147-149] 



Isothermals 



131 



"critical temperature" of the substance. So long as the temperature is kept 

 above the critical temperature, no pressure, however great, can liquefy the 

 substance. 



It is usual to speak of a gas, when below the critical temperature, as a 

 vapour. We therefore see that the line PP 2 in fig- 9 is the line of demarca- 

 tion between the gaseous and vapour states, and that PP l is the line of 

 demarcation between the gaseous and liquid states. We must now examine 

 the demarcation between the liquid and vapour states, which is at present 

 represented by the unstable region in which dp/dv is positive. If U is any 

 point in this region it is clear from physical considerations that there must 

 be some stable state in which the pressure and volume are those of the 

 point U. What is this state ? 



FIG. 9. 



Through U draw a line parallel to the axis of v. Let this cut any 

 isothermal in the points X, Y, Z, the two latter representing stable states- 

 liquid and vapour respectively. These states have the same pressure, so that 

 a quantity of vapour in the state Z can rest in equilibrium with a quantity 

 of liquid in state Y, By choosing these quantities in a suitable ratio, the 

 volume of the whole will be that represented by the point U. Here, then, 

 we have an interpretation of the physical meaning of the point U. As the 



92 



