( 790 ) 



elusions 1 ) which are irreconcilable with llio most obvious conception 

 of phenomena, then the limits allowed for making this definition, 

 are very narrow. 



Thus for a simple substance no other distinction will be possible 

 than by means of the isotherm of the critical temperature, and the 

 border curve (connodal curve on Gtibbs' surface), which is divided 

 into two branches by the critical state (plaitpoint of the connodal 

 curve), of which the branch with the larger volumes is to be defined 

 as gas branch, that with the smaller volumes as liquid branch 8 ). 

 Liquid phases are only those which by isothermic expansion may 

 pass into such as lie on the liquid branch of the connodal curve, 

 and also the metastable ') phases lying between the connodal and the 

 spinodal curves, which may he brought on the liquid branch of the 

 connodal curve by isothermic compression 



For binary mixtures the consideration of the ^/-surface of van der 

 Waals leads in many cases to definitions which are just as binding. 



1 ) So Thiesen's definition, Z.S. für compr. und (1. Gase 1 (1897) p. 86, 

 according to which e.g. strongly compressed hydrogen at ordinary temperatures 

 would have to be called a liquid. 



2) This is in harmony with the principle of continuity of phase along the 

 border earn' according to which a change of the character of the phases on a 

 border curve can only occur in a critical point. For substances which at tempe- 

 ratures near the critical one, in states represented by points on, or in the vicinity 

 of that branch of the connodal curve on Gibbs's surface which connects the 

 liquid states at low temperatures with the plaitpoint, should be associated to mul- 

 tiple molecules of which the volume is greater than the volumes of the composing 

 molecules together, this principle would admit the possibility that on the liquid 

 branch of the border curve liquid phases should occur with greater volume than 

 the coexisting gasphase. Such simple substances would then show the barotropic 

 phenomenon, till now only found for binary mixtures. There is nothing known 

 that points in the diiection, of making the existence of such simple substances 

 probable but there can be no more given a reason why it should be impossible. 

 [Added in the translation]. 



3 ) The metastable states have not been included in Boltzmann's definition 

 Gastheorie II, p. 45. 



4 ) We do not accept the principle of the distinction of Lehman, Ann. d. Phys. 

 22 (1907) p. 474 : "Erst die unterhalb der betrachteten Isotherme liegenden Kuiven, 

 welche in ihrem -S'-förmigen Teil unter die Abszissenachse hinunterreichen, ent- 

 sprechen wahrer (tropfbarer) Flüssigkeit, d. h. einem Zustand, der negativen Druck 

 zu ertragen im stande is", as depending on the meaning that the existence of 

 capillary surface tension in liquids which can form drops, would presuppose that 

 these liquids can bear external tensile forces, i. e. negative pressures without split- 

 ting up (cf. ibid p. ir2 in the middle, and p. 475 at the top). [Added in the 

 translation]. 



