BRIDGMAN. — THERMODYNAMIC PROPERTIES OF LIQUIDS. 81 



force also. The number of possible positions which the molecules 

 may assume in this attempt to adapt themselves to the diminished 

 space at their disposal may evidently be very great with molecules 

 of at all complicated shapes, so that there is here the possibility of 

 such very complicated dilatation curves as we actually have. 



Another possible explanation which in the end amounts to very 

 much the same thing for some purposes, is that of association. If we 

 suppose that association takes place with decrease of volume, and that 

 the amount of association is increased with increasing pressure, and 

 is decreased with rising temperature, then we have also the possibility 

 of decreasing dilatation with rising temperature and of increasing 

 dilatation with rising pressure. In this case the phenomena are 

 essentially similar to those of solidification under pressure of a mixture 

 of different liquids. Such a case has already been investigated for 

 kerosene, which shows the same general features as above. The 

 phenomena for kerosene are much simpler than for these licjuids, 

 however. It is evident that one simple association (such as single to 

 double molecules) is not sufficient of itself to explain the facts, but 

 if we assume several associated molecules of varying degrees of com- 

 plexity, and that the relative numbers of these change with pressure 

 and temperature, the explanation would account for several of the 

 facts actually found. But reasons will be given in the next section, 

 for supposing that association cannot have a very large part in the 

 phenomena at high pressures. 



We now turn our attention to Figure 32, which gives in one diagram 

 the average dilatation between 20° and 80° for all twelve liquids. The 

 origin of each of these curves has been displaced downwards one unit 

 with respect to the one above it. The origin is so located that the 

 value of the thermal dilatation for each of the liciuids at 12000 kgm. 

 is between 0.0002 and 0.0003. The scale of the drawing is indicated 

 at the side. The immediately striking feature is that the curves are 

 nearly equi-spaced at the higher pressures. Approximate equal 

 spacing of the curves would of course be a consequence of their all 

 approaching zero, but this is not the entire effect by any means. For 

 instance, the ratio of the dilatation of ether to that of amyl alcohol 

 at low pressure is 1.50 while at 12000 kgm. pressure it has dropped to 

 1.03. (In the computation for ether, the initial dilatation at 20° was 

 used.) This is only the first example of many we shall meet tending 

 to show that at high pressures liquids lose the individual differences 

 which characterized them at low pressures, and become more alike. 



The very gradual change of dilatation with pressure at the high 



