396 Prof. R. Clausius on the Behaviour of Carbonic Acid 



certain volume onwards, at which condensation begins, subse- 

 quent diminution of volume takes place without increase of 

 pressure, and only when the volume has become much smaller 

 and the entire mass is liquid, does the pressure again com- 

 mence to increase as the volume is diminished, the increase 

 then being rapid. The portion of the curve corresponding to 

 the occurrence of condensation is a straight line, met at both 

 ends by the continuously curved portions of the curve. A 

 slight curvature delineated by Andrews at one extremity of 

 the straight line appears to depend on a slight mixture of air, 

 and hence may here be left out of consideration. 



Two years afterwards James Thomson*, whose ingenious 

 speculations have already so much contributed to the enlarge- 

 ment of mechanical and physical science, supplemented An- 

 drews's curves by adding, at the places where in them a 

 straight line is found, a curved line which joins on to the two 

 curved portions of the curve given by Andrews in a conti- 

 nuous manner and represents a gradual transition from the 

 gaseous to the liquid state, in which the entire quantity of the 

 substance is found continuously in a similar state — a kind of 

 transition which is theoretically thinkable, but cannot actu- 

 ally occur, because it contains intermediate states in which 

 no stable, but only unstable, equilibrium exists. In fig. 1 



Fig. 1. 



Volume. 

 * Proc. Roy. Soc. November 1871. 



