CHAPTER XL 



CHANGE OF STATE. LIQUID VAPOUR (continued). 



Indicator Diagram Critical Point Critical Constants Equation of Van der 

 Waals Liquefaction of Gases. 



The Indicator Diagram for Water-Steam. The indicator pressure 



volume diagram gives us a convenient graphic method of representing 

 many of the facts already mentioned in connection with the water- 

 steam change of state, and it also serves as au introduction to other facts 



200000 912000 1100000 1300000 



FIG. 108. Indicator Diagram for Water-Steam. 



now to be described. In this diagram volumes are measured from left to 

 right, and pressures from below upwards, starting from two lines OP, OV 

 at right angles, Fig. 108. 



Suppose we take 1 gramme of steam at 0, and, say, 1 mm. mercury 

 pressure, it will have a volume of rather more than 900,000 c.c. This 

 will be represented by a point a on the diagram, a distance proportional 

 to rather more than 900,000 from OP, and a distance proportional to 

 1 mm. from OV. If we gradually increase the pressure, decreasing the 



185 



