CHANGE OF STATE LIQUID VAPOUR. 



191 



accordance with Boyle's law. The volume of unit mass, termed the 

 critical volume, was about T ^ of the volume at the same temperature 

 and 1 atmosphere. 



Above the critical isothermal, the only survival of the vaporous con- 

 dition is found in the fact that on the right-hand part of each curve, 

 pv decreases as p increases to a certain point. After this it increases 

 as p increases, and this may be regarded as a survival of the liquid 

 condition. The point of minimum value of pv may be considered to 

 represent what in the lower isothermals is a change of state. 



Regnault (whose work has been verified by Amagat) found that with 

 pressures up to 20 atmospheres, pa for air and nitrogen decreased as p 

 increased, while it increased for 

 hydrogen. (See Properties of 

 Matter, p. 124.) 



Andrews found that it was 

 quite possible to take a quantity 

 of carbon dioxide gas round from 

 the undoubtedly gaseous to the 

 undoubtedly liquid condition 

 without any condensation. The 

 mode in which this was effected 

 may be understood from Fig 114. 



Starting with a quantity 

 of gas, say at 13, represented 

 by A, let it be heated at con- 

 stant volume till it reaches, 

 say, the 40 isothermal at B. 

 Let it then be compressed at 

 40 until the volume is less 

 than that occupied by the 

 liquid when just entirely con- 

 densed at 13, this being re- 

 presented by 0. Let it then 

 be cooled at constant volume to 

 D on the 13 isothermal. It is 

 certainly gaseous at A, certainly 

 liquid at D, for, on increasing 

 the pressure at the former point, condensation occurs, and on removing 

 the pressure at the latter point, ebullition occurs. But throughout the 

 passage from A to D the substance remains homogeneous. 



Andrews made the suggestion that the term gas should be restricted 

 to the condition of a substance above the critical point, that the sub- 

 stance below that point and to the right of the border curve should be 

 termed vapour, while to the left of the border curve it is liquid. 



Determinations of the critical constants of temperature and pressure 

 and, in some cases, volume have been made for a large number of sub- 

 stances by Andrews and others experimenters. Some have used 

 Andrews' method more or less modified, but various other methods have 

 been employed. For example, Cailletet and Collardeau (Ann. de Chimie 

 et de Physique, 6, xxv. p. 519), experimenting on water, heated the sub- 

 stance in a steel tube, and observed the point at which the vapour 



FIG. 114. A, B, C, D, path from gas to liquid 

 without discontinuous change of state. 



