Mixtures with Maximum or Minimum Vapour-Pressure. 11 1 



as is the cause o£ a maximum with normal mixtures, but to 

 the association at low temperatures of the molecules of the 

 component which has the higher vapour-pressures at high 

 temperatures, and the abnormally low vapour-pressure exerted 

 by this substance in consequence. This abnormality naturally 

 makes the vapour-pressures of the mixtures appear relatively 

 high, and may even produce a maximum. In that case there 

 would be no reason to expect a minimum critical temperature. 



Other exceptional cases due to association will naturally 

 suggest themselves, and instances will be discussed below. 



The above conclusions for normal substances are, to a 

 certain extent, confirmed by the results contained in former 

 papers, to which the reader may here be referred. The ex- 

 periments described in this paper were undertaken with a 

 view to completing the comparison with theory, and obtain- 

 ing further information on the behaviour of similar mixtures. 

 Three different combinations were examined and will be 

 taken up successively. 



1. Propyl Alcohol and Water. 



In this mixture it was our object to follow the maximum 

 in the vapour-pressure discovered by Konowalow * up to the 

 critical point, to ascertain whether the maximum shifts to 

 any extent to mixtures of different composition, whether it 

 disappears or not, and finally whether the critical tempera- 

 tures showed a minimum or not. 



The propyl alcohol obtained from Kahlbamn was found 

 to be sufficiently pure to be used without further purification. 

 Three mixtures were investigated, the first containing ap- 

 proximately 2-J per cent, of water (mixture 1) ; the second 

 and third both approximately 25 per cent, (mixtures 2 and 3) . 

 This is approximately the maximum mixture at the boiling- 

 point. Some of our results are shown in TablesT.-V. (p. 118) , 

 and the diagram (fig. 1) . As we could not observe the beginning- 

 of the condensation of the mixtures in our tubes, we always 

 took the pressure at a small volume near the end of conden- 

 sation : the figure thus gives condensation lines instead of 

 narrow loops. 



The vapour-pressures of water used in the figure are due 

 to Batelli t ; at temperatures above 250° the values are 

 somewhat higher than those found by Cailletet and 

 Colardeau J. 



Our value for the critical temperature of propyl alcohol 

 agrees well with that found by Ramsay and Young §, viz., 



* Wied. Ann. xiv. p. 34. f Ann. Chimie et PJn/s. [7] iii. p. 412. 



X Ibid. [6] xxv. p. 527. § Phil. Trans, clxxx. p. 137. 



