Transformation from the Liquid to the Gaseous State. 237 



into spherical drops, so that a stage is ultimately reached in 

 which the mass consists of a system of spherical drops sur- 

 rounded by their own vapour (fig. 4). 



The state of affairs is now reversed, for instead of having 

 vapour in contact with a concave liquid surface, and therefore 



Fig. 4. — Liquid drops surrounded by vapour. 



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at a pressure less than w , the normal saturated vapour-pressure, 

 we have saturated vapour in contact with convex liquid sur- 

 faces, and therefore at a pressure ot, greater than w . 



Hence, in this limit, we may take the pressure on the 

 enclosing piston to be that of the saturated vapour, namely -or, 

 the mass will be subject to an external pressure greater than 

 -bjo, namely p = vr, and this brings us into the region ON 

 (fig, 2) of the isothermal which lies above the normal pressure 

 line BD. In this it is assumed that the mass is largely in 

 the condition of saturated vapour, and that the liquid exists as 

 a system of equal spherical droplets, swimming in their own 

 vapour. 



If the drops were of different radii equilibrium would be 

 impossible, as evaporation would take place at the surfaces of 

 the smaller drops, and condensation at the surfaces of the 

 larger. This instability is made evident by the equation 



■ = ®o + 



2T 



P2 



r P2—P1 



which shows how the vapour-pressure increases as the radii 

 of the liquid drops diminish, and when the drops are small, -or 

 may exceed ix by a considerable quantity. 



There is a limit, however, beyond which, if the radii of 



