Maximum or Minimum Vapow'-Pressure. 123 



larger quantity o£ acetone, in a closed tube without mer- 

 cury. These tubes were closed at one end, provided with 

 a small bulb at the top and drawn out. They were filled 

 by exhaustion and boiling at the air-pump, and then sealed. 

 In order to obtain the critical phenomena about the middle 

 of the tube, we found that the tubes had to contain as 

 nearly as possible two-fifths of their volume of liquid at the 

 ordinary temperature *. The following values were obtained: — 



o 



Chloroform . . . 262' 6 5 



1st mixture . . . 260*5 



2nd mixture . . . 258*4 



Our previous results were thus confirmed as regards the 



temperatures, and there is no reason to doubt the accuracy 



of the above conclusions. 



In chloroform and acetone we have therefore a combi- 

 nation which has no maximum or minimum in the critical 

 temperature or in the vapour-pressures in the critical re- 

 gion, but in which a minimum vapour-pressure appears at 

 low temperatures. This cannot be accounted for by the 

 probable but certainly only slight abnormality of acetone. 

 On the contrary, this abnormality would tend to make the 

 vapour-pressures of the mixtures appear higher. The low 

 pressures of the mixtures might indicate a strong attrac- 

 tion of the two substances ; but according to theory this 

 attraction would have produced a maximum in the critical 

 temperatures. As such a maximum does not appear, we 

 are led to the conclusion that the low vapour-pressures at 

 low temperatures are due to an abnormal increase in the 

 mutual attraction of the two substances at lower tempera- 

 tures, and therefore probably to mutual association of the 

 two kinds of molecules. Of course this conclusion is only 

 legitimate on the assumption that the equation used by 

 van der Waals is a sufficiently correct test of normality or 

 abnormality. 



The non-existence of a maximum critical temperature 

 thus suggests the probability of mutual association. With 

 methyl ether and hydrochloric acid, where a maximum 

 critical temperature exists, as pointed out in the paper on 

 these mixtures t, we had no means of distinguishing between 

 association and mutual attraction. But a great deal more 

 evidence will be required before this distinction is capable 

 of being clearly drawn. 



* In Ostwald's l Physico-chemical Measurements' (1894, p. 114) the 

 prescription is to fill the tubes to two-thirds of their volume. This 

 quantity is much too large, at least with an air-free liquid. The correct 

 amount is given by Nadejdine, Exner's JRepertoruim, xxiii. (1887) p. 617. 



t Phil. Mag. [6] i. p. 597. 



