BRIDGMAN. — THERMODYNAMIC PROPERTIES OF LIQUIDS. 79 



At higher pressures there is in many cases, though not in all, a 

 tendency for the effect to reverse again, that is, for the dilatation to 

 again become greater at the higher temperature. The pressure of 

 this second reversal is in the vicinity of 9000 or 10000 kgm. This 

 fact also is indicated by the Cp curves. 



A comparison of the curves for the different liquids shows a few 

 features of interest. In the group of the five alcohols, isobutyl 

 stands out as being the simplest, there being none of the crossing and 

 recrossing which the others exhibit at the higher pressures. In this 

 respect the four normal alcohols are much alike in their high pressure 

 complications. That isobutyl alcohol should be different from the 

 other alcohols was not anticipated before these experiments were 

 made, since it seemed probable that at high pressure the effect of 

 structural differences in the molecule would be eliminated. The 

 effect of structural difference is also shown by a comparison of isobutyl 

 alcohol and ether, since these two have the same formula, C4Hi30. 

 The high pressure effects are more complicated for ether. Acetone 

 is peculiar in the wide divergence of the curves at the maximum 

 pressure. It does not show any unusual effects in the neighborhood 

 of the freezing point. It will be seen later that the specific heats are 

 the quantities most susceptible to irregularities at the freezing point. 

 Carbon bisulphide is remarkable for the curve at 20°, which shows a 

 large increase of dilatation between 6000 and 9000 kgm., and also 

 in this region shows a much greater dilatation than the curves for 

 the higher temperatures. Phosphorus trichloride is the only one of 

 the twelve liquids which behaves approximately as had been expected, 



since it shows little irregularity at the high pressures, and (^) 



nearly vanishes. The curves for the three ethyl halogen compounds 

 do not show any particular progressive change of character such as 



one might expect, except with regard to the pressure at which f t-^ ] 



reverses in sign. This is shown best on the diagram for Cp. The 

 pressure for reversal is lower than for most of the other liquids, and 

 becomes less as the molecular weight of the compound increases. 

 This may be because the increased molecular weight produces an 

 increase in the cohesional force of attraction, and a consequent in- 

 crease in the internal pressure, with the same effect as an increase of 

 the external pressure. Ethyl iodide is remarkable for the low value of 

 the dilatation at 80° at 6000 kgm. The minimum as 0.00017, and is 

 lower than for any of the other liquids at the highest pressures. 



