( 35 ) 



realised, among others, for mixtures of He(//,) and H, (a,). In these 

 papers, particularly in the last, a particular kind of plaitpoint line 

 has been repeatedly mentioned, viz. one passing from the critical 

 temperature 1\, called "third" by me (Keesom's Tk, n ), to the highest 

 of the two critical temperatures T a (Keesom's 1\). 



Now the theoretical possibility of such a course of the plaitpoint 

 line, i.e. of one of its two branches, has been first brought to light 

 by me in a series of Discussions on this subject '). Not only for the 

 special case b l = b i , for which among others, tig. 1 of June 21, 1905 

 holds, but for all possible cases (see specially Teyler I and II). We 

 found that such a course will always be found, when the ratio of 



2\ 

 the two critical temperatures 6 = — - is larger than the value of this 



-* i 

 ratio, for which the plaitpoint line has a double point. This type was 



called type I by me. (see also tig. 1 of Oct. 25, 1906). 



The case that a plait starts from C to C\, or also at the same 

 time from C, to C (when there is a minimum temperature in the 

 plaitpoint line) is not new (see K. 0. and Keesom, p. 78S below), 

 but has been before described and calculated by me in all particulars. 



The double point in the plaitpoint line, discovered by me in 1905 

 (June 21), did not only give the key to the possibility of such a 

 course, which had already been ascertained for mixtures of water 

 and ether, of ethane and methylalcohol ') ; but also the connection 



i) These Proc. May 25, 1905, p. 046-057 ; Ibid. June 21, 1905, p. 33-48 ; Ibid. Aug 

 17, 1905, p. 144—152 (Gf. also Arch. .Veil. 1905, p. 373-413); Ibid. Jan 25, 1906, p. 

 57S-590 (Also Arch. Néerl. 1900, p. 224— 238); Ibid. Oct. 25, 1906, p. 226— 235. 

 Further Arch. TEyLER (2) X, Premiere partie, p. 1 — 26 (1905); Ibid. Deuxièmc 

 partie, p. 1—54 (1906). Henceforth 1 shall refer to papers in these Proceedings 

 by mentioning the date, to papers in the Arch. Teyler by putting Teyler 1 or II. 



-) I do not quite understand why in cases as for He -f- H 2 the plait considered 

 is particularly called a "gaspluit". With exactly the same right the two coexisting 

 phases might be called liquid phases, expeeially at the higher pressures in the 

 neighbourhood of the point C . With reference to water-ether, etc. we speak of a 

 gas phase and a liquid phase before the three phase equilibrium is reached, i. e. at 

 higher temperatures: and when at lower temperatures the equilibrium mentioned 

 has established itself, of two liquid phases. The "gas phase" is then determined 

 by the branch plait of the original transverse plait (which latter has now the 

 peculiar shape directed towards C in the neighbourhood of the axis x = 0. But 

 I acknowledge that this is perfectly arbitrary, it being difficult to indicate where 

 the pressure is high enough on such a plait to justify us in speaking of liquid 

 phases. Would it not be better to follow here van der Waals' terminology, and 

 speak of fluid phases, and to call the two phases liquid phases at temperatures 

 where the three phase equilibrium is found'? Otherwise in this latter case — keeping 

 to K. 0. and Keesom's terminology — we should have to speak of three coexisting 

 gas phases, a rarefied one and two very dense ones, which latter, however, we 

 should never refer to as gas phases in the perfectly identical case of water + ether. 



3* 



