the Mutual Solubility of Liquids. 187 



that this diagram holds for mixtures of ethane with ethyl, 

 propyl, isopropyl, and normal butyl alcohols. For ethane 

 and methyl alcohol the lower critical temperature, if it 

 exists, was not reached in our experiments. Very probably 

 fig. 4 is also the true diagram for some of the mixtures men- 

 tioned above which have a lower critical point, although they 

 may equally well belong to the type of fig. 3, with the addition 

 of a lower critical point for the liquids. 



The solubility-curve does not completely represent the 

 phenomena with which we are dealing, so that it is 

 necessary to use other diagrams, especially the p-t and v-x. 

 We shall use these later, particularly the v-x diagram, 

 because it is the one that is used in the theory of van der 

 Waals regarding mixtures of two substances. In all our 

 experiments we have been guided by this theory. Its value, 

 it seems to us, is largely independent of the special form 

 of the equation of condition on which it is based. For the 

 purpose of disentangling the complicated relations between 

 composition, pressure, volume, and temperature, the ^-surface 

 with its plaits and plait-points is an indispensable guide; and, 

 so far no phenomena have been discovered which are not 

 fully accounted for by the possible position, growth, and decay 

 of the plaits. 



Our first experiments were made with ether and water. 

 The questions we wished to solve for this combination were : — 

 What are the pressures of the three phases at temperatures 

 above the boiling-point ? And secondly, Do the liquids mix 

 at a certain temperature, or is a critical point between the 

 vapour and the upper liquid reached ? In order to obtain 

 correct values for the vapour-pressures it was necessary to 

 mix pure water and pure ether, and to exclude all other 

 substances, such as air. The water used was distilled, free 

 from organic matter, and left no residue on evaporation. 

 The ether was from Kahlbaum, and was always freshly 

 distilled from sodium before being used. It did not act 

 on mercury even when wet. It did not give iodoform 

 with caustic potash and iodine, nor did it dissolve fuchsin ; 

 the boiling-point was constant at 34°' 35 C. (760 mm.). The 

 mercury was treated with nitric and sulphuric acids, and 

 distilled in vacuo. In order to introduce the liquids air-free 

 into the compression-tube, the principle of the method used by 

 Ramsay and Young was applied. Our arrangement was as 

 follows : — The tube of a Cailletet apparatus, but with a narrow 

 tube at the bottom instead of a wide one, had sealed to it, at 

 this end, a bulb, and to this bulb a second one at right angles 

 to the direction of the tube. Into the former bulb some 



