and Electronic Potential Ei 



W- 



257 



more kinetic energy at any point than just suffices to carry 

 the com^t to infinity, the parabola is the orbit of infinite 

 range with zero velocity at infinity, the ellipse the orbit of: 

 finite range because nowhere is the kinetic energy equal to 

 that acquired by falling from rest at infinity. By this con- 

 sideration of orbits we can give a dynamical definition of the 

 states of vapour and liquid. In a vapour the relative orbit 

 of two neighbour molecules is an arc of a curve of infinite 

 range open at infinity, in a liquid the relative orbit is one of 

 finite range. At the critical point the orbit is a transitional 

 form, like the parabola, between these two, being of infinite 

 range but closed at infinity. Liquefaction is the gathering 

 together of neighbours of relative orbits of finite range, 

 evaporation is the segregation of neighbours whose relative 

 orbit is of infinite range. When a liquid is in contact with 

 its vapour we have just seen from statical considerations that 

 the surface layer of molecules in the liquid is in tension, as 

 if at less pressure, while the surface layer of molecules of 

 vapour is in compression, as if at higher pressure than 

 prevails away from the surface. Let us state the facts with 

 the aid of the James Thomson ABCD curve, which replaces 



Fix. 2. 



the straight line AD of condensation at constant saturation 

 pressure. The state of the surface layer of the liquid is 

 expressed by a point between A and B, let us say at B. The 

 state of the surface layer of vapour is represented by a point 

 between D and C, let us say at C. The states represented 

 by points between B and C are unstable. The two surface 

 layers of molecules consist of subsaturated liquid and super- 

 saturated vapour. The two layers might be replaced by a 

 single homogeneous medium occupying the same space in a 

 state represented by the point E, the instability being inter- 

 preted as a continual change of state, both condensation and 

 evaporation, occurring at the transition from liquid to vapour. 



