120 Prof. Kuenen and Mr. Robson on Mixtures with 



would intersect, if the propyl-alcohol curve were a little- 

 longer. This irregularity would not occur with normal sub- 

 stances, and is due to the very high abnormality of water,. 

 which reduces its pressure abnormally as the temperature 

 falls. This low vapour-pressure of the water makes those 

 of the mixtures appear relatively high. 



In a somewhat crude form our explanation is this : if the 

 water molecules did not associate, water would have higher 

 vapour-pressures than propyl alcohol. Adding water to 

 propyl alcohol, therefore, raises the vapour-pressure until, 

 when the density of the water in the mixture becomes suf- 

 ficiently high, the association begins to make itself felt, and 

 the vapour-pressure passes through a maximum and falls 

 towards the pressure of water itself. It is thus unnecessary 

 to assume a specially small mutual attraction between the 

 two kinds of molecules, and there is no reason to expect the 

 critical temperature to fall when water is added to propyl 

 alcohol. 



As regards the shifting of the maximum, our experiments, 

 as far as they go, do not seem to show a considerable change 

 in the composition of the maximum mixture. On the con- 

 trary, the almost perfect parallelism of the vapour-pressure 

 curves for the two mixtures, 2 and 3, seems to show that the 

 change is very small, and it looks altogether as if the 25 per 

 cent, mixture was still approximately the maximum mixture 

 in the critical region *. 



It follows, from the position of the critical curve in the 

 diagram, that all the mixtures between propyl alcohol and the 

 maximum mixture ought to show retrograde condensation of 

 the second kind. It was impossible to confirm this completelv 

 in the experiments, but what was observed is in accordance 

 with the theoretical conclusion. When the mixture 1 was 

 in the critical condition, i. e. at its plaitpoint, compression 

 invariably brought back the liquid surface, and made the 

 liquid increase until it filled the tube. This phenomenon 

 shows that the plaitpoint in the volume-composition diagram 

 is placed on the side of the large volumes; and this agrees 

 with the theory and with the existence of retrograde conden- 

 sation of the second kind, which, owing to gravitation and 

 slight impurities, could not itself be observed. 



We did not consider it worth while repeating our experi- 

 ments with purified alcohol, as a special experimental diffi- 

 culty prevented us from obtaining accurate results or going- 

 into the various questions more thoroughly. We refer to the 

 action of water on glass, which we observed in 1898 f. and 



* Compare Ramsav and Young, Phil. Trans, clxxx. p. 157. 

 t Phil. Mag. [5] xlviii. p. 188. 



