which dip into the thermostat. That this method of 

 computing the results is preferable to the graphical one previously 

 mentioned is shown by the fact that this method gives very much 



