328 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Of course the experiments at low pressures showed very much less 

 hysteresis, in fact it was so small as to be almost imperceptible. The 

 effect of hysteresis was eliminated as far as possible by using for the 

 displacement at any pressure the mean of the results with increasing 

 and decreasing pressure. The hysteresis was so constant that it 

 would probably have been sufficient to have used consistently the 

 results either at increasing or decreasing pressure. The actual pro- 

 cedure has, therefore, the weight of two independent determinations. 

 In the determinations of thermal dilatation, on the other hand, the 

 hysteresis effects were so much smaller, that except for one run initially 

 to show that there was no effect of this kind, the readings were always 

 made either only with increase or only with decrease of temperature 

 for any mean pressure, never with both increase and decrease. 



The Data. 



Three independent sets of experiments were performed to give the 

 change of volume with temperature and pressure over the entire range; 

 namely the isothermal compressibility at pressures over 2500 kgm., 

 the isothermal compressibilit}' and the thermal dilatation at pres- 

 sures below 2500 kgm., and the thermal dilatation at pressures over 

 2500 kgm. This is the actual order of experiment, but for the pur- 

 poses of presentation it will be better to use the natural order, pro- 

 ceeding from low to higher pressures. 



Compressibility at Low Pressures. 



The method with the present form of apparatus is not very sensitive 

 at the low pressures, and not many measurements were made over 

 this range. Two sets of determinations of compressibility were made, 

 the first at 20°, 40°, 60°, and 80°, and the second at only 20° and 80°. 

 Here, just as for the measurements at the higher pressures, there is 

 always sufficient friction in the packing after the pressure has once 

 been applied not to permit of close enough approach to the zero to 

 make an extrapolation back to the zero justifiable. And if the extra- 

 polation to the zero is to be made from the readings during first appli- 

 cation of pressure, special effort has to be made to design the washers 

 so as to avoid small initial distortions. For this reason only the 

 second of the above sets could be used by extrapolation back to the 

 zero of pressure. The readings of volume at 20° and 80° were corrected 

 back to 40° from the thermal dilatation as determined by this same set 

 of experiments, so that we have from the above two values for the 



