CHANGE OF STATE LIQUID VAPOUR. 



187 



may be done even when some water is above the mercury. The water is 

 then at a negative pressure, showing that its isothermal may be prolonged 

 even below the line OV. Worthington has succeeded in subjecting 

 alcohol to a negative pressure or pull of seventeen atmospheres, and has 

 measured its expansion under this pull (Properties of Matter, p. 123). 



But not only may the water-lines be prolonged below the normal turn- 

 ing point, but, as described on page 168, if air containing water vapour 

 is filtered through cotton-wool into a clean flask, so that it is quite free 

 from dust, then it may be either compressed beyond, or cooled below, 



938 



- 760 



\\tf 



FlG. 109. Indicator Diagram showing Prolongations of Water and of Steam 

 beyond Normal Turning Points. 



the normal condensing-point of the vapour present. The curves ah, a'b' t 

 may, therefore, be prolonged as represented in Fig. 109 by bf, b'f. Just 

 as the change from water to steam appears to require some gas nucleus, 

 so does the change from gas to water seem to require some liquid or 

 solid nucleus. Professor James Thomson suggested that the cui'ves ce, 

 bf ultimately turn round and join, as represented in Fig. 110. 



If this is the case we can hardly hope to experiment on the state re- 

 presented by gJi, as it would represent that of a substance increasing 

 in volume with increase of pressure, evidently an unstable condition. 

 But if we could imagine it possible to take the substance round the 

 cycle ckhgbhc in a " reversible " manner (chap, xvii.) then the total work 

 done is zero since the temperature is uniform and the area ckh must 

 therefore equal the area hgb. 



