64 POPULAR LECTURES AND ADDRESSES. 



APPENDIX C. 



ON THE EQUILIBRIUM OF VAPOUR AT A 

 CURVED SURFACE OF LIQUID. 



\_A paper read before the Royal Society of Edinburgh and 

 published in Proc. R. S. E. February jth, 1870 (vol. vii. 

 pp. 63-68).] 



IN a closed vessel containing only a liquid and 

 its vapour, all at one temperature, the liquid rests, 

 with its free surface raised or depressed in capil- 

 lary tubes and in the neighbourhood of the solid 

 boundary, in permanent equilibrium according to 

 the same law of relation between curvature and 

 pressure as in vessels open to the air. The per- 

 manence of this equilibrium implies physical 

 equilibrium between the liquid and the vapour in 

 contact with it at all parts of its surface. But the 

 pressure of the vapour at different levels differs 

 according to hydrostatic law. Hence the pressure 

 of saturated vapour in contact with a liquid differs 

 according to the curvature of the bounding surface, 

 being less when the liquid is concave, and greater 

 when it is convex. And detached portions of the 

 liquid in separate vessels all enclosed in one con- 

 taining vessel, cannot remain permanently with 

 their free surfaces in any other relative positions 



