314 



HEAT. 



right of B', while BN cuts the lower isothermal in the mixture part 

 to the left of B". 



Usually for saturated vapours the specific heat is negative, but for 

 ether vapour (Clausius's Mechanical Theory of Heat, p. 136) 

 it is positive at ordinary temperatures. Hence an adiabatic 

 expansion of ether vapour does not produce condensation. 

 Alteration of Vapour-Pressure with Curvature of 



Liquid Surface. Lord Kelvin (Proc. R.S.E., February 7, 

 1870) pointed out that the vapour pressure of a liquid with 

 a curved surface must be different from that of the same 

 liquid with a plane surface. Suppose that a capillary tube 

 (Fig. 178) dips in a liquid contained in a closed vessel, and 

 that no gas other than the vapour of the liquid is present 

 above the free surface. Further, suppose that the liquid 

 rises in the tube, and is in equilibrium at the level A. 

 Then there must be equilibrium between liquid and vapour 

 at A, as well as at the plane surface B ; that is, the concave 

 curved surface is in equilibrium with the vapour at a 

 pressure less than that at the plane surface B by the 

 weight of unit column of vapour of height AB. For if not, 

 suppose the vapour rising from A can have a pressure 

 greater than the pressure of the vapour at the same level 

 FIG. 178. outside the tube. The space above A will not be saturated, 

 and circulation will take place through continuous evapora- 

 tion from A and condensation at B, and work can be obtained. We may 



make the idea more definite by imagining 



a small engine above the capillary surface, 



as in Fig. 179, the whole being maintained 



at one constant temperature. If the pressure 



of the vapour from A exceeds the pressure 



of the surrounding vapour, let evaporation 



take place from A at the maximum pressure, 



and let the vapour go through the valve V 



and push against the piston P with this 



pressure. When some quantity of vapour 



has passed through V, let V be closed and 



let P move forward with isothermal expan- 

 sion of the vapour between it and V till 



there is the same pressure on both sides of 



the piston. The excess of vapour pushed in 



front of P will produce condensation at B. 



Then let a valve be opened in P, and let 



P move back to V. It is now ready for 



another stroke. This process can be carried 



on endlessly, and the piston does work while 



the substance is carried through a cycle which 



can be made perfectly reversible by merely 



reversing the action of the valves. But in 



such a cycle no balance of work can remain over, so that our supposition 



of difference of pressure between the vapour from A and its surroundings 



must be false. 



FIG. 179. 



