( 502 ) 



appearance of the longitudinal plait does not cause any irregularity *). 

 The component with the higher critical temperature (7a) is chosen 

 as first component; its critical temperature is, accordingly, denoted 

 by Tt v The special case that Tj, 2 = 0, is that of a gas without 

 cohesion with molecules having a certain extension. The investigation 

 of the tf'-surfaces becomes simpler for this case. For the present it 

 seems probable to us that helium still possesses some degree of 

 cohesion. We will, however, in a following communication compare 

 the case of a gas without cohesion with what the observations yield 

 concerning mixtures with II,.. 



§ 2. Barotropic pressure <nt< I barotropic concentration. We shall call 

 v and x of the gas phase v g and x g , of the liquid phase vi and #/. 

 At a temperature T a little below T^ , we shall always have 

 Ug^>vi. For then the plait extends only little on the ^-surface (see 

 tig. 1), the plait point is near the top of the connodal curve, which 

 is turned to x = l, and all the projections of the connodal tangent- 

 chords deviate little in direction from the v-axis, the angle with 



•>'>/ -'7 ~ 



the ?>axis, &,, = arc tg — , increases regularly if we go from x = 



Vg— vi 



along the connodal curve (o the plaitpoint, but it has but a small value, 



when Tt — T is small. Only when we take for T a value a certain 



amount lower than l\-\ , the plait extends sufficiently on the ip-surface 



n 

 to allow that v g = vi and 6 == — . 



If at a suitable temperature T we have substances as mentioned 

 at the beginning, as e. g. helium and hydrogen at the boiling-point 

 of hydrogen, we shall find the projection of a connodal tangent- 

 chord denoting the equilibrium considered in the xv projection of the 

 gas-liquid-plait on the tf>surface for T , to reach it we shall have 

 to ascend from x = along the connodal curve up to a certain value 

 of the pressure of coexistence p, before 8, which itself is zero for 



x = 0, can become — . A pressure of coexistence p = pi,, under 



which v g = vi at the temperature T, we call a barotropic pressure 

 for that temperature, the corresponding concentrations of liquid and gas 

 phase the barotropic concentration of the liquid and of the gas phase 

 at that pressure and that temperature. For when v g — Vi with increasing 

 pressure of coexistence p passes through zero at p = pb, we find 

 in equilibria with pressures of coexistence above and below the value 

 pi the phases to have changed positions under the influence of gravity. 



l ) This will be treated in a following communication. 



