CONSTITUTION OF BODIES. 617 



has reached a certain limit. The liquid and the gaseous portions of the sub- 

 stance are then in equilibrium. If the volume of the vessel be now made 

 smaller, part of the gas will be condensed as a liquid, and if it be made larger, 

 part of the liquid will be evaporated as a gas. 



The processes of evaporation and condensation, by which the substance 

 passes from the liquid to the gaseous, and from the gaseous to the liquid state, 

 are discontinuous processes, that is to say, the properties of the substance are 

 very different just before and just after the change has been effected. But this 

 difference is less in all respects the higher the temperature at which the change 

 takes place, and Cagniard de la Tour in 1822* first shewed that several sub- 

 stances, such as ether, alcohol, bisulphide of carbon, and water, when heated 

 to a temperature sufficiently high, pass into a state which differs from the 

 ordinary gaseous state as much as from the liquid state. Dr Andrews has 

 since t made a complete investigation of the properties of carbonic acid both 

 below and above the temperature at which the phenomena of condensation and 

 evaporation cease to take place, and has thus explored as well as established 

 the continuity of the liquid and gaseous states of matter. 



For carbonic acid at a temperature, say of C., and at the ordinary pressure 

 of the atmosphere, is a gas. If the gas be compressed till the pressure rises 

 to about 40 atmospheres, condensation takes place, that is to say, the substance 

 passes in successive portions from the gaseous to the liquid condition. 



If we examine the substance when part of it is condensed, we find that 

 the liquid carbonic acid at the bottom of the vessel has all the properties of 

 a liquid, and is separated by a distinct surface from the gaseous carbonic acid 

 which occupies the upper part of the vessel. 



But we may transform gaseous carbonic acid at 0" C. into liquid carbonic 

 acid at C. without any abrupt change, by first raising the temperature of 

 the gas above 30.92 C. which is the critical temperature, then raising the 

 pressure to about 80 atmospheres, and then cooling the substance, still at high 

 pressure, to zero. 



During the whole of this process the substance remains perfectly homo- 

 geneous. There is no surface of separation between two forms of the substance, 

 nor can any sudden change be observed like that which takes place when the 

 gas is condensed into a liquid at low temperatures ; but at the end of the 



* Annales de Chimie, 2 me s^rie, xxi. et xxn. 

 t Phil. Trans. 1869, p. 575. 



VOL. II. 78 



