4,52 Drs. Ramsay and Young on 



be applied. It is not to be supposed that at any given volume 

 only one of these factors is operative ; the actual size of the 

 molecules exerts its influence even at large volumes, and the 

 cohesion does not disappear, but no doubt immensely increases, 

 as the volume is reduced, even when that reduction requires 

 rise of pressure Still, a mental picture of the process may, 

 we think, best be attained by directing attention to cohesion, 

 when volume is being decreased with fall of pressure, and to 

 the influence of the actual size of the molecules when volume 

 is small. 



When a liquid is converted into gas, heat is absorbed, or 

 work is done on the liquid. We have previously (loc. cit.) 

 given tables showing the heats of vaporization of ether at 

 various temperatures. Our experiments have confirmed the 

 prediction that the heat of vaporization of stable liquids 

 decreases with rise of temperature, and in all probability 

 becomes zero at the critical temperature. Now the volume of 

 a fluid may be changed, either keeping the pressure constant 

 or allowing it to vary during the operation ; but if the initial 

 pressure and final pressure are the same, the variation of 

 pressure during the operation does not affect the total work 

 done. A liquid may be changed into saturated vapour at any 

 given temperature in the usual manner, when the intermediate 

 states are represented by non-homogeneous mixtures of liquid 

 and saturated vapour. The area enclosed between the vapour- 

 pressure line and lines drawn vertically from its terminal points, 

 cut by the line of zero pressure (or pressure x change of 

 volume), represents graphically the external work performed 

 in evaporating a liquid. If, however, the change of condition 

 be not abrupt, but continuous, the area enclosed by the iso- 

 thermal below the vapour-pressure line must be equal to that 

 above the vapour-pressure line (see Plate VII.). If this were 

 not the case, the amount of work required to effect the con- 

 tinuous change would differ from that required for the abrupt 

 change of state. 



Now it is evident that a slight alteration in the position of 

 the vapour-pressure line would have great influence on the 

 relative areas enclosed by the isothermal above and below the 

 vapour-pressure line ; and it may also be seen that, when these 

 areas are rendered equal by a horizontal line, the position of 

 that horizontal line must represent the true vapour-pressure. 



We have determined the position of the horizontal line in 

 the following manner : — 



Knowing approximately the position of the vapour-pressure 

 line at a given temperature, three pressures were chosen — the 

 highest above and the lowest below the experimentally deter- 



