Young — Vapour-Pressures, Sfc, of Thirty Pure Substances. 407 



The specific volume of saturated vapour is Vv = Rf'i- 

 2. Sealed-Tube Method. — It has ah-eady been shown (p. 403) that when two 

 tubes are employed the volume of a gram of liquid is given by the equation 



Vv'Vl - VvV{ 



mi\ - m i\ 

 The ratio of the volume of a gram of saturated vapour to that of liquid may 

 be calculated from the equation 



v^m - ir„'m 



vim - Villi 

 and, as before, Ts = RVj. 



In the case of substances which do not attack mercury, the volume of a 

 gram of liquid may be determined quite satisfactorily at all temperatures by 

 means of the pressui'e-apparatus ; and tlie values so obtained may be used in 

 determining the volumes of a gram of saturated vapour by the sealed-tube 

 method. One tube only is then required for the determinations. 



As before (p. 403). R = —!l— 



Villi - Vi 



where Vi is now known, and F» = RVi. 



The Critical Constants. 

 Critical Temperature. 



The critical temperature may be defined in various ways. When the 

 unsaturated vapour of a pure substance is compressed at a constant tempe- 

 rature far below the critical temperature, tlie volume diminishes and the 

 pressure rises until the condensing point is readied ; the vapour is tlieu said 

 to be saturated. When the volume is further diminished, liquefaction gra- 

 dually takes place, and the pressure remains perfectly constant so long as 

 both the liquid and the vapour phases are present. When liquefaction is 

 complete, the pressure rises again ; but the volume diminishes very slightly. 



At a higher temperature, but still below the critical temperature, the 

 changes observed are similar in kind ; but the volume of saturated vapour is 

 smaller and that of the liquid greater, and the liquid is more compressible. 

 As the temperature is raised the volume of saturated vapour continues to 

 diminish, and that of the liquid to increase, until a temperature is reached at 

 which they are equal. This is the critical temperature. Wlien a gas is 

 compressed at its critical temperature, the liquid phase never appears ; but 

 within certain limits of volume a peculiar mist or opalescence is visible ; and 

 the fluid under these conditions is far more compressible than an ordinary 



3k2 



