( 79Ï ) 



When discussing this we shall leave out of account the raseofsolid 



states of' aggregation and three phase equilibria. 



In the first place gas states are all the states on the ^-surfaces 

 on which there are no plaits. As criterion to divide states which 

 belong to the stable or metastable l ) region of ^'-surfaces which show 

 plaits, into gas states and liquid states, analogy with the simple 

 substance indicates their relation with the connodal curves of those 

 plaits while for the metastable states the help of spinodal curves is 

 to be called in. 



For this first of' all the distinction between the two branches of 

 the connodal curve of a plait is required. For in the first place 

 we shall have to give the same name to each of the two branches 

 of a connodal curve separated by one or two plaitpoints throughout 

 its length 2 ). 



Now, on account of' rhe existence of' the barotropic phenomenon we 

 cannot simply call gas branch of the connodal curve that at which 

 one of the isopiestically connected states has the smallest density 3 ). 

 It is therefore the question to indicate if possible on each branch a state 

 whose nature is already known through the definition holding for 

 simple substances or for those which behave as such when splitting 

 up into two phases. In this different cases are to be distinguished. 



For the case that the considered plait 4 ) extends from one of the 

 side planes a: = or x = l over the ^-surface, follows from the 

 definition of gas phase and liquid phase of a single substance that 

 the branch of' the connodal curve from the gas state of the pure 

 substance to the plaitpoint is to be called gas branch, and also that 

 the branch from the liquid phase of the simple substance to the 

 plaitpoint is to be called liquid branch. The gas branch and the 

 liquid branch of the spinodal curve may be distinguished in the 

 same way as those of the connodal curve. 



Let us restrict ourselves for the present to the distinction of gas 

 and liquid in this case. In the first place we make use for this 

 purpose of the isomignic (Comm. N°. 96£) compression and expansion. 



J) It follows from the nature of the case that unstable states have not to be 

 considered here. 



2 ) Gf. p. 790 footnote [2]. 



3 ) Even it if we wish to leave gravity out of account, and pay only attention to the 

 molecular volume of the phase, the barotropic phenomena have yet called attention 

 to the possibility that we may find the gas volume first larger and then smaller 

 than the liquid volume when passing along the same connodal curve. 



4 j The case of the two plaits at minimum critical temperature is comprised 

 in this. 



