BRIDGMAN. — MERCURY UNDER PRESSURE. 



417 



as a result that the tangent at the origin as determined here gives a 

 rise of pressure of 195.5 kgm./cm.^ per degree, against 19G.5 found 

 before, a difference of 1/2 per cent. The two series are not really so 

 much different as this would indicate, because the smaller initial slope 

 of this present series is unduly inflaenced by the very evident greater 



G 7 8 3 

 PRESSURE, KGM/CM* X \ol 



Figure 20. The equilibrium curve of liquid and solid mercury obtained 

 by the change of volume method. 



irregularity of the low-temperature points, and is compensated for by 

 the greater departure from linearity. If we are willing to admit that 

 the departure from linearity of the resistance measurements is probably 

 more nearly correct, and apply this departure to the seven determina- 

 tions by the present method above zero, we shall find as the average 

 slope 196.3 kgm./cm.^ at the origin, agreeing within 1/10 per cent of 

 the former value. In the rest of the work of this paper the best value 

 for the slope at the origin will be taken as 196.4 kgm./cm.'^ per degree, 

 and the departure from linearity given by the resistance measurements 

 will be taken as most probably correct. 



The changes of volume corresponding to these equilibrium pressures 

 were all determined with the same set up of apparatus as already ex- 

 plained. These results are given in Table XL, and plotted against 

 temperature on an enlarged scale in Figure 21. The observed values 

 were obtained graphically from figures like Figure 19, drawn on a very 

 much enlarged scale. For this purpose, it was not necessary to plot 

 the actual pressures, but the position of the slider on the bridge wire 

 on which the resistance measurements were made was used instead. 

 This enables smaller changes of pressure to be measured with accuracy 

 than could be done in the other way. The absolute gauge against 



VOL. XLVII. — 27 



