Maximum or Minimum Vapour-Pressure. 119 



262°-7, but our pressures are all a little higher than their*, 

 the difference increasing with the temperature up to the 

 critical point, where we find 51*51, their value beino- 50'16. 



This difference is probably due to a trace of water in our 

 alcohol (see below) . It will be seen (1) that the maximum 

 vapour-pressure remains up to the critical point, and (2) that 

 the critical temperatures have no minimum. 



The very small difference in the critical temperatures of 

 pure propyl alcohol and of the first mixture leaves some 

 doubt as to whether there might not actually be a small 

 minimum close to propyl alcohol (especially since readings 

 were taken with different thermometers, one having been 

 broken). But even if it did exist, which is very unlikely, 

 the minimum would be very little pronounced, and would 

 not belong to the mixture of maximum vapour-pressure at 

 low temperatures. The fact that Ramsay and Young found 

 a lower critical temperature for water-free propyl alcohol 

 supports the conclusion that there is no minimum. 



This deviation from the normal case, as deduced from the 

 theory and as realized by mixtures of ethane and carbon 

 dioxide (see above), makes it probable that the maximum 

 vapour-pressure in the combination of propyl alcohol and 

 water is not due to a small attraction between the two kinds 

 of molecules, but to association of molecules. The chief cause 

 i- probably the very high association of the water molecules 

 amongst themselves. That this association has begun even 

 at high temperatures appears from the fact that the 

 vapour-pressure curve of water, which is considerably below 

 the curve of propyl alcohol at low temperatures, approaches 

 the latter as the temperature rises, so that it looks as if they 



