BRIDO^IAN. — THEiniODYNAMIC PROPERTIES OF WATER. 327 



dently given directly from the curves for coniprossihility at constant 

 temperature. The sIoj)e of this curve changes somewliat with the 

 temperature, so that a correction shouKl he apphed for this, but the 

 change is so sHght at the higher pressures that for this i)urpose the 

 compressihihty can he assumed constant. At the lower pressures, 

 below 2500 kgm., the change cannot be neglected, and another 

 method of computation must be applied. 



The thermal dilatation at low pressures was determined by taking 

 directly the difference between the isothermals traced out at different 

 temperatures. This method is not applicable at high pressures be- 

 cause the irregularities of isothermals traced at different times is 

 sufficient to make their difference an inaccurate measure of the slight 

 change of volume with temperature, but at the low pressures, the 

 errors introduced by hysteresis and other irregular action of the steel 

 cylinder are so slight that the method may be used directly to give the 

 value of the compressibility, and by taking the differences, the value of 

 the thermal dilatation. In fact it would seem that the method would 

 be applicable with slight modifications to the determination of the 

 compressibility of a great variety of substances at Ioav pressures, and 

 it is very mucli more rapid than the methods hitherto used. 



A special setting up of the apparatus was necessary for the experi- 

 ments at low pressures, because in order to be able to reach low pres- 

 sure on release of pressure it is necessary that the friction in the 

 movable plug be not too high, and if the pressure has once been run 

 to so high a value as to upset the plug, the friction becomes so great as 

 not to permit release of pressure to much below 1500 kgm. P^or these 

 experiments, then, the plug was made initially a push fit for the hole, 

 by making it about 0.0015" smaller than when used for the higher 

 pressures, and in performing the experiment the pressure was never 

 pushed l:)eyond 2500 kgm. In other respects the experiments at low 

 pressures were the same as those at higher pressures. It was not 

 necessary to take quite so elaborate seasoning precautions at these 

 low pressures, however. 



With regard to the amount of hysteresis or elastic after-effects 

 to be met in the experiments, the difference of the displacment with 

 increasing or decreasing pressure usually amounted at the middle of 

 the range to 0.03 in. This amount was ver^' uniformly consistent, 

 indicating that the cylinder had really settled down to a steady be- 

 havior. The piston always returned to the starting point to within 

 the limits of accuracy of reading, indicating that there was no leak or 

 permanent set, or wearing of the packing in appreciable amount. 



