234 Mr. T. Preston on the Continuity of Isothermal 



remain in situ, except in so far as it might drift with currents 

 in the mass. The formation of bubbles, under these con- 

 ditions, would cause the mass to swell into a spongy condition 

 — a heterogeneous mixture of liquid and vapour, — in which, 

 if the equilibrium could be maintained, the volume and pres- 

 sure would vary according to laws very different from the 

 simple law of constant pressure which governs the transforma- 

 tion of ordinary boiling under the action of gravity (fig. 1). 



In order to determine, under these conditions, how the 

 pressure varies with the volume, at constant temperature, let 

 us consider the case of a mass of liquid in which a spherical 

 bubble of the vapour of the liquid has been formed, as shown 

 in fig. 3. For the sake of clearness, let the mass be enclosed 



Fig. 3. 



-Bubble 



surrounded by liquid. 



■ 





WMMMMMMtMM 



==== 



SHIP 



==^=^^=^== 



=irr= 



= == 



= - = 



— -= 



— — 



•— ~~~ 



























/ 





zr_rz=L 



=4 













j. 



























in a cylinder by means of a piston, so that the volume and 

 external pressure can be varied at pleasure, then, if p be the 

 pressure, applied through the piston (which we may term the 

 external pressure of the mass, in the ordinary sense), the 

 pressure at any point in the interior of the liquid will be 

 p + c, where c is a quantity depending on the surface film, and, 

 as it arises from the mutual attraction of molecules well 

 within each other's sphere of action, may be very large. But, 

 if ot be the vapour-pressure within the bubble, the relation 

 connecting p and ot is 



2T 





(i) 



where r is the radius of the bubble, and T the surface-tension 



