CRITICAL POINT 



is again negative, so that the left portions of n and in 



dV 



run like I. 



2 a 



But as soon as -TFO 



RT 



\v-br 



a horizontal tangent appears somewhere, as on the iso- 

 thermal II at A since -. is zero ; whilst if the temperature 



is still lower positive values of -^ arise, and the curve in 





has consequently a bend upwards towards the right as 

 at BC. 



Physically this isothermal in corresponds to the possibility 

 of three volumes (F, o, and E, Fig. 2) for one pressure. Of 

 these three that at o is 

 unstable since here for in- 

 creasing pressure there is 

 an increase in volume, and 

 consequently a further in- 

 crease in pressure ; hence 

 only two possibilities re- 

 main, corresponding to the 

 liquid volume F and the 

 vapour volume E, and the 

 empirical curve becomes 

 D F o E instead of D F B o c E, 



Fig. 2. 



since at E condensation occurs, and the volume is reduced 

 in presence of liquid and vapour together, by increase in 

 the quantity of the former at constant pressure till 

 finally at F only liquid remains, and the pressure begins to 

 rise again. 



Curve I therefore shows the continuous compression 

 without partial condensation ; curve II the limiting tempera- 

 ture for which at A a small reduction of temperature 

 causes separation into liquid and vapour, as in curve in ; 

 II therefore shows the critical temperature. 



It may be added that the position of EF is fixed by 



B 2 



