BRIDGMAN. — THERMODYNAMIC PROPERTIES OF LIQUIDS. 83 



with increasing molecular weight. But isobutyl alcohol shows the 

 effect of its different structure by a higher dilatation than its position 

 in the series would indicate. A comparison of isobutyl alcohol with 

 ether, its isomer, shows that although initially the dilatation of ether 

 is greater, at 12000 kgm. it has become less. This emphasizes again 

 that the structure of the molecule continues to play a part even at 

 high pressures. It is not to be wondered at in view of the suggested 

 explanation of the complicated nature of the dilatation curves. For 

 molecules of the same atomic formula, but of different structure, 

 are to be thought of as possessing different shapes, and it is at high 

 pressures that the effect of shape is greatest. 



Isothermal Compressibility. — The isothermal compressibility is 

 shown in Folder II; the values for the liquids separately at 20° inter- 

 vals in Figures 33 to 44, and in Figure 45 the average results over the 

 entire temperature range are collected into a single diagram for the 

 twelve liquids. 



The curves require a word of explanation. Up to 4000 kgm. the 

 curves for each liquid are drawn for the four different temperatures, 

 but at pressures higher than 4000 kgm. the curves would be so close 

 together, sometimes crossing each other, that it Avould have been very 

 confusing to draw them on the same scale. Therefore, at higher 

 pressures, the only complete curve given is for 40°, while in the upper 

 part of the diagram are shown on a larger scale the differences between 

 the compressibilities for 20° intervals. The zero of these difference 

 curves is drawn as a heavy line. Negative ordinates of the difference 

 curve 20°-40° indicate that the compressibility is less at 20° than at 

 40°; positive values for the difference curve 40°-60° indicate a greater 

 compressibihty at 60° than at 40°, and similarly positive differences 

 60°-80° mean a greater compressibility at 80° than at 60°. To find 

 the compressibility at 20°, one adds to the value obtained from the 

 40° curve the ordinates of the difference curve 20°-40° (this ordinate 

 is usually negative) ; the compressibility at 60° is found by adding to 

 the ordinate of the 40° curve that of the 40°-60° curve, and the com- 

 pressibility at 80° by adding to that at 60° (obtained as above) the 

 ordinate of the 60°-80° curve. A larger compressibility at 20° than 

 at 40° is indicated by the difference curve 20°^0° rising from below 

 and crossing the axis, while a smaller compressibility at 60° than at 40° 

 is similarly shown by the curve 40°-60° crossing the axis from above. 

 The mutual crossing of the difference curves, from this point of view, 

 is not especially significant; it is the crossing of the axis that counts. 

 The meaning of the mutual crossing of the curves 40°-60° and 60°-80°, 



