190 



HEAT. 



The critical isothermal just touches the vertex of this dotted curve, 

 and, being there nearly horizontal, a slight change in pressure corre- 

 sponds to a great change in volume. Andrews noticed that about this 

 point any change in pressure was accompanied by flickering movements 

 in the tube, showing considerable local alterations in density. 



FIG. 113. 



The Critical Constants, The vertex of this dotted curve may be 

 termed the critical point. It therefore represents a temperature of 

 30 '9 2, and a pressure, termed tha critical pressure, of about 73 or 75 

 atmospheres. The pressure could not be determined quite accurately, 

 as the air in the attached manometer did not change volume exactly in 



